A neurology resident notes that a patient has 'internuclear ophthalmoplegia (INO)' — on lateral gaze to the right, the left eye does not adduct but the right eye abducts with nystagmus. Where is the causative lesion?

• A Right medial longitudinal fasciculus (MLF)
• B Left CN III nucleus
• C Right CN VI nucleus
• D Left medial longitudinal fasciculus (MLF) ✓

Explanation

In INO, the lesion is in the MLF on the side of the adduction failure. For right gaze, the right CN VI nucleus activates the right lateral rectus and simultaneously sends a signal up the left MLF to the left CN III nucleus (medial rectus subnucleus) for adduction. If the left MLF is damaged, adduction of the left eye fails while the right eye abducts normally. Monocular nystagmus in the abducting eye is characteristic. The most common cause in young adults is multiple sclerosis; in older adults it is brainstem infarction.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.