Pupil irregularity can result from many factors that are either congenital or acquired. Congenital iris malformations such as coloboma or aniridia result in abnormal pupil shape and reactivity. Acquired conditions that damage the mechanical compliance of the iris stroma or iris musculature can result in an irregular pupil. Blunt trauma to the eye can cause focal tears in the sphincter muscle. An iridodialysis occurs when the outer edge of the iris is torn away from its ciliary attachment. Intraocular inflammation can damage the iris or cause it to adhere to the lens or cornea (synechiae). Neovascularization can also distort the iris and impair pupillary reactivity. A surgical procedure in the anterior segment may affect the shape or function of the pupil; cataract surgery is probably the leading cause of a misshapen pupil in an adult.
Two rare conditions of focal abnormality in iris innervation may cause pupillary irregularity:
Tadpole pupil. This disorder is attributed to a focal spasm of the iris dilator muscle, which produces a peaking of the pupil that lasts a few minutes. The phenomenon may occur numerous times over several days or a week and then disappear. It is idiopathic, but a small percentage of patients harbor an underlying sympathetic lesion; thus, pharmacologic testing for Horner syndrome is suggested.
Midbrain corectopia. In rare cases, eccentric or oval pupils are present in patients with rostral midbrain disease. The abnormality is presumably caused by incomplete damage of the pupillary fibers, leading to selective inhibition of iris sphincter tone.
Balaggan KS, Hugkulstone CE, Bremner FD. Episodic segmental iris dilator muscle spasm: the tadpole-shaped pupil. Arch Ophthalmol. 2003;121(5):744–745.
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.