Severe destruction of the neural integration mechanism for upward eye movements in bilateral internuclear ophthalmoplegia.

The visually elicited saccades and smooth pursuits of a 24-year-old man with bilateral internuclear ophthalmoplegia were recorded with an infrared limbus reflection technique. Although the amplitude-peak velocity relationship of saccades appeared to be normal, all the upward saccades were hypermetric and were followed by postsaccadic exponential drifts. The time constant of the postsaccadic exponential drifts was 248 +/- 19.5 ms (mean +/- SD). Upward pursuit eye movements were also severely disturbed, and only movements with a short duration (mean 113.8 ms) and low gain (mean 0.51) were frequently observed. Thus, the transformation of a pulse velocity signal for saccades and of a step velocity signal for smooth pursuits into step and ramp position commands, respectively, was almost completely lost for upward eye movements in this patient because of bilateral lesions of the medial longitudinal fasciculus.