Internal capsule lesion — MRCPsych Paper A MCQ

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ModerateNeuroanatomyInternal capsule lesionMRCPsych Paper A

A 56-year-old woman develops sudden weakness of the right lower face, arm and leg, with each region affected to a similar degree. Sensation, visual fields, coordination and language are intact. MRI shows a small left-sided deep cerebral infarct situated between the thalamus medially and the lentiform nucleus laterally. Which structure is most likely affected?

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Correct answer: C — Left posterior limb of the internal capsule

The correct answer is C, the left posterior limb of the internal capsule. A small lesion here can interrupt densely concentrated corticospinal and corticobulbar fibres, producing contralateral weakness of the lower face, arm and leg without sensory, visual, cerebellar or cortical deficits—a pure motor lacunar syndrome. The stated position between the thalamus and lentiform nucleus further localises the posterior limb. A corona-radiata lesion can cause contralateral weakness but lies superior to the internal capsule. A pontine lesion may cause hemiparesis but is not in the described cerebral location and may produce brainstem signs. A precentral-gyrus lesion is cortical and is more likely to show somatotopic imbalance or associated cortical features. A right internal-capsule lesion would cause left-sided weakness.

Reference: Jang SH et al. Distribution of the corticobulbar tract in the internal capsule. Journal of the Neurological Sciences, 2013. https://pubmed.ncbi.nlm.nih.gov/24034408/