Nystagmus
Nystagmus can present a difficult diagnostic challenge for even the most experienced clinician. The varied etiologic factors include genetic and developmental anomalies, as well as active pathology (which can occur at any age). When nystagmus is present, patients often experience considerable anxiety; while hoping for the best, they are forced to rely on the clinician’s knowledge, experience, and management skill. To properly manage nystagmus patients, it is necessary to describe/diagnose the relevant characteristics, classify the condition, identify the possible causes and associations, and determine and implement appropriate management.
This chapter describes examination techniques, differential diagnosis, and treatment for many of the common types of nystagmus seen in clinical practice. The examination and diagnostic emphasis is on determination of the underlying disorder, because nystagmus is typically not a discrete disease entity. In many instances, the clinician’s primary consideration is to identify and provide access to treatment of the underlying condition. When nystagmus treatment is indicated, therapy is designed to dampen the oscillations and reduce symptoms. In most cases, functional (and sometimes cosmetic) improvement is possible.
Virtually, all nystagmus patients should have aggressive management following a careful diagnosis. It is not acceptable to simply monitor these patients without treatment. It is possible to improve acuity, ocular motor control, cosmesis, and visual comfort using sequential considerations of (a) correction of the refractive error with spectacles or contact lenses; (b) prisms to improve fusion, induce convergence, and reduce a head turn; and (c) vision therapy to improve fusion capability and enhance stability and accuracy of fixation (foveations). Surgery and medication can also be recommended in certain cases.
Etiology and Prevalence
DEFINITION AND ETIOLOGY
Nystagmus (involuntary rhythmic oscillations of one or both eyes) may be the presenting sign of either a pathologic afferent visual pathway lesion or a disorder in ocular motor control.1 Nystagmus can be considered a disorder of the mechanisms that keep fixation/eye position stable.2 The pursuit, optokinetic, and vestibular systems act to maintain a steady image on the retina.3 Any lesion that creates an imbalance in these neurologic systems can make the eyes drift off target, requiring a saccadic movement to regain fixation; continual drifts and refixation saccades result in horizontal or vertical nystagmus. Torsional nystagmus results when there is instability of the vertical meridian of the cornea requiring continual repositioning.
The appearance of nystagmus causes considerable distress for parents and other family members. Its presence usually is interpreted as a sign of serious visual dysfunction or possibly brain damage. Patients with nystagmus challenge a clinician’s skills in differential diagnosis, sensitive communication, and management.
PREVALENCE
Nystagmus affects about 1 in every 5,000 to 10,000 persons.4 It is much more prevalent, however, in patients who have certain ocular or systemic health conditions. For example, a large percentage of persons with albinism have nystagmus,5 and 13% of cerebral palsy patients have nystagmus.6 Approximately 10% to 15% of visually impaired school-aged children have nystagmus.7 Nystagmus that starts before age 6 months is termed infantile nystagmus.8
Characteristics
Detailed observation and a comprehensive case history usually provide sufficient information to determine the type and cause of nystagmus. Complete description of nystagmus requires evaluation of postural affections, type
and direction of oscillations, amplitude and frequency of the nystagmus, symmetry between the movement of the two eyes, constancy, and the latent component, as well as evaluation of movements in all fields of gaze and with convergence (Table 18.1).
and direction of oscillations, amplitude and frequency of the nystagmus, symmetry between the movement of the two eyes, constancy, and the latent component, as well as evaluation of movements in all fields of gaze and with convergence (Table 18.1).
Table 18.1 CLINICAL CHARACTERISTICS OF NYSTAGMUS | ||||||||||||||||||||||||||||||||
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As in the strabismus examination, it is essential to initially make an overall observation of the patient’s general posture, head position, and facial characteristics. These observations can be made by visual inspection at three fixation distances in the primary position: (a) distance fixation (4 to 6 m); (b) near fixation (50 to 100 cm, simulating a common distance of social interaction); and (c) at the preferred reading distance. The same observations should be repeated with the patient assuming the preferred head position and at the computer if the patient spends much time on this task.
The importance of these observations is that they allow determination of the patient’s performance in real life as well as examination situations. Because the oscillations present in some forms of nystagmus decrease in certain fields of gaze, many patients have a habitual head tilt or face turn. The cosmesis of the nystagmus and other disfiguring conditions (e.g., facial asymmetries, ptosis, or strabismus) can be a major social concern of the parents or patient. In addition to influencing cosmetic and functional considerations, careful observation may provide diagnostic information concerning the cause of the nystagmus.
