Nystagmus is a rhythmic biphasic oscillation of the eyes, and the slow phase eye drift initiates the movement. Nystagmus should also be distinguished from other ocular oscillations or nystagmoid eye movements. These other ocular oscillations usually do not have a slow phase and often represent disorders of saccades. Characteristically, they interrupt foveal fixation, and although they can be distinguished clinically, some may be best characterized by eye movement recordings.
This chapter reviews the symptoms and examination of patients with nystagmus and details each of the important types of nystagmus and nystagmoid eye movements and their pathophysiology and management.
Nystagmus
Nystagmus may occur physiologically in response to environmental stimuli (optokinetic nystagmus (OKN)) or from rotation of the head (vestibular-ocular reflex (VOR)). It may also be pathologic and signify damage to the peripheral or central vestibular pathways or visual pathways. One method to organize the different types of nystagmus is to distinguish those cases with jerk properties from those with pendular waveforms and to consider the various types according to the patient’s age ( Table 17.1 ). In pendular nystagmus, the phases are of equal velocity, and there are no corrective saccades. Jerk nystagmus has a slow (pathologic) phase followed by a fast (corrective, position reset) phase in the opposite direction. Entities such as congenital nystagmus and spasmus nutans are more likely to present in childhood.
| Type | Childhood | Any Age |
|---|---|---|
| Jerk | Latent | Vestibular |
| Peripheral and central | ||
| Gaze-evoked | ||
| Physiologic | ||
| Pathologic | ||
| Brun’s | ||
| Dissociated | ||
| Periodic alternating (PAN) | ||
| Downbeat | ||
| Upbeat | ||
| Convergence retraction | ||
| Epileptic | ||
| Drug-induced | ||
| Optokinetic | ||
| See-saw | ||
| Pendular | Spasmus nutans | Oculopalatal |
| Monocular nystagmus due to visual deprivation/loss | Oculomasticatory myorhythmia | |
| See-saw | ||
| Either pendular and/or jerk in the same patient | Congenital |
By convention, the direction of jerk nystagmus is named after the fast phase. Patients who have a slow phase to the left and a corrective phase to the right are said to have right-beating nystagmus. Frequently, jerk nystagmus associated with vestibular disorders will have a torsional component. Torsional nystagmus is named by whether the top pole of each eye beats toward the patient’s right or left shoulder ( Fig. 17.1 ). Pendular nystagmus has no fast phase, so it is usually described by its vector—whether it is primarily torsional, vertical or horizontal (although convergent-divergent, see-saw and other forms exist). Occasionally, nystagmus results from a combination of vertical and horizontal phases to produce a circular, elliptical, or oblique waveform.
Most forms of nystagmus can be interpreted at the bedside. However, eye movement recordings can more accurately characterize the pathophysiologic substrate of the nystagmus. For example, the slow-phase velocity is linear with vestibular nystagmus, increasing in congenital nystagmus, and decreasing in gaze-evoked nystagmus (brainstem/cerebellar).
Symptoms Associated With Nystagmus
The accompanying symptoms may be extremely helpful in discerning the various forms of nystagmus. Nystagmus may be asymptomatic or produce a jumping of the visual environment called oscillopsia. Oscillopsia commonly occurs with acquired nystagmus, and with jerk nystagmus, the environment is perceived to move in the direction of the fast phase. Patients with oscillopsia and dizziness or vertigo are likely to have a vestibular disorder. In contrast, patients with congenital nystagmus typically have a reduction in visual acuity without oscillopsia.
Examination
The examination begins by observing the patient in primary gaze, with attention to the direction and quality of nystagmus or nystagmoid movements, and whether there is tilting or turning of the head, or ptosis. Then the eyes are viewed in all positions of gaze to ensure a normal range of eye movements and to observe whether the nystagmus changes direction or its amplitude in lateral and vertical gaze. Smooth pursuit and saccades should be tested (see Chapter 16 ). An abnormality of either the saccadic or the pursuit system suggests a central cause for the nystagmus. Ocular alignment should also be assessed.
