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1 Ear

1.1 Applied Anatomy and Physiology

1.1.1 Embryology

Inner ear: The sensory organs for hearing and balance develop from ectoderm. The membranous labyrinth develops from the ectodermal otic placode. Embryonic mesenchymal tissue surrounding the membranous labyrinth is converted into cartilage and also, by a process of vacuolization, into a fine reticular network that forms the inner layer of the perilymphatic space. The outer layer of the cartilage forms the labyrinthine capsule.

Middle ear: The eustachian tube and the mucosa of the middle ear arise from a diverticulum of the first pharyngeal pouch (endoderm).

The malleus and incus develop from Meckel cartilage, which emerges from the first branchial arch and is supplied by the trigeminal nerve. The stapes develops from the second branchial arch and is supplied by the facial nerve.

Myxomatous embryonic connective tissue lies between the ectodermal and endodermal ingrowths and makes a preformed middle ear cavity. If this myxomatous tissue does not involute properly after birth, the epitympanic recess remains as a narrow cleft. This is easily occluded by inflammation and creates a predisposition for chronic ear disease to develop.

External ear: The external meatus and the tympanic membrane develop from an ectodermal diverticulum between the first and second branchial arches. Developmental disorders may therefore cause deformities of both the external and middle ears. Bilateral lesions causing severe conductive deafness or a psychologically unacceptable deformity should be corrected, for both esthetic and functional reasons (see ▶ p. 53 and ▶ p. 106) ( ▶ Fig. 1.1 and ▶ Fig. 1.2).

1.1.2 Basic Anatomy

The hearing and balance systems consist of the peripheral receptor apparatus (i.e., the ear in the strict sense), neurological pathways, and centers in the central nervous system. Two main subdivisions can therefore be distinguished:

Peripheral part:

• 
  

  The external, middle, and inner ear.

  
  
• 
  

  Vestibulocochlear nerve with its two parts, the cochlear and the vestibular divisions.

Central part:

• 
  

  Central auditory pathways.

  
  
• 
  

  Subcortical and cortical auditory centers.

  
  
• 
  

  Central balance mechanism.

The anatomic boundary between the peripheral and central parts is the point of entry of the eighth cranial nerve into the brainstem (the cerebellopontine angle [CPA]), at which point the peripheral part of the vestibulocochlear nerve passes into the central part, interspersed with glial cells. In functional terms, however, the peripheral neurons end in the primary centers.

1.1.2.1 External Ear

The auricle consists of a framework of elastic cartilage covered by skin ( ▶ Fig. 1.3), located between the temporomandibular joint anteriorly and the mastoid process posteriorly. The skin adheres tightly to the perichondrium on the anterior surface, but is more loosely attached posteriorly. For this reason, contusions of the anterior surface often lead to detachment of the skin-perichondrial layer and to the formation of a hematoma (see ▶ p. 60).

The external meatus is ≈3 cm long, consisting of an outer cartilaginous part and an inner bony part. The cartilaginous meatus is curved and lies at an angle to the bony part. The tympanic membrane and the middle ear lying beyond it are thus protected from direct trauma.

The cartilaginous part is attached firmly to the rim of the bony meatus by connective tissue. The bony canal is covered by a thin layer of skin that adheres to the periosteum. It contains no accessory structures, in contrast to the cartilaginous part of the meatus, which has numerous hair follicles and ceruminous glands that form wax (epidermis scale, sebaceous matter, pigment) (see ▶ p. 61).

The external meatus narrows medially. Foreign bodies may therefore become impacted at the junction of the cartilaginous and bony meatus. The meatal cartilage does not form a closed tube, but rather a channel closed superiorly by fibrous tissue. The cartilage contains several fissures (Santorini fissures), which provide a pathway for the spread of severe bacterial infection to the parotid space, the infratemporal fossa, and the base of the skull.

The auricle and the cartilaginous meatus have very rich lymphatic drainage to an extensive regional lymphatic network consisting of parotid, retroauricular, infra-auricular, and superior deep cervical nodes. Infections of the external meatus with regional lymphadenitis can thus cause extensive swelling in these areas.

The sensory innervation is supplied by the trigeminal, great auricular, and vagus nerves and the sensory fibers of the facial nerve. Irritation of the posterior meatal wall stimulates the vagus and induces the cough reflex. Hypoesthesia of the posterosuperior meatal wall occurs with facial nerve impingement from a vestibular schwannoma (see the discussion of Hitselberger sign, ▶ p. 14 and ▶ p. 103).

Relations ( ▶ Fig. 1.4): The cartilaginous meatus abuts anteriorly on the parotid gland, allowing the spread of infection or malignant tumors.

The posterosuperior wall of the bony meatus forms part of the lateral attic wall (the partition between the external auditory meatus and the attic), the mastoid antrum, and the adjacent pneumatic system of the mastoid process. A middle ear infection can thus break through into the external auditory meatus, causing swelling of the posterosuperior wall or a fistula in acute mastoiditis. Destruction of the lateral attic wall by cholesteatoma may also lead to an open communication between the external auditory meatus and the attic or mastoid antrum. The anterior wall of the bony meatus forms part of the temporomandibular joint. There is therefore a risk of fracture resulting from a blow to the chin.

1.1.2.2 Middle Ear and Pneumatic System

The middle ear cavity consists of an extensive pneumatic system aerated by the eustachian tube. It has the following components:

• 
  

  Eustachian tube.

  
  
• 
  

  Tympanic cavity.

  
  
• 
  

  Mastoid antrum.

The eustachian tube consists of a mobile, cartilaginous portion (two-thirds) suspended from the skull base, and a bony portion (one-third). The bony portion, together with the tensor tympani muscle, forms the musculotubal canal in the temporal bone.

This canal lies adjacent to the internal carotid artery. The funnel-shaped pharyngeal ostium of the cartilaginous part (the torus tubarius) lies in the nasopharynx. The bony end opens into the middle ear.

The junction between the two parts of the tube is very narrow. This isthmus is the site of predilection for inflammatory stenosis of the tube. The tube serves to equalize the pressure between the middle ear and the nasopharynx, and thus to equalize the pressure on each side of the tympanic membrane (see ▶ p. 7 and ▶ p. 37). An increase in pressure in the tympanic cavity is usually compensated for passively via the eustachian tube to the nasopharynx, whereas a decrease in pressure usually requires active ventilation from the nasopharynx along the tube to the middle ear cavity. The tube opens and closes in response to movements of the neighboring muscles and by differences of air pressure between the nasopharynx and the middle ear cavity that tend to equalize spontaneously. The principal closing mechanism is elastic recoil of the cartilage of the tube and the valvular action of the pharyngeal ostium of the tube. The tube opens by contraction of the tensor palati and levator palati muscles. The mechanism is partially under the control of voluntary muscle, but the reflex movements on yawning and swallowing and the muscle tone are under autonomic control. Tension opposing the opening muscles is provided by the elastic recoil of the tubal cartilage and the pressure of the peritubal tissues—i.e., the pterygoid muscles, Ostmann fatty bodies, the venous and lymphatic plexus of the tubal mucosa, and the pterygoid venous plexus.

The middle ear cavity is an air-containing space lying between the external ear and the inner ear. It is divided into three parts ( ▶ Fig. 1.5):

1. 
   

   Epitympanic recess or attic.

   
   
2. 
   

   Mesotympanum.

   
   
3. 
   

   Hypotympanic recess.

There are two narrow zones within the middle ear cleft. First, there is an anatomic constriction between the epitympanum and mesotympanum that can lead to retention of secretions in inflammation and to deficient aeration of the attic. This is due to the considerable narrowing of this area caused by the head of the malleus, the body of the incus, numerous ligaments, nerves (the chorda tympani), and mucosal folds and pockets. This is one of the causes of chronic inflammation of the epitympanum (chronic epitympanitis), which is one of the causative factors for epitympanic cholesteatoma (see ▶ p. 76). A second narrow zone lies at the junction of the attic and the mastoid antrum (the aditus ad antrum). This may be blocked by granulation tissue in chronic inflammation, leading to deficient aeration or drainage of the mastoid cell system.

The hypotympanum is closely related to the bulb of the internal jugular vein.

Tympanic membrane: The lateral wall of the middle ear cavity is formed by the tympanic membrane. The tympanic membrane consists of the pars tensa and the pars flaccida. The pars tensa forms the stiff vibrating surface of the membrane and is attached to a fibrous ring (the annulus fibrosus), lying in the tympanic sulcus of the tympanic part of the temporal bone. The pars flaccida is the superior part of the membrane in the area of the tympanic notch (Rivinus notch) where the annulus fibrosus ends ( ▶ Fig. 1.6).

The microscopic cross-sectional appearance of the tympanic membrane is shown in ▶ Fig. 1.7. The epithelial or cuticular layer (the stratum corneum) is similar in structure to the skin of the external auditory meatus. Close to the tympanic annulus is the marginal zone of the tympanic membrane. This section shows extremely active proliferation due to papillary ingrowths into the stratum germinativum. This is another important factor in the genesis of cholesteatoma (see ▶ pp. 76–84).

The keratinizing squamous epithelium regenerates through migration of the epidermis from the center of the tympanic membrane to the periphery—in contrast to superficial desquamation, as occurs in normal skin. Migration of the outer epidermal layer forms an important part of the self-cleansing mechanism of the external meatus; this can be observed clinically in the movement of a blood clot from the tympanic membrane to the external meatus.

The lamina propria has an external radial layer of fibers and an internal circular layer: this is evident during myringotomy. The annulus fibrosus forms a thickening of the edge of the tympanic membrane and is formed by both layers of fiber. A lamina propria can also be seen in the pars flaccida, but it lacks the characteristic radial and circular structure described above, which provides the normal pars tensa with the necessary functional tension.

The middle ear, or tympanic cavity, is empty except for air. Only the epitympanic recess contains solid structures—the ossicular chain and the chorda tympani. The ossicular chain consists of three separate bones connecting the lateral and medial wall of the middle ear. The medial wall of the middle ear also forms the lateral wall of the labyrinthine capsule. The malleus is the most lateral of the ossicles. Its inferior portion, or handle, is incorporated into the eardrum, while the superior portion, or head, is located in the anterior portion of the attic. The incus is connected to the head of the malleus by a genuine articulation surrounded by a joint capsule. The long process of the incus ends in the lenticular process, which bends medially to articulate with the head of the stapes. The lenticular process is covered by cartilage to form the incudostapedial joint ( ▶ Fig. 1.8 and ▶ Fig. 1.9).

