The Parks three-step test is used to identify the paretic muscle in vertical strabismus. In a patient with right hypertropia that increases on left gaze and on head tilt to the right, which muscle is paretic?

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

Explanation

The Parks three-step test: Step 1 — determine which eye is hypertropic (right eye is higher = right hypertropia). Step 2 — the hypertropia increases on left gaze, implicating either the right SO or left IO (depressors on left gaze). Step 3 — hypertropia increases on right head tilt (Bielschowsky head tilt test), implicating the right SO (intortors of the right eye are the right SO and right SR; when tilted right, SO and SR are stimulated; if SO is paretic, hypertropia increases). Convergence of all three steps identifies right superior oblique palsy — the most common acquired cranial nerve IV palsy (trochlear nerve).

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.