Observation of the characteristics of nystagmus should be made under magnification using the slit lamp, a high plus lens or loop and a penlight, or an ophthalmoscope. Slit lamp inspection, which is preferable, is usually possible with children aged 2 years or older. An assistant can attract a young child’s attention with sounds or small toys.
Primary diagnostic characteristics include the type and direction of oscillations. Nystagmoid eye movements are typically classified clinically into two broad categories: jerk and pendular.
Jerk movements have both quick and slow components (Fig. 18.1A). There may be intervals, known as foveations, in which the eye movement is relatively slower for a short duration as the target crosses the fovea and then the velocity increases. At some point, there is a fast corrective saccade to bring the eyes back on target. If the foveations are of sufficient duration (approximately 60 ms) and accuracy, visual acuity
may not be significantly impaired.9 Differential diagnosis of patients who can be helped with therapy will thus depend on determining the accuracy and duration of the foveations. Patients with short-duration inaccurate foveations are much more likely to have a satisfactory increase in acuity with a vision therapy program.
▪ Figure 18.1 A: Eye movement traces for a patient with jerk nystagmus. There is a fast phase (saccade) toward the top of the figure and then a drift away from the target. B: The eye movement trace shows pendular nystagmus. There are approximately equal velocity movements in both directions, with a fairly smooth transition between directions of movement (sinusoidal).
Pendular movements consist of to-and-fro eye movements of approximately equal velocity in each direction (Fig. 18.1B). A pendular waveform can be sinusoidal (smooth transition to the opposite direction) or triangular (an abrupt direction shift). Because patients with pendular nystagmus do not typically have appreciable foveations, a vision therapy program that can develop accurate moderate duration (60 ms) foveations will provide a welcome acuity increase.
Occasionally, the waveform (and sometimes the direction of the waveform) alternates between jerk and pendular as a function of position of gaze or time. This is more common in patients with infantile nystagmus. Research10 shows that “horizontal” nystagmus typically includes a torsional component and that, in many patients with infantile nystagmus, the waveform is actually seesaw, with a large horizontal component, a smaller torsional one, and an even tinier vertical component.11 There are also patients who alternate the direction of the fast phase of the waveform, generally as a function of time (periodic alternating nystagmus [PAN]). Figure 18.2 shows many of these variations, which can easily be missed if eye movements are not inspected under magnification or evaluated with eye position detectors.
Nystagmus is described clinically with reference to the horizontal (x), vertical (y), and rotary (z or anterior polar) axes, because the eyes may move in one, two, or all three directions. Most of the time the clinical description is of the most visible movement (e.g., jerk right) because the other, more complex components (vertical and torsional) are not easily recognized without eye movement recordings. Jerk nystagmus is characterized by the direction of the fast component (right, left, up, or down) in addition to description of the type of movement (e.g., jerk right nystagmus). Describing jerk nystagmus based on the direction of the quick phase can be misleading because the slow phase of nystagmus usually reflects the underlying abnormality. The eyes drift from the target because of a neurologic imbalance in the position maintenance (fixation) system, and the quick phase of nystagmus is a correcting saccade to return the fovea to the target.12
The amplitude and frequency of the nystagmus are additional diagnostic features. The amplitude can be estimated using a millimeter ruler or a reticule in the magnifier used to evaluate the type of nystagmus. As the patient fixates a target at 6 m, measurement of the overall excursion is made by holding the ruler in front of the eye with the best acuity (either eye, if the movements are conjugate). One millimeter of movement at the plane of the cornea translates to about 22 Δ (12 degrees of visual angle).13 Therefore, if 2 mm of movement is noted, the eyes are moving approximately 24 degrees.
 ▪ Figure 18.2 A: The eye movement trace shows jerk nystagmus, with fairly long foveations indicated by the flat portion of the trace. B: The eye movement trace shows pendular nystagmus, with foveating saccades seen at the top of the waveform. C: This patient’s eye movements are an example of periodic alternating jerk nystagmus. Note the center of the trace, where the patient’s nystagmus switches direction. D: Pendular and jerk nystagmus are superimposed for this patient’s waveform. |
One Hertz (Hz) (1 cycle/second) means that the waveform completes one full rotation in 1 second. Frequencies greater than 2 Hz are considered fast, and frequencies less than 1 Hz are considered slow. Frequencies slower than 2 Hz can be timed with a stopwatch as the oscillations are counted. More rapid frequencies can be estimated with observation under low magnification of the slit lamp.