The mucosa that lines the middle ear space consists of stratified cuboidal epithelium, which changes to pseudostratified ciliated epithelium around the mouth of the eustachian tube. A few goblet cells and submucosal glands are normally present. The submucosa is very thin, so that the mucosa lies directly on the periosteum, forming a tightly bound unit called the mucoperiosteum. In pathologic conditions such as tubal occlusion or chronic otitis media, the structure of the mucosa changes considerably to show hyperplasia of the glands, proliferation of the goblet cells, edema of the submucosa, vascular buds, and transformation of the flattened cuboidal epithelium to columnar epithelium.

The middle ear mucosa forms several pouches and folds (Prussak space, Tröltsch pouch), which narrow the junction between the attic and the rest of the middle ear and between the attic and the antrum. The epitympanic recess may remain as a narrow cleft with development, and if chronic hyperplastic inflammation follows an infection, the “mesenchyme” can completely obliterate the epitympanum. Ventilation and drainage of the attic is then impeded by thickened masses of inflammatory tissue, despite normal tubal function. Deficient aeration and drainage of this small space favors the development of chronic epitympanitis and plays a considerable role in the pathogenesis of chronic otitis media (see ▶ pp. 75–76), especially attic cholesteatoma.

The arterial blood supply originates from the basilar artery (the labyrinthine artery), the maxillary artery (the middle meningeal and tympanic arteries), and the stylomastoid artery. Venous drainage is via the middle meningeal veins, the venous plexus of the internal carotid artery and pharynx, and venous connections into the bulb of the internal jugular vein.

The nerve supply of the mucosa is provided from two sources: the tympanic branch of the glossopharyngeal nerve (cranial nerve IX) and the auriculotemporal branch of the trigeminal nerve (cranial nerve V).

Pneumatic System of the Temporal Bones

The air-containing cells of the mastoid process are continuous with the air in the middle ear. These multiple interconnecting spaces arise from the mastoid antrum, and the extent to which they are pneumatized is extremely variable. On the one hand, pneumatization may be well developed, extending to the temporal and occipital bones and the origin of the zygomatic arch. Acute infections of the mastoid may cause inflammatory swellings in these regions. At the other extreme, in a poorly pneumatized mastoid, the mastoid process may consist exclusively of compact bone, with the pneumatized cells lying in the immediate vicinity of the antrum.

The mastoid process begins to develop after birth as a small tuberosity, which is pneumatized synchronously with the growth of the mastoid antrum. In the first year of life it consists of cancellous bone, so that true mastoiditis cannot occur. Between the second and fifth years of life, as pneumatization proceeds, it consists of mixed cancellous and pneumatic bone. Pneumatization is complete between the sixth and twelfth years of life ( ▶ Fig. 1.10 and ▶ Fig. 1.11).

Principle of pneumatization (the concept of biological mucosal competence): Bone is destroyed by an enzymatic lacunar osteoclastic process. The resulting bony spaces are lined by continuous ingrowth of mucoperiosteum from the antrum. A system of hollow cavities results, consisting of numerous spaces lined by mucosa and communicating with each other.

Normal tubal function is a prerequisite for biologically active, healthy middle ear mucosa, and thus for the normal process of pneumatization. The process of pneumatization can be related to the biological competence of the middle ear mucosa. The mucosa may be described as biologically normal or as inferior, depending on the degree of pneumatization. Good pneumatization indicates biologically competent middle ear mucosa, whereas restricted pneumatization indicates biological incompetence of the middle ear mucosa. Biologically incompetent middle ear mucosa may be due to two possible mechanisms—(1) a defective enzyme system that is impairing normal pneumatization, and/or (2) a deficient local immune system in the respiratory mucosa and middle ear mucoperiosteum that predisposes to chronic or recurrent otitis media.

The better pneumatized the temporal bone is, the easier it is for infection to break through the thin cortical bone. When there is poor pneumatization (known as a dangerous mastoid process), the inflammatory process may be concealed in the depths and lead to unexpected complications.

1.1.2.3 Inner Ear

The inner ear, or labyrinth, is embedded in the temporal bone and is divided into two functionally separate receptor mechanisms:

1. 
   

   The vestibule and semicircular canals (the vestibular end organ).

   
   
2. 
   

   The cochlea (the acoustic end organ).

The labyrinth can also be divided morphologically into bony and membranous parts.

Bony labyrinth: This is formed by the labyrinthine capsule, which develops by periosteal and enchondral ossification. In systemic bone diseases (e.g., Paget disease and osteodystrophy) and in localized bone disease (e.g., otosclerosis), the bony labyrinth shows characteristic histopathological and chemical abnormalities. These conditions demonstrate continuous bone remodeling.

The oval and round windows form the bony and membranous openings to the labyrinth from the middle ear cavity, and are closed by the stapes footplate and round window membrane, respectively (see ▶ p. 4).

Membranous labyrinth and inner ear fluids ( ▶ Fig. 1.12): The membranous labyrinth develops from the ectodermal otic placode. It encloses a hollow system filled with endolymph. This passes via the endolymphatic duct to end in a blind sac, the endolymphatic sac, in the posterior cranial fossa. The sac lies in the epidural space on the posterior surface of the petrous pyramid, close to the sigmoid sinus.

The perilymphatic system forms a hollow space consisting of the scala tympani and the scala vestibuli. The system communicates directly with the subarachnoid space in the jugular foramen via the cochlear aqueduct. Perilymph separates the membranous labyrinth from the internal layer of the labyrinthine capsule. Perilymph is the immediate substrate of the cochlear and vestibular sensory cells. The origin of perilymph is a matter of controversy; it may form from filtration of perilymphatic capillary blood and/or through diffusion of cerebrospinal fluid.

Endolymph is a filtrate of perilymph that has completely different concentrations of sodium and potassium, which are kept constant by the epithelium of the stria vascularis (see ▶ p. 12). The electrolyte composition of the endolymph regulates the volume of the fluid circulating in the endolymphatic system. The basis of the electrolyte exchange system, which maintains a constant ion concentration, is the cellular potassium-sodium exchange pump found in the stria vascularis, the utricle, and the saccule. There is also passive diffusion between the endolymphatic and perilymphatic spaces, with potassium-sodium ion exchange in the endolymphatic sac. Functional disturbances of this electrolyte regulation system lead to a disorder of the middle ear known as Ménière disease (see ▶ p. 109).

Vestibular-Semicircular Canal System

The anatomical fine structure of the balance mechanism system is shown in ▶ Fig. 1.13a, ▶ Fig. 1.14, and ▶ Fig. 1.15. It consists of the utricle and saccule enclosing the static maculae with the sensory end organs for the reception of linear acceleratory stimulation. These consist of supporting cells and hair cells, which have cilia embedded in a gelatinous mass consisting of sulfomucopolysaccharides. On their surface lie the otoliths (or statoconia), which consist of rhomboid calcium carbonate crystals ( ▶ Fig. 1.13b). Linear acceleration changes the otolith pressure, deflecting the sensory hairs. This stimulates the sensory cell by altering the resting potential.

The three semicircular canals arise from the utricle and have a pear-shaped expansion at one end called the pars ampullaris, enclosing the sensory cells, which are stimulated by angular acceleration ( ▶ Fig. 1.16). The sense organs consist of an ampullary crest (crista ampullaris), on which sensory hair cells are arranged in such a way that their cilia extend to the cupula, which reaches to the roof of the ampulla. The cupula acts as a mobile partition that closes off the pars ampullaris and is relatively impervious to endolymph ( ▶ Fig. 1.15).

Cochlea (Acoustic End Organ)

The macroscopic and microscopic structure of the bony and membranous cochlea are shown in ▶ Fig. 1.17 and ▶ Fig. 1.18.

Functional structure of the organ of Corti: The basilar membrane supports the sensory apparatus of the organ of Corti. It stretches between the bony spiral lamina and the lateral cochlear wall and forms the border to the scala tympani. Surrounded by supporting cells, there are two types of receptor cells: one row of inner and three rows of outer hair cells, totaling ≈16,000 sensory cells. The hair cells have fine cilia on their free surfaces, with approximately 80 cilia per cell. So-called tip links, ≈10 μm thick, extend from the tips of the small cilia to the longer, very fine protein strings. There are ion channels where the tip links connect to the cilia, providing the basis for transduction of the sound stimulus to a receptor potential. Lying on top of the organ of Corti is the gelatinous tectorial membrane. The cilia of the outer hair cells lie below the tectorial membrane, while the cilia of the inner hair cells do not insert into the tectorial membrane. The hair cells are secondary sensory cells and have no nerve cell processes. They receive fibers from the spiral ganglion. Approximately 90% of the nerve fibers extend to the inner hair cells, and each inner hair cell is connected to many afferent fibers, each of which undividedly connects to an individual hair cell. The remaining 10% of the nerve fibers are widely dendritic and innervate the outer hair cells. There are ≈30,000 to 40,000 axons that lead from the spiral ganglion to form the vestibulocochlear nerve ( ▶ Fig. 1.19).

Central Connections of the Cochlea

The cochlear division of the eighth cranial nerve (pars cochlearis) is formed by the bipolar neurons of the spiral cochlear ganglion. It runs through the internal auditory meatus, unites with the vestibular division, crosses the CPA, and enters the brainstem at the lower border of the pons, at which point the central auditory pathway begins ( ▶ Fig. 1.20).

The central auditory radiation incorporates the strict tonotopic arrangement, as does the auditory cortex. The cochlea is thus represented unrolled, as it were, from the basal turn to the helicotrema. The auditory cortex is considerably larger than the area of Heschl transverse striations, since these represent only the primary auditory field (AI) in which the auditory radiation ends. The secondary acoustic field (AII) and the posterior ectosylvian gyrus, like the visual cortex, include secondary integration areas such as the Wernicke speech center. Numerous commissural systems allow fibers to be exchanged between the two halves of the brain. These are very important for directional hearing.

Central Connections of the Vestibular System

The bipolar neurons of the vestibular ganglion send out their peripheral processes as two divided neural bundles—a superior division to the sensory cells in the macula of the utricle, the lateral and superior semicircular canals; and an inferior division to the posterior semicircular canal and the macula of the saccule ( ▶ Fig. 1.21).

