What are the typical signs of trochlear nerve (CN IV) palsy?

• A. Vertical diplopia that worsens on looking down and in; head tilt to the opposite shoulder; weakness of downward intorsion.
• B. Lateral deviation of the eye with horizontal diplopia.
• C. Ptosis with preserved movement.
• D. Convergence insufficiency with normal vertical alignment.

Answer: (A)

Trochlear nerve palsy affects the superior oblique muscle, which normally depresses and intorts the eye when it is adducted. When this nerve is impaired, the eye loses those actions, so you get vertical diplopia that becomes worse when looking down and in (toward the nose). The affected eye also shows weakness of downward movement and intorsion, leading the patient to adopt a compensatory head tilt away from the involved side to reduce the diplopia. This combination—vertical diplopia that worsens on downward gaze and adduction, with head tilt to the opposite shoulder and reduced downward intorsion—fits CN IV palsy best.

Other patterns don’t match as well: horizontal diplopia with lateral eye deviation points toward a lateral rectus or abducens/III issue rather than a purely intorsional, depressor deficit; ptosis with preserved movement suggests a different cranial nerve palsy (often III) rather than isolated trochlear dysfunction; convergence insufficiency with normal vertical alignment would not produce the characteristic vertical diplopia and compensatory head tilt seen in CN IV palsy.