Because damage to peripheral or central vestibular pathways may cause nystagmus, evaluation of the VOR and vestibular system can offer important clues. In a normal individual, when a low-amplitude, rapid impulse of the head to the right is performed, and the patient is asked to fixate on the examiner’s nose, the eyes quickly move to the left (the right horizontal semicircular canal (SCC) is stimulated), and fixation of the target (nose) is well maintained ( Fig. 17.2 and ). In a patient with right unilateral vestibular loss, with a rapid head impulse to the right, the eyes will initially move with the head to the right, and a leftward corrective saccade will be necessary for the eyes to refixate on the target. This is a positive head impulse test (HIT) to the right side and suggests a peripheral vestibular localization with rare exceptions.
Under certain conditions, suppression of the VOR is necessary so that foveation of a target can be maintained despite a head movement. The cerebellum is primarily responsible for suppression or cancellation of the VOR (see Fig. 2.29 ). In this test, combined eye–head movements are assessed by having the patient focus on an outstretched thumb while being rotated in a chair. During this maneuver, the eyes should be stable, and the appearance of saccades suggests that the VOR cannot be adequately suppressed due to cerebellar dysfunction ( ). Abnormalities in VOR suppression and pursuit are almost always seen together unless there is bilateral vestibular loss and no VOR to suppress—in this case, VOR suppression will be less impaired than pursuit.
Caloric stimulation is another method of assessing the VOR and provoking nystagmus. The test procedure itself and mechanism are discussed in detail in Chapter 2 (see Fig. 2.41 ).
These and other methods used to localize a central or peripheral vestibular process are outlined in Tables 17.2 and 17.3 . These assess the VOR, including consequences of static (spontaneous nystagmus) and dynamic (+ ipsilateral HIT) VOR imbalance and the localizing value of provocative neurovestibular maneuvers.
| Provocative Maneuver | Procedure | When to Perform | Physiology/ Interpretation | Relevant Conditions | Notes |
|---|---|---|---|---|---|
| Removal of fixation | Occlusive ophthalmoscopy (see Fig. 17.3 ); Frenzel goggles (see Fig. 17.4 ) | When UVL is suspected; to better evaluate any nystagmus; with persistent or transient oscillopsia | Removal of fixation accentuates or brings out peripheral vestibular nystagmus | UVL as in vestibular neuritis; microsaccadic oscillations otherwise not visible on external examination | With a direct ophthalmoscope, the disc moves opposite to that observed due to its location posterior to the axis of rotation for horizontal and vertical vectors. |
| Dynamic visual acuity | Establish baseline binocular visual acuity; remeasured with 2–3 Hz horizontal and vertical oscillations | When UVL or BVL is suspected | UVL: may lose 2 or less lines of visual acuity BVL: typically lose 4 or more lines | Gentamicin-induced BVL, sequential vestibular neuritis, bilateral Ménière’s; Wernicke’s encephalopathy; CANVAS | Test uses visual acuity chart; distance is better than near. |
| Head impulse test (HIT) | With patient fixating upon examiner’s nose, a rapid horizontal head impulse is applied (see Fig. 17.2 ), and catch-up saccade indicates a hypoactive response | When UVL or BVL is suspected | Positive test when a catch-up saccade is noted | A peripheral sign, used to differentiate vestibular neuritis from stroke in the AVS; any condition causing UVL or BVL | Rarely, a central lesion can cause a positive HIT. |
| Valsalva/tragal compression | Valsalva against closed glottis; Valsalva against pinched nostrils; compression of the tragus or application of positive external auditory canal pressure | When Tullio phenomenon or autophony a is present | With a fistula, abnormal pressure or sound-induced endolymph flow and excitatory (pinched nose Valsalva) or inhibitory (regular Valsalva) nystagmus in plane of AC | Superior canal dehiscence syndrome | Valsalva can also increase ICP and may exacerbate or bring on nystagmus in Chiari. |
| Vibration | Applied to right and left mastoid, vertex | When UVL or BVL is suspected | Excitatory stimulus to the vestibular system; transiently increases vestibular asymmetry if UVL, causing nystagmus with slow phase toward weak side | Vestibular neuritis or other UVL | If vertical nystagmus is seen, suspect a lesion of the central vestibular pathways. |
