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INO – MLF Lesion Anatomy — SCE Neurology MCQ

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ModerateMultiple Sclerosis & CNS InflammationINO – MLF Lesion AnatomySCE Neurology

A 50-year-old man with MS develops acute onset of bilateral horizontal diplopia with right medial rectus weakness on left gaze (right eye cannot adduct) and nystagmus of the abducting left eye. Convergence is preserved. What is the diagnosis and where is the lesion?

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Reveal the answer and explanation

Correct answer: E — Right internuclear ophthalmoplegia (INO) — lesion in the right medial longitudinal fasciculus (MLF) in the brainstem

Right INO results from a right MLF lesion disrupting the signal from the left CN VI nucleus (which contains interneurons projecting to the right CN III nucleus via the MLF). This causes failure of right eye adduction on LEFT gaze, with nystagmus of the abducting left eye. Convergence is preserved (CN III nucleus itself is intact — the deficit is in the internuclear pathway). INO in a young patient is most commonly caused by MS. A: CN III palsy would also affect pupil, ptosis, and vertical movements. C: CN VI palsy causes abduction failure, not adduction. D: Left CN III would affect the LEFT eye. E: MG has fatigable weakness.

Reference: Clinical Neuroanatomy; NICE NG100