A patient complains of diplopia that is worst when looking down and to the left. Bielschowsky's head tilt test is positive on tilting to the right shoulder. Parks three-step test identifies a right superior oblique palsy. What is the action of the superior oblique muscle in the primary position?

• A Primary action: depression; secondary: intorsion; tertiary: abduction
• B Primary action: intorsion; secondary: depression; tertiary: abduction ✓
• C Primary action: elevation; secondary: extorsion; tertiary: adduction
• D Primary action: abduction; secondary: depression; tertiary: intorsion

Explanation

The superior oblique's primary action in the primary position is intorsion (incycloduction), with secondary depression, and tertiary abduction. This is because the muscle's line of pull from the trochlea passes posterolaterally; when the eye is in the primary position, intorsion is the dominant action. Testing depression is best when the eye is adducted (when intorsion and abduction become minimal), which forms the basis of the Parks three-step test. Right SO palsy causes hypertropia of the right eye that worsens in left gaze and on right head tilt.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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