Additional features of nystagmus that should be routinely investigated include variations in different fields of gaze, changes with occlusion, and changes with visual demand. Frequently, patients are seen who have nystagmus waveform changes (pendular to jerk) when changing fixation between left or right gaze. Such patients may have a habitual head turn or tilt favoring one position of fixation. The amplitude of nystagmus may decrease with convergence, and the eye movements should be observed during testing of the near point of convergence. Many patients with nystagmus have a latent component revealed by covering one eye. In patients with latent nystagmus, the fast phase of the waveform is toward the uncovered eye, and the amplitude and frequency often increase. A latent component usually indicates a congenital rather than an acquired condition, and treatment that restores or enhances binocularity often reduces the nystagmus and improves acuity. Contrary to clinical wisdom, objective measurements do not indicate that the severity of infantile nystagmus increases with visual demand. However, these studies do not deny that nystagmus increases because of stress.14,15
Some patients have nystagmus that is characterized by variations in conjugacy, constancy, and symmetry between the eyes. Prolonged observation may be required to evaluate these features. For example, spasmus nutans can be constant or intermittent, appearing to start and stop randomly.16 It may be present more in one eye than in the other and only during purposeful fixation. The rapid pendular oscillations are typically conjugate but may have unequal amplitudes in each eye. Occasionally, patients are seen who appear to have monocular spasmus nutans; close inspection of the “quiet” eye usually indicates a (very small) conjugate movement.
Clinical Evaluation
Most of the equipment and procedures used for evaluation of patients with nystagmus are used in many clinical vision examinations. However, these procedures must be modified to obtain the clinically relevant information; it is awareness of how and when to modify the usual clinical evaluation that makes clinical care of nystagmus patients an art.
PATIENT HISTORY
Onset, Associated Conditions, and Symptoms
The usual case history must be expanded to include a number of additional questions when evaluating patients with nystagmus. These include questions relating to the onset (i.e., at birth, before 6 months of age, or specifically when) and association of the onset with any infection, drugs or medications, metabolic disease, or trauma. Answers to such questions can help establish the often elusive cause of nystagmus. Information must be sought about observations by the parents or patient concerning variability of the frequency, amplitude, position of gaze, or time characteristics of the nystagmus. Symptoms should be assessed, especially those related to visual acuity, visual discomfort, periodic blurring of vision, or oscillopsia. Except for the severely visually impaired, children with congenital nystagmus seldom complain of visual symptoms.
Neurologic and Developmental Factors
General questioning concerning neurologic and developmental factors should be added to questions about the features of the nystagmus. Investigate general neurologic signs or symptoms, including dizziness, local pains, numbness, poor balance, ringing of the ears, seizures, uncoordinated movements and gait irregularities, weakness, or other recent unusual symptoms. By the age of 7 or 8 years, most children can be quite accurate in their answers. In young children, the Denver Development Test can be used to evaluate developmental status, including language acquisition, social development, and gross and fine motor coordination and perception.*
Genetic Factors
Many of the motor and sensory causes of nystagmus are inherited17; as interest in genetic research increases, more genes may be identified that are associated with infantile nystagmus.18 To determine a possible hereditary nature, the family history should investigate at least three generations for nystagmus and other visual disorders. This information will help determine the hereditary pattern of a disorder. When a family inheritance pattern is apparent, referral to a genetic counselor is an important part of management.
REFRACTIVE ERROR
The clinical impression that patients with nystagmus have a higher prevalence of significant refractive error is not well documented. However, in most cases, it is reasonable to assume that a significant refractive error is present until examination provides contrary findings. Patients who have nystagmus resulting from a sensory etiology sometimes have considerable improvement in ocular motor control and binocular fusion simply from correcting a significant refractive error.
