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Chapter 202 – Nystagmus and Saccadic Intrusions and Oscillations

Chapter 202 – Nystagmus and Saccadic Intrusions and Oscillations

ROBERT D. YEE

DEFINITION

• Fixation instabilities that usually are involuntary and rhythmic. Nystagmus arises from slow eye movement instability. Saccadic intrusions and oscillations result from saccadic eye movement instability.

KEY FEATURES

• Decreased visual acuity.

• Oscillopsia.

ASSOCIATED FEATURES

• Central nervous system abnormalities.

• Strabismus.

• Albinism.

INTRODUCTION

Nystagmus, saccadic intrusions, and saccadic oscillations are fixation instabilities that usually are involuntary and rhythmic. They may impair vision, and many are signs of neurological disorders. By recognizing the specific type of instability, the ophthalmologist can localize central nervous system (CNS) lesions; determine whether laboratory and other tests, such as magnetic resonance imaging (MRI), are needed; and sometimes recommend treatment.

EPIDEMIOLOGY AND PATHOGENESIS

The application of bioengineering principles, the use of electronic recordings of eye movements in humans, and neurophysiological studies in animals have led to many hypotheses about the pathophysiology of nystagmus. Abnormalities of the vestibulo-ocular, otolithic-ocular, smooth pursuit, optokinetic, vergence, and eccentric gaze-holding systems have been postulated.[1] However, the causes of most types of nystagmus are still not known.

OCULAR MANIFESTATIONS

Nystagmus is caused by an abnormality in a slow eye movement system or in the system that holds eccentric gaze. Abnormal slow eye movements take the eyes away from the intended direction of gaze. Fast eye movements or slow eye movements in the opposite direction carry the eyes back. Nystagmus waveforms can be jerky or pendular ( Fig. 202-1 ). If the corrective movements are reflex saccades, the waveform describes a jerk. The slow movements are called slow components or slow phases, and the saccades are called fast components or fast phases. The direction of jerk nystagmus often is designated by the direction of the fast

Figure 202-1 Nystagmus waveforms. The horizontal dashed lines indicate the intended position of gaze. A, Jerk nystagmus with slow components of constant velocity. B, Jerk nystagmus with slow components of exponentially increasing velocity. The flat, slow component portions near the intended gaze position follow the fast components and represent extended foveation periods typical of congenital nystagmus. C, Jerk nystagmus with slow components of exponentially increasing velocity. Extended foveation periods follow slow movements that bring the eye toward the intended gaze position. D, Pendular nystagmus. Note that foveation periods are brief compared with those in B and C. E, Jerk nystagmus with slow components of exponentially decreasing velocity.

components; for example, fast components to the right indicate “right-beating” jerk nystagmus. If the corrective movements also are slow eye movements, the waveform is pendular.

Saccadic oscillations are caused by abnormalities in the saccadic eye movement system. Abnormal saccades move the eyes away from the intended direction of gaze, and corrective saccades carry the eyes back. In saccadic intrusions, such as square-wave jerks and macrosquare-wave jerks, brief pauses occur, or intersaccadic intervals, between the opposing saccades ( Fig. 202-2 ). In ocular flutter and opsoclonus, no intersaccadic intervals occur.

Normal visual acuity requires a stationary retinal image on the fovea. If fixation instabilities cause movement of the retinal image across the fovea at speeds of a few degrees per second or greater, visual acuity is diminished. Therefore, many types of nystagmus and saccadic oscillations without intersaccadic intervals cause deceased visual acuity. During volitional saccades, images move across the retina, but there is no sensation of movement of the visual surround. In contrast, most types of nystagmus and saccadic oscillations without intersaccadic intervals cause illusory, back-and-forth movements of the visual surround, called oscillopsia.

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Figure 202-2 Saccadic intrusions and oscillations. Dashed lines indicate the intended gaze position. A, Square-wave jerks with intersaccadic intervals. B, Macrosquare-wave jerks with intersaccadic intervals. C, Single saccadic pulse and double saccadic pulses. D, Ocular flutter with no intersaccadic intervals. E, Macrosaccadic oscillations following a refixation saccade.

DIAGNOSIS

Most types of nystagmus and saccadic instabilities can be detected and identified, without the aid of eye movement recordings and other specialized equipment, by careful attention to characteristics of the oscillations. While the patient fixates on a stationary target at distance and near, the following questions should be addressed:

• Are the eye movements that move the eyes away from the target slow eye movements (nystagmus) or saccades (saccadic instabilities)?

• Do slow movements occur in one direction and fast movements in the opposition direction (jerk nystagmus), or are the opposing movements of equal speed (pendular nystagmus)?

• What is the direction of the instability (horizontal, vertical, oblique, or torsional)?

• What is the effect of blocking fixation? Does it increase the nystagmus intensity (vestibular nystagmus), or does it decrease the intensity (congenital nystagmus)?

Frenzel goggles to block fixation or electronic equipment to record eye movements in the dark usually are not readily available. Viewing the fundus of one eye with a direct ophthalmoscope while the patient covers the other eye blocks fixation and magnifies motion of the fundus caused by eye movements. The fundus moves in the direction opposite to that of the eye. The direct ophthalmoscope is an excellent instrument with which to detect small-amplitude oscillations such as voluntary “nystagmus” and superior oblique myokymia.

Further questions to address are:

• What is the effect of different gaze positions?

• Does eccentric gaze change the intensity or the direction of the instability?

• Is the instability present only in eccentric gaze?

• Are the oscillations in both eyes symmetrical, or are they asymmetrical with different amplitudes or directions in each eye?

• If no instability occurs in the sitting upright position, is it present in other positions of the body and head (positional vestibular nystagmus)?

With the answers to these questions and information from the patient’s history and other physical findings, the ophthalmologist

Figure 202-3 Identification of types of nystagmus.

Figure 202-4 Identification of types of vestibular nystagmus.

almost always can identify the fixation instability. Figures 202-3 to 202-6 present flowcharts that use this information to identify the types of nystagmus.

DIFFERENTIAL DIAGNOSIS

Vestibular Nystagmus

The characteristics and localizations of several types of vestibular nystagmus are shown in Table 202-1 .

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Figure 202-5 Identification of types of gaze-evoked nystagmus.

Figure 202-6 Identification of types of dissociated nystagmus.

