What are the pupillary findings in unilateral 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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Pupillary dilatation and sluggish or absent reaction to light results from involvement of parasympathetic fibers that originate in the Edinger-Westphal subnucleus of the third cranial nerve complex, as follows:

  • Fascicular or peripheral nerve involvement caused by compressive lesions often affects these autonomic fibers because they are situated very superficially within the nerve trunk.
  • This contingent of fibers are medially placed as the nerve exits the brainstem and gradually migrate to a more inferior and lateral position as they proceed anteriorly along the nerve.

If there is partial preservation of parasympathetic pupillary innervation with third cranial nerve disorders, the pupil on the involved side may react to light nearly as briskly as the pupil of the other eye, as follows:

  • Distinguishing pupillary involvement caused by third cranial nerve lesions from ocular sympathetic palsy (Horner or Claude Bernard syndrome) or physiological anisocoria requires detailed examination of pupil size in dim and in bright, ambient light. Third nerve palsy mydriasis can coexist with Horner syndrome–related miosis (eg, cavernous sinus lesion), and the two pupillary effects may obscure the diagnosis or cancel out the anisocoria in ambient lighting.
  • With sphincter weakness due to parasympathetic involvement in third cranial nerve lesions, the involved pupil is larger than the fellow pupil. However, with concomitant Horner syndrome or aberrant regeneration of the pupil, the pupil may be smaller rather than larger in a third nerve palsy.
  • The size difference between the 2 pupils (anisocoria) is greater in bright light (when the sphincter is called upon to act most strongly) and lesser in dim light (when the sympathetically innervated dilator fibers are the dominant contributors to pupil size). The reverse is true for ocular sympathetic lesions, in which the pathological pupil is the smaller one and the difference (anisocoria) is greater in dim light than in bright light. The anisocoria remains the same in dim and bright light in simple central anisocoria, also called physiological anisocoria.

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