In internuclear ophthalmoplegia (INO), the lesion is in the medial longitudinal fasciculus (MLF). The affected eye (ipsilateral to the MLF lesion) shows failure of adduction. The characteristic finding in the contralateral abducting eye is:

• A Complete horizontal gaze palsy
• B Upbeat nystagmus
• C Convergence spasm
• D Monocular horizontal nystagmus (abducting nystagmus) ✓

Explanation

In INO, the ipsilateral eye (same side as MLF lesion) fails to adduct on conjugate gaze to the opposite side. The contralateral eye shows monocular abducting nystagmus — nystagmus only in the abducting eye. This is believed to result from compensatory increase in innervation to the abducens nucleus to overcome the adduction deficit, leading to overshoot/nystagmus. Convergence is typically preserved in INO (differentiating from 3rd nerve palsy). Bilateral INO ('WEBINO' syndrome) in young patients strongly suggests multiple sclerosis.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.