Normal pupil findings can give a false sense of security in a presumed cranial nerve III palsy patient: A unique case report

Abstract 

Background

A cranial nerve (CN) III palsy can be caused by a life-threatening aneurysm, typically of the posterior communicating artery. The “rule of the pupil” states the majority (96%) of such cases will present with abnormal pupil findings caused by compression of the fibers, located superficially along CN III. Careful examination of the size and reactivity of the pupils is critical.

Case Report

This case describes the complex management of a 79-year-old man with sudden-onset ptosis and diplopia consistent with a superior division CN III palsy. The pupils were normal, but because of the incomplete nature of the palsy the “rule of the pupil” could not be applied, and an aneurysm remained a threat. After urgent testing, microvascular infarct was deemed the cause. However, the palsy failed to resolve after 3 months. Further investigation found myasthenia gravis as the final diagnosis.

Conclusions

Several important points are outlined regarding CN III palsies. First, lack of pupil involvement in an incomplete palsy is not equivalent to pupil sparing in a complete palsy. Second, after initial diagnosis, if subsequent findings are no longer consistent, further investigation is warranted. Lastly, myasthenia gravis should be considered as a differential in apparent palsy cases.

Keywords: Partial third-nerve palsy, Aneurysm, Pupillary disorders, Myasthenia gravis