Parks 3-step test identifies a paretic cyclovertical muscle. The three steps are: (1) hypertropia in primary position, (2) side of gaze increasing the deviation, (3) head tilt increasing the deviation. In a right hypertropia that increases in left gaze and on right head tilt, the paretic muscle is:

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

Explanation

Applying the Parks 3-step test: Step 1 — right hypertropia implicates right depressors (RSO, RIR) or left elevators (LSR, LIO). Step 2 — increases in left gaze: right eye depressors active in left gaze are RSO (since SO acts in adduction), and left eye elevators in left gaze are LIO. Common muscle: RSO. Step 3 — increases on right head tilt: the intorting muscles of the right eye (RSR and RSO) are activated; since RSO is already paretic, an RSO palsy worsens on right tilt. All three steps converge on right superior oblique palsy.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.