Internuclear ophthalmoplegia (INO) results from a lesion of the medial longitudinal fasciculus (MLF). The MOST common cause of bilateral INO in a young adult is:

• A Brainstem infarction (vertebrobasilar stroke)
• B Miller Fisher syndrome
• C Multiple sclerosis (MS) — demyelinating plaque in the MLF ✓
• D Wernicke's encephalopathy

Explanation

Bilateral INO in a young adult is multiple sclerosis until proven otherwise. MS demyelinating plaques in the MLF disrupt conjugate horizontal gaze by preventing the adducting eye from crossing the midline while the abducting eye shows nystagmus. The mnemonic is: in right INO (right MLF lesion), the right eye cannot adduct on left gaze while the left eye abducts with nystagmus. Brainstem stroke causes unilateral INO in older individuals. Miller Fisher syndrome causes ophthalmoplegia with ataxia and areflexia. Wernicke's causes bilateral horizontal gaze palsies and nystagmus rather than typical INO.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.