▃Pupil Anomalies

Pupil regulates the amount of light, increases the depth of focus, and reduces various optical aberrations. It is regulated by two muscles– sphincter pupillae (supplied by parasympathetic system) and dilator pupillae (supplied by sympathetic system).

Normally, pupils constrict equally. Retina and optic nerve provide the afferent signals, while the oculomotor nerve provides the efferent component to both direct and consensual light reactions. Pupillary disorders may involve:

•The afferent pathways.

•The efferent pathways: It may be parasympathetic or sympathetic. Disorders of parasympathetic system include III nerve palsy, while the interruption of sympathetic nerve supply to the eye causes Horner’s syndrome.

■Afferent Pupillary Defect

It may be absolute or relative. The relative afferent pupillary defect is the most common abnormal pupillary finding which was described by R. Marcus Gunn. Kestenbaum named the findings after Marcus Gunn and Levatin introduced the “swinging flash light test.”

Absolute Afferent Pupillary Defect (Amaurotic Pupil)

It is caused by complete optic nerve lesion, so there is no light perception in involved eye, that is, the involved eye is completely blind. Characteristic features of absolute afferent pupillary defect are as follows:

•Both pupils are equal in size in diffuse illumination.

•Near reflex is normal in both eyes.

Characteristic reactions are listed in Table 23.1.

Relative Afferent Pupillary Defect (Marcus Gunn Pupil)

Relative afferent pupillary defect (RAPD) is caused by an incomplete optic nerve lesion or severe retinal disease. It is never caused by a dense cataract. The causes include:

•Unilateral optic neuropathy: Traumatic and ischemic (arteritic and nonarteritic).

•Optic neuritis.

•Glaucoma: Normally, glaucoma is a bilateral disease; if one optic nerve has particularly severe damage, a RAPD can be seen.

•Compressive damage to optic nerve as in optic nerve tumor or thyroid orbitopathy.

•Radiation optic nerve damage.

Due to incomplete optic nerve lesion, pupils respond weakly to the stimulation of involved eye and briskly to that of normal eye (Table 23.2).

Swinging flash light test: In this test, each eye is stimulated in rapid succession. The light source is alternatively switched from one eye to the other and back. This compares the direct and consensual pupillary constriction of each eye to look for a difference in the afferent conduction between them, which is called a RAPD. It relies on a comparison between the two eyes and is looking for (and can only detect) an asymmetrical abnormality in the afferent pathway (Fig. 23.1).

■Efferent Pupillary Defect

The physiology behind a normal pupillary constriction is a balance between sympathetic and parasympathetic systems. The sphincter pupillae encompass the pupil and is innervated by parasympathetic system which leads to pupillary constriction.

The dilatation is controlled by the dilator pupillae (in the peripheral 2/3rd of iris), which is innervated by sympathetic system.

Etiology

•Brain stem lesions.

•III nerve lesion (fascicular).

•Ciliary ganglion lesion.

•Iris damage.

Characteristic features of efferent pupillary defect are:

There is no response of involved eye, while there is constriction and dilatation in normal eye, so anisocoria prevails. In normal eye, direct and consensual light reflexes are present. In involved eye, direct and consensual light reflexes are absent. Near reflex is also absent. Pupil is fixed and dilated.

■Horner’s Syndrome (Oculosympathetic Paresis) (AN31.3)

Horner’s syndrome results from damage to sympathetic innervation of the eye and is characterized by miosis, partial ptosis, loss of hemifacial sweating as well as enophthalmos.

Etiology

It is caused by an interruption of the sympathetic nerve supply to the eye. Causes of Horner’s syndrome are depicted in Fig. 23.2.

Clinical Features

The classical signs of Horner’s syndrome are as follows:

•A constricted pupil (due to unopposed action of sphincter pupillae with resultant anisocoria, Table 23.3).

•Mild ptosis (due to weakness of Muller muscle).