TABLE 202-1 — CHARACTERISTICS AND LOCALIZATIONS OF VESTIBULAR NYSTAGMUS

Nystagmus

Characteristics

Localization

Spontaneous peripheral vestibular

Jerk, horizontal, small torsional, inhibited by fixation

Labyrinth, eighth nerve (acute)

Central vestibular (fixation) nystagmus

Jerk, pendular, horizontal, vertical, torsional, not inhibited by fixation

Brainstem, cerebellum

Sustained positional vestibular

Jerk, horizontal, small torsional, direction fixed, direction changing (static positioning)

Labyrinth, eighth nerve or brainstem, cerebellum

Benign paroxysmal positional

Jerk, dissociated upbeat, latency, not inhibited by fixation, fatigue (Nylen–Barany maneuver)

Posterior vertical canal

Central paroxysmal positional

Jerk, symmetric, upbeat, downbeat

Brainstem, cerebellum

Peripheral Vestibular Nystagmus

Peripheral vestibular nystagmus is caused by an acute imbalance of tonic innervation to the brainstem from the vestibular labyrinths and the eighth nerves. Destructive disorders, such as labyrinthitis and vestibular neuritis, decrease innervation from the affected ear and produce jerk nystagmus with slow components toward that ear and fast components beating toward the opposite side. Irritative disorders, such as Meniere’s disease, increase innervation from the affected ear and generate jerk nystagmus with fast components toward that ear and slow components toward the opposite ear.

Because the vestibular nerve conveys tonic innervation from a horizontal semicircular canal, a pair of vertical canals, and otoliths (saccule and utricle), the nystagmus is mainly horizontal but has vertical and torsional components as well (rotary nystagmus). The slow component has a constant-velocity waveform. Gaze in the direction of the fast component increases the nystagmus intensity (amplitude X frequency), and gaze in the direction of the slow component decreases the intensity (Alexander’s law). Nausea and vertigo with the sensation of rotation of the environment or self-rotation in the direction of the fast component are usually present. Tinnitus, hearing loss, and ear pain also may be present. The nystagmus intensity is high during the first few days but spontaneously decreases. At this time, fixation might inhibit the nystagmus. However, blocking fixation reveals the nystagmus. Imbalance of tonic inputs from the otoliths can cause a transient skew deviation (hypotropic eye ipsilateral to the damaged ear).

Head-Shaking Nystagmus

Rapid head oscillations can produce head-shaking nystagmus. The head is shaken horizontally and vigorously by the patient for 10–15 seconds, and then fixation is blocked. In patients who have peripheral vestibular lesions, a transient, horizontal jerk nystagmus with the fast components to the side opposite the damaged side is induced. Vertical head-shaking can produce a less intense horizontal nystagmus with fast components beating toward the damaged side. In patients who have central vestibular lesions, horizontal head-shaking can induce a downbeat nystagmus or a horizontal nystagmus.

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Central Vestibular Nystagmus

Lesions of the vestibular nuclei, the cerebellum, or the connections between the vestibulocerebellum (flocculonodular lobes) and the brainstem can cause central vestibular nystagmus. In contrast to peripheral vestibular nystagmus, fixation does not greatly inhibit the nystagmus, which leads to the synonymous term fixation nystagmus. Central vestibular nystagmus can be purely horizontal, torsional, or vertical, because horizontal and vertical vestibulo-ocular pathways begin to separate in the vestibular nuclei. Jerk nystagmus in primary gaze that is predominantly torsional is associated with lesions of the vestibular nuclei on the side contralateral to the fast component.[2]

Positional Vestibular Nystagmus

Positional vestibular nystagmus is not present in the sitting upright position but is induced by the supine and lateral positions or by rapid movements of the head and body into head-hanging positions. Fixation suppresses the nystagmus when the cause is a peripheral vestibular lesion but does not suppress it when a central vestibular lesion is present. The nystagmus direction can remain the same in the right and left lateral positions (direction fixed), or it can change (direction changing). The fast components may beat toward the down ear (geotropic) or toward the up ear (apogeotropic). Both peripheral and central vestibular lesions can cause direction-fixed and direction-changing positional nystagmus.

Benign Paroxysmal Positional Nystagmus

Rapid positioning of the head and body into the right or left head-hanging position (Nylen-Barany or Dix-Hallpike maneuver) induces benign paroxysmal positional nystagmus (BPPN). After a delay of 1–2 seconds, an intense vertical nystagmus develops. Fixation does not suppress the nystagmus, and the patient usually complains of vertigo after the maneuver. Characteristic binocular asymmetry exists in which the nystagmus primarily upbeats in the higher eye (i.e., the eye opposite to the head-hanging position) and is oblique and torsional in the lower eye. The asymmetry is explained by the primary and secondary actions of the vertical extraocular muscles stimulated by the posterior semicircular canals (contralateral inferior rectus and ipsilateral superior oblique). The nystagmus dies away over several seconds. Repetition of the maneuver soon after the initial positioning generates a less intense nystagmus (fatigue).

The cause of BPPN is otoconia that have become dislodged from the otoliths (utricular macule) and either are attached to the cupula of a posterior semicircular canal (cupulolithiasis) or freely move in that canal (canalithiasis). Endolymph flow in the posterior canal produces an abnormally prolonged deflection of the hair cells in the crista of the canal. Positional exercises, such as the Epley maneuver, can move the granules back into the utricle and eliminate the positional nystagmus and vertigo.[3] Canalithiasis and paroxysmal positional nystagmus of the horizontal and anterior posterior canals can occur spontaneously or can be produced by repositioning for the posterior canal form of BPPN.[4] The Nylen-Barany maneuver can induce paroxysmal positional nystagmus other than BPPN, such as downbeat nystagmus and other types of central vestibular nystagmus.[5] Therefore, the typical features of BPPN must be present to confirm the diagnosis; it can result from viral labyrinthitis, head injury, and infarction of the inner ear. Most often it is an isolated disorder in the elderly.

Congenital Nystagmus

Congenital nystagmus is one of several common types of nystagmus that occur in children ( Table 202-2 ); it is a high-frequency, horizontal nystagmus that begins in the first few months of life.

