Internuclear ophthalmoplegia — MRCPsych Paper A MCQ

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ModerateNeuroanatomyInternuclear ophthalmoplegiaMRCPsych Paper A

A 67-year-old man develops sudden-onset horizontal diplopia. His pupils are equal and reactive, and there is no ptosis. On attempted left gaze, the right eye fails to adduct while the left eye abducts with nystagmus. Eye movements on right gaze are normal. Which neural structure is most likely affected?

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Correct answer: D — Right medial longitudinal fasciculus

The correct answer is D, the right medial longitudinal fasciculus (MLF). The MLF carries internuclear fibres from the contralateral abducens nucleus to the ipsilateral oculomotor nucleus, coordinating horizontal conjugate gaze. A right MLF lesion therefore causes impaired adduction of the right eye during left gaze, with compensatory nystagmus of the abducting left eye: a right internuclear ophthalmoplegia. A right oculomotor nerve lesion could impair adduction but would commonly produce additional third-nerve signs and does not explain abducting nystagmus. A left abducens lesion would impair left-eye abduction. A left paramedian pontine reticular formation lesion would cause failure of both eyes to look left. An optic tract lesion causes a contralateral homonymous visual-field defect rather than an ocular-motility disorder.

Reference: Virgo JD, Plant GT. Internuclear ophthalmoplegia. Practical Neurology. 2017;17(2):149-153. https://pubmed.ncbi.nlm.nih.gov/27927777/