Anatomy of the trochlear nerve

1. Big picture

The trochlear nerve, cranial nerve IV, is a small but very exam-important ocular motor nerve. It innervates only one extraocular muscle: the superior oblique muscle. Its lesion causes a characteristic pattern of vertical and torsional binocular diplopia, especially when the patient looks downward and inward, for example while reading or walking downstairs.

For the exam, the key is not only to know the anatomy, but to connect it to clinical localization:

Trochlear nucleus → superior oblique muscle → intorsion and depression of the adducted eye → lesion causes hypertropia, extorsion, vertical/torsional diplopia, and compensatory head tilt to the healthy side.