A 22-year-old man presents with vertical diplopia that is worst when descending stairs. On examination, the affected eye elevates (overaction of the unopposed inferior oblique) when the head is tilted toward the same side. The nerve injured is:

• A Oculomotor nerve
• B Trochlear nerve ✓
• C Abducent nerve
• D Frontal division of trigeminal nerve

Explanation

The superior oblique depresses the adducted eye and intorts it. Trochlear palsy produces vertical diplopia worst on looking down and inward, classically while walking downstairs. Patients compensate by tilting the head away from the affected side; tilting toward the affected side worsens separation because it demands more depressed intorsion from the weak muscle. Abducent palsy causes horizontal diplopia from lateral rectus weakness, and oculomotor palsy causes ptosis with a down-and-out pupil.

Reference: Gray's Anatomy, 42nd ed.

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Written and medically reviewed by the StethoPrep medical team.