A 34-year-old woman has bilateral horizontal diplopia worse on lateral gaze. On testing the right eye adducts poorly during attempted left gaze, but converges normally when following a near target; the abducting left eye shows nystagmus. Where is the lesion?

• A Left medial longitudinal fasciculus
• B Right medial longitudinal fasciculus ✓
• C Right abducens nucleus
• D Right oculomotor nucleus

Explanation

Internuclear ophthalmoplegia results from a lesion of the medial longitudinal fasciculus on the side of the impaired adduction; here the right eye fails to adduct, localising to the right MLF, typically demyelination in a young patient. Preserved convergence distinguishes it from a third nerve palsy, and the abducting nystagmus in the contralateral eye completes the classic triad of INO.

Reference: Adams and Victor's Principles of Neurology, 12th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.