Which cranial nerve is the only one to emerge from the dorsal side of the brainstem?

The trochlear nerve is the only cranial nerve to emerge from the dorsal side of the brainstem.

The trochlear nerve, designated cranial nerve IV, emerges from the back of the midbrain just below the inferior colliculi. Its fibres cross completely before leaving the brainstem, another distinctive anatomical feature. After travelling around the brainstem, the nerve enters the orbit and supplies the superior oblique muscle of the eye.

The superior oblique muscle helps depress and rotate the eye, particularly when the eye is turned inward. Damage to the trochlear nerve can cause vertical or torsional double vision. Patients may compensate by tilting their head, a clinical sign associated with trochlear nerve palsy.

Other eye-movement cranial nerves emerge from the ventral or lateral brainstem. The oculomotor nerve is cranial nerve III, the abducens is VI, and the trochlear nerve is IV. Remembering the nerve number alone is not enough to identify its unusual dorsal exit.