Voluntary upward gaze is controlled by the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). A unilateral riMLF lesion produces:

• A Bilateral loss of upward gaze
• B Ipsilateral loss of upward saccades and torsional nystagmus ✓
• C Contralateral downward gaze palsy
• D Convergence retraction nystagmus alone

Explanation

The riMLF in the midbrain tegmentum is the saccade generator for vertical and torsional eye movements. A unilateral riMLF lesion causes ipsilateral loss of upward saccades (vertical saccades are yoked but generated by one side) and torsional nystagmus. Bilateral damage (as in dorsal midbrain or Parinaud syndrome) causes loss of upward gaze with convergence retraction nystagmus, light-near dissociation, and lid retraction. A single riMLF lesion produces predominantly ipsilateral upward saccade paresis.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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