Pearls and Oy-sters: Central fourth nerve palsies

Lesions of the fourth (trochlear) cranial nerve cause vertical or oblique diplopia by impairing the ability of the superior oblique muscle to intort and depress the eye. This binocular diplopia worsens in downgaze and lateral gaze away from the affected eye. Because intorsion is necessary to maintain fusion in ocular counter-roll, this diplopia also worsens with head tilt toward the affected eye.1,2 The authors thank Dr. Laura Balcer for assisting in the production of the figures.