| Head shaking | 2 Hz passive horizontal and vertical rotations for 10–15 cycles back and forth | When UVL, BVL, or central vestibular lesion suspected | Transiently increases vestibular asymmetry if UVL, causing nystagmus with slow phase toward weak side | Vestibular neuritis or other UVL | If vertical nystagmus is seen, suspect a lesion of the central vestibular pathways. |
| Hyperventilation | 40–60 seconds of hyperventilation | When a peripheral or central vestibular lesion is suspected | Increases CSF pH, increases ICP | UVL; acoustic neuroma; vascular compression of VIIIth nerve; cerebellar/ cervicomedullary disease (DBN) | Spontaneous nystagmus from a demyelinating plaque, as in MS, may improve or resolve transiently with hyperventilation. |
| Positional | Dix–Hallpike (see Table 17.3 ); supine roll test; straight head hanging | When positional vertigo (mainly BPPV) or central vestibular disease is suspected | Localization of side and specific SCC can be made based on pattern of nystagmus | BPPV, cervicomedullary/ cerebellar disease (mainly DBN) | Central cause must be excluded if there is no latency/ fatigability, there is an atypical pattern of nystagmus, or it is refractory to properly performed repositioning maneuvers. |
| Subjective visual vertical (SVV) | By orienting a vertical line to patient’s subjective vertical where environmental visual cues are removed, this is compared with the actual vertical meridian and measured in degrees | When a peripheral or central vestibular (utricle-ocular pathway) lesion is suspected | Measures patient’s perception of subjective vertical to true earth vertical; assesses the utricle-ocular pathways | UVL or peripheral utricle lesion, ipsilesional deviation; in the direction of the OTR (perceptual/ subjective manifestation of utricle injury) in brainstem/ cerebellar lesions | Tilt is common in SVV with an acute brainstem lesion; caudal to ponto-medullary decussation of utricle-ocular fibers (e.g., lateral medullary lesion)—ipsiversive SVV tilt and OTR; rostral to decussation (e.g., medial longitudinal fasciculus lesion)—contraversive SVV tilt and OTR. |
a Autophony is the ability to hear internal sounds such as eye movements, heartbeat, or joint movements, usually due to superior canal dehiscence syndrome.
| Findings | Peripheral | Central |
|---|---|---|
| Latency | Present | Absent |
| Duration | <1 minute | >1 minute |
| Fatigability | Yes | No |
| Nystagmus waveform | Upbeat and torsional in the posterior canal variant; pure horizontal—geotropic or apogeotropic—in the horizontal canal variant | Pure torsional; pure vertical; can have horizontal canal mimics (usually apogeotropic) that are central |
The optokinetic flag (see Fig. 2.30 ) may be helpful in the diagnosis of congenital nystagmus and convergence retraction nystagmus. Normally, the eyes follow (using smooth pursuit) then beat away (using saccades) from the direction of the moving tape. Patients with congenital nystagmus may have a reversal of the normal response in which the eyes beat in the direction of the moving tape. Patients with a dorsal midbrain syndrome often display a characteristic eye movement abnormality, particularly in attempted upgaze, known as convergence retraction nystagmus (saccades). This nystagmus may be elicited by moving the OKN tape downward to induce upward saccades. Other clinical uses are described later in this chapter.
Accurate assessment of afferent visual function is also paramount, because long-standing vision loss and pendular or jerk nystagmus (or mixed pendular and jerk waveforms) suggest congenital sensory nystagmus. The Heimann–Bielschowsky phenomenon (monocular pendular nystagmus) may be associated with optic nerve disease, severe amblyopia, dense cataract, high myopia, or other ophthalmic conditions and is usually monocular (or worse in the eye with poorer vision) and vertical. The examiner should be aware that in patients with congenital nystagmus, visual acuity should be tested with each eye separately then with both eyes open, in case there is superimposed latent nystagmus. The fundus examination might reveal a congenital optic disc abnormality, pigmentary retinopathy, or foveal hypoplasia (albinism) in congenital sensory nystagmus. More often than not, localization of nystagmus to the visual or vestibular pathways (central or peripheral) is possible by evaluation of the visual system, cranial nerves, and cerebellar function.
