Oculomotor (III) Nerve Palsy
Oculomotor (III) nerve palsy produces:
- Ptosis: weakness of levator palpebrae superioris (LPS),
+/− Müller’s muscle;
- Mydriasis: impaired parasympathetic outflow to the pupil ("inter- nal ophthalmoplegia");
- Diplopia: weakness of medial rectus (MR), inferior rectus (IR), superior rectus (SR), and inferior oblique (IO) muscles causing the eye to point "down and out" (external ophthalmoplegia); the pres- ence of intorsion confirms integrity of superior oblique muscle/trochlear (IV) nerve function.
These changes may be complete or partial.
Pathological correlates of third nerve palsy may occur anywhere from the brainstem to the orbit:
- Intramedullary (brainstem):
Nuclear: very rare; SR subnucleus lesion causes bilateral denervation; other clinical signs may be expected, such as pupillary (Edinger-Westphal nucleus) and medial longitudinal fasciculus involvement
Fascicular (within substance of midbrain): all muscles or specific muscles involved, + other clinical signs expected, such as contralateral ataxia (Claude’s syndrome), hemiparesis (Weber’s syndrome)
Subarachnoid space: peripherally located pupillomotor fibers often spared by ischemic lesions, but not by spaceoccupying lesions (e.g., aneurysm), however the distinction is not absolute
Cavernous sinus: III runs over trochlear nerve; other oculomotor nerves +/− trigeminal nerve often affected
Superior orbital fissure: superior division/ramus to SR, LPS; inferior to MR, IR, IO; selective involvement (divisional palsy) may occur; proptosis with space occupying lesions Orbit: paresis of isolated muscle almost always from orbital lesion or muscle disease
Oculomotor nerve palsies may be distinguished as "pupil involving" or "pupil sparing" (q.v.), the former implying a "surgical", the latter a "medical" cause, but this distinction only holds for complete palsies. Incomplete palsies are more likely to be of "surgical" origin (e.g., posterior communicating artery aneurysm). Imaging is the appropriate management if in doubt. Transtentorial (uncal) herniation due to raised intracranial pressure may, particularly in its early stages, cause an oculomotor nerve palsy due to stretching of the nerve, a "false-localizing sign."
Brazis PW. Subject review: Localization of lesions of the oculomotor nerve: recent concepts. Mayo Clinic Proceedings 1991; 66: 1029-1035 Coles A. The third cranial nerve. Advances in Clinical Neuroscience & Rehabilitation 2001; 1(1): 20-21