Manifest latent and congenital nystagmus waveforms in the same subject A need to reconsider the underlying mechanisms of nystagmus

Abstract

We report a unique case of a horizontal manifest latent nystagmus (MLN) that is converted to a congenital nystagmus (CN) on covering the subjectþs only seeing eye. The subject is a 65-year-old female who, at five days after birth, had her right eye ennucleated. A prosthetic eye was fitted soon after. Horizontal eye movements were recorded using an IRIS (Skalar Medical) infrared system. The effect of visual feedback on primary and secondary gaze, smooth pursuit, optokinetic responses, and the role of selective attention on fixation stability were examined. On primary gaze with the left eye open and under normal viewing conditions, the patient exhibited a typical MLN waveform. The slow phase was a decreasing velocity and the fast phase beat to the left. On covering the left eye, the nystagmus slow phase immediately increased in velocity and the fast phase beat to the right. Removal of visual feedback (darkness or stabilising the retinal image) also brought about a change from MLN to CN. Both the MLN and CN states were strongly influenced by gaze. The decreasing velocity MLN slow phase was also seen to change to a grossly extended slow phase during periods of visual disengagement. On occasions, these oscillations became a short burst of right-beating CN whenever the patient lost interest in the visual task. This is the first report of an adult subject who can exhibit either of two separate and sustained nystagmus states, an MLN or a CN, whenever the only eye is covered. Two explanations are offered to account for the presence of the two separate waveforms. Central to our interpretation is that the neural integrator has an eccentric null and that it is under a variable gain control. Furthermore, we propose that future models of nystagmus generation will need to include retinal image and attention control loops.