TABLE 202-2 — CHARACTERISTICS AND LOCALIZATIONS OF NYSTAGMUS IN CHILDHOOD

Nystagmus

Characteristics

Localization

Idiopathic congenital

Complex waveforms, jerk (increasing velocity slow components), pendular, horizontal, null zone, (face turn, head nodding, no oscillopsia)

Coexisting ocular, visual pathway lesions (not pathogenetic)

Latent/manifest latent

Jerk (decreasing velocity slow components), horizontal, fast components beat toward fixing eye

Coexisting infantile esotropia

Spasmus nutans

Pendular, horizontal, small vertical, torsional, dissociated, high frequency, (torticollis, head nodding), onset in first year, resolution in 1–2 years

No signs of visual pathway lesions

Monocular visual loss

Pendular, vertical, horizontal, monocular, high frequency, intermittent, (occasional head nodding)

Gliomas of optic nerve, chiasm or third ventricle, and other causesof visual loss

It rarely is vertical. Square-wave jerks have been described before the onset of nystagmus and in unaffected parents of patients who have hereditary congenital nystagmus. Congenital nystagmus is not pathogenetically associated with other CNS disorders, although it is found frequently in patients who have certain systemic and ocular disorders that impair vision, such as oculocutaneous albinism and ocular albinism. It can be an X-linked recessive, autosomal dominant, or autosomal recessive disorder. Two families who had autosomal dominant congenital nystagmus were found to have a gene that localized to chromosome 6p12[6] and a translocation between chromosomes 7 and 15, respectively.[7] Congenital nystagmus associated with visual disorders has been called sensory defect nystagmus, and congenital nystagmus with no associated ocular abnormalities has been called motor defect nystagmus. However, the nystagmus characteristics are precisely the same, and the ocular defects and decreased vision probably do not cause the nystagmus.

The nystagmus waveforms are pendular, jerk, or a combination of the two, and many are complex. Occasionally, a congenital nystagmus patient does not have a history of nystagmus from childhood; in such cases, electronic eye movement recordings that reveal one of the complex waveforms can be valuable in differentiating congenital nystagmus from acquired nystagmus. The slow components of the jerk nystagmus often have curved trajectories, with exponentially increasing velocities (see Fig. 202-1 ). Often brief intervals occur when the retinal image is relatively stationary on the fovea, called extended foveation periods, which allows better visual acuity. Unlike in vestibular nystagmus, the effort to fixate increases the nystagmus intensity, while staring and blocking fixation decrease the nystagmus. In contrast to patients who have acquired types of nystagmus, patients who have congenital nystagmus rarely spontaneously complain of oscillopsia.

Patients who have congenital nystagmus often have a direction of horizontal eccentric gaze, called the null zone, in which nystagmus intensity is low, foveation periods are long, and visual acuity is best. A habitual head turn places the eyes in the null zone. High-frequency, low-amplitude head nodding is seen commonly. The head nodding usually does not improve vision. Congenital nystagmus remains horizontal in vertical gaze and usually is decreased at near with convergence. The dampening of

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nystagmus with convergence improves vision, which is one reason why many children who have congenital nystagmus do not need schoolbooks with large-size print. In one form of the nystagmus blockage syndrome, excessive convergence produces an esotropia, fixation of the distant target with the adducted eye, a decrease in nystagmus, and improved vision.[8] In another form, a switch occurs from a congenital nystagmus waveform to a manifest latent nystagmus (MLN) waveform (see the next section) when the adducted eye fixates. In such patients, vision is better with the latter nystagmus.

Latent and Manifest Latent Nystagmus

Latent nystagmus is always associated with strabismus, usually infantile esotropia. In true latent nystagmus, no nystagmus is present with both eyes open. When each eye is occluded, a horizontal jerk nystagmus occurs, the slow components of which are toward the occluded eye and the fast components of which beat toward the uncovered, fixing eye. The shift of fixation is the stimulus for the nystagmus. Electronic eye movement recordings have shown that true latent nystagmus is rare. In most instances, a low-intensity jerk nystagmus (MLN) exists that beats toward the fixing eye without occlusion. Nystagmus intensity increases with occlusion of the nonfixing eye, and the jerk nystagmus reverses direction when the eye that preferentially fixes is occluded. Gaze in the direction of the fast component increases nystagmus intensity, and gaze in the opposite direction decreases the intensity (Alexander’s law). Patients who have MLN can have a habitual face turn toward the direction of the fast component, which places the eyes in the opposite direction and improves vision. Congenital nystagmus patients who have jerk nystagmus also can show reversal of the nystagmus direction when each eye is occluded, as a result of a shift in the position of the neutral point. However, electronic recordings show that congenital nystagmus slow components have exponentially increasing velocities, whereas latent nystagmus slow components have exponentially decreasing velocities (see Fig. 202-1 ). Rarely, patients who have congenital nystagmus also have MLN.

Gaze-Evoked Nystagmus

Several types of gaze-evoked nystagmus exist that are present in eccentric gaze but not in primary gaze ( Table 202-3 ). In gaze-paretic

TABLE 202-3 — CHARACTERISTICS AND LOCALIZATIONS OF GAZE-EVOKED NYSTAGMUS

Nystagmus

Characteristics

Localization

Physiologic, endpoint

Jerk, small amplitude, intermittent, extremes of horizontal and up gaze

Physiologic

Gaze-paretic (symmetric)

Jerk (decreasing velocity slow components) at 30° eccentric gaze

Nonlocalizing (drugs, mental fatigue)

Gaze-paretic (asymmetric)

Jerk (decreasing velocity slow components), horizontal, at 30° eccentric gaze, larger amplitude toward side of lesion

Lesions of brain-stem, cerebellum, cerebral hemisphere

Rebound

Jerk, horizontal, decreases and direction can reverse in eccentric gaze, transient jerk nystagmus on return to primary gaze, fast components beating-toward eccentric gaze

Cerebellum

Myasthenia gravis

Jerk, horizontal or vertical, gradual onset in prolonged eccentric gaze

Myoneural junction (fatigue— increasing transmission block)

nystagmus, no nystagmus occurs in primary gaze, but a jerk nystagmus occurs in about 30° of eccentric gaze. The slow components move the eyes toward primary gaze and have waveforms with exponentially decreasing velocities (see Fig. 202-1 ). Fast components beat toward the intended eccentric gaze position. The drift toward primary gaze results from impairment of gaze-holding mechanisms that involve the nucleus prepositus hypoglossi and medial vestibular nucleus (the “neural integrator”) and their connections with the flocculonodular lobe of the cerebellum. The eye position signal cannot hold the eyes eccentrically in the orbits, so they drift back toward primary gaze.

Normal, physiological, endpoint nystagmus is present in the extremes of horizontal and upward gazes of about 45–50°. Therefore, nystagmus at only 30° is likely to be a pathological finding. Endpoint nystagmus is irregular and might be slightly dissociated (larger amplitude in the abducting eye), which mimics the dissociated nystagmus associated with internuclear ophthalmoplegia. However, the other eye movement abnormalities associated with internuclear ophthalmoplegia are absent.

