A 40-year-old woman presents with sudden diplopia on looking to the right. Examination shows right eye abduction weakness with right beating nystagmus when the left eye (intact) adducts. Convergence is preserved. The MOST likely localization is:

• A Right abducens (VI) nerve palsy
• B Right internuclear ophthalmoplegia — right MLF lesion
• C One-and-a-half syndrome — right paramedian pontine lesion
• D Left internuclear ophthalmoplegia (INO) — left MLF lesion ✓

Explanation

Internuclear ophthalmoplegia (INO) is caused by a lesion in the medial longitudinal fasciculus (MLF). In left MLF INO: the left eye cannot adduct (left eye adduction failure on right gaze), and the right eye shows abducting nystagmus as a compensatory overshoot. The diplopia and the described pattern — right abduction nystagmus with the left eye failing to adduct — localizes to the left MLF. Convergence is preserved because the convergence pathway is separate from the MLF. Right abducens palsy causes isolated right eye abduction failure without the adducting nystagmus pattern in the fellow eye.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.