The central processes combine to form the vestibular division of the eighth cranial nerve, which unites in the internal auditory meatus with the cochlear division to form the vestibulocochlear nerve which has a common nerve sheath. The vestibular division sends ascending fibers to the vestibular centers after it has entered the medulla oblongata. The secondary vestibular pathway is connected to the spinal cord by the vestibulospinal tract. Its fibers end at the spinal intermediate neurons and activate the alpha and gamma motor neurons of the extensor muscles. They are therefore the antagonists of the pyramidal pathway and mainly produce flexor inhibition and activation of extensors. They form part of a phylogenetically old antigravity system that serves to maintain balance. In addition, there are important ascending pathways to the cerebellum, the reticular formation (a multisensory integration center), and the centers for the eye muscles (where the oculomotor muscles are coordinated), via the medial longitudinal bundle.

A vestibulocortical connection is provided via the thalamus. Vestibular stimulation is projected to a small area in the ventral postcentral somatosensory region, near the visual area. This region represents a primary vestibular cortical area.

1.1.2.4 Facial Nerve

The seventh cranial nerve carries motor fibers for the mimetic muscles of the face, and afferent sensory taste fibers and visceroefferent secretory neurons in a separate nerve bundle, the intermediate nerve. The nerve also contains the sensory fibers that supply the posterior wall of the external auditory meatus. This explains the reduced sensation of this area of skin in patients who have a vestibular schwannoma (Hitselberger sign) ( ▶ Fig. 1.22).

The motor fibers originate from the facial motor nucleus in the floor of the fourth ventricle, run round the abducens nucleus (the internal “genu”), and exit at the lower border of the pons, together with the visceroefferent fibers of the intermediate nerve arising from the superior salivatory nucleus. The gustatory fibers insert into the subcortical taste centers in the nucleus of the solitary tract. All of these branches form the nervus intermediofacialis, which runs first in the internal auditory meatus (the meatal segment). It enters the bony canal immediately adjacent to the labyrinth (the labyrinthine segment) and runs to the hiatus in the canal for the facial nerve. At this point, the greater superficial petrosal nerve divides off from the main trunk. This branch goes to the lacrimal gland and also supplies fibers to the glands of the nasal mucosa. The first “genu” of the facial nerve lies at the level of the geniculate ganglion. The nerve then turns into the horizontal tympanic segment before it passes at the level of the entrance to the mastoid antrum, the second “genu,” into the vertical mastoid segment. In this area, it branches to the stapedius muscle and the chorda tympani, which contains taste fibers for the anterior two-thirds of the tongue and carries visceroefferent fibers for the sublingual and submandibular glands. After leaving the mastoid process through the stylomastoid foramen, it divides into five extratemporal branches—the temporal, zygomatic, buccal, marginal mandibular, and cervical—to the platysma. These branches are highly variable (see ▶ pp. 127–131).

The facial nerve is surrounded by a tough fibrous sheath in its course through the temporal bone. Its individual fascicles are embedded in a well-developed epineurium of loose connective tissue that encloses the vessels and nerves. The fiber bundles are enclosed in a perineurium. When injuries to the nerve are being repaired, the epineurium has to be resected from the stump, and a perineural suture has to be used so that the site of anastomosis can be adapted precisely, to prevent the formation of a scar tissue neuroma due to connective-tissue infiltration of the anastomosis (see ▶ p. 131).

1.1.3 Physiology and Pathophysiology of Hearing and Inner

1.1.3.1 Physiology of Hearing: Middle and Inner Ear

The functions of the various parts of the ear are as follows:

• 
  

  The external and middle ear transport the stimulus.

  
  
• 
  

  The cochlea distributes the stimulus.

  
  
• 
  

  The function of the outer hair cells is mechanoelectric transduction.

  
  
• 
  

  The inner hair cells transform the stimulus.

Stimulus Transport

In the external auditory meatus, the resonance effect lowers the hearing threshold to between 2,000 and 3,000 Hz, the main range of speech frequencies.

The tympanic membrane is a sound pressure receptor and transformer.

The ossicular chain is responsible for impedance adaptation between the middle ear, in which the medium is air, and the inner ear in a fluid medium, as well as pressure transformation. The pressure enhancement is 1:17, due to the ratio between the surface of the tympanic membrane and the stapes footplate. The ratio due to the mechanical advantage of the incudomalleolar joint is 1:1.3. The total pressure on the stapes footplate is therefore increased 22 times (see ▶ Fig. 1.9).

The physical movements of molecules that we perceive as sound set the tympanic membrane in motion. The frequency of the motion is the same as that of the vibrations of the air, and its amplitude is proportional. The transmission of sound waves from the air medium to the fluid medium in the perilymphatic and endolymphatic space requires a relative increase in power, due to the increase in density—i.e., impedance adaptation through sound pressure transformation (impedance = acoustic resistance).

For normal transmission of sound to the inner ear, the tympanic membrane has to be in a normal position and have normal mobility, and the air pressure in the outer and middle ears has to be similar. Measuring the impedance at the tympanic membrane can provide information about the functioning of the sound transmission apparatus, and this method—known as impedance audiometry—is used for clinical investigations (see ▶ p. 37). Sound energy reaches the cochlea first via the sound transmission apparatus of the middle ear (air conduction) and second through the bone of the skull, which is set in motion in a sound field. The sound energy is thus transmitted directly to the cochlea via the labyrinthine capsule (bone conduction).

Audiometry is used to measure the hearing threshold for both air and bone conduction (see ▶ pp. 27–33).

Stimulus Distribution

The main function of the cochlea is mechanical frequency analysis, which depends on its hydrodynamics. Periodic movements at the stapes are converted into aperiodic movements to produce a traveling wave on the basilar membrane ( ▶ Fig. 1.23). Since the inner ear fluids are not compressible, volume displacement at the stapes footplate leads to an equal volume displacement at the round window, and this produces a bulging of the round window membrane that is equal in extent to the depression of the stapes footplate. This volume displacement, produced by periodic vibrations of the stapes footplate, leads to displacement of the cochlear duct (scala media, Löwenberg scala; the space surrounded by the basilar membrane and Reissner membrane, between the scala vestibuli and scala tympani) (see ▶ Fig. 1.17a, b). This initial displacement forms a wave motion that proceeds along the partition to the helicotrema. This is an aperiodic vibration, or traveling wave. The wavelength becomes shorter as the wave approaches the helicotrema, but the amplitude becomes greater. The amplitude reaches a maximum at one specific point and then immediately begins to fall sharply, before dying away toward the helicotrema. The traveling wave causes a displacement between the tectorial membrane and the basilar membrane at its point of maximal amplitude, so that the cilia of the hair cells are displaced at this point, forming the sensory stimulus for these mechanoreceptors (see ▶ Fig. 1.18c and ▶ Fig. 1.24b).

The frequency-dependent development of the maximal amplitude on the traveling wave induces a corresponding frequency-dependent localized stimulus on the basilar membrane in the sensory cells of the organ of Corti that lie at the point of maximal amplitude. An initial analysis of the sound is thus achieved in accurately defined frequency stimulus patterns (Békésy dispersion or traveling wave theory).

The maximum displacement of the traveling wave lies at a different point for each frequency: it is nearer the helicotrema for the lower frequencies and nearer the stapes footplate for the higher. The tonotopic arrangement of the cochlea means that every frequency is thus represented at a particular point on the basilar membrane ( ▶ Fig. 1.24). Since the distribution of the maximal amplitude across the basilar membrane determines the point of excitation of the organ of Corti and thus the activity of the afferent nerve fibers in the cochlear nerve, the traveling wave hypothesis is also a “one-point” hypothesis, as suggested by Helmholtz. Each point on the basilar membrane therefore corresponds to a specific frequency.

Mechanoelectric Stimulus Transduction

The cilia of the outer hair cells are bent to the greatest extent when the wave motion approaches the maximum range. A force pushing on the tip links causes the ion channels to open and changes the receptor potential. The outer hair cells carry out active, oscillating extension and thus locally intensify the travelling wave ( ▶ Fig. 1.25a).

Stimulus Transformation

The actively intensified vibrations of the inner hair cells also cause the cilia of the inner hair cells to bend, which results in opening of the ion channels. An influx of Ca^{2 +} causes a basal discharge of glutamate as a transmitter, and the afferent nerve fibers of the vestibulocochlear nerve are consequently stimulated ( ▶ Fig. 1.25b).

Otoacoustic Emissions

Active contractions of outer hair cells have natural modes of vibration and are subject to distortion. In this phenomenon of normal hearing, sounds emitted by the cochlea occur at certain frequencies as spontaneous otoacoustic emissions (SOAEs). Evoked otoacoustic emissions (EOAEs) can be recorded in the external auditory canal after induction by external acoustic stimuli (see ▶ p. 40).

1.1.3.2 Physiology of Hearing: Retrocochlear Analysis of Acoustic Information

The electrical stimulus pattern of sensory cells in the organ of Corti is converted in the peripheral cochlear neuron into the action potential pattern of the vestibulocochlear nerve. The sound stimulus—which has many parameters, such as frequency, intensity, temporal pattern, and the periodicity of the action potentials—has to be encoded to allow the information to be analyzed in the central nervous system.

Sound frequency and sound intensity coding play a very important role in the central analysis of the acoustic signal.

• 
  

  Sound intensity coding occurs through frequency modulation. With increasing sound intensity, the number of spikes in the sensory cell discharge increases.

  
  
• 
  

  In sound frequency coding, specific sensory cell groups in the organ of Corti are stimulated depending on the sound frequency. Tonotopicity (see below) allows these locally circumscribed stimulus patterns, produced on the basilar membrane, to be conducted by the vestibulocochlear nerve to the higher centers without distortion.

Tonotopy is a point-to-point connection between the sound receptors and the neurons analyzing the signal. Each cochlear neuron has what is known as a best frequency—i.e., it responds only to an acoustic stimulus that has a frequency identical to the frequency assigned to it.

The acoustic system can process the duration, intensity, and frequency parameters of the acoustic signal in the following ways:

• 
  

  With increasing intensity and constant frequency, the action potential rate in the nerve fibers increases, and the number of stimulated afferent neurons also increases, corresponding to the extent of the deflected area of the basilar membrane.

  
  
• 
  

  At constant intensity and variable frequency, the deflected area of the basilar membrane is displaced into the appropriate segment of the organ of Corti within the cochlea, so that frequency is determined by point analysis. In addition, changes occur in the periodicity of the action potential series within the individual nerve fibers, which are analyzed by means of periodicity analysis. This provides another means of frequency determination.