•Absence of facial sweating (anhydrosis), if the lesion is below the superior cervical ganglion because the sudomotor fibers supplying the skin of the face run along the external carotid artery.

•Slight enophthalmos.

•Pupillary reactions are normal to light and near.

■Pupillary Light-Near Dissociation

The term pupillary light-near dissociation refers to any situation in which pupillary near reaction is present and the light reaction is absent. It is seen in Argyll Robertson (AR) pupil and Adie’s tonic pupil. It is possible to distinguish between the two types of pupil. The accommodation response in AR pupils is brisk and immediate. The near response in tonic pupils is slow and prolonged.

Etiology

Light-near dissociation may be unilateral or bilateral and the causes are as listed in Table 23.4.

Argyll Robertson (AR) Pupil

It is a bilateral small pupil which responds to accommodation reflex but does not react to light. So, the pupil shows light-near dissociation and does not respond to both direct and consensual reactions.

Etiology

It is caused by neurosyphilis and attributed to a dorsal midbrain lesion. Neurosyphilis leading to accommodation reflex present (ARP) causes damage in the area of pretectal to the internuncial neuron. The pupillary near reflex pathway is more ventral than pupillary light reflex pathway, which is why the lesion interrupts the light reflex pathway but spares the near reflex pathway. It results in light-near dissociation. Characteristic features of AR pupil are as follows:

•Involvement is usually bilateral but asymmetrical.

•The pupils are small in size (<2 mm) and irregular in shape.

•The light reflex is absent but near reflex is present (ARP).

•The pupils are difficult to pharmacologically dilate. They dilate poorly with mydriatics like atropine.

Adie’s Tonic Pupil

Etiology

It is due to damage to the ciliary ganglion or postganglionic parasympathetic fibers, following a viral illness, for example, herpes zoster ophthalmicus (HZO). Characteristic features of Adie’s tonic pupil are:

•It typically affects younger women (third or fourth decade) and is unilateral in 80% cases.

•Pupil is large and there exists a difference in the size of the pupils (anisocoria). There is abolition of light reflex with retention of near reflex (i.e., light-near dissociation). The near reflex is slow and tonic, that is, once the pupil has constricted, it remains small for an abnormally long time (tonic pupil).

•On slit lamp examination, vermiform movements of the pupillary border are typically seen.

Pharmacological Testing

It helps in the diagnosis of Adie’s tonic pupil. There is cholinergic supersensitivity of the denervated muscle. In 80 to 90% of patients with Adie’s tonic pupil, 0.125% Pilocarpine causes denervation super sensitivity (i.e., Adie’s tonic pupil constricts with 0.125% Pilocarpine, while normal pupil does not). The concentration is too weak to cause constriction of the normal pupil (0.125% pilocarpine is prepared by diluting one part 1% Pilocarpine with 7 parts balanced salt solution).

Adie’s syndrome is applied when both pupil and associated hyporeflexia are present. The association includes diminished deep tendon reflexes of the knee and ankle– Holmes–Adie syndrome.

Wernicke’s Hemianopic Pupil

It is caused by division of the optic tract which results in a contralateral homonymous hemianopia. The pupils fail to react light when a narrow pencil of light is shone onto the nonseeing part of the retina (i.e., temporal retina of affected side and nasal retina of opposite side), but they do react if it falls onto the seeing retinal areas (i.e., nasal retina of affected side and temporal retina of opposite side).

■Abnormalities of Size

The pupil may be small (miotic) or large (mydriatic) (Table 23.5).

▃Disorders of Cranial Nerves

The cranial nerve (CN) palsies are of neuro-ophthalmological significance. These may be:

•Nuclear ocular motor palsies that occur at the level of ocular motor nucleus.

•Fascicular nerve palsies, which are caused by lesions of the fascicle of nerve, travel through the brain stem from the nerve nucleus to its exit into the subarachnoid space.

