Effects of visual fixation and convergence on periodic alternating nystagmus due to MS

We studied a young woman with multiple sclerosis, who developed periodic alternating nystagmus (PAN) with a period of oscillation of about three minutes. Neither visual fixation of a stationary target nor large-field optokinetic stimulation substantially influenced the cycle of PAN. During convergence, induced by fixation of a near target, PAN was suppressed by over 70%. Treatment with baclofen abolished her nystagmus, but optokinetic and pursuit responses remained impaired. Convergence during viewing a near target did not increase the response (gain) of her vestibulo-ocular reflex. We postulate that visual drives were able to suppress PAN independently of any effects on vestibular responses and were prevented from exerting effects on velocity storage and vestibular gain adjustment by demyelinating lesions affecting her pontine nuclei and cerebellar circuits.