Frequency analysis by means of local pattern scanning, intensity perception by frequency modulation, and time-periodicity analysis by combined evaluation of the time and place pattern also provide information that passes to the higher auditory centers as a result of tonotopicity ( ▶ Fig. 1.26).

1.1.3.3 Pathophysiology of Hearing Disorders

Conductive or middle ear hearing loss is caused by lesions of the stimulus transport organ. A characteristic symptom of this type of hearing loss is that bone conduction functions better than air conduction. The depression of the hearing threshold for air conduction is associated with an increase in acoustic impedance, as seen with stapes fixation due to otosclerosis.

Sensory hearing loss is caused by lesions in the stimulus transformation organ and/or in the vestibulocochlear nerves, and is therefore better known as sensorineural hearing loss. Noise-induced hearing loss and age-related hearing loss (presbyacusis) are caused mainly by mechanical overloading of the cochlear amplifier system of the outer and inner hair cells.

Disorders of sound perception are caused by lesions in the subcortical or cortical auditory centers and by pathologic processes involving the central auditory pathway. As a result, the acoustic signals are falsely coded, stimulus patterns are wrongly analyzed, and acoustic information can no longer be integrated. The patient can then hear but not understand.

Central hearing disorders are characterized by a loss of the integrative functions of the auditory centers. Differences in level of tone, differences in loudness, and temporal differences of acoustic stimulus pattern can no longer be analyzed. Redundancy is also reduced—i.e., the information content is reduced due to loss of secondary and tertiary cochlear neurons. These disorders affect the understanding of speech (whereas hearing of pure tones may be preserved), directional hearing, and speech intelligibility.

Recruitment: In certain forms of unilateral sensorineural deafness, the loudness perception rises quickly with increasing loudness intensity, so that despite different hearing thresholds both ears hear the tone at the same loudness once a certain threshold is reached. This phenomenon is called recruitment. The pathophysiologic basis for recruitment is loss of the cochlear amplifier mechanism, with abnormal sound processing dynamics. Positive recruitment can generally be regarded as a sign of a cochlear lesion, whereas absent recruitment indicates a retrocochlear lesion localized to the first or second neuron.

1.1.3.4 Physiology of the Balance System

Balance is maintained by coordination of visual kinesthetic and vestibular regulatory mechanisms. These serve for spatial orientation, upright posture, and gait. Control of all the static and motor muscle groups allows the body to counteract the influence of weight and centrifugal forces ( ▶ Fig. 1.27).

The main functions of the vestibular system are:

• 
  

  To send information to the central nervous system about the action of linear and angular acceleratory forces.

  
  
• 
  

  Coordination: Movement is coordinated by continuous control of the tone of the skeletal muscles. Information from the vestibular sensory receptors is coordinated and integrated with information from the visual system. Spatial orientation is also ensured.

The potential difference between the sensory cells and the extracellular fluid forms the physiologic basis for normal functioning of the vestibular sense organ. A constant discharge of action potentials passes along the vestibular nerve fibers, even when the end organs are at rest (resting activity). As in the cochlea, a transduction channel in the vestibular hair cells is opened by a force pushing on the tip links, allowing an influx of ions and causing the receptor potential to change. Depending on the direction of the ciliary deflection of the sensory hair cells, the resting activity is altered by an increase in the discharge frequency (depolarization) or by inhibition (hyperpolarization) ( ▶ Fig. 1.28). Modulation of resting activity thus allows the body to sense movement both in one direction and also in the opposite direction using a single receptor.

Function of the Otolith Organ: Linear Acceleration Measurement

Linear acceleration is the sensory stimulus for the horizontally orientated macula of the utricle and the vertical macula of the saccule. Shearing forces occur during linear acceleration that shift the otoliths from their base, causing shearing of the hair cells (see ▶ Fig. 1.12b) and providing an adequate stimulus for the sensory cells. The resulting neuronal impulses release the maculoocular reflex, producing compensatory eye movements that ensure optimal static positioning of the eyes during linear movement. The maculospinal reflex is also evoked, which influences the musculature of the trunk and limbs via the motor anterior horn cells in the spinal cord to ensure that the position of the body remains stable during linear movement. The otolith apparatus also has another important function: due to the continuous effect of gravity, the otoliths exert constant pressure on the underlying sensory cells, even at rest. This pressure influences the resting activity of these mechanoreceptors. Linear acceleration (e.g., a fall, rapid lowering of the head, air travel, or fast movement in an elevator) changes this resting activity, thus guaranteeing continuous spatial orientation during vertical movement.

Function of the Semicircular Canals: Angular Acceleration Measurement

Positive or negative angular acceleration causes endolymphatic movement within the semicircular canals lying in the plane of the centrifugal force. The stimulus always affects the semicircular canals on both sides; the cupula is displaced toward the utricle on one side (ampullopetal stimulation) and in the opposite direction on the other side (ampullofugal stimulation). As a result, resting activity increases in the semicircular canal in which the cupula is deflected in an ampullopetal direction (depolarization effect), whereas activity decreases in the contralateral canal (hyperpolarization effect). This rule applies only to the horizontal canals, since ampullofugal deflection causes depolarization in the vertical semicircular canals. This is the neurophysiologic basis for the stimulating mechanism of the vestibulo-ocular reflex (VOR).

The VOR also serves for spatial orientation. In addition, it assists in stabilizing the retinal image of the visual environment and induces vestibular nystagmus. Every movement of the head causes slow, conjugated movement of the eyes in the opposite direction, to stabilize the field of vision on the retina for as long as possible during the movement. Two modifiable parameters determine the progress of the VOR: the position of the head and the position of the eyes. The difference between these is the angle of vision (see ▶ Fig. 1.16).

Conjugated eye movements due to the VOR, with typical slow and fast components, are classified as vestibular nystagmus (see ▶ p. 43).

The intervertebral joints of the cervical spine and the deep muscles of the neck contain mechanoreceptors, which are connected to the reticular formation by afferent fibers and from there to the vestibular and oculomotor centers. The function of these receptors is to provide continuous information about the position and movement of the head and to allow coordination of eye movements through the cervicoocular pathway.

The central vestibular system includes the cerebellum and the reticular formation of the brainstem—i.e., it is integrated into the centers for multisensory data analysis. This allows multisensory control and coordination of posture, movement, and oculomotor functions.

1.1.3.5 Pathophysiology of Functional Disorders

Vestibular disorders become manifest through:

• 
  

  Vertigo: partial or complete loss of spatial orientation (e.g., apparent movement of the environment as a result of spontaneous vestibular nystagmus).

  
  
• 
  

  Disturbed balance, with an inability to maintain balance, stand upright, or walk properly (ataxia) ( ▶ Fig. 1.29).

  
  
  
  

  Fig. 1.29 Pathogenesis of disorders of orientation and balance. Disorders of proprioceptive information: loss of control over the ability to stand upright and walk straight causes a balance disorder. Disorders of visual information: loss of optical control of the visual field occasionally leads to dizziness due to a discrepancy between visual and vestibular information, causing disorientation. Disorders of vestibular information are due to involvement of the spatial orientation and stabilization of the gaze axis, leading to contradictory vestibular visual and kinesthetic information and dizziness. If central compensation of the loss of vestibular function is also absent, there is an additional balance disorder.

  
  

  

Vestibular disorders may be peripheral, caused by sudden unilateral failure of one labyrinth, or by a unilateral lesion of the vestibular nerve. They may also be central, caused by a lesion in the vestibular centers or their central connections to the cerebellum and reticular formation.

Every functional disturbance in a vestibular end organ causes unequal activity in the higher vestibular centers. This central imbalance initially produces a disturbance of vestibular information. The multisensory spatial orientation is therefore no longer capable of functioning, since vestibular information on the one hand and visual somatosensory information on the other hand contradict each other. This causes a disturbance of orientation, which in turn causes dizziness. If the central imbalance in the two vestibular centers influences the main neighboring coordination centers for eye movements in the reticular formation of the brainstem, spontaneous abnormal eye movements occur that have the characteristics of nystagmus ( ▶ Fig. 1.30).

Peripheral functional failure is compensated centrally by adjustment of the difference in neuronal activity in the vestibular centers and by substitution of visual and somatosensory regulatory mechanisms for the loss of peripheral vestibular function. This process is called central vestibular compensation. Central vestibular disorders are only incompletely compensated by the above mechanisms (or not at all), since the multisensory connections to the vestibular centers are damaged.

1.2 Methods of Investigation

1.2.1 Inspection, Palpation, Otoscopy

1.2.1.1 Investigation of the External Ear

The physician should look for redness, swelling, ulceration, tumors, malformations, fistula, or retroauricular scars.

Palpation

The mastoid process should be palpated with both hands to search for swelling and for sensitivity to pressure on the surface of the mastoid process and at its apex. The auricle is examined for pain when pressure is applied to the tragus or when the auricle is pulled. Finally, the regional lymph nodes in the preauricular and postauricular areas and the upper deep cervical chain are examined.

Otoscopy

The external auditory meatus and the tympanic membrane are examined, and if a perforation is present, the middle ear is also examined.

Indirect illumination with a head mirror is a difficult method of investigation for the nonspecialist, as correct adjustment of the light source and head mirror require time and practice, especially when patients are being examined in bed (see Fig. 2.16).

The electrical otoscope is more widely used, as it is easier to handle. It consists of a combination of an interchangeable ear speculum with a small built-in light source and a magnification attachment providing a magnification of 1.5 to 2 × ( ▶ Fig. 1.31a).

The otomicroscope provides a magnification of 6 to 12 × and is indispensable for accurate examination of the meatus, tympanic membrane, and parts of the middle ear in cases of perforation ( ▶ Fig. 1.32).

The otoendoscope provides a wide-angled and magnified view over the tympanic membrane, allowing complete investigation of the annulus and anterior tympanomeatal angle. Rigid scopes with 0-degree and 30-degree views are used ( ▶ Fig. 1.33). Digital recordings of findings can be managed by small cameras and LED sources in ear speculums ( ▶ Fig. 1.31b).

Technique of otoscopy: The cartilaginous part of the external meatus is stretched by pulling the auricle upward and backward. The speculum is then introduced into the long axis of the bony meatus. The instrument is held with one hand, so that the other hand remains free for handling instruments such as cotton-wool probes, hooks, an aspirator, and aural forceps ( ▶ Fig. 1.34). The speculum has to be introduced carefully, and the end of it should not be moved abruptly, as its opening has relatively sharp edges. The wall of the bony meatus is particularly sensitive and easy to injure, and contact with it should therefore be avoided.