Symmetrical gaze-paretic nystagmus in which the nystagmus intensity is the same in right gaze and left gaze usually is not a localizing sign. It is produced by mental fatigue; CNS depression from barbiturates, tranquilizers, anticonvulsants, alcohol, and other drugs; and disorders of the cerebral hemispheres, brainstem, and cerebellum. Asymmetrical, horizontal, gaze-paretic nystagmus often is lateralizing. A lesion of the brainstem or cerebellum is generally on the side of greater nystagmus intensity.

Myasthenia gravis can produce a horizontal or upbeat gaze-paretic nystagmus. Initially, little or no nystagmus exists, but as the extraocular muscles fatigue, nystagmus develops. In horizontal gaze, the amplitude of the fast component in the abducting eye is often larger than that in the adducting eye as a result of the greater fatigue of the medial rectus muscle. Normal subjects can have an endpoint nystagmus of very small amplitude that increases with fatigue.

Rebound Nystagmus

Rebound nystagmus is a type of horizontal, gaze-paretic nystagmus in which the jerk nystagmus gradually decreases in amplitude as the eyes remain in eccentric gaze for many seconds. In some instances, the nystagmus direction actually reverses (centripetal nystagmus); for example, it becomes left-beating in right gaze. On return to primary gaze, a jerk nystagmus occurs that beats in the direction opposite to that of the previous gaze-paretic nystagmus. The secondary nystagmus decreases and disappears after several seconds. Rebound nystagmus usually is associated with disorders of the cerebellum. Vertical rebound nystagmus occurs less often. Normal subjects can have a few beats of rebound nystagmus after prolonged eccentric gaze if no fixation target is present on return to primary gaze (lights turned off).

Alternating Nystagmus

The direction of jerk nystagmus changes spontaneously in alternating nystagmus ( Table 202-4 ). In periodic alternating nystagmus, a repetitive cycling of right-beating and left-beating nystagmus occurs in primary gaze. The amplitude of nystagmus gradually increases and decreases over a period of about 90 seconds, followed by a short period of about 10 seconds in which there is no nystagmus, small-amplitude vertical or torsional nystagmus, or square-wave jerks (null period). Nystagmus that beats in the opposite direction increases and decreases over 90 seconds and is followed by a null period. The cycle continues and is not affected by other eye movements, except for strong rotational vestibular stimuli, which can reset the cycle. During periods of jerk nystagmus, patients have horizontal oscillopsia and blurred vision. They might spontaneously turn their heads in the direction of the fast component. This moves the eyes to a position of minimal nystagmus and better vision (null position).

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The null position moves gradually to the right, back to primary gaze, to the left, and back to primary gaze. This type of alternating nystagmus is almost always associated with cerebellar disorders. Ablation of the nodulus and uvula in the monkey produces periodic alternating nystagmus.

Alternating nystagmus also occurs in congenital nystagmus, in MLN, and in association with severe binocular visual loss from many causes (e.g., chronic papilledema, vitreous hemorrhage, cataract). In congenital nystagmus and MLN, the change in nystagmus direction can be caused by a shift of fixation from one eye to the other. However, congenital nystagmus and periodic alternating nystagmus can coexist, for example, in patients with albinism.[9] The periods of alternating nystagmus are not as symmetrical or regular as in periodic alternating nystagmus associated with cerebellar disorders, although shifting of the null positions also occurs.

Upbeat Nystagmus

Upbeat nystagmus that is present only in upgaze and is associated with symmetrical, horizontal, gaze-paretic nystagmus is usually a type of gaze-paretic nystagmus that might not have a localizing significance. However, upbeat nystagmus in primary gaze is caused by lesions that affect the brainstem, especially the lower pontine tegmentum (see Table 202-4 ). [10] Lesions of the medulla, midbrain, thalamus, and cerebellum also can cause upbeat nystagmus. Common causes of these lesions are multiple sclerosis, infarction, intra-axial tumor, Wernicke’s encephalopathy, brainstem encephalitis, and cerebellar degeneration. Rarely, upbeat nystagmus can be a form of congenital nystagmus and might be seen as a transient finding in normal infants. Patients with upbeat nystagmus may have slow components with constant velocity, decreasing velocity, or increasing velocity waveforms. Nicotine can produce a small-amplitude upbeat nystagmus seen in the dark in normal subjects.

Downbeat Nystagmus

Downbeat nystagmus in primary gaze usually is caused by a structural lesion in the posterior fossa at the level of the craniocervical junction (see Table 202-4 ). The nystagmus intensity characteristically increases in horizontal eccentric gaze and may be increased by convergence. Convergence also can convert an upbeat nystagmus in primary gaze to a downbeat nystagmus. Lesions of the cerebellum and pons are associated most often with downbeat nystagmus, and the most common causes are infarction, cerebellar degeneration, multiple sclerosis, and congenital malformations. [11] Downbeat nystagmus may be part of an

TABLE 202-4 — CHARACTERISTICS AND LOCALIZATIONS OF OTHER TYPES OF FIXATION NYSTAGMUS

Nystagmus

Characteristics

Localization

Periodic alternating

Jerk, horizontal, in primary position, regular phases of right-beating, null, left-beating (shifting null position)

Cerebellar nodulus and uvula

Alternating (irregular)

Jerk, horizontal, in primary position, variable, asymmetric phases

Congenital nystagmus, severe, binocular visual loss

Upbeat

Jerk, fast components beat upward

Only in up gaze—part of symmetric gaze—paretic nystagmus; in primary gaze—lower pons

Downbeat

Jerk, fast components beat downward, vertical intensity increases in horizontal gaze

Cerebellum, lower brainstem

acquired syndrome in adulthood consisting of cerebellar ataxia, lower brainstem dysfunction, or cranial nerve palsies caused by Arnold-Chiari malformations (types 1 and 2). Although such malformations are not the most common cause of downbeat nystagmus, a magnetic resonance study of the posterior fossa should be obtained, because surgical decompression can diminish the nystagmus and the other abnormalities in the syndrome. Rarely, a variety of other disorders can cause downbeat nystagmus, including lithium toxicity, magnesium deficiency, vitamin B12 deficiency, midbrain infarction, brainstem encephalitis, Wernicke’s encephalopathy, increased intracranial pressure with hydrocephalus, syringobulbia, cerebellar tumor, and anticonvulsant medication. Downbeat nystagmus has been reported to occur as an inherited congenital disorder. The slow components can have constant velocity, increasing velocity, and decreasing velocity waveforms.

