Home › Body Atlas › Nerves › Trochlear Nerve (CN IV)

Trochlear Nerve (CN IV)

nervus trochlearis

The trochlear nerve (CN IV) is the smallest cranial nerve and the only one to exit the dorsal brainstem surface, arising from the trochlear nucleus in the dorsal midbrain tegmentum at the level of the inferior colliculus, decussating in the superior medullary velum, and winding around the brainstem to enter the lateral wall of the cavernous sinus. It traverses the superior orbital fissure outside the annulus of Zinn to supply the superior oblique muscle of the eye — the only extraocular muscle supplied by a nerve from the contralateral brainstem nucleus.

Region: Head & Skull

Clinical Relevance

Clinical Notes

Trochlear nerve palsy (CN IV) is the most common cause of vertical diplopia, producing weakness of the superior oblique muscle (which intorts and depresses the eye in adduction). Patients characteristically tilt their head away from the affected side to compensate for the extorsion defect. CN IV palsy may be congenital (most common) or acquired from head trauma (dorsal midbrain injury during deceleration), microvascular ischaemia (diabetic), or cavernous sinus lesions. The classic sign is positive Bielschowsky head tilt test. Acquired non-traumatic CN IV palsy in a patient over 50 is almost always microvascular.

Pathology

Common Injuries & Conditions

Superior Oblique Palsy from Trochlear Nerve Trauma

Head trauma damaging the dorsal midbrain as the trochlear nerve decussates in the superior medullary velum produces CN IV palsy with ipsilateral superior oblique weakness and contralateral tilt head compensation; bilateral CN IV palsy from severe trauma produces V-pattern exotropia with cyclotropia; prism glasses or superior oblique tuck surgery corrects the deviation.