In infants and young children, the auricle is pulled downward and backward to allow the speculum to be introduced. The short cartilaginous part of the external meatus is reduced to a cleft, which can only be entered with a narrow speculum with a small lumen, making otoscopy difficult. The head has to be immobilized, either by an assistant or using a headrest on the patient’s chair, to prevent unnecessary movements that can cause pain ( ▶ Fig. 1.35).

Wax and other material obstructing the view into the external auditory meatus has to be removed using the following methods:

• 
  

  By syringing for foreign bodies, wax, and exudate.

  
  
• 
  

  With the hook, ring curette, or Jobson Horne probe, for hard wax.

  
  
• 
  

  With microsuction under microscopic view, for exudate or fluid wax.

  
  
• 
  

  With a cotton-wool probe for exudate.

The ear is syringed with tap water at body temperature. Hard wax should be softened for at least several days prior to syringing, with softening drops such as 3% hydrogen peroxide, 5% sodium bicarbonate, soft soap, olive oil, or a commercial preparation. Syringing is now performed with automated syringing equipment that regulates the pressure of the water to protect the eardrum from perforation.

It is important to obtain a history of any previous perforation, as syringing may rupture a thin scar. In the United States, failure to take a history can result in a malpractice suit.

Mistakes to be avoided:

• 
  

  A speculum that is too narrow and that penetrates too deeply into the sensitive bony meatus.

  
  
• 
  

  Introducing the speculum in the wrong direction, e.g., from above downward.

  
  
• 
  

  Not introducing the speculum far enough, causing its opening to be blocked by otic hairs.

  
  
• 
  

  Unsatisfactory cleaning of the external meatus, so that a proper view of the tympanic membrane is not obtained.

Normal Otoscopic Appearance

Characteristics of the tympanic membrane: The pars tensa is grayish-yellow. The cutis layer is often slightly injected. The surface is smooth and without any relieving features, apart from the handle of the malleus. The membrane is moderately translucent and is only transparent in scarred areas. A tympanic membrane showing the properties described above is described as normal. The mobility of the tympanic membrane can be assessed using a pneumatic otoscope ( ▶ Fig. 1.36).

The tympanic membrane is moved back and forth with positive and negative pressure while it is in the field of vision. Atrophic parts flutter, and the movement of the pars tensa may be limited by scar tissue. In the presence of a perforation, the remnants of the tympanic membrane are completely immobile.

Appearance of a Pathologic Tympanic Membrane

• 
  

  Injection of the vessels and inflammation are seen in otitis externa (occasionally), myringitis, and otitis media.

  
  
• 
  

  Hemorrhage is red if fresh, or brownish if old. Blood vesicles are seen in influenzal otitis, and the hemotympanum is dark blue.

  
  
• 
  

  Serous exudate: A fluid level can be seen, and there are air bubbles in the fluid. The tympanic membrane looks like oiled silk when there is a complete middle ear effusion. A blue tympanic membrane or “blue drum” is seen in advanced stages of otitis media with effusion.

  
  
• 
  

  Retraction of the tympanic membrane as a result of decreased pressure in the middle ear: The short process of the malleus protrudes externally, and there is displacement of the manubrium of the malleus posteriorly and superiorly, causing an apparent shortening of the malleus handle. The triangular light reflex is fragmented, or disappears entirely.

  
  
• 
  

  Bulging due to the formation of exudate behind the tympanic membrane, at times with an irregular surface, which may be papillary, with an opaque surface.

  
  
• 
  

  Atrophy of the tympanic membrane with retraction pockets results from chronic inflammation and reduced pressure. The site of predilection is the posterosuperior quadrant.

  
  
• 
  

  Thickening of the tympanic membrane, as a result of degenerative changes or as the result of inflammation, produces a surface that is dark and lacking in luster.

  
  
• 
  

  Scars of the tympanic membrane: These may be thickened areas, with or without calcium deposits or atrophic areas.

  
  
• 
  

  Tympanic membrane perforations: These may be either central or peripheral, mesotympanic or epitympanic. Central or mesotympanic defects are the result of chronic mucosal inflammation (see ▶ p. 75), whereas peripheral or epitympanic perforations are usually associated with a cholesteatoma (pp. 76–84).

  
  
  
  

  
  

  Note: A tympanic membrane that has a surface with an opaque and dull appearance—as a result of inflammatory infiltration of the pars tensa, with hyperemia, edema, formation of bullae, desquamation of the epidermal layer, and distortion of the characteristic appearance of the handle of the malleus—is designated as abnormal or pathologic.

1.2.2 Diagnostic Imaging

The position of the petrosal temporal bone inside the skull base generates overlapping artifacts during radiological imaging. Special radiographic images of both temporal bones are therefore essential for comparison.

1.2.2.1 Computed Tomography

High-resolution CT images of the petrosal bone in axial ( ▶ Fig. 1.37) and coronal views ( ▶ Fig. 1.38), with slice thicknesses of 1 and 2 mm, have replaced conventional radiographic imaging in image-guided diagnosis.

With the multislice spiral CT technique it is possible to obtain multiplanar reconstructions from the primary data set of one axial scan in high-resolution, which allows three-dimensional interpretation of the middle and inner ear structures. The axial sections should be selected in a plane roughly parallel to infraorbital-meatal line and planned from the upper edge of the petrous bone to the mastoid tip determined by obtaining a lateral scout. Images should be displayed in a “large bone window” (width>4,000 HU, center about 700 HU). CT clearly depicts anatomical structures and a comparison of both right and left sides, and has replaced the use of plain films. CT scans are used to evaluate developmental, traumatic, inflammatory, and neoplastic processes of the petrous bone, e.g., in facial palsy and hearing loss. CT images precisely show bony destruction and soft tissue enlargement associated with cholesteatoma, mastoiditis, or tumors as well as fluid retention, fractures, or otosclerotic foci.

Intravenous contrast medium administration is only necessary in exceptional cases, e.g., consideration of a tumor or inflammatory conditions when an additional MRI examination is contraindicated. The evaluation of skull base abnormalities is best accomplished by conducting a combination of a CT to determine the bony involvement and/or an MRI to determine the neurovascular involvement, respectively.

1.2.2.2 Angiography

CT-angiography (CTA) or MR-angiography (MRA) is performed for diagnostic purposes of vascular structures or vascular tumors (glomus tumor) of the petrous temporal bone. The latter is either done as a 3D-time-of-flight-sequence (no contrast administration needed) or as a contrast-enhanced MRA. Thanks to the thin-slice technique, views can be taken in all three planes without secondary reconstruction using a computer and without the need for special positioning of the patient.

The digital subtraction angiography is restricted to interventional techniques such as embolization.

1.2.2.3 Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is often complementary to CT for the assessment of inflammatory, traumatic, or neoplastic pathologies in the temporal bone and skull base. It should be considered the first modality of choice in retrocochlear sensorineural hearing loss, facial nerve palsy, and vertigo.

It is routinely used with T 1-weighted and T 2-weighted spin-echo sequences, producing thin-slice images, sometimes with contrast administration (gadolinium diethylenetriamine penta-acetic acid, Gd-DTPA). Using high-resolution T 2-weighted sequences such as the Constructive Interference in Steady State (CISS) sequence—a three-dimensional magnetic resonance sequence that displays cerebrospinal fluid spaces—even details of the membranous labyrinth and neuronal structures of the inner ear canal and CPA become visible.

Very small intra- and extrameatal vestibular schwannoma (acoustic neuromas) can be clearly demonstrated and measured ( ▶ Fig. 1.39). The volume-rendering technique can even produce three-dimensional images of the entire inner ear, as well as of the topological relationships in the entire temporal bone. In addition, it provides virtual endoscopic visualization of the inner ear region.

The T 1-weighted imaging sequence with contrast administration (Gd-DTPA) is highly sensitive for the delineation of pathologic lesions (e.g., tumors and inflammatory conditions). The MRI should also cover a fast spin echo (FSE) T 2 sequence for an overview of the skull. Diffusion-weighted MRI is a specific technique in which the restricted motion of water molecules within a voxel of tissue is visible. This is particularly useful for detecting cerebral ischemia, local recurrence, or residual cholesteatoma; cholesteatomas as small as 2 mm can be detected. DWI images with b-value 1,000 s/mm² (b-factor is a measure of the strength of the respective diffusion weight) will show a cholesteatoma as a hyperintense area. The corresponding apparent diffusion coefficient (ADC) map should show a low signal in the same area, confirming the presence of diffusion restriction.

These visual characteristics are similar to the epidermoid cyst, a histologically identical lesion—in contrast, granulation tissue, fibrous tissue, cholesterol granuloma, or serous fluid show high signal in the ADC map which means no diffusion restriction.

1.2.3 Functional Assessment of the Eustachian Tube

Tests of tubal function are always necessary in all patients with middle ear hearing loss, particularly before an operation to improve hearing.

1.2.3.1 Qualitative Assessment of Tubal Function

Valsalva test: This test is used to demonstrate normal tubal patency without the need for any external aids. Failure of the test does not prove pathologic occlusion of the tube, but further functional tests may be required.

After taking a deep breath, the patient pinches his nose and closes his mouth in an attempt to blow air into his ears.

Otoscopy shows bulging of the tympanic membrane, and auscultation reveals crackling.

Toynbee test: This test is used to confirm normal tubal air patency with a simple and safe method. During swallowing, pressure in the middle ear falls if the nose is closed off. This can be seen on otoscopy as a drawing-in of the tympanic membrane.

Tympanometry: Impedance audiometry assesses eustachian tube, air passage, and function.

1.2.4 Hearing Investigations

1.2.4.1 Testing Hearing without an Audiometer

Hearing Threshold for Whispered Voice and Conversational Speech

Two-syllable words are articulated at a decreasing distance from the patient until the test words can be correctly repeated. The distance is recorded in meters. Alternatively, the examiner can say numbers or words at a fixed distance with decreasing loudness. When severe unilateral deafness is being assessed, and also when the hearing distance for conversational speech is being measured, it is necessary to mask the contralateral ear. Each ear is tested separately, with the better ear being tested first. The contralateral ear canal is closed with a finger.

Requirements:

• 
  

  A sufficiently large, quiet room (6 m long).

  
  
• 
  

  Good acoustic properties (no smooth walls with distorting echoes).

Tuning Fork Tests

A C fork with a frequency of 512 Hz is used.

