Congenital nystagmus (CN, or infantile nystagmus syndrome) is usually recognized in the first few months of life; there may be a family history of the disorder. Patients with CN usually do not have oscillopsia. Congenital nystagmus may occur in the presence of poor vision or good acuity. Reduced acuity may be due to the nystagmus itself or related to an afferent visual pathway disorder. Therefore, the ophthalmologist must determine if there is evidence of damage to the visual pathways. 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. Congenital nystagmus often occurs with such conditions as ocular albinism, achromatopsia, Leber congenital amaurosis, and aniridia. Frequently, electrophysiologic testing (ERG, VEP) is warranted.
Congenital nystagmus is almost always conjugate and horizontal, features that are maintained 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 other posture to improve vision by placing the eyes in the null position. Visual attention and fixation usually amplify CN (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 CN are
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
a unique pattern in which the velocity of the slow-phase movement increases exponentially with distance from fixation; this requires eye movement recordings (see Fig 9-1)
To summarize, the features of CN include
jerk and/or pendular pattern
presence with or without normal acuity
conjugate horizontal eye movements that remain horizontal in up- and downgaze
presence of a null point
increasing velocity of slow phase
accentuation by distant fixation; diminishment by convergence
15% of patients with strabismus
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abolished in sleep