A 28-year-old woman reports vertical diplopia that is worst when walking down stairs. On examination there is weakness of downward gaze when the right eye is adducted, and tilting the head toward the right worsens the diplopia. Which muscle is affected?

• A Right superior rectus
• B Right superior oblique ✓
• C Right inferior oblique
• D Right inferior rectus

Explanation

The superior oblique depresses the adducted eye and intorts it. Weakness produces difficulty descending stairs and compensatory head tilt away from the affected side; tilting the head TOWARD the affected side worsens the separation of images (positive Bielschowsky test). The inferior rectus depresses the abducted eye, so it cannot explain diplopia maximal on attempted downgaze in adduction. Inferior oblique elevates the adducted eye, and superior rectus elevates the abducted eye.

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

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