Dissociated Nystagmus

Several types of dissociated nystagmus occur in which the eye movements are strikingly disconjugate ( Table 202-5 ). Nystagmus might be present in only one eye (spasmus nutans, optic glioma, uniocular visual loss, and superior oblique myokymia), larger in one eye than the other (abduction nystagmus in internuclear ophthalmoplegia and ocular myoclonus), or in different directions (see-saw nystagmus and ocular myoclonus). Dissociated nystagmus present in the primary position is often pendular and is often jerk nystagmus in eccentric gaze.

ACQUIRED PENDULAR NYSTAGMUS IN ADULTS.

Acquired pendular nystagmus usually has horizontal, vertical, and torsional components and is often disconjugate. Lesions of the pons, medulla, midbrain, and cerebellum, often caused by multiple sclerosis or infarction, produce oscillations with a typical frequency of 3–4?Hz. MRI studies show large or multiple lesions, which suggests that more than one pathway must be damaged to produce pendular nystagmus.[12] A head tremor might be present. The nystagmus trajectory also can be elliptical or circular. When acquired pendular nystagmus is associated with similar movements of the soft palate, tongue, facial muscles, pharynx, and larynx, it is called ocular myoclonus; this syndrome has also been called oculopalatal myoclonus. The cause usually is an infarction that affects the structures of Mollaret’s triangle and their connections (red nucleus in the midbrain, inferior olive in the medulla, and contralateral dentate nucleus of the cerebellum).

TABLE 202-5 — CHARACTERISTICS AND LOCALIZATIONS OF DISSOCIATED NYSTAGMUS

Nystagmus

Characteristics

Localization

Acquired pendular in adults

Pendular, horizontal, vertical, torsional, disconjugate (coexisting palatal myoclonus)

Brainstem, cerebellum

Superior oblique myokymia

Pendular, jerk, torsional, vertical, high frequency, small amplitude, monocular

Trochlear nucleus

See-saw

Pendular, vertical, torsional, rising eye intorts, falling eye extorts; rarely jerk

Midbrain (interstitial nucleus of Cajal)

Abducting ‘nystagmus’ of internuclear ophthalmoplegia

Jerk, horizontal, decreasing velocity slow components, larger in abducting eye in

Medial longitudinal fasciculus in pons, midbrain horizontal gaze

Abducting nystagmus of myasthenia gravis

Gaze-paretic nystagmus in horizontal gaze, greater paresis of medial rectus muscle

Myoneural junction— myasthenia gravis

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Hypertrophy of the inferior olive and the pendular oscillations begin several months later. Extensive hemorrhage in the pons can produce a large-amplitude, vertical, pendular nystagmus and bilateral horizontal gaze palsies.

SPASMUS NUTANS.

Spasmus nutans occurs in the first year of life and is a triad of pendular nystagmus, head nodding, and torticollis. The nystagmus often is dissociated, and in individual patients it can vary from conjugate to disconjugate to monocular over a few minutes. Its direction is primarily horizontal, but it can have vertical and torsional components. In most patients, the syndrome seemingly resolves spontaneously over 1–2 years. However, electronic eye movement recordings show that a small-amplitude, intermittent, dissociated, pendular nystagmus can persist at least until age 5–12 years.[13] Characteristically, the nystagmus frequency is higher (3–11?Hz) and its amplitude more variable than in congenital nystagmus.

Head nodding is found in most patients who have spasmus nutans. It induces vestibulo-ocular responses that transform the nystagmus into larger-amplitude, slower, binocularly symmetrical, pendular oscillations with improved vision. Spasmus nutans must be differentiated from other disorders that cause head nodding and nystagmus, such as visual loss in children, intracranial tumors, and congenital nystagmus. Children with bilateral vision impairment can have rapid, horizontal, pendular head oscillations; horizontal or vertical nystagmus; and intermittent head tilting during attempts to fixate.[14] The nystagmus can be pendular or jerk, with slow components of constant, increasing, or decreasing velocity.[15] The head shaking seems to be a voluntary, learned adaptation that can improve vision. The diagnostic signs from careful examination of eye and head movements, including electronic recordings, can differentiate spasmus nutans from congenital nystagmus but do not reliably separate spasmus nutans from nystagmus and head nodding due to CNS lesions.[16] Visual loss, optic atrophy, abnormal growth and development, signs and symptoms of CNS disorders, or an older age of onset warrants MRI studies.[17] Some clinicians obtain neuroimages for all patients who have spasmus nutans. Others do not, because the prevalence of CNS tumors in patients without other signs of CNS masses is low.[18]

OPTIC GLIOMA IN INFANTS.

Tumors of the optic nerve, optic chiasm, or third ventricle can produce a high-frequency, pendular nystagmus in infants. Its direction is usually vertical, and it is often monocular. A careful examination to detect visual loss, optic atrophy, and signs of neurofibromatosis type 1 is required to differentiate this type of pendular nystagmus from spasmus nutans. MRI of the orbits and brain is warranted.

MONOCULAR VISUAL LOSS AND BILATERAL VISUAL LOSS.

Children who have monocular visual loss from causes other than optic nerve glioma can have a monocular, high-frequency, small-amplitude, pendular nystagmus.[19] They do not have intracranial tumors, spasmus nutans, or signs of damage to the optic nerve or optic chiasm. The nystagmus can disappear after successful treatment for the monocular visual loss. Adults who have acquired, severe monocular visual loss (e.g., dense cataract) can have a very low-frequency, irregular, vertical drift and jerk nystagmus (Heimann-Bielschowsky phenomenon), which can also be abolished with recovery of vision. Bilateral blindness results from a number of causes and can produce large-amplitude oscillations with small-amplitude ones superimposed. Both oscillations are horizontal and vertical and can have jerk and pendular waveforms. The direction of the jerk nystagmus varies over time (shifting null position). Vestibulo-ocular responses are impaired; volitional saccades and the fast components of vestibular nystagmus may be absent. Head nodding is usually present. Children who have congenital stationary night blindness and rod monochromatism may have small-amplitude, high-frequency, disconjugate, pendular nystagmus similar to that seen in spasmus nutans.

SUPERIOR OBLIQUE MYOKYMIA.

