The rostral interstitial nucleus of the medial longitudinal fasciculus
(riMLF) and the interstitial nucleus of Cajal (iC) are involved in th
e generation of vertical and torsional saccades and gaze holding. Midb
rain lesions involving either riMLF or iC produce a tonic torsional oc
ular deviation to the contralesional side. Lesions of the iC are assoc
iated with an ipsilesional torsional nystagmus (TN). For the first tim
e, we describe a patient with a unilateral midbrain lesion showing a c
ontralesionally beating torsional nystagmus. This nystagmus was probab
ly caused by a lesion involving the riMLF.