Pathophysiology of Nystagmus
Nystagmus may result from imbalance or a deficit in visual fixation; the vestibular system; smooth pursuit; vergence; and the optokinetic and neural integrator pathways. Failure of visual fixation, vestibular imbalance, or impairment of the gaze-holding mechanisms are the most common causes of nystagmus. Impaired visual fixation, due to disorders of the visual pathways, for instance, may cause the eyes to drift from an object of regard and cause nystagmus.
The VOR allows images to remain steady on the fovea while the head is moving. Under normal conditions, the VOR generates slow eye movements that are opposite the direction of the head movement. The VOR pathways originate in the three SCCs, synapse in the vestibular nuclei, and travel through the brainstem and cerebellum, ultimately connecting to the ocular motor nuclei subserving horizontal or vertical/torsional movements depending on horizontal or anterior/posterior SCC input, respectively.
The neural integrator is a network of cells responsible for maintaining the eyes in eccentric gaze, by opposing the normal orbital elastic forces that drive the eyes back to midline . When it is defective, the eyes slowly drift back to the midline, and a corrective saccade is generated to return the eyes back to eccentric gaze. This is called gaze-evoked nystagmus. The horizontal neural integrator complex consists of the nucleus prepositus hypoglossi (NPH) and medial vestibular nucleus (MVN), while vertically the interstitial nucleus of Cajal (inC) is the primary structure. The cerebellar flocculus and paraflocculus play a role in both horizontal and vertical gaze holding.
Types of Nystagmus
In the following discussion of the various entities, those presenting predominantly in childhood are reviewed first, followed by those without any particular age predilection (see Table 17.1 ). In a young child with nystagmus, the most important distinction is between congenital nystagmus and spasmus nutans ( Table 17.4 ).
| Features | Congenital Nystagmus | Spasmus Nutans |
|---|---|---|
| Waveform | Mixed waveform (pendular and/or jerk); usually horizontal | Pendular; usually horizontal |
| Symmetry | Symmetric conjugate ocular oscillations | Monocular or asymmetric ocular oscillations |
| Amplitude | Low or high | Low |
| Frequency | Low | High |
| Other features | Horizontal in upgaze Null point | Head turn or tilt Head bobbing |
| Types | Motor (no vision loss) Sensory (vision loss) | Benign Suprasellar mass-related Retina-related |
| Onset | 2–4 months | 4–14 months |
| Course | Waveform may change | Benign type typically resolves by 5 years |
| Other ophthalmic findings | Esotropia, latent nystagmus in some cases | Amblyopia, refractive error |
Congenital Nystagmus
Congenital (or “infantile”) nystagmus is most frequently characterized by its early onset in life, conjugacy, a common pattern of mixed pendular and jerk waveforms, horizontal direction in upgaze, presence of a null point, and lack of oscillopsia. It does not localize to any particular lesion in the central nervous system (CNS), and it may be associated with either normal or reduced vision. Congenital nystagmus may be recognized rarely at birth but much more commonly arises in the second through fourth months of life when visual fixation normally develops. Conjugate oscillations with several types of waveforms can be seen. Classically, congenital nystagmus is a horizontal nystagmus with a pendular waveform, but it may be torsional or rarely vertical in nature. Patients with congenital nystagmus may also demonstrate jerk properties, either in primary or in horizontal endgaze. In the jerk waveforms, the eyes drift during an increasing velocity slow phase, and a subsequent saccade brings the eyes back to foveation. Thus, a mixed waveform consisting of pendular nystagmus in primary position and upgaze and a jerk component in horizontal endgaze is classically seen in congenital nystagmus.
In addition, the waveform at onset in young infancy may differ from what is seen later in the same child at 1 year of age. In one recognized pattern, a young infant first exhibits a large amplitude but low frequency strictly pendular eye oscillation, in so-called triangular wave nystagmus, named after the shape of the waveform ( ). Then the eye movement disorder gradually converts to a smaller amplitude jerk nystagmus or the mixed pendular and jerk nystagmus described previously some time before 1 year of life ( ). The waveform progression reflects the maturation of the visual system.
A reversed optokinetic response may be seen in which the fast phase of the response abnormally moves in the same direction of the moving OKN stimuli. Congenital nystagmus also is one of the few types of nystagmus that usually remains horizontal in vertical gaze. It is this property that helps distinguish congenital nystagmus from gaze-evoked nystagmus. Other forms of nystagmus that remain horizontal in vertical gaze include peripheral vestibular nystagmus and periodic alternating nystagmus (PAN).