Superior oblique myokymia is a very high-frequency, torsional, and oblique oscillation of one eye that causes monocular oscillopsia and, occasionally, vertical diplopia. Careful observation of the conjunctival blood vessels with a slit lamp or of the fundus with an ophthalmoscope reveals the extremely high-frequency, low-amplitude, pendular oscillations, as well as the occasional jerk nystagmus and tonic intorsion and infraduction that produce diplopia. Electromyogram of the superior oblique muscle has shown abnormal discharges at a frequency of 35?Hz. Superior oblique myokymia usually occurs in otherwise healthy adults, can remit spontaneously, and may recur. Rarely, it is associated with brainstem disorders, such as multiple sclerosis or a pontine tumor.

SEE-SAW NYSTAGMUS.

See-saw nystagmus is a disconjugate, vertical, pendular nystagmus. In one half of a cycle, the rising eye also intorts and the falling eye extorts. The movements are reversed in the other half cycle. See-saw nystagmus is caused most often by large parasellar tumors that cause bitemporal hemianopsia (optic chiasm) and impinge on the third ventricle. Less often, head trauma and infarction of the upper brainstem are the causes. Congenital forms occur, including those in infants who have albinism. In the congenital forms, the rising eye extorts and the falling eye intorts. See-saw nystagmus might be caused by damage to otolithic pathways involving the interstitial nucleus of Cajal, which participate in the ocular tilt reaction. Stereotactic ablation of the interstitial nucleus of Cajal, clonazepam, and baclofen abolish the nystagmus. Rarely, see-saw nystagmus has a jerk waveform, in which case it arises from a unilateral midbrain lesion. The lesion hypothetically damages the interstitial nucleus of Cajal (torsional eye velocity generator) and spares the adjacent rostral interstitial nucleus of the medial longitudinal fasciculus (MLF; torsional fast component generator).[20]

ABDUCTING NYSTAGMUS IN INTERNUCLEAR OPHTHALMOPLEGIA.

In internuclear ophthalmoplegia, horizontal gaze in the direction opposite to the lesion in the MLF in the midbrain or pons induces a jerk nystagmus in the abducting eye and a smaller (or no) nystagmus in the paretic, adducting eye. This abducting nystagmus is the most common type of dissociated nystagmus. It might be simply a gaze-paretic nystagmus with superimposed paresis of the medial rectus muscle ipsilateral to the MLF lesion. However, in many patients, the speed of the exponentially velocity-decreasing waveform of the centripetal slow component is much higher than that found in gaze-paretic nystagmus. The abducting saccade has a characteristic overshooting waveform with a rapid, postsaccadic drift. The hypermetria may be a consequence of an adaptive increase in innervation in response to the weakness of adduction. The saccadic pulse is increased, but the step is not increased proportionately (pulse-step mismatch) or is absent, which results in a rapid centripetal drift. Therefore, the abducting nystagmus might be caused by a train of hypermetric saccades.[21] Physiological endpoint nystagmus also can be dissociated slightly (larger amplitude in the abducting eye), but the other ocular motor abnormalities that are characteristic of internuclear ophthalmoplegia are absent. These abnormalities consist of limitation of adduction, slow adducting saccades, hypermetric abducting saccades, upbeat nystagmus, and skew deviation.

Down Syndrome

Patients who have Down syndrome frequently have nystagmus. The nystagmus types include dissociated pendular nystagmus, horizontal nystagmus of high frequency, and small-amplitude and latent nystagmus or MLN.[22]

Convergence-Retraction Nystagmus and Convergence Nystagmus

Voluntary or reflex upward saccades in Parinaud’s syndrome (dorsal midbrain syndrome) are hypometric and show simultaneous adduction and retraction of both eyes. Co-contraction of antagonist muscles causes the retraction. Optokinetic stimuli that move

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downward elicit upward, reflex saccades and the pattern of convergence-retraction nystagmus. Voluntary convergence can induce downbeat nystagmus and can convert an upbeat nystagmus into a downbeat nystagmus. Pendular convergence nystagmus with alternating convergence and divergence is rare. Convergence-divergence oscillations have vertical and torsional components and can be increased with convergence.[23] Convergence nystagmus has been caused by an Arnold-Chiari type 1 malformation, which resolved with surgical decompression of the foramen magnum.[24] It can also be caused by Whipple’s disease, which produces contractions of the masticatory muscles (ocular masticatory myorhythmia) and a vertical gaze palsy.[25] Antibiotics can resolve the oculofacial-skeletal myorhythmia.

Lid Nystagmus

Upward twitches of the upper eyelids (lid nystagmus) sometimes can exceed the amplitude of the upward fast components in upbeat nystagmus. Wallenberg’s syndrome (lateral medullary syndrome) has a variety of ocular motor abnormalities, [26] including horizontal, gaze-paretic nystagmus with lid nystagmus. Convergence can induce lid nystagmus in patients who have lesions in the medulla or cerebellum.

Epileptic Nystagmus

Involuntary head turns, tonic deviation of the eyes, and nystagmus can be caused by a variety of seizures. In general, when an epileptic focus occurs in the parietal-temporal-occipital lobe, the eyes deviate to the contralateral side, and a horizontal jerk nystagmus is seen with fast components beating toward that side. The fast and slow components are confined to the contralateral field of gaze, and the nystagmus might occur as a result of activation of cortical saccadic regions in the cerebral cortex.[27] Ipsiversive deviation of the eyes and nystagmus with ipsiversive slow components might arise from an epileptic focus in the temporal-occipital cortex that activates a cerebral cortical area for smooth pursuit.[28] Pendular, torsional, or convergence nystagmus also can occur with epilepsy.

Ocular Bobbing

Stupor and coma are associated with several ocular abnormalities, including ocular bobbing. Intermittent, irregular, conjugate, downward saccades are followed by slower, upward drift movements. Patients who have ocular bobbing have extensive damage to the pons from hemorrhage or compression or have toxic or metabolic encephalopathies. Several variants of ocular bobbing exist. In inverse bobbing, or ocular dipping, downward, slow movements are followed by upward saccades back toward the primary position. In converse bobbing, or reverse ocular dipping, large-amplitude, upward saccades are followed by downward drifts.

