The Ear

The Ear



Basic concepts



Anatomy and physiology


Structurally, the ear has three parts (Fig. 1.1):






The middle ear


The middle ear is an air-containing space connected to the nasopharynx via the Eustachian tube. It acts as an impedance matching device to transfer sound energy efficiently from air to a fluid medium in the cochlea (Fig. 1.3). The middle ear space, including the mastoid air cells, is closely related to the temporal lobe, cerebellum, jugular bulb and labyrinth of the inner ear. The space contains three ossicles (the malleus, incus and stapes) which transmit sound vibrations from the eardrum to the cochlea. The middle ear also contains two small muscles and is traversed by the facial nerve before it exits the skull.





Symptoms and signs





Clinical tests of hearing


A sympathetic approach is important to overcome embarrassment or denial with potential hearing problems. Whisper and voice tests are of little value unless performed in a quiet room with a sound pressure level meter placed near the patient. The following tests are more useful.



Tuning fork tests


Tuning fork tests distinguish between conductive and sensorineural hearing loss, but are of limited value in children. Two tests are usually employed using a 512 Hz fork. The fork is sounded by striking the tines against the patella or elbow.





Audiometry, vestibulometry and radiology


Until the advent of audiometers, hearing was tested exclusively by the examiner’s voice and tuning forks. Modern hearing tests are performed in soundproofed rooms using high-precision electronic audiological equipment. All tests require cooperation from the patient. Those tests requiring patient response are termed subjective tests, while those not requiring patient response are termed objective.



Audiometry






Vestibulometry


The vestibule has three parts: the utricle, the saccule and the semicircular canals (see Fig. 1.1, p. 2). Each vestibule tonically discharges information to the brain regarding head position, and linear and angular acceleration. This information is part of the general proprioceptive input (joint, tendon, skin and ocular inputs). Dysequilibrium may be the result of an abnormal input from any part of the proprioceptive sensors, or a dysfunction of the central nervous connection secondary to disease, e.g. ischaemia or demyelination.


Stimulation or destruction of one labyrinth produces nystagmus (involuntary eye movements), hallucinations of movement (vertigo) and a feeling of nausea. Nystagmus associated with peripheral vestibular disease is usually horizontal in direction. The direction of nystagmus is named according to the fast component. Nystagmus in directions other than horizontal is associated with central vestibular disorders.



Tests


The functional status of the peripheral vestibular system can be tested in a variety of ways. These include:




Positional test


From an erect sitting position on a couch, the patient lies flat with the head turned to one side and below horizontal (Fig. 1.8). The onset of any vertigo is noted and the eyes are observed for nystagmus. The feeling of movement and the nystagmus, if present, are allowed to settle before the patient sits upright. The manoeuvre is repeated with the head to the opposite side. This test may help to distinguish vertigo caused by peripheral (otological) as opposed to central pathologies.








Radiology


Imaging of the temporal bone is now done with computed tomography (CT). Bone is black on magnetic resonance imaging (MRI), therefore MRI is used to image temporal bone. Plain radiographs only provide evidence of gross disease and their interpretation may be difficult. Clouding of mastoid air cells may be seen in acute mastoiditis. Diseases that produce significant bony erosion, e.g. carcinoma of the middle ear, may be shown on plain X-rays.


Modern CT scanning and MRI are now widely used to provide information on otitis media with complications, and in the diagnosis of acoustic neuromas (Figs 1.9 and 1.10). MRI is particularly useful in assessing the extent of vascular lesions such as glomus jugulare tumours, and in visualizing the acoustic nerve.






Hearing loss – general introduction and childhood aetiology



General introduction


A hearing loss, as mentioned previously, can be conductive, sensorineural or mixed. Any disease affecting the outer or middle ear will produce a conductive deafness. Sensorineural loss results from damage to the cochlea or eighth nerve. The degree of hearing loss can be quantified on an audiogram with the thresholds of hearing quoted in decibels (p. 4).


Table 1.2 lists the most common causes of hearing loss. Most of those leading to a conductive deafness will be evident from history, otoscopy, tuning fork tests and audiometry. However, the aetiology of sensorineural loss is frequently unclear. In these cases, specific points in the history should be determined. These points are listed in Table 1.3.


