Congenital Nystagmus (Infantile Nystagmus Syndrome)

1) Congenital motor nystagmusCongenital motor nystagmus is a binocular conjugate nystagmus that is usually horizontal and commonly remains so on upgaze and downgaze (uniplanar). Congenital motor nystagmus can be pendular, jerk. circular, or elliptical. and more than I type may exist in the same individual. The characteristic waveform of congenital motor jerk nystagmus is a slow phase with an exponential increase in velocity. A null point. or neutral zone, may be present; this is a gaze position in which the intensity of oscillations is diminished and the visual acuity improves. If the null point is not in primary position, anomalous head postures may be adopted to damp the nystagmus and provide the best visual acuity. Head bobbing or movement may also be present at first, although this usually decreases with age. Oscillopsia is rare. See Table 25-4. Congenital motor nystagmus is damped by convergence and therefore is often as sociated with esotropia. This combination of nystagmus and esotropia has been termed nystagmus blockage syndrome. a distinction from those cases in which esotropia and nystagmus happen to coexist. Patients with nystagmus blockage syndrome characteristically present with an esotropia that "eats up prism" on attempted measurement and exhibit an increased jerk nystagmus on attempted lateral gaze. Purely congenital motor nystagmus is not associated \vith other central nervous system abnormalities. Visual function can be near normal. Patients develop a preferred gaze position to utilize a null location, thereby increasing foveation time, during which their acquisition of visual information occurs. This head position becomes more obvious as the child reaches school age. Approximately two-thirds of these patients exhibit a paradoxical inversion of the OKN response. Normally, if a patient with right jerk nystagmus views an OKN drum rotating to the patient's left (eliciting a pursuit left, jerk right response), the right jerk nystagmus will increase. However, patients with congenital motor nystagmus exhibit either a damped right jerk nystagmus or possibly even a left jerk nystagmus. This paradoxical response occurs only in congenital nystagmus. Table 25-4 Characteristics of Congenital Nystagmus Bilateral

