A 34-year-old man with demyelinating disease has impaired adduction of the LEFT eye on attempted right lateral gaze, with coarse nystagmus of the abducting RIGHT eye. Convergence is intact. The lesion lies in which structure?
- A Left abducens nucleus
- B Right oculomotor nucleus
- C Left medial longitudinal fasciculus ✓
- D Left paramedian pontine reticular formation
Correct answer: C. Left medial longitudinal fasciculus
Explanation
Internuclear ophthalmoplegia results from a lesion of the medial longitudinal fasciculus, which carries fibres from the contralateral abducens nucleus to the medial rectus motor neurons of the oculomotor nucleus. The side named corresponds to the eye that fails to adduct, so a left MLF lesion blocks left adduction on right gaze. Intact convergence distinguishes this from a genuine third nerve palsy and confirms the supranuclear nature of the lesion. A PPRF lesion would cause a horizontal gaze palsy of both eyes together.
Reference: Snell's Clinical Neuroanatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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