JERK-WAVE-FORM SEE-SAW NYSTAGMUS DUE TO UNILATERAL MESO-DIENCEPHALIC LESION

See-saw nystagmus is an uncommon but highly characteristic eye movemen t disorder comprising intorsion and elevation of one eye, with synchro nous extorsion and depression of the other. It generally has a pendula r waveform and is due to a midline, extrinsic, suprasellar mass lesion compressing or invading the brainstem bilaterally at the meso-diencep halic junction. This report deals with the clinical and MRI findings i n three patients (and binocular three-dimensional quantitative oculogr aphic findings in one patient) with a jerk waveform see-saw nystagmus due in each case to a unilateral meso-diencephalic lesion. In each pat ient the torsional component of the nystagmus fast phases rotated the upper poles of the eyes toward the side of the lesion. Jerk see-saw ny stagmus can be clinically indistinguishable from pendular see-saw nyst agmus and from the torsional-vertical nystagmus which occurs with medu llary lesions. We propose that jerk see-saw nystagmus is due to unilat eral inactivation of the torsional eye-velocity integrator thought to be in the interstitial nucleus of Cajal, with sparing of the torsional fast-phase generator thought to be in the adjacent rostral interstiti al nucleus of the medial longitudinal fasciculus.