Manifest latent and congenital nystagmus waveforms in the same subject - Aneed to reconsider the underlying mechanisms of nystagmus

We report a unique case of a horizontal manifest latent nystagmus (MLN) tha t is converted to a congenital nystagmus (CN) on covering the subject's onl y seeing eye. The subject is a 65-year-old female who, at five days after b irth, had her right eye ennucleated, A prosthetic eye was fitted soon after . Horizontal eye movements were recorded using an IRIS (Skalar Medical) inf rared system. The effect of visual feedback on primary and secondary gaze, smooth pursuit, opto-kinetic responses, and the role of selective attention on fixation stability were examined. On primary gaze with the left eye ope n and under normal viewing conditions, the patient exhibited a typical MLN wave form. The slow phase was a decreasing velocity acid the fast phase bea t to the left. On covering the left eye, the nystagmus slow phase immediate ly increased in velocity and the fast phase beat to the right. Removal of v isual feedback (darkness or stabilising the retinal image) also brought abo ut a change from MLN to CN. Both the MLN and CN states were strongly influe nced by gaze. The decreasing velocity MLN slow phase was also seen to chang e to a grossly extended slow phase during periods of visual disengagement, On occasions, these oscillations became a short burst of right-beating CN w henever the patient lost interest in the visual task. This is the first rep ort of an adult subject who can;exhibit either of two separate and sustaine d nystagmus states, an MLN or a CN, whenever the only eye is covered. Two e xplanations are offered to account for the presence of the two separate wav eforms, Central to our interpretation is that the neural integrator has an eccentric null and that it is under a variable gain control. Furthermore, w e propose that future models of nystagmus generation will need to include r etinal image and attention control loops.