A 26-year-old woman reports horizontal double vision worse on looking to either side. On right gaze, the right eye abducts fully with nystagmus while the left eye fails to adduct. On left gaze, the left eye abducts with nystagmus while the right eye fails to adduct. Convergence is preserved. Where is the lesion?

• A Bilateral abducens nuclei
• B Left oculomotor nucleus
• C Bilateral medial longitudinal fasciculi ✓
• D Bilateral medial rectus muscles

Explanation

Internuclear ophthalmoplegia is caused by a lesion of the medial longitudinal fasciculus, interrupting fibers from the abducens nucleus to the contralateral medial rectus subnucleus, producing impaired adduction with abducting nystagmus. Preserved convergence localizes the lesion above the oculomotor nuclei and rules out a third nerve palsy. Bilateral INO in a young woman is classic for multiple sclerosis demyelination.

Reference: Adams and Victor's Principles of Neurology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.