Conjugate

Horizontal

Uniplanar

Worsens with attempted fixation

Improves with convergence

Null point often present with head position

Two-thirds of patients have "inverted" OKN response

Oscillopsia usually not present

2) Congenital sensory nystagmus Congenital sensory nystagmus, another form of congenital nystagmus, is secondary to a bilateral, pregeniculate, afferent visual pathway abnormality. Inadequate image formation results in fa ilure of development of the normal fixation reflex. If this is present at birth, the resulting nystagmus begins in the first 3 months of life. Its severity depends on that of the vision loss. The nystagmus is typically horizontal and uniplanar and has a waveform identical to that of congenital motor nystagmus. Pendular nystagmus is most common and on lateral gaze, the nystagmus rnay become jerk. Searching, slow, wandering conjugate eye movements may also be observed. Searching nystagmus, defined as a roving or drifting, typically horizontal, movement of the eyes without fixation, is usually observed in children whose vision is worse than 20/200. Pendular nystagmus occurs when the visual acuity is better than 20/200 in alleast [ eye. Jerk nystagmus is often associated with visual acuity between 20/60 and 201 [00. The associated bilateral, pregeniculate afferent abnormality may be obvious, slich as in children with bilateral cataracts or corneal opacities. The ocular abnormality may be more subtle, such as in children with optic nerve hypoplasia or foveal hypoplasia. A child with congenital sensory nystagmus from a retinal dystrophy may have mild vascular attenuation, optic disc pallor, or a completely normal retinal appearance. An electroretinogram may be required for diagnosis. See Table 25- [ for the ocular conditions that can give rise to sensory nystagmus.3) Periodic alternating nystagmus.Periodic alternating nyslagmus (PAN; centml vestibular instability nystagll1us) is an unusual form of congenital motor jerk nystagmus that periodically changes direction. The motion typically starts with a jerk nystagmus in [ direction that lasts for 60-90 seconds and then slowly begins to damp, until it reaches a period of no nystagmus that lasts from 10 to 20 seconds, followed by nystagmus that jerks in the opposite direction in a repeating process. Some children adopt an alternating head position to take advantage of the changing null position~ The cause of congenital PAN is unknown, but this condition has been associated with oculoclltaneolls albinism. 4) Latent nystagmus.Latent nystagmus (fusion maldeve!opmenl nystagmus syndrolne) is a congenital conjugate horizontal jerk nystagmus that is a marker of fusion maldevelopmen!. Latent nystagmus occurs in children with decreased fusion, which results from either early onset strabismus or decreased vision in [ or both eyes. When [ eye is occluded, a jerk nystagmus develops in both eyes, with the fast phase directed toward the uncovered eye. ThLlS, a left jerk nystagmus of both eyes occurs when the right eye is covered, and a right jerk nystagmus of both eyes occurs when the left eye is covered. This nystagmus is the only form that reverses direction as driven by a change in fixation. Similarly, the null point and corresponding head position reverse with a change in fixation (Fig 25-2). The null point is with the fLxating eye in adduction. Therefore, occlusion of the right eye induces a left jerk nystagmus with a null point in right gaze (left head turn). Occlusion of the left eye induces a right jerk nystagmus with a null poiJlt in left gaze (right head turn). Asymmetries in amplitude, frequency, and velocity of the nystagmus also can be present, depending on which eye is covered. Latent nystagmus usually is noted in early childhood, particularly in patients with congenital esotropia and dissociated vertical deviation. The cause is unknown but may be related to the mechanism of vestibular nystagmus, as well as fusion maldevelopment. Because latent nystagmus is induced when an eye is covered. binocular visual acuity is better than monocular, and occlusion must be avoided during monocular tests of vision. Use of polari Zing lenses and a polarized chart, blurring of the non tested eye with a +5.00 D sphere, or repositioning of the occluder several inches in front of the eye not being tested can be effective. Latent nystagmus is damped by fusion and increased with disruption of fusion (such as ocular occlusion). Latent nystagmus may become manifest (manifest latent nystagmus) when both eyes are open but only 1 eye is being used for vision (ie, the other eye is suppressed or amblyopic). Occlusion of the preferred eye results in a change in the direction of the jerk nystagmus. Electronystagmographic evaluation of latent and manifest latent nystagmus reveals a similar waveform with an exponential decrease in velOCity of the slow phase-a pattern opposite that of congenital motor nystagmus, which shows an exponential increase in slow-phase velOCity (Fig 25-3). Acquired Nystagmus

1) Spasmus nutans Spasmus nutans (spasmus nutans syndrome) is an acquired nystagmus that occurs in children during the first 2 years of life, presenting as a triad of nystagmus, head nodding, and torticollis. The nystagmus generally is bilateral and of small amplitude and high frequency (shimmering); however, it can be monocular, asymmetric, and variable in different gaze positions. It can be horizontal, vertical, or rotary and is occasionally intermittent. The head nodding and torticollis appear to be compensatory movements that reduce the frequency and asymmetry of the nystagmus and therefore maximize vision. Spasmus nutans occasionally can be familial and has been present in monozygotic twins. It usuaLly disappears by age 3- 4 years. It is a benign idiopathic disorder in most cases, but nystagmus characteristic of spasmus nutans has been associated with chiasmal or suprachiasmal tumors and retinal dystrophies sllch as congenital stationary night blindness in children. Neuroradiologic investigation is warranted if spasmus nutans occurs with evidenc~ of optic nerve dysfunction (optic disc pallor, relative afferent pupillary defect). As subtle optic disc paLlor and relative afferent pupillary defects are difficult to detect in children, some investigators believe that neuroimaging is indicated for all children with spasmus nutans, although this view is controversial. Electroretinography should be considered in children with spasmus nutans and clinical signs of a retinal dystrophy such as myopia or paradoxical pupils.2) See-saw nystagmusSee-saw nystagmus (vision loss nystagmus), an unusual but dramatic type of nystagmus. has both vertical and torsional components. The name derives from the action of the familiar playground device. If2 eyes were placed on a see-saw, one at either end, they would "roll down the plank" as the see-saw rose, with the high eye intorting and the low eye extorting. As the direction of the see-saw changed, so would that of the eye movement. Thus, the eyes make alternating movements of elevation and in torsion followed by depreSSion and extorsion. This type of nystagmus is often associated with a lesion in the r0stral midbrain or the suprasellar area. In children, the most likely associated intracranial tumor is a craniopharyngioma. Confrontation visual fields may elicit a bitemporal defect. Neuroradiologic evaluation is necessary. The treatment for see-saw nystagmus is removal of the inciting calise.