Weber test: This test is based on binaural comparison of bone conduction. The tuning fork is placed in the center of the skull at the hairline. A patient with normal hearing or with symmetrical hearing loss localizes the tone either in the center of the head or equally in both ears. A patient with unilateral conductive hearing loss (middle ear) localizes the tone in the affected ear, whereas a patient with unilateral inner ear deafness localizes the sound in the healthy ear.

Theoretical explanations:

• 
  

  In middle ear disorders, the mobility of the ossicular chain is reduced and it thus transmits less sound energy than it does in normal physiologic conditions (Mach sound wastage theory).

  
  
• 
  

  Pathologic processes in the middle ear cause an increase in the mass of the sound conduction apparatus, so that increased forces are exerted at the oval window, due to inertia. This leads to greater stimulation of the inner ear (inertia theory) ( ▶ Fig. 1.40).

  
  
  
  

  Fig. 1.40 The Weber test. A vibrating tuning fork is placed on the midline of the skull. (a) Equal loudness perceived in both ears means symmetrical hearing. (b) Lateralization of sound to the affected ear (right) is present in cases of conductive hearing loss. (c) In cases of sensorineural hearing loss, the sound is lateralized to the better ear (left). (d) Correct orientation of the tuning fork.

  
  

  

Rinne test: This test is based on monaural comparison of air conduction with bone conduction. If air conduction is better than bone conduction, Rinne test is positive. This is the finding in normal hearing or sensorineural hearing loss (inner ear). If bone conduction is better than air conduction, Rinne test is negative. This is found in conductive or middle ear hearing loss.

The patient is asked whether the tuning fork placed in front of the ear is heard better than when it is placed behind the ear, on the mastoid process, without striking it again. If the patient cannot decide with certainty, the decay period of the tuning fork should be determined precisely for both air and bone conduction separately ( ▶ Fig. 1.41).

Gellé test: This can be used to test the mobility of the ossicular chain in cases of otosclerosis (see ▶ p. 90) and fixation of the incus. The test has now been replaced by impedance audiometry (see ▶ p. 37) ( ▶ Fig. 1.42).

1.2.4.2 Audiometry: Fundamental Physical and Acoustic Concepts

See ▶ Table 1.2  to ▶ Table 1.7 .

1.2.4.3 Pure-Tone Audiometry

An audiometer is an electric tone generator used to determine the hearing threshold for pure tones—i.e., tones free of harmonics within a frequency range from 125 to 12,000 Hz.

The hearing threshold is measured for both air and bone conduction in decibel steps. The normal hearing threshold is indicated by a straight line at 0 dB. Hearing loss is measured in decibels relative to this threshold for all frequencies and is recorded on an audiogram ( ▶ Fig. 1.43).

The decibel (dB) is a relative value that compares one sound pressure to another. The reference point in audiometry is the human hearing threshold of 1,000 Hz. The sound pressure necessary to produce the subjective impression of hearing at a threshold of 1,000 Hz is 20 μPa (2 × 10⁻⁴ μbar) (see ▶ Table 1.3 ). This is the average value for young individuals with normal hearing and is the reference point for the physical or absolute measurement of the hearing threshold in decibels (sound pressure level, SPL). The relative hearing threshold for pure tones is a simpler method of demonstrating and describing the hearing threshold. The reference point is no longer the absolute sound pressure, but the just-audible threshold of hearing, measured in dB (hearing level, HL). This makes it possible to use a coordinate system with a horizontal zero line. The absolute hearing threshold is curved in comparison with the relative hearing threshold. The reason for this is that greater sound pressure is needed at high and low tones to produce a similar sensation of sound near the threshold than for the central part of the frequency range around 1,000 Hz ( ▶ Fig. 1.44).

A disorder of sound conduction can be identified by assessing the difference between the hearing threshold for air and bone conduction, in the same way as with tuning fork tests.

Relationship between Air Conduction and Bone Conduction

Normal conduction of sound to the inner ear via the sound-conducting apparatus is defined as air conduction (conduction via earphones). Sound is also conducted via the bones of the skull to the inner ear, either via the middle ear (osteotympanic or craniotympanic bone conduction) or by direct transmission via the labyrinthine capsule (osteal or cranial bone conduction) (conduction via a vibrator).

Conductive hearing loss results from an increase in impedance ( ▶ Fig. 1.45a–d). If the elastic recoil due to air in the middle ear and mastoid process increases, mobility in the middle and low tones decreases at constant mass and tension. The resonance point of the middle ear is displaced to the upper frequencies. Conductive hearing loss is characterized by greater loss of hearing for air conduction in the lower frequencies—as seen, for example, in ossification of the stapes annulus in otosclerosis ( ▶ Fig. 1.45c; see also ▶ Fig. 1.106). The conduction system is increasingly damped by the increase in mass and tension, and the resonance point of the middle ear is thus displaced into the lower frequencies. The resulting hearing loss affects air conduction in the middle and higher frequencies to a greater extent, similar to that seen with fluid in the middle ear and impacted wax ( ▶ Fig. 1.45b).

Conductive hearing loss independent of frequency is caused by simultaneous elastic stiffening and dampening of the sound conduction apparatus. This may occur in advanced otosclerosis, in middle ear cholesteatoma with destruction of the ossicular chain, in tympanosclerosis, and in congenital anomalies. A flat air conduction curve is found in such cases.

This rule applies with a few unimportant exceptions (e.g., bony closure of one or both windows).

The audiometric characteristic of all forms of sensorineural hearing loss (inner ear and retrocochlear hearing loss) is that the thresholds for air and bone conduction coincide ( ▶ Fig. 1.46a–d). Supplementary suprathreshold tests have to be performed to differentiate inner ear hearing loss from retrocochlear hearing loss.

Demonstration of Recruitment

Patients with inner ear hearing loss showing recruitment often have difficulty in hearing relatively soft tones. In contrast, they hear loud conversational speech as well as individuals with normal hearing. They find excessive loudness upsetting due to distortion and painful sensations, as the threshold of discomfort is exceeded. In inner ear deafness, recruitment occurs in the frequency range of the damaged hair cells, which require a considerably higher sound pressure in comparison with the normal hair cells to produce a response. The resulting reduction of the dynamic hearing range has extremely deleterious effects as far as hearing of speech is concerned (see ▶ Fig. 1.44 and ▶ pp. 124–127).

The following tests are used:

Fowler test: Principle—This test is based on a subjective comparison of loudness between the right and left ears. A tone of the same frequency and loudness is presented alternately. A recruitment phenomenon is present if a difference in the hearing threshold between the two sides disappears as the loudness of the test tone increases ( ▶ Fig. 1.47).

Tone intensity difference threshold (Lüscher test): Principle—The threshold of intensity difference in decibels at the same distance above the hearing threshold is smaller in an ear affected with recruitment than in the normal ear.

Short increment sensitivity index (SISI) test: Principle—A test tone is produced 20 dB above the patient’s threshold and is increased by 1 dB every 5 seconds with a duration of 0.2 second. Negative results are obtained in retrocochlear lesions with pathologic fatigue. The score is greater than 80% in patients with cochlear hearing loss showing recruitment.

Demonstration of Pathologic Fatigue

Pathologic auditory fatigue is a sign of a retrocochlear hearing loss. It can be demonstrated using the technically simple tone decay test and the Békésy test. These two methods have now been largely replaced by measurement of auditory evoked potentials (AEPs; see ▶ p. 39), which allows objective testing of auditory functions and considerably more accurate diagnosis of retrocochlear hearing disorders.

1.2.4.4 Speech Audiometry

Speech audiometry is an integral part of audiometric methods of investigation. The ability to hear and understand speech is more important in human communication than the ability to hear pure tones. Speech audiometry therefore has both diagnostic and therapeutic significance. To understand the results of speech audiometry, it is necessary to know the frequencies contained in speech. The fundamental vocal frequencies for men (125 Hz) and women (250 Hz) are shown in a tone threshold audiogram in ▶ Fig. 1.48.

The loudness of speech is perceived as an acoustic image, the frequencies of which extend from 100 to 8,000 Hz. Hearing loss for speech is assessed using two-syllable test words, and maximum discrimination is also measured using one-syllable test words ( ▶ Fig. 1.49).

Speech audiometry is not performed in the same way as testing of the vocal speech (see ▶ pp. 143–145)—i.e., with an increasing distance between the patient and the sound source—but rather by varying the loudness as measured in decibels, i.e., with a speech sound level above 20 μPa ( ▶ Table 1.2 ).

The speech or test material is recorded on a disk and is presented to the patient either using earphones or in a free field using a loudspeaker with varying loudness levels. The percentage of numbers, words, or sentences understood correctly at each loudness level is then assessed.

The dependence of speech comprehension on the loudness level is tested using speech audiometry. In the standardized test (e.g., the Freiburg speech test), multisyllabic numbers are first used. This can provide a rapid rough estimate of the extent of hearing loss.

An individual with normal hearing understands 50% of numbers presented at 18.5 dB. This normal value forms the basis for assessing hearing loss for numbers. The patient’s ability to comprehend monosyllabic words is also tested.

These words are considerably more difficult to understand than multisyllabic numbers. The purpose of the monosyllable test is to assess percentage comprehension and ultimately to achieve 100% comprehension values, if possible, by increasing the loudness level. Normal individuals hear 100% of monosyllables at 65 dB, and in favorable conditions at 50 dB, whereas 100% speech comprehension cannot be achieved even in normal individuals at a sound pressure level of less than 50 dB.

Speech audiometry allows quantitative measurement of hearing. The speech audiogram indicates the percentage of syllables, words, or sentences that the individual has heard correctly in each test series. The result of a speech audiogram depends not only on hearing, but also on higher cognitive functions such as memory, language comprehension, and motor speech. Other factors that influence the results include whether the patient’s mother tongue is being used and the patient’s vocabulary range.

Comparison of Pure-Tone and Speech Audiograms

Discrepancies between the results of pure-tone and speech audiometry are mainly found in retrocochlear hearing disorders. In such cases, hearing for speech is considerably worse than hearing for pure tones. The pathophysiologic basis for this is described on ▶ p. 18.

Diagnosis of central hearing disorders is based on tests of the central understanding of speech. The classic methods of testing hearing fail in such cases due to the phenomenon of redundancy. This is the safety margin within the auditory pathways, which can transmit and analyze billions of information units, whereas only 100 are necessary for recognizing and decoding acoustic information. A disorder of the central summation and integration capacity can only be demonstrated with difficulty—e.g., by distorting speech by filtering out high frequencies and inserting periodic interruptions of the speech signal, or with binaural application of garbled test words, reducing the information content of normal speech to a minimum (Feldmann dichotic speech test).

