Dysfunction of the third cranial nerve (oculomotor nerve) can result from lesions anywhere along its path between the oculomotor nucleus in the midbrain and the extraocular muscles within the orbit. The diagnosis and management of third nerve palsy varies according to the age of the patient, characteristics of the third nerve palsy, and the presence of associated signs and symptoms.
The manifestations and diagnosis of third nerve palsy in adults will be reviewed here. Third nerve palsy in children and other causes of diplopia, ptosis, and anisocoria are discussed separately. (See "Third cranial nerve (oculomotor nerve) palsy in children" and "Overview of diplopia" and "Overview of ptosis" and "Approach to the patient with anisocoria".)
The third cranial nerve supplies the levator muscle of the eyelid and four extraocular muscles: the medial rectus, superior rectus, inferior rectus, and inferior oblique. These muscles adduct, depress, and elevate the eye. The superior oblique muscle is innervated by cranial nerve IV and the lateral rectus muscle by cranial nerve VI. In addition, the third cranial nerve constricts the pupil through its parasympathetic fibers that supply the smooth muscle of the ciliary body and the sphincter of the iris.
The third nerve begins as a nucleus in the midbrain that consists of several subnuclei that innervate the individual extraocular muscles, the eyelids, and the pupils. Each subnucleus, except the superior rectus subnucleus, supplies the ipsilateral muscle.
●The superior rectus subnucleus innervates the contralateral superior rectus muscle