Which of the following is NOT a typical sign of trochlear nerve (CN IV) palsy?

• A. Vertical diplopia that worsens on looking down and in
• B. Head tilt to the opposite shoulder
• C. Weakness of downward intorsion
• D. Ptosis

Answer: (D)

Trochlear nerve palsy affects the superior oblique muscle, whose normal actions are to intort the eye and, especially when the eye is adducted, depress it. When this muscle is weak, the eye tends to drift upward (hypertropia) and extort, producing vertical or oblique diplopia that worsens when looking down and inward. The usual compensatory mechanism is a head tilt toward the shoulder opposite the affected eye, which helps align the visual axes and reduce double vision. Weakness also strips away the downward intorsion component that the superior oblique provides. Ptosis, however, is not a typical feature because eyelid elevation is controlled by the oculomotor nerve, not the trochlear nerve. So this particular sign doesn’t fit CN IV palsy.