Adaptive mechanisms in patients with congenital nystagmus include preferring a head turn or tilt at an angle where the best vision and least amount of oscillopsia are achieved. In this null point or quiet position of the eyes, the nystagmus is most diminished. In addition, convergence can damp congenital nystagmus. In the most common example of the nystagmus blockage syndrome, an individual with congenital nystagmus, attempting to suppress it by converging, develops an esotropia with a head turn. Attempts at fixation may also exacerbate congenital nystagmus.
Despite the fact that their eyes are almost constantly in motion, most patients with congenital nystagmus do not complain of oscillopsia. In congenital nystagmus, visual sampling occurs only during brief foveation periods, allowing optimal viewing of a visual target without a sense of motion despite the abnormal eye movement. Many also have only slightly reduced visual acuities, especially when their visual pathways are normal. Explanations for these observations have included a reduced sensitivity to retinal image motion, adaptation to retinal image motion, information sampled only when the eyes are moving relatively slowly during the short foveation periods mentioned previously, and the use of extraretinal information to cancel the effects of eye movements. When oscillopsia occurs later in life in a patient with congenital nystagmus, breakdown in motor or sensory status due to a decompensated strabismus, refractive error, return of an eccentric null position, acute illness, or worsening of the underlying disorder of the visual pathways should be considered, as should the development of new neurologic disease.
Rarely patients with congenital nystagmus have alternating epochs of right then left beating nystagmus. This so-called congenital PAN can be observed in patients with albinism, but other patients with or without sensory deficits may also display this type of nystagmus. In this form of congenital nystagmus, patients will adapt a slow back-and-forth head posture associated with the alternating null point.
Occasionally patients with congenital nystagmus may also exhibit head oscillations. Whether the head movements are an adaptive strategy to improve vision, as in spasmus nutans, or may represent an effect of a common disordered neural mechanism is uncertain. In addition, strabismus and high refractive error, including astigmatism, are frequently associated with congenital nystagmus. Rarely, congenital-type nystagmus (i.e., congenital nystagmus waveform with eye movement recordings) may emerge later in life, and this is likely to represent a benign process.
Etiology . In one approach to patients with congenital nystagmus, patients are subdivided into those with relatively normal vision (congenital motor nystagmus) versus those with vision loss (congenital sensory nystagmus). While likely an oversimplification and discouraged by some experts, this distinction is nevertheless extremely helpful in the clinic for both the physician and the patient’s family for understanding the mechanism, deciding upon the evaluation, and predicting the visual outcome for a child with congenital nystagmus. The waveforms of the two types are indistinguishable at the bedside.
Congenital nystagmus may occur in isolation with relatively normal vision but completely normal fundus and neurologic examinations. These patients have congenital motor nystagmus, which may be considered to be an efferent pathway disorder, perhaps involving the ocular motor systems involved in visual fixation. Although usually sporadic, congenital motor nystagmus can be inherited in an autosomal-dominant, autosomal-recessive, or X-linked fashion (e.g., the FRMD7 gene). Most such patients have nearly normal visual acuities which remain relatively stable over their lifetime. However, subtle retinal abnormalities have become increasingly appreciated through the use of high-resolution optical coherence tomography (OCT).
Many patients with congenital nystagmus have a component of afferent pathway dysfunction. Common causes of this congenital sensory nystagmus include congenital optic disc abnormalities such as optic nerve hypoplasia or atrophy, ocular albinism, retinal dystrophies such as congenital stationary night blindness or Leber’s congenital amaurosis, or cataracts. Their visual prognosis depends on whether the underlying disorder is static or degenerative.
