The internuclear ophthalmoplegia (INO) is a disorder of rapid saccadic eye movements which results from a lesion between:

• the VIth nerve nucleus in the pons, and
• the IIIrd nerve nucleus in the midbrain

The nerve bundle which usually connects these nuclei is called the medial longitudinal fasciculus (MLF).

A lesion of the right MLF causes a right INO which is the failure of the right eye to adduct on saccadic lateral gaze to the left.