Saccadic Intrusions

Reflex saccades to objects that enter the visual field are mediated through pathways from the visual association areas of the parietal lobes and temporal lobes, and from ocular motor fields in the frontal lobes. They project to the superior colliculi and the saccade-related areas of the brainstem. Normally, reflex saccades to these sites can be inhibited voluntarily. Pathways from the frontal lobes to the basal ganglia (pars reticularis of the substantia nigra) and superior colliculus might be important for the inhibition of reflex saccades. Patients who suffer frontal lobe diseases, including Alzheimer’s disease, Huntington’s disease, progressive supranuclear palsy, and schizophrenia, have inappropriate saccades that interrupt fixation. These saccadic intrusions have been called the “visual grasp reflex” (see Fig. 202-2 and Table 202-6 ).

TABLE 202-6 — CHARACTERISTICS AND LOCALIZATIONS OF SACCADIC INTRUSIONS AND OSCILLATIONS

Type

Characteristics

Localization

Square-wave jerks

Horizontal, 1–5°, 200?msec intersaccadic intervals

Not localizing

Macrosquare-wave jerks

Horizontal, 10–40°, 100?msec intersaccadic intervals

Cerebellum

Macrosaccadic oscillations

Horizontal saccadic dysmetria, series of hypermetric saccades, 200?msec intersaccadic intervals

Cerebellum

Voluntary “nystagmus”

Horizontal, high frequency, low amplitude, intermittent, no intersaccadic intervals

Volitional

Saccadic pulses

Horizontal, single or double saccades with no steps

Cerebellum, lower brainstem

Ocular flutter

Horizontal, large amplitude, no intersaccadic intervals

Cerebellum, lower brainstem

Opsoclonus

Multidirectional, large amplitude, linear and curvilinear trajectories, no intersaccadic intervals

Cerebellum, lower brainstem

SQUARE-WAVE JERKS.

Normal subjects have infrequent, small-amplitude (less than one to a few degrees), horizontal saccades that move the eyes away from the fixation target and then back to the target, called square-wave jerks (see Fig. 202-2 ). Pause occurs between the to-and-fro saccades (intersaccadic interval) of about 200?msec, which allows sufficient foveation time for normal visual acuity with no oscillopsia. The frequency of square-wave jerks increases in the dark. Larger (1–5°) and more frequent (>2?Hz) square-wave jerks are abnormal and are associated with cerebellar disorders, progressive supranuclear palsy, Huntington’s disease, and schizophrenia. They occur sporadically or in bursts.

MACROSQUARE-WAVE JERKS.

Macrosquare-square wave jerks are horizontal and large (10–40°) and have intersaccadic intervals of about 100?msec. They are found in cerebellar disorders (e.g., multiple sclerosis and olivopontocerebellar atrophy) and occur sporadically or in bursts.

MACROSACCADIC OSCILLATIONS.

Macrosaccadic oscillations are a type of saccadic dysmetria. A hypermetric saccade overshoots the target and is followed by a series of hypermetric, corrective saccades that straddle the target and gradually decrease in size until the target is fixated. The intersaccadic intervals are 200?msec long. Macrosaccadic oscillations are associated with cerebellar disorders.

VOLUNTARY “NYSTAGMUS.”

Normal subjects can voluntarily produce bursts of high-frequency (10–20?Hz), small-amplitude (a few degrees), horizontal, saccadic oscillations, called voluntary “nystagmus.” This is not a true nystagmus, because it consists of to-and-fro, back-to-back saccades. Because no intersaccadic intervals occur, visual acuity is poor, and oscillopsia is present during the oscillations. Voluntary “nystagmus” cannot be sustained for more than several seconds; subjects show signs of intense effort, such as squinting, facial muscle contractions, and convergence.

SACCADIC PULSES.

Saccadic pulses are saccadic intrusions in which saccades move the eyes away from the fixation target, followed by a rapid drift back to the target (glissade). They represent saccadic pulses without steps; they can occur singly, in a series, or in a train (saccadic pulse train) that mimics nystagmus (abducting nystagmus of internuclear ophthalmoplegia). Saccadic pulses occur in normal subjects, patients who have myoclonus, and patients who have multiple sclerosis. Double saccadic pulses are pairs of saccadic pulses that move in opposing

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directions and occur back-to-back with no intersaccadic intervals. They are part of a continuum of other saccadic oscillations with no intersaccadic intervals (ocular flutter and opsoclonus).

OCULAR FLUTTER.

Ocular flutter consists of bursts of moderately large-amplitude, horizontal, back-to-back saccades without intersaccadic intervals. Blurred vision and oscillopsia usually are present. Ocular flutter can occur in the primary position and after a refixation saccade (flutter dysmetria); it is associated with the same disorders of the brainstem and cerebellum that produce opsoclonus (see next section). Eyelid blinks induce bursts of large-amplitude flutter in neurodegenerative disorders and a few beats of low-amplitude flutter in normal subjects.

OPSOCLONUS.

In opsoclonus, a series of large-amplitude, back-to-back, multidirectional saccades interrupt fixation. The directions of the to-and-fro saccades can be horizontal, vertical, or oblique; their trajectories can be linear or curvilinear, and the frequency is high (10–15?Hz). The chaotic appearance of the oscillations has led to the use of the term “saccadomania.” In its severe form, opsoclonus is nearly continuous and persists even in some stages of sleep. With improvement, or in its milder form, the oscillations are intermittent. During fixation, saccadic burst cells in the pontine paramedian reticular formation (horizontal saccades) and in the rostral interstitial nucleus of the MLF (vertical saccades) are inhibited by tonic activity in pause cells in the nucleus raphe interpositus in the midbrain. Pause cell activity is momentarily inhibited during saccades, which allows the burst cells to fire and generate the saccadic pulse signal. An abnormal decrease in pause cell activity as a result of direct damage to these cells or abnormal input to them from other neurons might produce opsoclonus and ocular flutter.

