This retrospective, population-based study assessed the incidence and etiologies of acquired third nerve palsy among newly diagnosed patients. Investigators identified all newly diagnosed cases of acquired third nerve palsy from 1978 through 2014 using the Rochester Epidemiology Project, a record-linkage system of medical records for all patient-physician encounters in Olmsted County, Minnesota.

After adjusting for age and sex, the annual incidence of acquired third nerve palsy was 4.0 per 100,000. There was a higher incidence of microvascular nerve palsy, and lower incidence of aneurysm than previously reported in non-population-based studies.

While pupil involvement was more common in compressive lesions, this condition was noted in some cases of microvascular third nerve palsies. Pupil sparing was seen in some cases of compressive lesions, including aneurysm.

A limitation of the study is that Olmsted County has primarily a white population, therefore, the results may not extrapolate well for more diverse populations. There is also the evolution of neuroimaging over the course of the study.

In acquired third nerve palsy, pupil involvement does not exclude microvascular third nerve palsy, and lack of pupil involvement does not rule out compressive third nerve palsy. Thus, neuroimaging is recommended for all new patients without an obvious cause.