Determining the refractive error in patients with nystagmus can be a trying experience. The most efficacious procedure is cycloplegic retinoscopy, usually using 1% cyclopentolate. The accuracy of retinoscopy can be enhanced by directing fixation to a gaze position that gives maximum ocular stability. Most patients turn their heads to the null position automatically, but sometimes a parent or assistant may have to hold the patient’s head in the proper position. Use trial lenses or a lens bar so that the refractive error can be measured directly along the optical axes and the two eyes can be quickly compared. Sometimes determination of astigmatism can be difficult. Use of a Placido disc or illuminated keratoscope can help identify corneal astigmatism or detect an optical irregularity when the eyes are directed to the position of least nystagmus. It is always wise to reevaluate the refraction on subsequent visits.
VISUAL ACUITY
Visual acuity in patients with nystagmus varies from nearly normal to severely impaired, depending on the etiology, associated conditions, and waveform. Nystagmus caused by disturbances of motor coordination usually results in less severe disturbances than nystagmus secondary to sensory conditions such as albinism, aniridia, or congenital cataracts. Indeed, some patients with motor etiology have normal or near-normal acuity. For these patients, nystagmus may be a cosmetic problem without significant visual deficit. Visual acuities of adult patients without major sensory impairment typically are not worse than 20/80.
Acuity testing of patients who read optotypes can proceed in the standard manner. It is often more reliable to determine line or single-letter acuity with children rather than to assess full-chart Snellen acuity. When visual acuity is reduced, a Bailey-Lovie chart (with logarithmic proportionally spaced letters) or low vision chart may be needed (Fig. 18.3). Determine acuity thresholds at distance (6 m) and at near (40 cm) for each eye
independently, as well as for binocular viewing. Allow the patient to assume the preferred head position for distance and near testing.
independently, as well as for binocular viewing. Allow the patient to assume the preferred head position for distance and near testing.
 ▪ Figure 18.3 The Bailey-Lovie acuity chart has logarithmic proportional spacing. Because each line has the same number of test characters with the same spacing, the Bailey-Lovie chart is ideal for evaluation of patients with nystagmus or amblyopia. |
 ▪ Figure 18.4 Occasionally, preschool children are not able to accurately respond when Snellen letters are used. In these instances, a handheld acuity test card can be substituted. |
Preschool children may need to be evaluated with handheld figure cards (Fig. 18.4), and it may not be possible to determine the exact clinical acuity for children younger than 2 years. However, an indication of visual ability can be determined by comparing visual and hand-eye behavior with each eye. For example, if an infant objects to having a particular eye covered, amblyopia or some other deficit of monocular visual acuity should be suspected. When deficient visual acuity is suspected in these patients, a visual evoked potential acuity threshold should be considered.
Monocular acuity differences are frequently seen in patients with nystagmus. Clinical differentiation needs to be between amblyopia (functionally reduced acuity in one eye) and latent nystagmus (in which the nystagmus increases in amplitude with occlusion). If there is high astigmatism, anisometropia, or strabismus—and amblyopia is suspected—a comparison of the acuity thresholds may help make a differential diagnosis. Contour interaction bars may be useful because they tend to depress the acuity threshold more in an amblyopic eye.19
Latent nystagmus is characterized by an occlusion-induced increase in jerk nystagmus that has a fast phase in the direction of the uncovered eye (slow phase toward the nose of the uncovered/viewing eye). When nystagmus increases with occlusion, assess acuity using a method that does not disassociate the eyes, such as the vectographic adult slide, or use a plus lens and blur one eye while measuring the acuity of the other eye. Use enough plus (usually between +2.00 and +5.00 D) to blur the eye without totally disrupting binocular vision, so that the latent nystagmus does not become manifest.
BINOCULAR VISION AND OCULAR MOTILITY
It is often difficult to assess binocular vision in a patient whose eyes are constantly moving. However, binocular evaluation is important because sensory and motor fusion can stabilize nystagmus in patients who have latent or manifest latent nystagmus.
Preliminary Testing
Preliminary testing is performed in the usual manner. Changes in the type, frequency, and amplitude of nystagmoid movements can be evaluated as the patient follows a penlight into the nine diagnostic fields of gaze. Gross convergence ability can be investigated using the penlight, although a small letter, picture, or toy provides a better accommodative stimulus (patients with infantile nystagmus often have a decrease in the nystagmus at near with convergence20 which does not lead to an immediate improvement in acuity).21 Investigating pupil
responses using a penlight in dim room illumination will reveal the direct, consensual, and accommodative reflexes, and a swinging light test will detect the presence of an afferent pupillary defect.
responses using a penlight in dim room illumination will reveal the direct, consensual, and accommodative reflexes, and a swinging light test will detect the presence of an afferent pupillary defect.