•Isolated and multiple cranial nerve palsies.

Nuclear and fascicular nerve palsies are often associated with other neurological signs because of many structures located nearby. Damage to nuclei and nerves results in specific deficits, depending on where the pathology occurs. Therefore, neuroanatomy of midbrain and pons along with knowledge of cranial nerve pathways is helpful in localizing lesions.

■Oculomotor Nerve (IIIrd Cranial Nerve) and its Lesions (AN31.5)

Oculomotor nucleus is located in the midbrain, ventral to aqueduct of sylvius at the level of superior colliculus and composed of subnuclei (or cell masses), subserving the individual extraocular muscles (EOM). These subnuclei are central caudal subnucleus, paired bilateral subnuclei, and Edinger westphal nucleus (EWN).

Central caudal subnucleus is unpaired central nucleus and innervates both levator palpebrae superioris (LPS) muscles. Paired bilateral subnuclei innervates left and right medial recti (MR), inferior recti (IR), superior recti (SR), and inferior oblique (IO) muscles. Each subnucleus innervates the ipsilateral corresponding EOM except SR subnucleus which innervates contralateral SR. Thus, right SR subnucleus innervates left SR muscle and vice versa.

EWN is an additional, bilateral subnucleus which provides parasympathetic input to the ciliary muscle in ciliary body, leading to accommodation and sphincter pupillae resulting in miosis, that is, constriction of pupil.

Course of IIIrd CN is explained in Flowchart 23.1 and depicted in Fig. 23.3.

Lesions of IIIrd CN

IIIrd CN may be affected at various sites through its course and present with distinct ocular and associated neurological signs and symptoms. IIIrd CN lesions may be involved at the level of nucleus, fascicle, subarachnoid space, cavernous sinus, and orbit (Table 23.6).

In partial (incomplete) IIIrd CN palsy, individual muscles or group of muscles are selectively affected. It may involve superior or inferior divisions. Superior division innervates SR and LPS muscles. Inferior division innervates MR, IR, IO, and sphincter pupillae.

Aberrant Regeneration (Misdirection) of IIIrd CN

It refers to erroneous connections (abnormal connections) found on examination after recovery of IIIrd CN from damage for example, fibers originally destined for MR or IR muscle now supply LPS muscle also.

Aberrant regeneration may occur after IIIrd CN palsies result from acute traumatic and compressive (aneurysms and tumors) lesions but never after ischemic or microvascular injury because in traumatic and compressive lesions, endoneural nerve sheath may be breached which, in turn, leads to aberrant regeneration; however, in ischemic injury, endoneural nerve sheath remains intact.

Pupillary Involvement

In compressive lesions, superficially located pupillary fibers are susceptible to damage involving the pupil.

In ischemic disease (diabetes and hypertension), ischemia of main trunk of IIIrd CN spares the superficial pupillary fibers. Thus, pupil is spared in IIIrd CN palsy.

Therefore, a normal pupil implies infarction, while a dilated pupil implies compression of IIIrd nerve (Rule of pupil).

■Trochlear Nerve (IV CN) and its Lesions (AN31.5)

Three important features of trochlear nerve are:

•It is the only CN that exits at the dorsal aspect of brain stem. All others exit ventrally.

•It undergoes an immediate decussation and thus innervates contralateral SO muscle.

•It has the longest intracranial course.

Trochlear nucleus is located in the midbrain at the level of inferior colliculus. It lies caudal to the IIIrd nerve nucleus and ventral to the aqueduct of Sylvius. It receives input from vestibular nucleus, medial longitudinal fasciculus (MLF), and rostral interstitial MLF (riMLF). Course of IVth CN is explained in Flowchart 23.2 and depicted in Fig. 23.4.

Lesions of IVth CN

As the trochlear nerve exits the brainstem dorsally, so IVth nerve fasciculus is quite short. Therefore, isolated lesions that affect only nucleus or fascicle of IVth CN are very rare and usually involve both the nucleus and fasciculus.