Multiple potential mechanisms of congenital nystagmus have been proposed. In theory, if there is abnormal development of the (1) visual pathways, (2) ocular motor pathways, (3) extraocular muscle structure or innervation, or (4) ocular proprioceptive feedback mechanism, congenital nystagmus may be the result. In patients with congenital nystagmus, the extraocular muscles have a reduced nerve fiber and neuromuscular junction density, and other features are consistent with degeneration and regeneration and might relate to gaze-holding dysfunction. Theories related to aberrant ocular proprioception causing congenital nystagmus are somewhat controversial and have fallen out of favor. It is also possible that congenital nystagmus results from miswiring of the visual pathways or a maladaption to visual deprivation that leads to inadequate compensation for eye drifts. Alternatively, when humans normally pursue a target, the visual world sweeps across both retinas. Cortical binocular motion centers need to inhibit reflexive subcortical structures involved in OKN; otherwise foveal pursuit would be compromised. In congenital nystagmus, the optokinetic response may be inadequately suppressed.
Evaluation . The workup of a patient with congenital nystagmus requires a careful assessment of visual function and fundus appearance in order to establish whether the condition is sensory or motor. Moderately to severely decreased vision, paradoxic constriction of the pupils when the lights are turned off (see Chapter 13 ), oculodigital reflex, photophobia, or high myopia are suggestive of an underlying retinal disorder. Transillumination of the iris, seen best with a slit lamp, or foveal hypoplasia would be consistent with ocular albinism.
In most patients with congenital motor nystagmus, particularly when the child is developmentally normal, no further workup is necessary. These children can be followed conservatively. In those with congenital sensory nystagmus or when the examination alone is inconclusive, magnetic resonance imaging (MRI) of the brain, OCT, and electroretinography (ERG) are the tests that are most likely to provide useful diagnostic information.
Treatment . Treatment of congenital nystagmus first involves correction of any refractive error. Contact lenses may also damp congenital nystagmus, presumably by enhancing sensory feedback and reducing the abnormal eye movements. Topical brinzolamide was shown to improve CN waveform characteristics in the primary position null zone, as well as acuity measures.
Prism therapy may be used to shift the null point to the primary position or to induce convergence. Prisms over both eyes oriented with the apices in the direction of the preferred gaze may help to direct the line of sight toward primary gaze and thereby minimize abnormal head postures. This will move visualized objects toward the null point so the individual does not have to adapt a head turn. Seven diopter base-out prisms over both eyes can also be used effectively to induce convergence, but −1.00 lenses need to be added to compensate for the induced accommodation.
If prism therapy fails, eye muscle surgery (Anderson–Kestenbaum procedure) may be performed to reposition the eyes and move the null point into the straight-ahead position. In this procedure, for a patient with a face turn to the right (eyes shift to a null point in left gaze), the yoked left lateral rectus and right medial rectus muscles are weakened (recessed) and the left medial rectus and right lateral rectus muscles are strengthened (resected). Tenotomy, in which all four horizontal recti muscles are detached and then reattached at their original site, has been reported to improve foveation times and improve vision, but this is controversial. Tenotomy coupled with strabismus surgery may be a particularly effective treatment to improve visual function in patients with congenital nystagmus and ocular misalignment.
Both memantine and gabapentin may improve visual acuity and foveation times in congenital nystagmus. However, in our clinical experience, the response to these medications is mixed or limited by side effects. Botulinum toxin has also been used in the therapy of congenital nystagmus; however, its effect remains limited by its complications of ptosis and double vision and the need for repeat injections.
Latent Nystagmus
Latent nystagmus (or fusion maldevelopment nystagmus syndrome) is thought to be generated by unequal visual input into both eyes. There is increasing evidence that latent and congenital nystagmus exist together on a spectrum, on which unilateral vision loss causes the former and bilateral vision loss leads to the latter. Latent nystagmus may be explained by a failure to suppress a subcortical monocular optokinetic predominance in the nasal direction as a result of impaired cortical binocular vision (mainly originating in the motion-sensitive visual area V5; see Chapter 9 ). For example, covering the left eye of a patient with latent nystagmus will stimulate the left nucleus of the optic tract (NOT) via connections originating in the right eye, thereby activating the vestibular system and causing a slow leftward (nasal) drift, and a fast (temporal) rightward phase ( Fig. 17.5 , ). Latent nystagmus is also maximal in intensity when the uncovered eye is abducted and decreases when the eye is placed in adduction. Patients with latent nystagmus may perform poorly on tasks requiring monocular viewing because of oscillopsia, and with both eyes open, vision may be much better. Commonly patients with CN will have superimposed latent nystagmus, causing further degradation of visual acuity with monocular viewing. Methods of measuring monocular vision include fogging the other eye with high plus lenses, testing with polarizing lenses, and using the red–green duochrome slide test.