Table 1.2 Aetiology of hearing loss





























Cause Conductive hearing loss Sensorineural hearing loss
Congenital Atresia of ear, ossicular abnormalities Prenatal: genetic, rubella
Acquired External: wax, otitis externa, foreign body Perinatal: hypoxia, jaundice
  Middle ear: middle ear effusion, chronic otitis (cholesteatoma, perforated drum), otosclerosis, traumatic perforation of drum (ossicular disruption) Trauma: noise, head injury, surgery
Inflammatory: chronic otitis, meningitis, measles, mumps, syphilis
Degenerative: presbyacusis
Ototoxicity: aminoglycosides, cytotoxics
Neoplastic: acoustic neuroma
Idiopathic: Ménière’s disease, sudden deafness

Table 1.3 Points to cover in clinical history of a patient presenting with hearing loss



In general terms, a conductive hearing loss is amenable to surgery. However, a common feature of sensorineural deafness is loss of hair cells from the organ of Corti. Hair cells are not replaced, thus sensory deafness is usually permanent. Sensorineural losses often display predominantly high tone loss on audiometry as the hair cells responding to high frequencies are most susceptible to damage.



Hearing loss in children


Deafness is an impairment to communication at any age, but children born with a hearing loss have a major handicap in developing communication. Therefore, early detection and management are required for adequate speech and language development.


The incidence of severe sensorineural deafness is about 1 in 1000. Half of these children have a hereditary type of deafness (Fig. 1.11). The others have hearing losses resulting from acquired causes. Even mild degrees of hearing loss, either conductive or sensorineural, can impair learning ability.



Childhood hearing loss should be suspected in certain groups of individuals (Table 1.4). Children falling into these risk categories should be referred to an audiological physician or otologist for audiometric assessment. This is often a multidisciplinary approach using teachers of the deaf and speech therapists in the same clinic. If a hearing loss can be overcome at an early age, particularly severe sensorineural losses, there is a greater chance the child can attend an ordinary school. Where a hereditary loss is confirmed, a geneticist may advise on risks to future children.


Table 1.4 The childhood groups at risk of suffering from hearing loss



History taking from the parents should concentrate on establishing the answers to specific questions, as well as making a general otological assessment (Table 1.5). Most hearing problems relate to middle ear disease. However, sensorineural deafness may coexist.


Table 1.5 History taking in childhood hearing loss



Children with a profound hearing loss should be fitted with an aid at the earliest possible opportunity after diagnosis. Great perseverance is needed with aiding, particularly in the first 2 years of life. Close observation by otologists, audiological physicians and teachers, with repeated assessments of hearing levels while aided, will give the best chance of normal development of speech and language.



Otitis media with effusion (glue ear)


Otitis media with effusion (OME) is the most common cause of acquired conductive hearing loss in children. The true incidence is unknown, but up to 60% of children in their first year may have middle ear effusions which are clinically asymptomatic. The peak clinical age group is 2–6 years, where about 30% of children suffer effusions. By the age of 11 the incidence has dropped to about 2%. There is a seasonal variation in the disease, associated with upper respiratory tract infections which are more common in October to March in the northern hemisphere.


The effusion in the middle ear may be serous, mucoid or thick (glue). The aetiology is usually Eustachian tube dysfunction, where normal ventilation of the middle ear is disturbed (Fig. 1.12). A diagnosis of chronic otitis media with effusion is made when fluid is present behind the eardrum for 12 weeks or more.







Hearing loss – adult aetiology


The most common causes of hearing loss in adults are wax impaction and presbyacusis. However, there are a number of other diseases in which hearing loss is the primary complaint, although often with other associated otological symptoms.



Conductive hearing loss


The aetiology of adult conductive hearing loss may be due to pathology of the ear canal, the eardrum or the middle ear.



Ear canal


Wax production varies between individuals and races. Blind attempts to remove wax with cotton buds usually result in impaction. Wax may be properly removed by syringing the ear or with a blunt hook (p. 25). Preliminary softening can be achieved with sodium bicarbonate eardrops three times a day, or hydrogen peroxide. Rarely, excessive accumulations of desquamated skin and wax in the deepest part of the external meatus can expand and erode the ear canal. This is termed keratosis obturans, and an anaesthetic may be required to remove it.


The external canal may be narrowed by bony exostoses predisposing to keratin accumulation (Fig. 1.16). These exostoses often occur in swimmers and require no treatment unless they cause external otitis or hearing deficits.




Eardrum and middle ear


Perforations of the eardrum can occur from trauma and acute or chronic otitis media (Fig. 1.17). The degree of hearing loss depends on the site of the perforation and the extent of middle ear disease.



Perforations from simple chronic otitis media where the mastoid is not diseased may be repaired by a tympanoplasty procedure using a graft (temporalis fascia). Ossicular discontinuity may also be treated surgically. Traumatic perforations, e.g. blow to the ear, invariably heal spontaneously if the ear is kept dry.