Damage within anterior medullary velum (anterior floor of IVth ventricle) causes damage to both IVth CN fascicle at their decussation and results in bilateral superior oblique (SO) palsy. So, when bilateral IVth CN palsy occurs, the site of injury is likely in the anterior medullary velum.

In cavernous sinus, IVth nerve palsy occurs in association with other CN palsies (IIIrd, Vth, and VIth) and oculosympathetic paralysis.

Since 1st division of 5th CN (trigeminal nerve) is also involved, pain may be a prominent feature. Most common cause of an isolated IVth CN palsy is trauma. Clinical features of trochlear nerve palsy are mentioned in Flowchart 23.3.

■Abducens Nerve (VIth CN) and its Lesions (AN31.5)

Abducens nucleus is located at the midlevel of pons, just ventral to the floor of 4th ventricle. It is closely related to the following:

•Paramedian pontine reticular formation (PPRF) (horizontal gaze center).

•MLF.

•1st order sympathetic fibers.

•Spinal tract of trigeminal nerve (Vth CN).

•Facial nerve (VIIth CN): Fasciculus of VIIth CN (facial nerve) wraps around the VIth nerve nucleus. So, isolated 6th nerve palsy is never nuclear in origin.

It contains two types of neurons:

•60% project directly to LR muscle via abducens nerve.

•40% are interneurons which project via MLF to contralateral MR subnucleus and cause adduction of contralateral eye.

Course of VIth cranial nerve is explained in Flowchart 23.4 and depicted in Fig. 23.5.

Lesions of VIth CN

VIth CN lesions may involve the nerve at the level of nucleus, fascicle, cavernous sinus, and orbit.

Nuclear VIth CN Palsy

A lesion in and around the 6th nerve nucleus involves the related structures. Damage to the 6th nerve nucleus or caudal PPRF (horizontal gaze center) causes ipsilateral horizontal gaze palsy. Associated deficits with ipsilateral horizontal gaze palsy (nuclear VIth CN palsy) may be:

•Ipsilateral lower motor neuron (LMN) facial nerve palsy (due to damage to facial nerve fasciculus).

•Ipsilateral preganglionic Horner’s syndrome (due to damage to 1st order sympathetic fibers traveling the pons).

•Ipsilateral facial analgesia (due to damage to spinal tract of Vth CN).

•Ipsilateral internuclear ophthalmoplegia (INO), due to involvement of ipsilateral medial longitudinal fasciculus. So, ipsilateral eye cannot adduct or abduct, while contralateral eye can only abduct. It is called one-and-a-half syndrome.

Fascicular VIth CN Palsy

•VIth nerve fasciculus, involving pyramidal tract, results in VIth nerve palsy with contralateral hemiplegia, since pyramidal tracts decussate in medulla to control contralateral voluntary movements (Raymond’s syndrome).

•Lesions involving VIth nerve fasciculus, VIIth nerve fasciculus, and pyramidal tract results in Millard–Gubler syndrome.

So, patients who have truly isolated VIth CN palsy, nuclear or fascicular involvement is unlikely.

VIth CN palsy at petrous apex results in Gradenigo syndrome which includes:

•VIth nerve palsy.

•Ipsilateral facial pain (Vth CN).

•Ipsilateral facial palsy (VIIth CN).

•Ipsilateral hearing loss (VIIIth CN).

Intracavernous VIth CN Palsy

In cavernous sinus, isolated VIth nerve palsy is rare. Lesions in cavernous sinus are associated with multiple CN palsies. VIth CN is more prone to damage than other nerves because VIth CN is most medially situated and runs through the middle of sinus in close relation to internal carotid artery while other CNs (IIIrd, IVth, 1st division of Vth CN) are protected within the wall of sinus.

Intraorbital VIth CN Palsy

VIth nerve palsy is associated with multiple CN palsies (IIIrd, IVth,).

