A patient with a focal lesion in the trochlear nucleus on the left side of the midbrain at the level of the inferior colliculus develops vertical diplopia. Which muscle shows weakness?
- A Left superior oblique
- B Right superior oblique ✓
- C Left inferior oblique
- D Right superior rectus
Correct answer: B. Right superior oblique
Explanation
The trochlear nucleus gives rise to axons that cross completely in the superior medullary velum before emerging dorsally below the inferior colliculus. It is the only cranial nerve whose fibres entirely decussate, so a nuclear lesion produces contralateral superior oblique palsy, whereas an intracranial fascicular or nerve lesion after decussation produces ipsilateral palsy. The diplopia worsens on looking down and inward.
Reference: Gray's Anatomy for Students, 4th ed.
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