Unequal pupils: Understanding the eye's aperture

BACKGROUND: Unequal pupils (anisocoria) may be physiological, pathological or pharmacological. Importantly, anisocoria can indicate underlying disease of the eye, orbit, brain, neck or chest. Examination of the pupils is therefore a crucial part of any eye examination. OBJECTIVE: As a clinician, it is important to determine whether a patient with anisocoria can be reassured or requires referral for further investigation. This review examines the anatomy of the pupillary pathway, and provides a structured approach to examination of the pupils. The aim is to provide clinicians with confidence when encountering patients with anisocoria. DISCUSSION: Anisocoria can imply serious underlying pathology, so accurate pupil testing and astute observation are paramount. This review discusses the differential diagnosis of a large pupil (anisocoria more obvious in the light) and a small pupil (anisocoria more obvious in the dark), and discusses the relevant afferent pupillary defect, in which there is no anisocoria but both pupils react differently depending on which eye is illuminated.