■Trigeminal (V) Nerve and Its Lesion

It is the largest and mixed (sensory and motor) CN.

Trigeminal nucleus extends through the whole of the midbrain, pons and medulla, and into the high cervical spinal cord. It consists of sensory nuclei (mesencephalic, principal sensory, and spinal nuclei of trigeminal nerve) and one motor nucleus (motor nucleus of the trigeminal nerve) in the upper pons, medial to the principal sensory nucleus (Fig. 23.6).

Course of Trigeminal Nerve

At the level of pons, the sensory root emerges from the sensory nuclei, and the motor nucleus continues to form the motor root (ventromedial to the sensory root). In middle cranial fossa, the sensory root expands into the trigeminal ganglion, located lateral to the cavernous sinus in a depression of temporal bone known as the trigeminal cave. The trigeminal ganglion (Gasserian ganglion) gives rise to three divisions (Fig. 23.7):

•Ophthalmic nerve (V1)– It exits via the superior orbital fissure and enters into the orbit. It carries sensory information from the scalp and forehead, upper eye lid, conjunctiva and cornea of the eye, nose (including the tip of nose except alae nasi), nasal mucosa, frontal sinuses, lacrimal gland, and parts of meninges. In the orbit, it divides into three branches: frontal, lacrimal, and nasociliary.

•Maxillary nerve (V2)– It leaves the skull via the foramen rotundum and supplies the face. It carries sensory information from the lower eye lid and cheek, nares and upper lip, upper teeth and gums, palate, roof of pharynx, and sinuses (maxillary, ethmoid, and sphenoid).

•Mandibular nerve (V3)– The motor root passes inferior to the sensory root (from the Gasserian ganglion) at the level of ganglion and both exit the skull via the foramen ovale. After leaving the skull, both roots unite to form a single trunk as the mandibular nerve. It has mixed sensory and motor fibers. It carries sensory information from the lower lip, lower teeth and gums, chin and jaw, except the angle of mandible, parts of external ear and meninges. The motor fibers supply the muscles of mastication.

Applied Aspect (Disorders of Trigeminal Nerve)

•Corneal reflex—The corneal reflex is the involuntary blinking of the eye lids (Flowchart 23.5).

Afferent limb: In the corneal reflex, the ophthalmic nerve acts as the afferent limb, which is stimulated by tactile, thermal, or painful stimulation of the cornea.

Efferent limb: In the corneal reflex, the facial nerve is the efferent limb causing contraction of the orbicularis oculi muscle.

If the corneal reflex is absent, it is a sign of damage to the trigeminal/ophthalmic nerve or facial nerve.

•Trigeminal neuralgia (Tic Douloureux)

Neuralgia is pain in the distribution of a nerve. Trigeminal neuralgia affects the sensory branches of the Vth cranial nerve.

It is usually idiopathic but may be due to:

•Demyelination of the nerve (multiple sclerosis).

•Petrous ridge compression.

•Post traumatic neuralgia.

•Intracranial tumors.

•Viral infections.

Clinically, it is characterized by pain in the distribution of one or more branches of the trigeminal nerve. The pain has the following clinical characteristics:

•Sudden.

•Unilateral.

•Intermittent paroxysmal.

•Sharp shooting.

•Rarely crosses the midline.

•Short duration.

•Mask-like face.

•Lancinating shock like pain elicited by slight touching.

Its treatment includes:

•Medical treatment.

•Surgical treatment.

Medical treatment includes:

•Oral carbamazepine 100 to 200 mg four times a day as first-line treatment.

•Baclofen and lamotrigine as second-line treatment.

•Others:

◊Clonazepam.

◊Oxcarbazepine.

◊Gabapentin.

◊Topiramate.

Surgical treatment: When drug fails to relieve the pain, surgery is indicated which includes:

•Percutaneous stereotactic radiofrequency thermal lesioning (RFL) of the trigeminal ganglion and/or root. This procedure selectively destroys nerve fibers associated with pain.

