occurs when the amplitude of oscillations differs in each eye, whereas disconjugate or disjunctive nystagmus occurs when the direction of the oscillations differs between the 2 eyes.

The amplitude of nystagmus often changes with gaze position. A few beats of nystagmus are normally present in the extremes of horizontal gaze (beyond 45°), especially in older patients. This finding should not be considered pathologic unless the nystagmus is persistent, asymmetric (eg, present to the left but not the right), or accompanied by other features. Assessment for nystagmus can be complemented by strategies that search for subtler, smaller-amplitude eye movements. Illuminated Frenzel (high-magnification) goggles are extremely useful in detecting eye movements, but a 20 D lens, slit lamp, or direct ophthalmoscope can also be useful to block the patient’s fixation and magnify abnormal eye movements. Ocular motor recordings (eg, electro-or video-oculography, infrared tracking, or electromagnetic scleral search-coil techniques) provide an objective and highly sensitive measure of eye movements but are rarely needed in standard clinical practice. The characteristics of eye movements can easily be recorded and communicated effectively using video recording and drawing. Video recordings of many of the nystagmus forms discussed in this chapter are available at h ttp://novel.utah.edu.

Leigh RJ, Zee DS. The Neurology of Eye Movements. 4th ed. Contemporary Neurology Series. New York: Oxford University Press; 2006.

Serra A, Leigh RJ. Diagnostic value of nystagmus: spontaneous and induced ocular oscillations. J Neurol Neurosurg Psychiatry. 2002;73(6):615–618.

Early-Onset (Childhood) Nystagmus

See BCSC Section 6, Pediatric Ophthalmology and Strabismus, for additional discussions of earlyonset nystagmus.

Infantile Nystagmus Syndrome (Congenital Nystagmus)

Infantile nystagmus syndrome (INS), or congenital nystagmus (CN), is often recognized in the first few months of life but may not become evident until several years of age; there may be a family history of the disorder. Children with INS usually do not have oscillopsia. True INS is not associated with afferent pathway disease, and visual acuity is proportional to the foveation period between the involuntary movements. Sensory nystagmus, in contrast, arises when central visual acuity is too poor to develop a stable fixational mechanism. Therefore, the ophthalmologist must determine if there is evidence of damage to the visual pathways in patients with early-onset nystagmus. In young children, it is important to detect any impairment of visual tracking (ie, determine that the eyes cannot follow visual stimuli equally) or optic atrophy. The presence of such abnormalities should prompt neuroimaging. Sensory nystagmus often occurs with such conditions as ocular albinism, achromatopsia, Leber congenital amaurosis, and aniridia. Frequently, electrophysiologic testing (eg, electroretinogram, visually evoked potential) is warranted.

Infantile nystagmus syndrome is almost always conjugate and horizontal, even in upgaze and downgaze. The nystagmus may be continuous or intermittent and can appear as jerk or pendular movements in different positions of gaze. There is frequently a null point, the field of gaze in which nystagmus intensity is minimal. If the null point is not in primary position, patients often adopt a head turn or posture to improve vision by placing the eyes in the null position. Visual attention and fixation

usually amplify INS (unlike the case with peripheral vestibular nystagmus, discussed later in the chapter), whereas convergence on a near target damps the amplitude of the nystagmus. Two characteristic signs of INS are

1.reversal of the normal pattern of optokinetic nystagmus characterized by the slow phase of eye movements moving in the direction opposite that of a rotating optokinetic drum

2.a unique pattern in which the velocity of the slow-phase movement increases exponentially with distance from fixation; determining this pattern requires eye movement recordings (see Fig 9-1)

To summarize, the features of INS include

jerk or pendular pattern

presence with or without normal visual acuity

conjugate horizontal eye movements that remain horizontal in upgaze and downgaze presence of a null point

no oscillopsia

increasing velocity of slow phase

accentuation by distant fixation and diminishment by convergence strabismus in 15% of patients

abolished in sleep

Abadi RV, Bjerre A. Motor and sensory characteristics of infantile nystagmus. Br J Ophthalmol. 2002;86(10):1152–1160. Hertle RW, Dell’Osso LF. Clinical and ocular motor analysis of congenital nystagmus in infancy. J AAPOS. 1999;3(2):70–79. Hertle RW; National Eye Institute Sponsored Classification of Eye Movement Abnormalities and Strabismus Working Group. A

next step in naming and classification of eye movement disorders and strabismus. J AAPOS. 2002;6(4):201–202.