Latent nystagmus is often a benign condition. It is usually unassociated with other neurologic abnormalities, except that patients with periventricular leukomalacia (PVL) seem more prone to developing latent nystagmus. Associated ophthalmologic abnormalities may include (1) congenital esotropia, (2) congenital nystagmus, (3) dissociated vertical deviation, (4) overaction of the inferior oblique muscle(s), and (5) monocular nasotemporal optokinetic asymmetry in which the OKN is more robust when the optokinetic stimulus is directed nasally with monocular testing. Latent nystagmus has a declining slow-phase waveform compared with the increasing slow-phase velocity characteristic of congenital nystagmus. In manifest latent nystagmus, a process that reduces acuity in one eye, such as a cataract, uncorrected refractive error, or amblyopia. In a patient with latent nystagmus, the eye movement disorder may be manifest when a process that reduces acuity in one eye, such as a cataract, uncorrected refractive error, or amblyopia, may cause nystagmus beating towards the other eye to be apparent even without monocular occlusion.
In general, latent nystagmus does not indicate a structural or progressive abnormality of the CNS. Neuroimaging is not indicated in latent nystagmus if the clinical examination shows the typical change in the direction of the waveform with occlusion and there is no history of prematurity.
Spasmus Nutans
Spasmus nutans is characterized by the triad of (1) torticollis, (2) head nodding (2–3 Hz), and (3) monocular or asymmetric nystagmus (see Table 17.4 ). The nystagmus is the hallmark, as both of the other features are not always present ( ). The disorder usually starts between the ages of 4 and 14 months. The nystagmus may last between 1 and 2 years, and the condition typically resolves clinically by 5 years. Spasmus nutans has a pendular waveform of low amplitude and high frequency (up to 15 Hz). However, the amplitude and frequency of the nystagmus may vary with gaze position. The nystagmus is often described as shimmering and horizontal, but it may be vertical or rotary in nature. Rarely the nystagmus may be convergent. Some patients may have subclinical nystagmus detected by eye movement recordings only.
Spasmus nutans is usually a benign condition. However, there are a number of reports that document a similar nystagmus with parasellar and hypothalamic tumors ( ), retinal disorders, and even a report where nystagmus resolved entirely before the diagnosis of chiasmal glioma. The nystagmus and head movements may be indistinguishable from idiopathic spasmus nutans, despite eye and head movement recordings. The most common associated neoplasm is an optic pathway glioma, and signs of afferent pathway dysfunction such as acuity loss, field defects or optic disc atrophy, or endocrinologic abnormalities including poor feeding or diencephalic syndrome (see Chapter 7 ) are usually but not always present. Spasmus nutans–like nystagmus and head movements have also been described in association with retinal diseases such as congenital stationary night blindness, rod–cone or rod dystrophy, Bardet–Biedl syndrome, spinocerebellar degenerations, or vermian agenesis. For unclear reasons, in demographic comparisons of patients with spasmus nutans and congenital nystagmus, the former is associated with lower socioeconomic status, parental drug and alcohol abuse, and African American or Hispanic ethnicity.
Evaluation and treatment . Patients with findings suggesting spasmus nutans should have brain MRI to exclude a mass lesion, since relying on an abnormal endocrinologic history or the presence of decreased vision or optic atrophy is problematic in young children, in whom the examination may be difficult. ERG should be considered when subnormal vision, an abnormal fundus examination, paradoxic pupillary reaction, severe myopia, or photophobia suggest a retinal disorder and the MRI is normal.
Idiopathic spasmus nutans requires no specific treatment, because the disorder usually spontaneously remits. However, careful follow-up by a pediatric ophthalmologist is necessary, because many children with idiopathic spasmus nutans will have amblyopia or strabismus in the eye with the nystagmus of greater amplitude. Refractive errors are also common. In some patients, subclinical nystagmus may persist up to the age of 12 years.