•Posterior fossa exploration and microvascular decompression (MVD) of the trigeminal root.

•Gamma knife radiation (GKR) to the trigeminal root entry zone.

■Facial (VIIth) Nerve and Its Lesions

It is a mixed nerve. The sensory component arises from the nucleus solitarius (located in medulla) and carries taste sensations from the anterior 2/3rd of the tongue. The motor component arises from the facial nucleus at the level of pons and innervates the muscles of facial expression. The parasympathetic component arises from the superior salivary nucleus and acts as preganglionic secretomotor component. It supplies lacrimal glands, and submandibular and sublingual salivary glands.

Course of Facial Nerve

The course of the facial nerve can be divided into two parts: intracranial and extracranial.

Intracranial Part

The nerve arises in the pons, as large motor and small sensory roots, and emerges from the brain stem between the pons and medulla. The nerve travels through the internal acoustic meatus in the petrous part of temporal bone. The roots leave the internal acoustic meatus and enter the facial canal (“Z”-shaped structure). Within the facial canal (Fig. 23.8):

•The two roots fuse to form the facial nerve.

•The nerve forms the geniculate ganglion.

•The nerve gives rise to three branches:

◊The greater petrosal nerve (parasympathetic fibers to glands).

◊The nerve to stapedius (motor fibers to stapedius muscle).

◊The chorda tympani (special sensory fibers to the anterior 2/3rd of tongue).

The facial nerve exits the facial canal via the stylomastoid foramen (located posterior to the styloid process of temporal bone).

Extracranial Part

After exiting the skull via the stylomastoid foramen, the facial nerve continues anteriorly and inferiorly into the parotid gland (parotid gland is innervated by the glossopharyngeal nerve, not facial nerve) and supplies the muscles of facial expression by splitting into five terminal branches (Fig. 23.9):

•Temporal branch.

•Zygomatic branch.

•Buccal branch.

•Marginal mandibular branch.

•Cervical branch.

Applied Aspect (Disorders of Facial Nerve)

Central connections of facial nerve nucleus: All voluntary movements depend upon excitation of LMN by upper motor neuron (UMN).

UMN controls LMN through two different pathways: pyramidal tract and extrapyramidal tract.

LMN functions as the final common pathway between the central nervous system (CNS) and skeletal muscles.

•The upper part of facial nucleus receives bilateral supranuclear (cortical) innervation.

•The lower part of facial nucleus receives contralateral supranuclear (cortical) innervation (Fig. 23.10).

The facial nerve has a wide range of functions. Thus, damage to the nerve can produce a varied set of symptoms, depending on the site of the lesion.

Facial nerve lesions may be supranuclear, nuclear, or peripheral type. Peripheral lesions occur due to the following reasons:

•Injury at internal acoustic meatus.

•Injury distal to geniculate ganglion.

•Injury at stylomastoid foramen.

Features of supranuclear type lesions (upper motor neuron lesion [UMNL]) are:

•Paralysis of the lower part of face contralateral (opposite side) to the lesion.

•Partial paralysis of the upper part of face.

•Normal taste and saliva secretion.

•Stapedius is not paralyzed.

Nuclear type lesions (lower motor neuron lesion [LMNL]) are found in pontine. Features of LMNL are:

•Paralysis of facial muscles ipsilateral (same side) to the lesion.

•Paralysis of lateral rectus (due to VIth nerve palsy).

Peripheral lesion: These may be intracranial or extracranial lesions. Intracranial lesions (proximal to the stylomastoid foramen) cause paralyzed or severely weakened muscles of facial expression. The other symptoms produced depend on the location of the lesion and the branches that are affected. The most common cause of an intracranial lesion of the facial nerve is middle ear pathology such as a tumor or infection. Extracranial lesions occur during the extracranial course of the facial nerve (distal to the stylomastoid foramen). Only the motor function of the facial nerve is affected; therefore, resulting in paralysis or severe weakness of the muscles of facial expression. There are various causes of extracranial lesions of the facial nerve, for example:

•Parotid gland pathology, for example, a tumor, parotitis, or surgery.