Motor Alignment
Angles of strabismus greater than 20 to 25 Δ are usually cosmetically evident, and comitancy can often be determined by direct observation of variations in the angle of deviation in different fields of gaze. Because of the constant eye movements, observations on the unilateral and alternate cover test are often equivocal when there is a small-angle strabismus or small heterophoria. A direct comparison of eye alignment can be gained using the Bruckner technique. This procedure is performed by simultaneously illuminating both eyes with an ophthalmoscope and observing the reflex at 0.5 m.22 If possible, observations should be made in the primary position and a position of gaze in which the oscillations are minimum. As the patient fixates the light, the clinician observes the corneal light reflexes within the red pupillary reflex. If the corneal light reflexes are in the same position and have symmetrical limits of movement within the red reflexes, binocular alignment is suggested. Strabismus or uncorrected refractive error causes unequal brightness of the red reflex, and, as in the Hirschberg test, when the corneal light reflexes are not symmetrically located, each 1 mm of difference between the eyes indicates 22 Δ of strabismus.13 To roughly measure the angle of deviation, prisms can be inserted in front of one eye until there is symmetry of the corneal light reflex movements (Krimsky technique).23
Although the presence of nystagmus may make observations less reliable, the cover test should be attempted in the primary position and the null position. Typically, a standard occluder is used, and the unilateral and alternate cover tests are performed using the conventional technique. However, when there is a latent component and nystagmus increases with occlusion, a trial case plus lens of about +5.00D may be substituted for an occluder. The plus lens “occluder” causes enough blur to ensure that fixation switches, but seldom increases the intensity of nystagmus.
Accommodative Assessment
Subjective assessment of accommodation may be difficult in patients with nystagmus because reduced visual acuity often causes increased difficulty in blur interpretation. However, accommodative function can be objectively evaluated using the monocular estimation method (MEM).24 The accuracy of the accommodative response can be determined by briefly placing a plus lens before the eye being tested and determining the lens that neutralizes movement of the reflex. The observed accommodative response at the reading distance is usually a fast “with” motion, indicative of a slight lag of accommodation. Moving the target closer until the fast “with” motion abruptly changes to a slow “with” motion will allow determination of the accommodative amplitude. Although reduced accommodative function is not unusual when there is reduced visual acuity, a large difference in MEM findings between the two eyes (in the presence of fully corrected distance refractive error for each eye) may indicate a third nerve lesion. Accurate objective measurement can be compromised by eye movements and inattention; thus, repeated observation of deficient responses is often necessary for accurate diagnosis.
Sensory Testing
Stereopsis and suppression testing can be very valuable when evaluating the nystagmus patient. Stereopsis testing is useful because there is probably a heterophoria when stereopsis is at least 100 seconds of arc.25 Suppression testing will often suggest which patients should have a vision therapy program. In general, heterophoric nystagmus patients who have suppression on the Worth four-dot test or on Mallett testing at near should undergo treatment to reduce suppression. Reduction in suppression with vision therapy can stabilize nystagmus and increase visual acuity in patients who have latent or manifest latent nystagmus.
OCULAR HEALTH AND SYSTEMIC HEALTH
Ocular Health
The ocular health examination is conducted in the usual manner.
Slit Lamp Evaluation
Most patients can cooperate for slit lamp examination. However, if a child is too young, direct ophthalmoscopy using a high plus lens can be substituted, or a handheld slit lamp can be used.
Ophthalmoscopy
Direct and indirect ophthalmoscopy with pupil dilation is needed to complete inspection of the posterior pole and retinal periphery. However, even full dilation sometimes is not sufficient to allow detailed evaluation when the
nystagmoid movements are too great. If the patient’s gaze can be directed to a null point, the fundus examination can generally be successful. When assessment at the null point is not satisfactory, fundus photographs will allow a detailed view because the strobe flash stops movement and provides a clear photographic record of the retinal features.
nystagmoid movements are too great. If the patient’s gaze can be directed to a null point, the fundus examination can generally be successful. When assessment at the null point is not satisfactory, fundus photographs will allow a detailed view because the strobe flash stops movement and provides a clear photographic record of the retinal features.
Tonometry
Air tonometry may yield the best result, if a reading can be attained. Unless the patient is under general anesthesia, applanation or indentation tonometry often yields poor results in nystagmus, because of the eye movement. Tactile (digital) pressures give a gross screening of interocular pressure, if the clinician has experience with this method.
