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Anisocoria, or unequal pupil sizes, is a common condition. Approximately 20% of the normal population have physiologic anisocoria. However, pathologic anisocoria indicates disease of the iris, parasympathetic pathway or sympathetic pathway. A systematic approach to the evaluation of anisocoria can help differentiate between etiologies that range from benign to life threatening.
The most important question in the evaluation of anisocoria is whether both pupils are normally reactive to light or is one (or both) poorly reactive. If both pupils are reactive, the smaller pupil is abnormal and the lesion is likely in the sympathetic pathway because pupillary constriction (parasympathetic pathway) is intact. If one pupil is poorly or non-reactive (and there is no relative afferent pupillary defect), the larger pupil is abnormal and the lesion is likely in the parasympathetic pathway.
DDx of anisocoria with normally reactive pupils:
- Physiologic anisocoria
- Horner syndrome
DDx of anisocoria with poorly or non-reactive pupil:
- Iris sphincter damage (traumatic mydriasis)
- Pharmacologic blockade
- Tonic pupil
- Cranial nerve III palsy
References
Kedar S, Biousse V, Newman NJ. Approach to the patient with anisocoria. In: UpToDate, Brazis, PW (Ed), UpToDate, Waltham, MA, 2014. (Accessed on November 12, 2014.)
Lam BL, Thompson HS, Corbett JJ. The prevalence of simple anisocoria. Am J Ophthalmol 1987;104(1):69-73.