3) Convergence-retraction nystagmus Convergence-retraction nystagmus (induced convergence-retraction) is part of the dorsal midbrain syndrome associated with paralysis of upward gaze, defective convergence, eyelid retraction, and pupillary light-near dissociation. In the pediatric age group, convergenceretraction nystagmus is commonly secondary to congenital aqueductal stenosis or a pinealoma. It is best elicited on attempted upgaze saccades Ceg, by asking the patient to track a downwardly rotating OKN drum). Co-contraction of all the horizontal extraocular muscles occurs, and the eyes are pulled into the orbit. In addition, the medial rectus muscles overpower the lateral rectus muscles, causing the eyes to converge on attempted upgaze (hence the term convergence-retraction). However, voluntary convergence is minimal. The abnormal eye movements in convergence-retraction nystagmus are actually sacca des. Therefore, convergence-retraction nystagmus is not a true nystagmus and is considered a disjunctive saccadic oscillation. 4) Opsoclonus.Opsoclonus is an extremely rare eye movement disorder that, like convergence-retraction nystagmus, is not a true nystagmus. Rather, it is a bizarre, rapid. and involuntary ocular oscillation. Opsoclonus can be present intermittently and often has a very-high-frequency, low-amplitude movement. The movements are so fast and chaotic that they are not easily confused \vith other forms of infantile nystagmus. Common causes of opsoclonus in children are acute postinfectious cerebellar ataxia and epidemic viral encephalitis. OpSOciOIlUS can also be a paraneopiastic manifestation of occult neuroblastoma.5) Downbeat nystagmus Downbeat nystagmus is a jerk nystagmus with the fast component downward. It obeys Alexander's law (see the section Nomenclature, earlier in this chapter) and is maximum in downgaze and down to the left and right. Downbeat nystagmus often has a null position in upgaze. When congenital, this condition is associated with good vision and normal neurologic findings, although a hereditary form of downbeat nystagmus may precede spinocerebellar degeneration. More commonly, downbeat nystagmus is acquired, secondary to structural abnornlalities such as the Arnold-Chiari malformation. In this condition, the cerebellar tonsils herniate through the foramen magnum, compressing the brain stem and resulting in downbeat nystagmus, Decompression of this area often results in complete resolution. Pharmacologic agents such as codeine, lithium, tranquilizers. and anticonvulsants may also cause this condition. 6) Monocular nystagmus Monocular nystagmus has been reported to occur in severely amblyopic and blind eyes. The oscillations are pendular, chiefly vertical, slow, small in amplitude, and irregular in frequency. 7) Dissociated nystagmus.Nystagmus only in the abducting eye (dissociated nystagmus) occurs in several conditions, the most familiar being internuclear ophthalmoplegia. Myasthenia gravis may simulate an internuclear ophthalmoplegia. Surgical weakening of the medial rectus muscle has been reported to cause a nystagmus of the contralateral abducting eye similar to that seen with internuclear ophthalmoplegia but without the slowing of saccades in the adducting eye that occurs in the latter condition.