Changes of nystagmus characteristics following acuity demands in idiopathic infantile nystagmus

Purpose. To investigate the changes of nystagmus characteristics following visual acuity demands in patients with infantile nystagmus (IN). Methods. IN patients were asked to determine the direction of the gap in Landolt C optotypes while binocular eye movements were recorded. Optotypes, rear-projected onto a tangent screen, ranged in size from 20/200 to 20/20 and were presented in a random order. Results. Responses as a function of acuity demand varied among subjects in four ways: 1) little change in nystagmus; 2) dampening of nystagmus, minimally associated with convergence; 3) increase of nystagmus intensity with convergence; and 4) decrease of nystagmus with convergence (nystagmus blockage syndrome (NBS)). In an NBS patient the convergence increased monotonically with acuity demands but the associated nystagmus dampened with the onset of convergence. During blockage, jerk nystagmus (slow phase peak velocity ∼50°/sec, foveation time ∼50 msec) gave way to small amplitude, slow pendular nystagmus (peak velocity ∼20°/sec). Monocular convergence began about 730 msec after the small optotypes appeared and developed over approximately 670 msec. In contrast, after the attempt to identify the gap direction, the convergence state quickly relaxed (∼90 msec). Conclusions. IN patients exhibit various responses to acuity targets, which in some cases seem to hinder high acuity vision but in others (NBS) enhance it (by reducing retinal slip, e.g.). In NBS, nystagmus dampening and convergence may not be casually related but share the same trigger.