A 28-year-old man reports vertical diplopia that is worst when reading in bed and when walking downstairs. He tilts his head toward the right shoulder to compensate. Examination shows weakness of left inward depression of the eye. Which muscle is affected and on which side is the responsible nucleus?

• A Left superior oblique, left trochlear nucleus
• B Right inferior rectus, right oculomotor nucleus
• C Left superior rectus, left oculomotor nucleus
• D Right superior oblique, right trochlear nucleus ✓

Explanation

The patient tilts the head away from the weak side, so head tilt to the right indicates a left trochlear palsy affecting the left superior oblique. Because trochlear fibers decussate completely in the anterior medullary velum before exiting dorsally, each trochlear nucleus innervates the contralateral superior oblique. Therefore the left superior oblique is supplied by the right trochlear nucleus. Superior rectus weakness would cause difficulty elevating the eye, not depression.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.