1.2.4.5 Objective Hearing Tests

Behavioral, pure-tone audiometry is based on a subjective response from the patient. In contrast, objective audiometry makes it possible to carry out testing without eliciting a patient response. This method uses tests based on involuntary physiologic reactions and “objective” parameters. These objective responses support the interpretation of pure-tone audiometry and are very important for audiometric diagnosis in infants, small children, and patients with mental and cognitive impairment.

Three main methods are used in objective audiometry:

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  Measurement of changes in the acoustic impedance of the tympanic membrane: impedance audiometry.

  
  
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  Measurement of acoustically evoked bioelectric responses of the cochlea, vestibulocochlear nerve and tract, or cerebral cortex: auditory evoked potentials (AEPs).

  
  
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  Measurement of spontaneous or acoustically evoked vibrations of the cochlea: otoacoustic emissions (OAEs).

Impedance Audiometry

This technique is part of the functional diagnosis of the sound conduction apparatus. It includes the following investigation methods:

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  Tympanometry: This involves recording the impedance (see ▶ p. 15) or indirect measurement of pressure in the middle ear, when the tympanic membrane is intact, by means of pressure in the external meatus. This is an indirect test of tubal function.

  
  
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  Measurement of the acoustic reflex: The change in impedance caused by the acoustic stapedial reflex is measured.

Technique

The external auditory meatus is closed by an air-tight plug, through which three tubes pass. One tube carries the test tone; the second is connected to the pressure regulator, which allows positive or negative pressure (± 400 mmH₂O) to be produced in the external auditory meatus. A microphone is connected to the third tube, allowing measurement of the sound pressure of the test tone reflected from the tympanic membrane as the impedance changes ( ▶ Fig. 1.50).

Tympanometry: Normally, there is no pressure differential between the two sides of the tympanic membrane, so that the acoustic resistance of the tympanic membrane is minimal. Recording the impedance of the tympanic membrane during a change in pressure in the external auditory meatus allows the pressure difference on the two sides of the tympanic membrane to be determined by measuring its compliance. The greater the pressure differential, the greater is the impedance of the tympanic membrane. Recording the impedance at pressures from −300 mmH₂O to + 300 mmH₂O produces a curve with a peak at zero for a normally mobile tympanic membrane. This represents the maximum flexibility, i.e., compliance, of the tympanic membrane, and thus minimal impedance. The apex of this curve is lower if the tympanic membrane is stiffened by scar tissue or damped by exudate in the middle ear. It becomes higher with increasing compliance due to atrophic scars of the pars tensa ( ▶ Fig. 1.51).

Stapedial reflex: The principle of this test is that a sound stimulus greater than 70 dB above the threshold induces a reflex contraction of the stapedius muscle. This causes a change of impedance at the tympanic membrane, which can be recorded graphically. The effect is absent when the tympanic membrane is immobile, when the ossicular chain is disrupted, and when the stapes is fixed in the oval window by otosclerosis. In simulated deafness, this reflex is activated by loudness approaching the norm. In this case, simulation can be assumed.

The stapedial reflex is an acousticofacial reflex. The afferent limb is the vestibulocochlear nerve and parts of the central auditory pathway up to the auditory centers. The efferent limb is formed by the connections between the auditory centers and the facial nucleus, and finally by the facial nerve. Measurement of the stapedial reflex is therefore very useful in topical diagnosis of facial paralysis.

Testing the threshold for the stapedial reflex is of considerable diagnostic importance for assessing the following hearing disorders: otosclerosis, recruitment (Metz recruitment is reduction of the difference between an elevated hearing threshold and the threshold for the stapedial reflex, with increasing hearing loss for high tones), retrocochlear deafness, and brainstem lesions.

The stapedial reflex is absent in:

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  Retrocochlear sensorineural deafness as a result of auditory fatigue—i.e., in vestibular schwannoma.

  
  
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  Otosclerosis and other middle ear diseases.

  
  
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  Facial nerve damage proximal to the point at which the stapedius muscle is innervated.

  
  
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  Brainstem lesions with damage to the central reflex arc.

Auditory Evoked Potentials (AEPs)

The patient is repeatedly exposed to an acoustic stimulus, either regularly or irregularly, and an electroencephalogram (EEG) is used to assess whether there is any change in brain activity. The AEPs are recorded from the scalp using needle or surface electrodes. As the amplitudes of the AEPs are very small relative to the total activity of the brain, averaging the potentials is necessary (see ▶ Fig. 1.49). Averaging means that the individual response, which is concealed on the EEG by the “noise” of brain activity, can be distinguished by mathematical analysis of numerous evoked individual potentials. The intermittent acoustic stimulus produces a uniform potential during a time interval that always occurs at the same time and can be amplified by repetitive summation of the EEG segment.

The properties and shape of AEPs depend partly on the time at which they occur after presentation of the acoustic stimulus, or their latency (in milliseconds). Several types of AEP can be distinguished on the basis of different sites of origin and latency.

Classification

Electrocochleography (ECochG): This measures the potentials arising in the cochlea and vestibulocochlear nerve. These potentials occur ≈1 to 3 milliseconds after the stimulus is presented. The two most useful diagnostic parameters are cochlear microphonics (CM) and the action potential of the vestibulocochlear nerve (PI).

Auditory brainstem response (ABR) audiometry (brainstem evoked response audiometry, BERA): This measures the potentials arising in the vestibulocochlear nerve and brainstem structures, with a latency of up to ≈10 milliseconds. The latency of individual potentials, particularly between potential peaks I and V, is very important for recognizing retrocochlear hearing disorders ( ▶ Fig. 1.52).

Auditory middle latency potential (AMLP) audiometry: This measures potentials with a latency of 10 to 100 milliseconds that originate in the thalamus and primary auditory cortex.

Cortical evoked potentials (CEPs): This measures potentials with a latency of 100 to 1,000 milliseconds, which express generalized higher-order cortical function.

Measurement of the ABR and ECochG are two of the most important diagnostic methods for accurate differentiation between cochlear and retrocochlear deafness. The latter is due to space-occupying formations in the CPA (e.g., vestibular schwannoma), a tumor of the posterior cranial fossa, or multiple sclerosis. AEP is also very useful for investigating deafness in infants and young children. It can also be used to assess residual function of the central nervous system in patients with severe head injuries, coma, or other conditions marked by a complete loss of consciousness. It does not, however, replace pure-tone audiometry or tympanometry (including the stapedial reflex), which still form the basis for audiometric evaluations. ABR is also tested intraoperatively in order to monitor hearing.

Auditory steady-state response (ASSR): This is an auditory evoked potential measurement, elicited with modulated tones that can be used to predict hearing sensitivity in patients of all ages. It is also an objective test used for evaluation of hearing ability in children who are too young for traditional audiometric testing. ASSR is used in the newborn when hearing screen results in the hospital are suspicious for hearing loss. The test is often performed under sedation or in natural sleep if the person is under 6 months of age. Similar to traditional recording montages used for ABR, two active electrodes are placed at or near vertex and at ipsilateral mastoid with a ground electrode sited low on the forehead. Unlike ABR settings, the high pass filter might be approximately 40 to 90 Hz and low pass filter might be between 320 and 720 Hz with typical filter slopes of 6 dB per octave. Repeated sound stimuli are presented at a high frequency rate.

Otoacoustic Emissions (OAEs)

OAEs are sound signals emitted from the inner ear in response to acoustic stimulation. The signals are vibrations produced by the biomechanical cochlear amplifier (see ▶ p. 16). They occur spontaneously or in response to an acoustic stimulus and are transmitted in retrograde fashion across the ossicles to the tympanic membrane. The membrane acts like a loudspeaker membrane, so that emitted vibrations can be measured as sound waves in the external ear canal. These active cochlear vibrations can be detected by a sensitive microphone.

OAEs are clinically important, as they reflect the functional integrity of the cochlea. OAE detection depends on normal middle ear function for good transmission to the tympanic membrane.

Classification

Spontaneous OAEs (SOAEs): Vibrations can arise spontaneously in the cochlea without any external stimulus. They are detectable as low-level, continuous tones in ≈50% of individuals with normal hearing.

Transient evoked otoacoustic emissions (TEOAEs): Emissions are detected in response to an acoustic stimulus (click) in individuals with normal cochlear function. An averaging technique is used (as in ABR; see ▶ p. 39). This measurement is used as an objective audiometric testing method ( ▶ Fig. 1.53a). TEOAEs occur in normal hearing and confirm cochlear integrity; they are absent in patients with middle ear disease or cochlear hearing loss with a threshold increase of ≈30 dB. The amplitude in infants with normal hearing is usually higher than in adults.

Distortion product otoacoustic emissions (DPOAEs): Acoustic distortions in the cochlear amplifier can be detected by stimulation with two continuous tones that have different, but adjacent, frequencies. DPOAEs are intimately linked to outer hair-cell function. This is another frequently used objective audiometric testing method ( ▶ Fig. 1.53b).

The most important application of OAEs is for screening cochlear function in newborns, infants, and small children. DPOAEs can be used to detect early discrete lesions of the outer hair cells, and they provide an important noninvasive screening method for cochlear impairment that can even be used without sedation or general anesthesia. OAEs can also be used to investigate nonorganic hearing loss, to objectify audiometric findings in adults, and to assess cochlear function in risk groups (ototoxic medication).

1.2.4.6 Hearing Tests in Infants and Young Children

Since even a completely deaf child passes through a period of crying and babbling, serious hearing loss only begins to be suspected when speech does not develop. Most children with hearing disorders are therefore presented to the general practitioner or otologist between the first and third years of life. As hearing is not an obvious condition in the newborn, it needs to be detected using screening. Every newborn should be screened on the second or third day after birth during the second routine examination. Eighty percent of all hearing problems can be detected using this method. The organization required for this method of screening depends on the local health care system.

Additional screening should be performed at routine pediatric visits or in a preschool medical examination.

Tests

Otoacoustic emissions (OAEs): If OAEs are present, the peripheral hearing is satisfactory, but this does not exclude a hearing disorder. The degree of any hearing loss cannot be determined.

Auditory brainstem response (ABR): If an ABR is not elicited, severe hearing loss is present. The hearing threshold can be determined when AEPs are measurable.

Auditory steady-state response (ASSR): Modified AEP measurement, elicited with modulated tones.