Adults may suffer with middle ear effusions, although less commonly than children. Investigations should rule out sinusitis, or nasopharyngeal tumours blocking the Eustachian tube (see Fig. 1.12, p. 7).


Otosclerosis is a disease where new bone growth occurs in the capsule of the inner ear. This may fix the footplate of the stapes. Hearing loss characteristically develops in the young adult and is usually conductive (p. 3), although the otoscopic appearance of the eardrum is normal. Pregnancy can accelerate the symptoms, suggesting a hormonal association with the disease. A family history is frequently elicited. Tinnitus may also be present.


Surgery for otosclerosis may restore normal hearing but also carries a small risk of total hearing loss. Use of a hearing aid has no complications, but is often refused (Fig. 1.18).




Sensorineural hearing loss



Presbyacusis (common)


Presbyacusis is a progressive loss of hair cells in the cochlea with age. Roughly 1% of cells are lost each year, and this affects the high-frequency part of the inner ear first (Fig. 1.19). It becomes clinically noticeable from the age of about 60–65 years. The degree of loss varies, as does the age of onset. Some patients with presbyacusis have recruitment (reduced dynamic range of hearing) which reduces effective amplification. The threshold for hearing and the uncomfortable level of sound are abnormally close (e.g. ‘Speak up, I can’t hear you … don’t shout so loud!’). Discrimination may also be affected (‘I hear you but can’t understand you’). There is no treatment to prevent this loss. When a significant social or work handicap is present, a hearing aid may be prescribed. This should be digital so that the pattern of amplification is tailored to the pattern of the individual’s hearing loss. Two hearing aids are better than one, because of binaural hearing.












Aids to hearing


Hearing loss is a major disability that can interfere with the social, work and educational spheres of a patient’s life. A 35 dB loss in the speech frequencies (500–2000 Hz) can result in major problems. Fortunately, the majority of sufferers may be helped by employing one or more of the remedies available (Table 1.6).


Table 1.6 Aids to hearing




Electronic hearing aids


An electronic hearing aid consists of an earpiece, an amplifier and a microphone. There is a volume control, and many hearing aids are fitted with a ‘T’ setting that allows the use of electromagnetic induction waves to provide sound and cut out extraneous background noise.


A variety of aids is shown in Figure 1.21. The majority of patients will be fitted with a postauricular hearing aid which is relatively unobtrusive. However, severe hearing loss may only be assisted by body-worn (BW) aids. It is possible to incorporate the aid into a spectacle frame if desired. Miniaturized aids can also be worn in the ear or inserted into the ear canal.



Fitting aids to both ears is preferable in most patients. It is vital to counsel the patient that discrimination may not necessarily be improved, but that amplification can provide benefit by better recognition of rhythms and phrases.



Problems with electronic hearing aids


To gain the maximum benefit from the aid, it is important to provide patients with training. It is a shock to many to learn that an aid cannot produce normal hearing. Patients with conductive hearing losses have better results with aids than those with sensorineural losses. This is due to the fact that many of the latter losses are associated with a phenomenon called ‘recruitment’, where loud sounds are heard exceptionally loudly so that the amplification from a hearing aid merely adds to the patient’s difficulties.


The common problems encountered with electronic aids are listed in Table 1.7. Probably the most frequent difficulty is with acoustic feedback. This produces the familiar high-pitched whistle and is particularly seen in patients who require high amplification, and in whom the ear mould allows sound to escape into the microphone. A similar event will occur if the mould is incorrectly inserted, as is frequently seen in elderly people suffering from arthritic joints.


Table 1.7 Common problems with electronic hearing aids





















Problem Cause
Feedback Badly fitting ear mould
Otorrhoea Ear infection
  Allergy to mould
No sound Dead battery
  Blocked tube

A persistent otorrhoea may occur due to allergy to the mould. Alternative non-allergenic material can be employed. In some patients this manoeuvre is simply ineffective and, in others, continued insertion of a mould produces otitis externa or a discharge from a mastoid cavity. Such cases may benefit from a bone conducting aid worn as a headband with the microphone abutting firmly onto the mastoid. However, these are cosmetically unsightly.


More recent alternatives are bone conduction aids that are anchored in the temporal bone. The external stimulator sets the aid in vibration either across the intervening skin or by a direct percutaneous attachment facility (Fig. 1.22; Fig. 1.53, p. 24). Such aids do not suffer the feedback problems of conventional air conduction aids and also have the advantage of greatly reduced background noise.


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Jun 5, 2016 | Posted by in OTOLARYNGOLOGY | Comments Off on The Ear

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