On the basis of our own experience and a review of the literature, brain tumors do not cause infantile nystagmus in children unless optic nerve atrophy or hypoplasia is inevitably present. With the rare exception of achiasma (in which infantile nystagmus is accompanied by seesaw nystagmus), there exist no data to support routine neuroimaging in infantile nystagmus.

Infantile Nystagmus

Clinical Features

Infantile nystagmus is an involuntary, conjugate, rhythmic, horizontal oscillation of the eyes. It may appear as a pendular or jerk nystagmus in primary gaze, and it usually has a torsional component (seen in recordings but not clinically obvious). The intensity of infantile nystagmus increases on lateral gaze and becomes right-beating in right gaze and left-beating in left gaze. The fact that infantile nystagmus appears to disobey Alexander’s law (which states that in peripheral vestibular nystagmus, the direction of the nystagmus increases in the direction of the fast phase and decreases in the direction of the slow phase) is often useful in distinguishing it from horizontal peripheral vestibular nystagmus and from manifest latent nystagmus (which obeys Alexander’s law under conditions of monocular fixation). Infantile nystagmus remains horizontal in upgaze, in contrast to acquired horizontal vestibular nystagmus, which becomes upbeating in upgaze.

Parents of the child with infantile nystagmus often report that the nystagmus becomes worse with attempted fixation or intense visual effort. It seems to be the visual importance of the task to the individual rather than the inherent difficulty of the visual demand that exacerbates the nystagmus.534,578 This history is useful in further distinguishing the nystagmus from peripheral vestibular nystagmus, which becomes worse with occlusion and is damped by fixation. To distinguish these two forms of nystagmus, the examiner can observe one optic disc with the direct ophthalmoscope while periodically occluding the other eye. Increased nystagmus intensity with occlusion suggests a peripheral vestibular nystagmus, while either no change or a decrease in nystagmus intensity suggests infantile nystagmus.365 Anxiety, anger, fear, excitement, or fatigue also increase the infantile nystagmus intensity and thereby degrade visual acuity.13

Unlike adult-onset nystagmus, it is rare for individuals with infantile nystagmus to experience oscillopsia (an illusory to and fro movement of the environment), although exceptions to this rule are well documented.14,155,156 The absence of oscillopsia in older children and adults with conjugate horizontal nystagmus should therefore suggest the

diagnosis of infantile nystagmus. Infantile nystagmus often damps during convergence, which results in a near visual acuity that is better than the distance acuity.492,557 Parents may report that children with infantile nystagmus view objects at a very close distance. This visual adaptation, which provides the combined benefits of axial magnification and convergence damping of the nystagmus, should not be discouraged.

Clinical observation and eye movement recordings have demonstrated that the amplitude, frequency, and waveform of the nystagmus can vary with eye position, giving rise to a region of gaze (referred to as the null zone) in which the nystagmus intensity (amplitude x frequency) of the oscillation is minimal.139,557 In most individuals with infantile nystagmus, the head position corresponds roughly to the minimal intensity zone of the nystagmus. When the angle of the null zone exceeds 15 degrees, however, the angle of the head turn may fall short of the null zone.208 In some children, the anomalous head position appears to be dictated by the velocity distribution of the slow phase (i.e., the percentage of time for which the slow phase is less than or equal to 10 degrees per second) and the nystagmus beat direction (which can be influenced both by the prior position of gaze and by the length of time a subject has maintained a fixed gaze position).9 The multiplicity of factors that influences the null zone in infantile nystagmus might explain why the anomalous head posture in infantile nystagmus has been observed to change with time.9

Bagolini et al45 have recognized that some individuals with infantile nystagmus use large head turns to place their eyes in extreme sidegaze and actively block their nystagmus. Unlike positioning the eyes in a null zone, in which electromyographic activity decreases, the mechanism of active blockage utilizes the increased electromyographic activity associated with lateral gaze innervation to damp infantile nystagmus. The same mechanism of active blockage occurs when infantile nystagmus is damped by convergence. Individuals with active blockage of nystagmus by sidegaze momentarily move their eyes into extreme lateroversion when they seek good vision. This maneuver may cause them to complain of discomfort brought about by the extreme torticollis that is necessary to block their nystagmus.45 Head turns associated with horizontal null positions are usually less extreme than those associated with active blockage, and the nystagmus associated with a null zone can often be observed to increase if the eyes are carried further into sidegaze.

Infantile nystagmus may be accompanied by head shaking during periods of intense fixation. Head shaking seems to be more common in children (who are presumably less concerned about their cosmetic appearance).149 Head oscillations were originally thought to be an adaptive strategy to cancel the effects of the ocular oscillations, but it is now clear that this is not the case.365 Older children and adults are aware of