In the Parks three-step test for diagnosing cyclovertical muscle palsy, a patient has: left hypertropia in primary position, worse on right gaze, and worse on left head tilt. The paretic muscle is:
- A Left superior rectus
- B Left superior oblique ✓
- C Right inferior oblique
- D Right superior rectus
Explanation
Applying the Parks three-step test: Step 1 — left hypertropia means either a depressor in the left eye (SO or IR) or an elevator in the right eye (SR or IO) is paretic. Step 2 — worse on right gaze: in right gaze, the left SR and left IR act; in left gaze, the right SR and right IR act. If worse on right gaze, the paretic muscle acts predominantly in right gaze → left IR or left SR. But from Step 1, we need a depressor in the left eye, so left IR is candidate... Step 2 narrows to either muscles acting in right gaze (LIR, LSR — but only depressors in left eye from Step 1 remains left IR, and elevator right eye from Step 1 remains RSR). Left IR worsens right gaze and RSR worsens left gaze; here worse right gaze → left SO or RSR. Step 3 — worse on left head tilt: left head tilt incyclotorts the left eye (SO + SR) and excyclotorts the right. If paretic muscle is left SO (normally incyclotorts), its absence worsens left head tilt hyperdeviation. The answer is left superior oblique (CN IV palsy).
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.