A 24-year-old woman complains of vertical diplopia, worst when descending stairs. Cover testing shows right hypertropia greatest on left gaze, and it increases when the head is tilted to the LEFT shoulder. Which muscle is paretic?
- A Left superior oblique ✓
- B Right superior oblique
- C Right inferior oblique
- D Right superior rectus
Correct answer: A. Left superior oblique
Explanation
Applying the three-step test: the hypertropia is in the RIGHT eye, implicating either its depressors (superior oblique, inferior rectus) or the LEFT eye elevators (superior rectus, inferior oblique). Hypertropia worsening on LEFT gaze narrows it to the right superior oblique or left superior rectus. Worsening on LEFT head tilt points to an intorter, because tilting left requires intorsion by the left superior oblique and left superior rectus. The common denominator across all three steps is the LEFT superior oblique.
Reference: Khurana's Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.