•Infection of the nerve, particularly by the herpes virus.

•Compression during forceps delivery– The neonatal mastoid process is not fully developed and does not provide complete protection of the nerve.

Facial nerve involvement in herpes zoster is known as Ramsay–Hunt syndrome (also termed as herpes zoster oticus). Facial neuropathy is associated with the presence of vesicles on pinna or external auditory canal.

If no definitive cause can be found, then the disease is termed Bell’s palsy.

Bell’s Palsy

It is less common in those younger than 15 years and in those older than 60 years. Although the cause of palsy is unknown, it may possibly be due to edema of VIIth nerve within the canal caused by viral-induced inflammation or ischemia.

Features of Bell’s palsy are (Fig. 23.11):

•Unilateral involvement.

•Inability to raise eyebrow.

•Inability to puff cheeks (no muscle tone).

•Drooping of angle of the mouth.

•Inability to close eyelid.

•Inability to wrinkle forehead.

•Loss of blinking reflex.

•Slurred speech and inability to smile.

•Mask like appearance of face.

•Loss/alteration of taste.

Management of Bell’s Palsy

Most cases of Bell’s palsy show spontaneous recovery. Eye care focuses on protecting the cornea from drying and abrasion due to problems with lid closure and the tearing mechanism by:

•Lubricating drops applied hourly during the day and a simple eye ointment at night.

•Tapping of lids at bedtime.

Medical treatment includes corticosteroids like prednisolone 1 mg/kg/day for 7 to 10 days. Corticosteroids combined with antiviral drug (acyclovir 400 mg five times/day) are better.

Surgical treatment includes facial nerve decompression, tarsorrhaphy (OP4.7), and botulinum toxin injection to induce ptosis.

▃Nystagmus

Nystagmus is a rhythmic, repetitive, and involuntary to-and-fro oscillation of the eyes. Repetitive movements which are not regular or rhythmic are called nystagmoid movements (not nystagmus).

To understand the mechanisms by which the nystagmus may occur, it is important to discuss the means by which the nervous system maintains position of the eyes. Foveal centration is necessary to obtain the highest visual acuity. Three mechanisms are involved in maintaining foveal centration of an object of interest:

•Fixation in primary position involves the detection of drift in foveal image and corrective eye movement to refoveate the image.

•Vestibulo-oculo reflex involves neural inter connections that maintains foveal fixation during the changes in head position. The proprioceptors of this reflex are the semicircular canals (SCCs) of the inner ear.

•The neural integrator is a gaze-holding network which consists of cerebellum, ascending vestibular pathways, and oculomotor nuclei.

A disorder affecting any of the three mechanisms that control eye movements may result in nystagmus.

Nystagmus can be described in terms of direction of nystagmus, amplitude, frequency, conjugacy, null point, and type of nystagmus.

Direction of nystagmus: It is described in terms of the direction of the fast component. It may be horizontal, vertical, or rotatory (torsional).

Amplitude: It is the excursion of the nystagmus and may be fine (<5°), medium (5°–15°), or coarse (>15°).

Frequency: It is the number of to-and-fro movements in one second (i.e., cycles/sec or Hertz, Hz) with slow, medium and fast being 1 to 2 Hz, 3 to 4 Hz and ≥5 Hz, respectively.

Conjugacy of nystagmus: Conjugate nystagmus is bilaterally symmetrical, that is, both eyes demonstrate the same movement, while in disconjugate (dissociated) nystagmnus, eyes have different movements, for example, one eye has horizontal nystagmus and the other eye has rotatory nystagmus. Dissociation of eyes usually indicates posterior fossa disease.

