Eye movement deficits after unilateral mesencephalic lesions

The effect of unilateral mesencephalic lesions on eye movements in experimental and clinical studies is compared, with an emphasis on torsional eye movements. Two main structures in the mesencephalon are involved in the premotor processing of torsional and vertical eye movements: the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal (iC). In experimental and clinical studies, a unilateral lesion of riMLF leads to a loss of torsional saccades to the ipsilesional side, and vertical saccades are slower. In addition, there is a torsional nystagmus with the fast phase beating to the contralesional side. Experimentally, an iC lesion leads to ipsilesional torsional nystagmus with a conjugate vertical component and a vertical gaze-holding deficit. Saccade velocity is not reduced. Clinically, there appears to be no report with a lesion confined to the iC. Clinical studies with ‘presumed’ iC lesion effects differ in several aspects from experimental stu...