When the magnitude of the anisocoria is equal in dim and bright light, this indicates that the relative function of the pupillary sphincter and dilator muscles is intact. This scenario is consistent with physiologic anisocoria, and the patient can be reassured about the benign nature of this phenomenon.
Physiologic anisocoria (also known as simple or essential anisocoria) is the most common cause of a difference in pupil diameter. About 20% of individuals have noticeably asymmetric pupils. Usually the difference in pupil diameter is less than 1.0 mm and can vary from day to day.
Sometimes physiologic anisocoria is more apparent in dim light than in bright light. Ptosis occurring on the side of the smaller pupil may create diagnostic confusion with Horner syndrome. Pseudo-Horner and true Horner syndromes are best differentiated through pharmacologic testing (see the section “Horner syndrome” in this chapter).
Lam BL, Thompson HS, Corbett JJ. The prevalence of simple anisocoria. Am J Ophthalmol. 1987;104(1):69–73.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.