What are the physical findings in fascicular subarachnoid third cranial nerve palsy (oculomotor cranial nerve palsy)?

Updated: Oct 08, 2018
  • Author: James Goodwin, MD; Chief Editor: Andrew G Lee, MD  more...
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Answer

Many patients who have had subarachnoid hemorrhage from rupture of a berry aneurysm have alteration of consciousness and may be difficult to examine. Even if they cannot co-operate with ocular motility assessment, the findings of a dense third cranial nerve palsy should be obvious. The eye on the involved side is deviated "down and out" from residual tone in the fourth cranial nerve (superior oblique muscle) and the sixth cranial nerve (lateral rectus muscle). [6, 7]

Usually, there is prominent ptosis also, but this may be difficult to observe if the patient is unconscious with eyes closed. Efforts should be made to arouse the patient at least to the point that there is some effort at eye opening, when the ptosis should be apparent.

The dilated, light-fixed pupil should be apparent on inspection without requiring any co-operation from the patient.

Pupillary involvement is the rule in third cranial nerve palsy resulting from posterior communicating artery aneurysm, with or without overt subarachnoid hemorrhage. In more than 95% of aneurysmal palsies, the pupil reacts sluggishly to light or is fixed and dilated, but, in 73%, of ischemic palsies, the pupil is spared. These observations have led to the pupil-sparing rule, which states that patients with third nerve palsies that spare the pupil do not have aneurysms and can be safely managed expectantly. Three important caveats are as follows: [8, 9, 10, 11, 12, 13, 14, 15]

  • Patients must have complete paralysis of the ocular muscles of the cranial nerve III and complete sparing of the pupil.
  • This rule should be applied sparingly to patients aged 20-50 years, an age group in whom ischemic infarcts are uncommon.
  • Patients should have isolated third nerve palsies.

This is probably because the pathophysiology of the third cranial nerve lesion involves leakage of blood from the aneurysm dome into the nerve across its outer margin. The pupil fibers are located very superficially and are nearly always involved in this process. On the other hand, pupil-sparing third cranial nerve palsy is a hallmark of ischemic lesions that tend to involve the central core of the nerve. This is the type of nonsurgical or medical third cranial nerve palsy that often results from microvascular disease and tends to resolve uneventfully within a few weeks.


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