Torsional deviations with voluntary saccades caused by a unilateral midbrain lesion

Three dimensional eye rotations were measured using the magnetic
search coil technique in a patient with a lesion of the right
rostral interstitial nucleus of the medial longitudinal fasciculus
(RIMLF) and in four control subjects. Up to 10° contralesional
torsional deviations with each voluntary saccade were revealed,
which also could be seen during bedside examination. There was no
spontaneous nystagmus. Based on MRI criteria, the lesion involved
the RIMLF but spared the interstitial nucleus of Cajal. To date,
this deficit has not been described in patients. Our results
support the hypothesis that the vertical--torsional saccade
generator in humans is organised similarly as in monkeys: each
RIMLF encodes torsional saccades in one direction, while both
participate in vertical saccades.