A patient with an isolated left trochlear nerve (CN IV) palsy displays which eye movement deficit and compensatory head posture?

• A Left eye cannot abduct; patient tilts head toward the left shoulder
• B Left eye cannot intort; vertical diplopia worse on right gaze and left head tilt; patient tilts head toward right shoulder ✓
• C Left eye has ptosis with dilated pupil; head thrust toward left
• D Left eye shows convergence failure; no compensatory head posture

Explanation

CN IV (trochlear) innervates the superior oblique, which intorts, depresses, and abducts the eye. A left CN IV palsy causes defective intorsion of the left eye, producing cyclovertical diplopia that is worst when looking toward the right (superior oblique's depression action is greatest in adduction — looking right brings the left eye into adduction) and worst with ipsilateral (left) head tilt (which demands left eye intorsion). The patient compensates by tilting the head toward the contralateral (right) shoulder to reduce the intorsion demand and eliminate diplopia. This pattern is the Parks-Bielschowsky three-step test for CN IV palsy.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.