Clinical Reasoning: An Unusual Cause of Diplopia and Ptosis in a 67-year-old Woman.

Diplopia is a relatively common chief complaint encountered in an outpatient neurology clinic, and carries a broad differential diagnosis. In this case, a 67-year-old woman presented with new horizontal, binocular diplopia and ptosis of eight-month duration, which persisted without significant progression. This case highlights the need for a comprehensive list of differential diagnoses for patients with acquired ophthalmoplegia and ptosis. Key learning points include an illustration of the stepwise diagnostic approach to evaluate for common etiologies, the importance of interpreting test results in the appropriate clinical setting, and the significance of recognizing specific signs and symptoms in achieving the correct diagnosis.