Pediatric audiology with behavioral tests: Subjective responses in pediatric audiometric testing are important methods and can be performed at virtually any age, but should be age-appropriate. The reliability of the test results is variable.

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  Reflex audiometry: Nonspecific responses to auditory stimuli, such as sucking responses, motor responses (Moro reflex, acousticopalpebral reflex), or breathing responses, can be elicited in normal infants from birth on. The reflexes can be stimulated only by a loud noise (nearly 80 dB).

  
  
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  Response audiometry: By the second half of the first year of life, acoustic stimuli evoke typical response patterns. A normally hearing infant turns his or her head toward the sound source that is out of the range of vision.

  
  
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  Distraction test: A tester attracts the child’s attention with a toy, and the examiner presents an acoustic stimulus invisible to the child and observes the reaction.

Visual reinforcement audiometry (VRA): An acoustic stimulus is combined with the activation of a moving toy. After conditioning, the child moves toward the toy when it hears the acoustic stimulus.

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  Play audiometry: As a variation of pure-tone audiometry, tasks and responses to tone testing are incorporated into a play setting—e.g., while playing, the child has to react when an acoustic stimulus is presented.

  
  
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  Pediatric speech audiometry: Children aged 3 to 4 years can be examined using audiometric speech tests specially designed for children (e.g., the Pediatric Speech Intelligibility Test).

1.2.5 Vestibular Function Tests

Investigations of the vestibular system comprise:

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  Case history and analysis of symptoms.

  
  
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  Testing of the vestibulospinal reflexes.

  
  
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  Testing for spontaneous and provoked nystagmus.

  
  
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  Experimental testing of the vestibular and optokinetic systems.

1.2.5.1 Case History

The subjective feeling of dizziness is generally regarded as being an expression of a disturbed neuronal discharge pattern in the cortical projection areas. A thorough case history is required in order to achieve a structured analysis, allowing differential-diagnostic classification of:

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  Peripheral vestibular dizziness.

  
  
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  Central vestibular dizziness.

  
  
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  Nonvestibular dizziness.

The case history should include questions about previous illnesses, medications, and noxae. Questions regarding the type of subjectively perceived dizziness, as well as its duration and intensity, are important. Dizziness-causing factors and secondary symptoms are also important details to clarify.

1.2.5.2 Vestibulospinal Reflexes

In peripheral vestibular lesions, the body’s center of gravity is usually displaced to the side on which the labyrinthine lesion is located. In central disturbances of balance, the pattern of unsteadiness of gait and the direction of falling are irregular. Body sways can also be registered on an electronic scale (posturography).

Romberg test: The patient is asked to stand with the feet together (touching each other) and to close the eyes. A check is made to see whether there is then any unsteadiness or a tendency to fall.

Blindfold gait and walking a straight line: Only gross abnormalities of gait are diagnostically important. The patient deviates to the same side as in the Romberg test.

Unterberger stepping test ( ▶ Fig. 1.54): Stepping on the spot with the eyes closed. Patients with peripheral disorders show rotation of the body axis to the side of the labyrinthine lesion; in central disorders, the deviation is irregular. Only deviations of more than 40 degrees are of diagnostic significance.

Static positional tests

See also ▶ p. 46.

Spontaneous deviation reaction, past pointing: Parallel displacement of both arms (with arms in the supine position) occurs in accordance with the vestibulospinal reflexes.

Spontaneous tone reaction in the arms: The arm on the side of the cerebellar lesion sinks as a result of loss of tone of the muscles ( ▶ Fig. 1.55).

Finger-nose pointing test: The index finger of the outstretched arm is brought to the tip of the nose with the eyes closed. Ataxia and disorders of coordination (overshooting) indicate an ipsilateral cerebellar lesion or a disorder of positional sense and deep sensation.

1.2.5.3 Spontaneous and Provoked Nystagmus

Nystagmus: This is a conjugated, coordinated eye movement around a specific axis; the movement consists of rhythmically alternating slow- and fast-beating phases. The direction of the fast component of the nystagmus determines the laterality of the nystagmus.

Tests

Observation with and without Frenzel glasses: This is used for the diagnosis of a spontaneous nystagmus. The patient is examined in a darkened room with + 15-diopter lenses that almost completely suppress optical fixation, so that the visual fixation suppression of the vestibular nystagmus is eliminated ( ▶ Fig. 1.56).

Direct gaze, with and without fixation, is used to recognize fixation nystagmus. Lateral gaze and gaze upward and downward are used to confirm gaze-directional or gaze-paretic nystagmus.

The direction (←), frequency (>>—), and amplitude (=) of the eye movements observed are recorded on a Frenzel chart ( ▶ Fig. 1.57).

Electronystagmography (ENG): The eye is a dipole in which the cornea is electropositive and the retina electronegative. The periocular electrical field therefore changes when the eyes move. This change in the corneoretinal potential is proportional to the amplitude, frequency, and speed of the nystagmus. It can be picked up and recorded by electrodes and analyzed. The direction of the eye movements is demonstrated by a positive or negative corneoretinal potential ( ▶ Fig. 1.58).

Video nystagmography (VNG): The eye movements are recorded by a touchless video camera. The position of the dark pupil of the eye can be recorded by a processor that analyzes the eye’s horizontal and vertical rotation.

Spontaneous Nystagmus (Jerk-Nystagmus)

This term includes all eye movements that have the character of nystagmus and are not induced by external stimulation of the vestibular and visual systems ( ▶ Fig. 1.59). The fast component usually beats toward the side of the functionally dominant vestibular center.

Three main forms of spontaneous nystagmus can be distinguished:

Spontaneous vestibular nystagmus: This disorder may be due either to a peripheral vestibular disorder, in which case the fast component of the nystagmus always beats toward the dominant labyrinth; or it may be caused by a central vestibular disorder. The inhibitory impulses on the vestibular center are suppressed (see ▶ p. 20). The nystagmus beats on the side of the lesion.

The possible trajectories of the spontaneous nystagmus are:

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  Horizontal.

  
  
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  Vertical.

  
  
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  Torsional.

  
  
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  Combination of all three.

For clinical prescription spontaneous (Jerk)-nystagmus is graded into three types:

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  Grade 1: present only when looking in the direction of the quick component.

  
  
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  Grade 2: also present when looking straight ahead.

  
  
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  Grade 3: present when looking in the direction of the quick component, when looking straight ahead, and when looking in the direction of the slow component.

Recovery nystagmus may be due either to a central compensatory process after a peripheral lesion, or to the recovery of peripheral function. In both cases, it is directed toward the side of the dominant vestibular center—i.e., in this case toward the affected ear.

Gaze-evoked and gaze-paretic nystagmus: This form of nystagmus is always induced by a central lesion. Often it beats to both sides and in both the horizontal and vertical planes. It only appears after deviation of the globe by more than 30 degrees for at least 30 seconds.

An exceptional form of toxic gaze-evoked nystagmus may occur after barbiturate or alcohol poisoning, due to release of the central inhibitory effect.

This form of nystagmus is due to a lesion affecting voluntary motor control of gaze, which in serious cases is accompanied by paralysis of gaze. Transitions from gaze-evoked to gaze-paretic nystagmus are fluid. The latter is characterized by a nystagmus to the side of the gaze paresis.

This is due to a congenital or acquired disorder (such as multiple sclerosis) of the gaze centers of the reticular formation of the pons (the center for horizontal gaze movement) and of the tegmentum of the midbrain (the center for vertical gaze movements). These centers are involved in central voluntary motor control of gaze (integration of voluntary gaze impulses and visual and vestibular afferents), binocular coordination via the medial longitudinal bundle (see ▶ Fig. 1.21), and the rhythm of nystagmus. Lesions in this area of the brainstem therefore lead to serious abnormalities of gaze movements and nystagmus—such as changes in the rhythm and form of beat, dissociation of movements of the right and left eyes, extinction of the fast phase of nystagmus, unilateral or bilateral enhancement of optokinetic nystagmus, gaze-evoked and gaze-paretic nystagmus, and internuclear ophthalmoplegia.

Fixation nystagmus: This form of nystagmus does not have any typical fast or slow components, but rather a pendular movement. It almost always occurs with binocular fixation, but may rarely be seen with monocular fixation. It is often congenital and may even be hereditary. Synonyms for it include congenital or hereditary pendular nystagmus.

The three main forms of spontaneous nystagmus should not be confused with the following:

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  End point nystagmus, a short-lived, nonpathologic, rapidly decaying beat at the extremes of gaze—i.e., more than 50-degree deviation.

  
  
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  Fatigue nystagmus, which occurs during prolonged lateral gaze due to fatigue of the lateral rectus muscle, similar to tremor in skeletal muscles. This is also nonpathologic.

  
  
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  Adjustment nystagmus, which is due to adjustment of movements of a nystagmoid character when fixing on an object in the visual field. There is a rapid beat that fatigues quickly. This, too, is nonpathologic.

Provoked Nystagmus

Unlike spontaneous nystagmus, this is exclusively a vestibular-induced nystagmus that only appears after specific stimuli, such as changes in the position of the body or of the head:

Caloric nystagmus: An irrigation with warm water (in the head-up supine position) causes endolymph in the horizontal canal to move toward the ampulla, exciting the hair cells and driving a slow phase eye movement away from the irrigated side. An irrigation with cold water inhibits the horizontal canal, producing slow phase toward the irrigated side.

Rotational nystagmus (VOR vestibular ocular reflex): A prolonged head rotation produces a slow phase in the direction opposite to the head movement interrupted by quick phases in the same direction as head movement. This serves to stabilize retinal images as head moves.

Optokinetic nystagmus: It is driven by prolonged full-field visual motion. It supplements the VOR to stabilize vision.

Frenzel glasses are used to investigate these conditions. The same criteria are used for assessing provoked nystagmus as for spontaneous nystagmus; however, the duration of eye movements is also taken into account. One of the following patterns of nystagmus may be seen:

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  Transitory nystagmus, which lasts less than 60 seconds.

  
  
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  Continually beating persistent nystagmus.

  
  
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  Head-shaking nystagmus—i.e., “release” spontaneous nystagmus of peripheral or central origin. This may be transitory or persistent.

Provocation Measures

Head-shaking: Spontaneous nystagmus can be provoked by gentle, passive, horizontal shaking of the patient’s head.

Positional testing (static): The nystagmus is induced by adopting various body positions in slow motion (supine, lateral decubitus, head-hanging). The vestibular apparatus and the otolithic organs in particular are exposed to various gravitational stimuli in the different positions ( ▶ Fig. 1.60a).

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