Congenital motor nystagmus
Congenital motor nystagmus (CMN) is a binocular, conjugate nystagmus with several distinctive features (Table 13-1; Video 13-1). It is often recognized in the first few months of life. CMN is not indicative of central nervous system abnormalities. Patients typically have nearly normal visual function. The nystagmus is uniplanar (ie, the plane of the nystagmus remains the same in all positions of gaze) and is most often horizontal. When CMN has a jerk waveform, it shows an exponential increase in velocity during the slow phase (Fig 13-1). A null point may be present, with right jerk nystagmus to the right of the null point and left jerk to the left of the null point. If the null point is not in primary position, the patient may adopt an abnormal head position to improve vision by placing the eyes near the null point. This head position becomes more pronounced as the child approaches school age. Head bobbing or movement may be present initially but usually decreases with age. Nystagmus amplitude may diminish with age. Oscillopsia is rare.
Infantile nystagmus syndrome (congenital motor nystagmus).
Courtesy of Agnes M.F. Wong, MD, PhD.
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CMN worsens with fixation and may worsen with illness or fatigue. Convergence damps (reduces the intensity of) the nystagmus. Thus, near visual acuity is often better than distance acuity. Occasionally, children with CMN overconverge to damp the nystagmus (nystagmus blockage syndrome), resulting in esotropia. Clinicians should take care not to confuse it with cases in which esotropia and nystagmus happen to coexist or cases of infantile strabismus with fusion maldevelopment nystagmus syndrome (latent nystagmus). Patients with nystagmus blockage syndrome characteristically present with an esotropia that “eats up prism” as the strabismic deviation increases upon attempted measurement, and with nystagmus that is least apparent when the deviation is largest.
Approximately two-thirds of CMN patients exhibit a paradoxical inversion of the optokinetic nystagmus (OKN) response, which is unique to CMN. Normally, when a patient with right jerk nystagmus views an optokinetic drum rotating to the patient’s left (eliciting a “pursuit left, jerk right” response), the intensity of the right jerk nystagmus increases. However, patients with CMN exhibit a damped right jerk nystagmus or possibly even a left jerk nystagmus when viewing an optokinetic drum rotating to the left.
Figure 13-1 Left jerk nystagmus. A, Electronystagmographic evaluation of infantile nystagmus syndrome (congenital motor nystagmus) shows an exponential increase in velocity during the slow phase. B, An exponential decrease in velocity during the slow phase is the waveform characteristic of fusion maldevelopment nystagmus syndrome (latent nystagmus).
Table 13-1 Features of Congenital Motor Nystagmus
X-linked mutations in the FERM domain-containing-7 gene (FRMD7) underlie many cases of typical CMN.
Congenital sensory nystagmus
Congenital sensory nystagmus is secondary to an early-onset, bilateral abnormality of the pregeniculate afferent visual pathway. Inadequate retinal image formation interferes with the normal development of the fixation reflex. If the visual deficit is present at birth, the resulting nystagmus becomes apparent in the first 3 months of life. Its severity is somewhat correlated with the degree of vision loss. The waveform of sensory nystagmus can be pendular or jerk and cannot be distinguished from that of CMN.
Searching, slow, or wandering conjugate eye movements may also be observed. Searching nystagmus—defined as a roving or drifting, typically horizontal movement of the eyes without fixation—is usually seen in children whose visual acuity is worse than 20/200. Pendular nystagmus typically occurs in patients with visual acuity better than 20/200 in at least 1 eye. Jerk nystagmus is often associated with visual acuity between 20/60 and 20/100.
Table 13-2 lists some conditions that are associated with congenital sensory nystagmus. The abnormality may be obvious, as with cataracts; subtle, as with optic nerve hypoplasia or foveal hypoplasia; or not visible on examination, as with some retinal dystrophies.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.