See-saw nystagmus in a patient with clinically definite MS.

See-saw nystagmus (SSN) is a relatively uncommon oculomotor disorder, most often associated with parasellar or chiasmal lesions, although it has also been described in several other conditions. To date, SSN has not been reported in clinically definite multiple sclerosis (MS). We present a patient with clinically definite MS who subsequently developed SSN. Possible mechanisms of SSN are discussed. MS should be considered in the differential diagnosis of SSN.