Opsoclonus often is associated with cerebellar ataxia and limb myoclonus. The disorders that cause opsoclonus damage the brainstem or cerebellum; they include benign brainstem encephalitis in children and adults following viral illnesses, myoclonic encephalopathy of infants (dancing eyes and dancing feet), paraneoplastic brainstem and cerebellar syndromes in children (neuroblastoma) and adults (small cell lung carcinoma, breast carcinoma, ovarian tumors), and multiple sclerosis.[29] [30] Opsoclonus and ocular flutter have been reported in association with drug toxicities, exposure to toxic chemicals, and hyperosmolar coma and as transient findings in normal neonates. Adrenocorticotropic hormone can diminish the saccadic oscillations of infantile myoclonic encephalopathy and neuroblastoma, and corticosteroids can be effective in paraneoplastic syndromes in adults. Some normal subjects can produce saccadic oscillations volitionally, including many of the characteristics of opsoclonus and ocular flutter.[31]

TREATMENT

Drug Treatment

The goal of treating vestibular nystagmus is mainly to diminish the associated vertigo. The large number of medications that are used is an indication that no optimal drug therapy exists for most patients. The classes of drugs include anticholinergics (scopolamine [hyoscine]), antihistamines (meclizine), monoaminergics (ephedrine), benzodiazepines (diazepam), phenothiazines (prochlorperazine), and butyrophenones (droperidol). Unfortunately, drowsiness from many of these drugs limits their efficacy for chronic, recurrent vertigo. An exception is acetazolamide, which is very effective for the treatment of familial periodic ataxia with nystagmus. [32]

The goal in the treatment of nonvestibular forms of nystagmus and saccadic oscillations is to improve vision by ameliorating the associated blurring and oscillopsia. As in vestibular nystagmus, many medications have been tried, but few have been found to be consistently effective.[33] Only a few double-blind studies have been carried out: anticholinergics for acquired pendular nystagmus,[34] and muscarinic antagonists for acquired pendular and downbeat nystagmus.[35]

Baclofen is an analog of ?-aminobutyric acid and was developed to treat skeletal muscle spasm. It consistently decreases the symptoms and signs of periodic alternating nystagmus.[36] To diminish the side effect of drowsiness, the initial dosage is low, 5?mg by mouth three times a day, and is increased gradually. Patients perceive a return of symptoms after a few hours. The drug is not taken at bedtime, because the beneficial effects are not appreciated. Some patients who have congenital nystagmus report that their vision improves with baclofen, and in a few patients, nystagmus has been found to decrease slightly with an increase in the null zone of least nystagmus intensity. Baclofen decreases the slow component velocity and oscillopsia in some patients who have upbeat and downbeat nystagmus.[37] Its therapeutic effect might result from augmentation of the physiological inhibitory effect of ?-aminobutyric acid on the vestibular nuclei in the vestibulocerebellum and on the velocity storage mechanism.

Clonazepam is an antiepileptic agent that has been shown to decrease downbeat nystagmus in some patients.[38] Its most common side effect is drowsiness. The initial dosage of 0.5?mg by mouth three times a day is increased gradually. Recently, gabapentin (900–1500?mg/day) has been shown to decrease, but not abolish, acquired pendular nystagmus in a few patients.[39] Adrenocorticotropic hormone can diminish ocular flutter and opsoclonus in infantile myoclonic encephalopathy and neuroblastoma. Corticosteroids can decrease these saccadic oscillations in paraneoplastic, cerebellar ataxia syndromes in adults. Carbamazepine, baclofen, and clonazepam have been used to treat superior oblique myokymia. Isoniazid in dosages of 800–1000?mg/day decreased acquired pendular nystagmus in two patients who had multiple sclerosis. [40]

Optical Treatment

In congenital nystagmus, convergence and eccentric gaze often decrease the nystagmus and improve vision. To induce convergence, 7D (prism) of base-out prism can be placed in each spectacle lens. If the patient is young, -1.00D can be added to the spherical correction. If the null zone is in horizontal eccentric gaze, the spectacle prism powers can be modified to incorporate a prism effect in which the eyes conjugately rotate toward the null zone (prism apices toward the null zone). Contact lenses have fewer optical aberrations and usually correct the refractive errors in patients who have congenital nystagmus more effectively than do spectacles. In addition, tactile sensory feedback from the contact lenses might diminish the nystagmus intensity. One congenital nystagmus patient reported transient oscillopsia when contact lenses were removed after a short therapeutic trial.[41]

The combination of a high plus spectacle lens and a high minus contact lens for one eye has been devised to stabilize retinal images in that eye and improve vision.[42] This combination places the image at the eye’s center of rotation. However, because vestibulo-ocular eye movements and volitional eye movements do not cause retinal image movement with these lenses, walking is difficult.

A patient who had ocular myoclonus was found to have vertical pendular nystagmus in one eye and horizontal pendular nystagmus in the other. Patching the eye that had vertical nystagmus caused an esotropia of that eye but resulted in disappearance of the horizontal nystagmus in the other.[43]

In patients who have MLN, spectacle treatment for an accommodative component of their esotropia can transform MLN to latent nystagmus or decrease MLN, each of which leads to an improvement of binocular visual acuity.[44]

Surgical Treatment

An eccentric null zone in congenital nystagmus often produces a habitual face turn. If marked, the face turn can cause difficulties

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when viewing at distance or reading; in addition, it can be a cosmetic problem. Resections and recessions of the four horizontal rectus muscles can move the null zone toward the primary position (Anderson-Kestenbaum procedure), which improves vision in the primary position.[45] [46] However, months after the surgery, the null zone may become eccentric again. If convergence decreases congenital nystagmus significantly, surgery to induce a greater convergence effort can be combined with the Anderson-Kestenbaum procedure.[47] Large recessions of all four horizontal rectus muscles are designed to symmetrically weaken the muscles and can reduce nystagmus intensity. Diplopia and limitation of ductions reportedly have not been significant problems.[48] [49] Occasionally, bilateral vertical rectus recessions or bilateral, combined vertical rectus recession-resections are performed to correct vertical head postures in congenital nystagmus.[50] Rarely, surgery of the oblique muscles is used to correct a head tilt.[51]

In patients who have strabismus and MLN, strabismus surgery can change the MLN to latent nystagmus and improve binocular visual acuity.[44] In Arnold-Chiari malformations, suboccipital decompression can diminish downbeat nystagmus if permanent damage to the midline cerebellum and lower brainstem has not occurred. Procedures to weaken the superior oblique and ipsilateral inferior oblique muscles have been used to treat superior oblique myokymia.

Other Treatments

A variety of other therapies have been used to treat nystagmus. Of these, the Epley maneuver for BPPN is by far the most effective.[3] Tactile stimulation of the face and neck,[52] auditory biofeedback, and acupuncture have been shown by electronic recordings to decrease congenital nystagmus. However, their efficacy outside of the laboratory setting has not been established. Retrobulbar injections or intramuscular injections of botulinum A toxin decrease nystagmus by paralyzing the extraocular muscles. They have been used to treat congenital nystagmus,[53] latent nystagmus,[54] and acquired nystagmus.[55] The paralysis is temporary, requiring repetition of the injection every few months. The side effects are diplopia, ptosis, filamentary keratitis, and increased nystagmus in the noninjected eye from plastic-adaptive changes in response to the paresis of the injected eye.[56] [57]

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