Spontaneous Nystagmus in the Dark in an Infantile Nystagmus Patient May Represent Negative Optokinetic Afternystagmus

Abstract

Abnormal projection of the optic nerves to the wrong cerebral hemisphere transforms the optokinetic system from its usual negative feedback loop to a positive feedback loop with characteristic ocular motor instabilities including directional reversal of the optokinetic nystagmus (OKN) and spontaneous nystagmus, which are common features of infantile nystagmus syndrome (INS). Visual input plays a critical role in INS linked to an underlying optic nerve misprojection such as that often seen in albinism. However, spontaneous nystagmus often continues in darkness, making the visual, sensory-driven etiology questionable. We propose that sensorimotor adaptation during the constant nystagmus of patients in the light could account for continuing nystagmus in the dark. The OKN is a stereotyped reflexive eye movement in response to motion in the surround and serves to stabilize the visual image on the retina, allowing high resolution vision. Robust negative optokinetic afternystagmus (negative OKAN), referring to the continuous nystagmus in the dark with opposite beating direction of the preceding OKN, has been identified in various non-foveated animals. In humans, a robust afternystagmus in the same direction as previous smooth-pursuit movements (the eye's continuous tracking and foveation of a moving target) induced by visual stimuli has been known to commonly mask negative OKAN. Some INS patients are often associated with ocular hypopigmentation, foveal hypoplasia, and compromised smooth pursuit. We identified an INS case with negative OKAN in the dark, in contrast to the positive afternystagmus in healthy subjects. We hypothesize that spontaneous nystagmus in the dark in INS patients may be attributable to sensory adaptation in the optokinetic system after a sustained period of spontaneous nystagmus with directional visual input in light.

Keywords: albinism; infantile nystagmus syndrome; optokinetic afternystagmus; optokinetic response; smooth pursuit; smooth pursuit afternystagmus.

Figure 1

Optical coherence tomography scan showed the foveal hypoplasia.

Figure 2

Visual evoked potential (VEP) topography revealed asymmetric response localization over the two cerebral hemispheres in the left and right eye pattern onset responses.

Figure 3

Optokinetic nystagmus (OKN) and afternystagmus of the infantile nystagmus syndrome patient. Plots on the left demonstrate eye position traces during optokinetic stimulation in (A) CW (clockwise) and (C) CCW (counterclockwise) directions. Plots on the right demonstrate eye position traces in darkness subsequent to the 10-min optokinetic stimulations in (B) CW (clockwise) and (D) CCW (counterclockwise) directions. OKN stim, optokinetic stimulation phase; post-stim, post-stimulation phase.

Figure 4

Optokinetic nystagmus (OKN) and afternystagmus of the healthy subject. Plots on the left demonstrate eye position traces during optokinetic stimulation in (A) CW (clockwise) and (C) CCW (counterclockwise) directions. Plots on the right demonstrate eye position traces in darkness subsequent to the 10-min optokinetic stimulations in (B) CW (clockwise) and (D) CCW (counterclockwise) directions. OKN stim, optokinetic stimulation phase; post-stim, post-stimulation phase.

Figure 5

Velocity distribution before, during, and after the optokinetic stimulation. The data numbers within each time window of 20 s were plotted against velocity ranks (every 1°/s). Plots on the top demonstrate the velocity distribution of the healthy subject (A,B) and, at the bottom, of the infantile nystagmus syndrome (INS) patient (C,D). The velocity distributions during the pre-stimulation phase are shown in (A,B) of the healthy subject and (C,D) of the INS patient. Plots on the left demonstrate the velocity distribution during and after the clockwise (positive) stimulation while the plots on the right demonstrate the velocity distribution during and after the counterclockwise (negative) stimulation. The velocity distributions during the pre-stimulation phase are shown as blue line (first 20 s), blue dashed line (second 20 s), and blue dotted line (third 20 s); during the final 20-s stimulation phase as black line; during the post-stimulation phases as red line (first 20 s), red dashed line (second 20 s), and red dotted line (third 20 s). 1st 20 s pre-stim = first 20 s of the pre-stimulation phase; 2nd 20 s pre-stim = second 20 s of the pre-stimulation phase; 3rd 20 s pre-stim = third 20 s of the pre-stimulation phase; final 20 s optokinetic nystagmus (OKN) stim = final 20 s of the optokinetic stimulation phase; 1st 20 s post-stim = first 20 s of the post-stimulation phase; 2nd 20 s post-stim = second 20 s of the post-stimulation phase; 3rd 20 s post-stim = third 20 s of the post-stimulation phase.