Monocular Nystagmus and Visual Deprivation (Heimann–Bielschowsky Phenomenon)
Children who develop severe visual loss in one eye may develop a slow monocular vertical oscillation of the eye known as the Heimann–Bielschowsky phenomenon. It may appear years after visual loss and can resolve if vision is restored in the affected eye. Since monocular pendular nystagmus can result from a chiasmal glioma and mimic spasmus nutans, its appearance in childhood or adulthood should prompt neuroimaging. Patients with the Heimann–Bielschowsky phenomenon usually have underlying optic nerve disease or amblyopia.
Some adult patients with severe monocular visual loss can develop a similar slow (1–5 Hz) (or, less commonly, fast) vertical pendular nystagmus in the affected eye ( ). The waveform is classically vertical, but horizontal or elliptical oscillations are possible. Again, we recommend neuroimaging in such patients.
One report suggested that the vertical oscillations of the Heimann–Bielschowsky phenomenon can be reduced by gabapentin. However, most patients with the abnormality do not experience bothersome oscillopsia, because the vision in the affected eye is usually diminished to some degree.
Most of the nystagmus types described here that occur at any age (see Table 17.1 ) are acquired forms of nystagmus. The one exception is physiologic endgaze or endpoint nystagmus. The acquired types of nystagmus are usually best classified on the basis of waveform, either jerk or pendular, and this section incorporates this distinction. Acquired jerk nystagmus typically results from imbalance of the vestibular system or dysfunction of gaze holding, and it is often only observed in eccentric gaze.
Vestibular Nystagmus
Vestibular nystagmus may result from dysfunction of the peripheral (labyrinth, vestibular nerve) or central vestibular pathways (root entry zone of VIIIth nerve, vestibular nuclei to ocular motor nuclei).
Peripheral vestibular nystagmus results from asymmetric SCC input. Complete unilateral loss of labyrinth function produces a mixed horizontal–torsional nystagmus that may be suppressed by visual fixation. In an acute destructive peripheral vestibular lesion (involving the labyrinth or vestibular–ocular nerve), the fast phase of the nystagmus is directed away from the lesion. Similarly, cold water in the ear, which causes ipsilateral vestibular dysfunction, produces nystagmus to the opposite side (“COWS”) (see Chapter 2 ). For instance, a patient with left vestibular neuronitis will have a fast component to the right; the torsional component derives from involvement of the anterior and posterior canals, and the torsional fast phase is directed toward the right shoulder. Unilateral peripheral vestibular lesions are common and produce a contralateral horizontal–torsional nystagmus with a linear slow phase, and the horizontal direction does not change with gaze position. Alexander’s law is obeyed, whereby the amplitude of nystagmus increases with gaze directed toward the side of the fast phase. Because vestibulospinal pathways are also affected, patients may experience ipsilesional veering or falling, opposite to the direction of their nystagmus.
Conversely, injury of the central vestibular pathways (brainstem, cerebellar) may cause pure horizontal, torsional, or vertical nystagmus. Such patterns are rarely attributed to peripheral vestibular injury, because very specific combinations of SCC injury are required. Violation of Alexander’s law, whereby nystagmus increases in intensity in the direction opposite to the fast phase, suggests a central vestibular lesion. While the SCCs are angular acceleration detectors, the otoliths (utricle and saccule) are linear acceleration detectors. Commonly with central vestibular lesions, there is concomitant involvement of SCC and utricular pathways. Utricular imbalance can cause an ocular tilt reaction (OTR) (skew deviation, ocular counterroll, and head tilt; see Chapter 15 ), tilt in the subjective visual vertical (SVV), or torsional or vertical nystagmus. Abnormal SVV is the perceptual consequence of utricular injury in which the patient’s subjective vertical is tilted to the right or left with reference to true earth vertical. This can be measured when the patient is asked to align a vertical line as accurately as possible without visual environmental or proprioceptive cues. The perceived vertical relative to actual vertical is measured in degrees, and the tilt occurs in the same direction as the OTR.
The diagnosis and treatment of various central and peripheral vestibular disorders are summarized in Tables 17.5 and 17.6 . These conditions are distinguished by the duration of the vertigo, associated symptoms, neurovestibular and ocular motor examination, and the accompanying signs on examination. Some additional details are described here.