Null point: It is the point or field of gaze in which nystagmus becomes very faint or absent. Such patients adopt a head posture to keep eyes in the null position.

Type of nystagmus: Could be jerk, pendular, or mixed type. Jerk nystagmus is saccadic with slow movement in one direction followed by fast movement in the reverse direction, which means that it is characterized by phases of unequal velocity. Pendular nystagmus is nonsaccadic and the velocity of nystagmus is equal in both directions, that is, it is characterized by phases of equal velocity. Mixed nystagmus involves pendular nystagmus in the primary position and jerk nystagmus in the lateral gaze.

■Classification of Nystagmus

Nystagmus could be classified into physiological and pathological type. Physiological type consists of end-gaze nystagmus, optokinetic nystagmus and vestibular nystagmus, while pathological could be early onset or acquired.

Physiological Nystagmus

It may be:

•End-gaze nystagmus (at extremes of gaze).

•Optokinetic nystagmus.

•Vestibular nystagmus: may be caloric nystagmus or rotational nystagmus.

End-Gaze Nystagmus

It is found in extreme positions of gaze and there is no nystagmus in the primary position. It is a jerk nystagmus with fast phase in the direction of gaze.

Optokinetic Nystagmus (OKN)

It is physiological nystagmus commonly known as rail-road nystagmus. It is a jerk nystagmus induced by moving repetitive targets across the visual fields or when a person in a mobile vehicle looks at an outside object. In the slow phase (pursuit movement), the eyes follow the target and are controlled by the ipsilateral parieto-occipito temporal (POT) junction. The fast phase (saccadic movements) occurs in the opposite direction, as the eyes fixate on the next target and are controlled by the contralateral frontal eye field in the frontal lobe (see Fig. 16.18). For example, if the OKN drum is moved from right to left, slow phase is to the left and controlled by the left (ipsilateral) POT region, while the fast phase is to the right and controlled by the left (contralateral) frontal lobe. It is useful for detecting malingerers and visual acuity testing in small children.

Vestibular Nystagmus

It may be physiological as well as pathological nystagmus.

Physiological Vestibular Nystagmus

Each SSC of membranous labyrinth contains endolymph which may move toward or away from its ampulla. Horizontal SCC are oriented 30° above horizon with ampulla anteriorly located. Anterior SCC are oriented in vertical plane and directed outward and forward at 45°. Posterior SCC are oriented in vertical plane and directed outward and backward at 45°. The nystagmus varies according to the SCC stimulated. Thus, for maximal effect of rotational forces on horizontal SCC, head is inclined forward 30°. When head is tilted forward at 30° angle, horizontal SCC of both sides are at horizontal plane. For maximal effect of caloric testing (irrigating the ear with cold or warm water), head is inclined backward 60°.

Vestibular nystagmus may be caloric nystagmus or rotational nystagmus. Movement of endolymph in a SCC may be caused by cold or warm water (caloric nystagmus). SCC can also be stimulated by rotation, with the head in a suitable position (rotational nystagmus). Movement of endolymph toward ampulla results in stimulation of that ampulla. Movement of endolymph away from ampulla results in inhibition of that ampulla.

Caloric nystagmus: Vestibular nystagmus may be elicited by caloric stimulation with head tilted back 60° as described below. On irrigation of external ear (say left) with warm water, endolymph rises toward ampulla. Left SCC is stimulated, sending impulse to the left vestibular nucleus. It results in excitation of right PPRF, leading to slow eye movement to the right and fast phase to the left. So, warm water causes fast phase to the same side.

On irrigation of external ear (say left) with cold water, endolymph moves away from ampulla. Thus, inhibition of left SCC and vestibular tone from right SCC dominates, which sends impulse to the right vestibular nucleus. It results in excitation of left PPRF, leading to slow eye movement to the left and fast phase to the right. So, cold water causes fast phase to the opposite side.

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