Visual Fields
Visual field testing is generally difficult with nystagmus patients because of the frequent changes in fixation. Monocular confrontation perimetry, using targets approaching from various angles in the periphery, is a reasonable technique to screen for gross field defects. Automated perimeters can be used, as long as the clinician realizes the limitations on interpreting the results because of the variable fixation.
Oculomotor Movement Systems
Because nystagmus can result from an imbalance or disorder in one or more of the systems that maintain foveal fixation, it is important to investigate the optokinetic and vestibular systems.
Optokinetic Nystagmus
As part of nystagmus evaluation, it is important to determine whether optokinetic nystagmus (OKN) is functional. True OKN is accompanied by a sensation of self-rotation or movement, and precise OKN testing requires a moving stimulus that fills the field of vision. This cannot be achieved using handheld drums or tapes that test the pursuit system; rather, the patient must sit inside a large, rotating, patterned optokinetic drum. The drum is rotated opposite to the nystagmus direction to investigate the integrity of OKN responses. Thus, if there is horizontal nystagmus, the drum should be slowly rotated vertically. A reduced OKN response suggests a lesion involving that system.
Vestibular System
The visual system is limited by relatively slow (about 70 ms) retinal processing and cannot act rapidly enough in the presence of head movements to maintain a steady retinal image. The vestibuloocular reflex (VOR), which depends on motion sensors in the labyrinthine semicircular canals and has a latency of only 10 ms,26 stabilizes gaze to maintain clear vision during head movements. Vestibular nystagmus is accentuated when fixation targets are removed, so the patient should be observed with the eyes open and watching the lid motion with the eyes closed. If the amplitude of the jerk nystagmus increases with the eyes closed, vestibular nystagmus is suspected. Head shaking can also be used to increase vestibular nystagmus. After the head has been vigorously shaken for 10 seconds, the eyes are observed under magnification for an increase in nystagmus. The headshake test should be repeated with horizontal and vertical shaking.
Evaluation of VOR function involves (a) caloric testing to detect dynamic imbalances and (b) rotational testing, which generally gives more accurate and reproducible results than caloric tests.27 In caloric testing, which induces nausea in some people, the patient is positioned with the head elevated at an angle of 60 degrees. Water is slowly poured into one ear, with the head turned to one side, and the direction of the fast phase of induced nystagmus is observed. If cold water stimulates the right ear, the fast phase of induced nystagmus moves toward the left; however, if warm water is used in the same ear, then the fast phase is to the right (cold opposite, warm same—COWS). If differences are observed between the two eyes, a lesion in the vestibular track should be suspected.
Eye Movement Evaluation
If possible, the eye movement patterns should be evaluated using infrared or video-based eye movement monitors.27 Although this testing is not routinely performed by most clinicians, it is very helpful for diagnostic and treatment purposes. Parameters to evaluate include the type of waveform, the length of time and accuracy of foveations, and the velocity of the nystagmus. Refer to Figure 18.2 for the appearance of the eye position records for some of the important characteristics of nystagmus.
Specialized Testing
Computerized tomography (CT) scans and magnetic resonance imaging (MRI) are not routine procedures for the diagnosis of patients with congenital nystagmus. These procedures should be suggested in acquired nystagmus where active brainstem, cerebellar, or cortical lesions may be present.28
Differential Diagnosis
Once the examination is complete and the characteristics of the nystagmus and associated conditions are noted, the proper diagnosis must be made so that appropriate management can begin. This section reviews the common types of nystagmus seen in clinical practice. The clinical observations are summarized in Table 18.2.
INFANTILE NYSTAGMUS
Nystagmus that is present at (or shortly after) birth is referred to as infantile nystagmus. Infantile nystagmus affects males twice as frequently as females.29 Although this common form of nystagmus has variable presentations, it has certain characteristic clinical features (Table 18.2). Strabismus may be difficult to detect because of the nystagmus; as a result, the incidence of strabismus may be underestimated, although it is estimated to be about 40% to 50%.30 Most of the time, the nystagmus and strabismus are part of the underlying systemic or visual defect, but the etiology of strabismus can be independent of the nystagmus.
Table 18.2 CHARACTERISTICS OF SELECTED FORMS OF NYSTAGMUS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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