The third cranial nerve innervates all the extraocular muscles except the lateral rectus and superior oblique (LR6, SO4). It also controls the levator palpebrae muscle and supplies parasympathetics to the pupillary constrictor and ciliary muscles. Therefore, the clinical examination is notable for a dilated and unreactive pupil, limited extraocular movements, and ptosis. The eye rests in a position of abduction because of unopposed action of the lateral rectus.
Third-nerve dysfunction can result from lesions anywhere along its path from the oculomotor nucleus in the midbrain, within the subarachnoid space, traversing the cavernous sinus, and terminating in the extraocular muscles within the orbit. Contralateral hemiparesis suggests brainstem involvement (Weber syndrome). Pathology within the subarachnoid space causing a third-nerve palsy includes compression of the nerve by a posterior communicating artery aneurysm, uncal herniation, or compressive neoplasm.
Pathology within the cavernous sinus causing a third-nerve palsy includes carotid artery aneurysm, cavernous sinus thrombosis, and carotid-cavernous fistula. Third-nerve lesions here are often accompanied by lesions involving the fourth, fifth (ophthalmic branch), and sixth cranial nerves. Orbital pathology such as inflammation, trauma, or neoplasm should be suspected when orbital findings such as chemosis, conjunctival injection, or proptosis are seen.
Isolated third-nerve palsies (“pupil-sparing”) are usually caused by microvascular ischemia. Diabetes, hypertension, and advanced age are risk factors. These typically present with intact pupillary function probably because of the superficial location of the pupillomotor fibers.
Management and Disposition
In patients with brainstem involvement, CT or MRI is indicated. Associated fever, headache, and altered consciousness should prompt CT scanning and subsequent spinal tap. MRI with gadolinium is preferred for evaluation of the cavernous sinus, and CT scanning is recommended for suspected orbital pathology.
Of particular concern is the sudden onset of third-nerve palsy accompanied by a “thunderclap” headache, stiff neck, and depressed level of consciousness. Even those with “pupil-sparing” should be evaluated as a neurosurgical emergency with emergent neuroimaging to evaluate for aneurysm and uncal herniation. If subarachnoid hemorrhage is not found and suspicion of aneurysmal leak remains high, a lumbar puncture should be considered.
In the setting of head trauma and oculomotor palsy, the workup should proceed expeditiously, with measures to reduce intracranial pressure.
Patients with the abrupt onset of a “thunderclap” headache and third-nerve palsy require immediate evaluation for an aneurysm. The posterior communicating artery is a common site.
In patients over 50 with third-nerve palsies whose pupil is unaffected (“pupil-sparing”), the etiology is usually hypertensive or diabetic vascular disease.
Mild or moderate pain is common in ischemic lesions.
Third-nerve palsy is unlikely to cause isolated mydriasis. Other etiologies such as tonic pupil, iris sphincter damage, and pharmacologic mydriasis are more likely.
Eighty percent of carotid-cavernous fistulas result from trauma, and may present weeks after minor trauma. Findings include an ocular bruit, pain, pulsatile exophthalmos, and chemosis.
Third-Nerve Palsy. This composite shows the defects of a left third cranial nerve palsy. Conjugate eye movement is present only when the affected eye gazes laterally to the affected side (intact lateral rectus). When gaze is directly ahead, exotropia is seen secondary to the unopposed lateral rectus muscle of the affected side. (Photo contributor: Frank Birinyi, MD.)
Isolated Third-Nerve Palsy. This 60-year-old woman with diabetes mellitus presents with an isolated third-nerve palsy on the right. She is attempting to look leftward. Her symptoms began as an isolated pain above her right orbit 10 days prior at which time her double vision with leftward gaze began. (Photo contributor: Lawrence B. Stack, MD.)
Third-nerve palsy is demonstrated in the right eye in this patient. Ptsosis is also seen.