Fusional Maldevelopment Nystagmus Syndrome (Latent Nystagmus)

Fusional maldevelopment nystagmus syndrome, or latent nystagmus (LN), is an early-onset, horizontal jerk nystagmus that is accentuated by or appears only with monocular viewing conditions. Eyes with LN are stable until 1 eye is occluded, thereby eliminating binocular fixation. The fast phase beats toward the viewing eye and away from the occluded eye (the slow phase of the viewing eye is toward the nose). Hence, the fast phase reverses direction each time the eyes are alternately covered. The use of a standard occluder when measuring visual acuity in LN patients therefore often degrades acuity. Partial optical blurring of 1 eye (with a high-plus lens or filter) may permit better visual acuity measurements in the fellow eye without inducing nystagmus. Latent nystagmus is almost always associated with esotropia and, frequently, with dissociated vertical deviation.

Clinical characteristics of LN include

conjugate jerk nystagmus

beginning or accentuation when binocular fusion is disrupted

direction changing with monocular occlusion: fast phase beats toward the viewing eye; slow phase, toward the nose

congenital esotropia usually present subnormal stereopsis

may coexist with INS

Nystagmus with characteristics of LN that is present when both eyes are open is known as manifest latent nystagmus (MLN). Because most patients with LN have esotropia (which may be a subtle microtropia), MLN is initiated when the esotropic eye is physiologically suppressed. In other words, the nystagmus spontaneously and intermittently develops whenever suppression occurs (ie, the clinician may not need to occlude an eye to induce this form of nystagmus). Both LN and MLN are benign entities. When studied with eye movement recordings, LN usually has a constant-velocity slow phase, in contrast with the increasing exponential waveform of CN.

Dell’Osso LF, Schmidt D, Daroff RB. Latent, manifest latent, and congenital nystagmus. Arch Ophthalmol. 1979;97(10):1877– 1885.

Monocular Nystagmus of Childhood

Monocular nystagmus of childhood is a rare but important form of nystagmus that manifests early in life; its causes range from benign to sightand life-threatening. The eye movements are usually in the same eye at all times, vertical or elliptical, and of small amplitude. Monocular nystagmus in an eye with longstanding poor vision is often referred to as the Heimann-Bielschowsky phenomenon; it may occur with a variety of underlying pathophysiologies, including optic neuropathy and amblyopia. It may become manifest in adulthood with highly asymmetric vision loss and may not remit even if the visual problem (eg, dense cataract) is corrected. Monocular vertical nystagmus in an infant, often found in concert with an afferent pupillary defect and optic atrophy, is suggestive of an optic nerve or chiasmal tumor (glioma), and therefore neuroimaging is warranted.

Spasmus Nutans

Spasmus nutans is a disorder that often develops in the first year of life, manifesting as intermittent, binocular, very small amplitude, high-frequency, horizontal, pendular nystagmus. The nystagmus may be dissociated, even monocular, and the amplitude and phase relationships of the eye movements may frequently vary between the eyes.

Small-amplitude vertical nystagmus may be present as well. Spasmus nutans is accompanied by head nodding, which is often subtle. Many patients have an abnormal head posture, or torticollis. There is an association between spasmus nutans and African American ethnicity, Hispanic ethnicity, and low socioeconomic status.

In general, spasmus nutans is distinguished from INS by abnormal head movements and head posture, the intermittent and variable nature of the nystagmus, and the relatively high frequency of eye movements. However, the nystagmus of spasmus nutans is sometimes monocular and thus virtually impossible to distinguish from the more ominous conditions of monocular nystagmus of childhood. Therefore, patients with presumed spasmus nutans should undergo neuroimaging to exclude a glioma of the anterior visual pathway and other parasellar or hypothalamic tumors.

Similarly, some authors have reported the association of a spasmus nutans–like syndrome with retinal dystrophies (eg, congenital stationary night blindness), and consideration should be given to electroretinographic studies in such cases. Lack of the expected resolution of spasmus nutans or development of any other neurologic problems should likewise prompt appropriate evaluation, including neuroimaging.

Spasmus nutans itself is a benign disorder, and patients generally have no other neurologic