Triangle Pupils: Causes, Variations, and Examination Findings

Unusual pupil shapes can provide important clues about underlying eye conditions. While round pupils are the norm, variations such as triangle-shaped pupils may indicate structural or developmental anomalies.

Physical Characteristics

The pupil’s shape is primarily dictated by the iris, which regulates light entry into the eye. In most individuals, the pupil remains circular due to the coordinated action of the sphincter and dilator muscles. A triangular pupil suggests an alteration in iris structure, which may be congenital or acquired.

This irregularity can result from asymmetrical iris tension, connective tissue abnormalities, or adhesions that distort its contour. Anterior segment dysgenesis, improper embryonic development, trauma, surgery, or pathological processes like synechiae—where the iris adheres to the cornea or lens—can also contribute. The extent of the distortion may fluctuate with lighting conditions as the pupil reacts to illumination changes.

A triangular pupil may also present with additional structural anomalies, such as irregular pigmentation, abnormal pupillary margins, or asymmetric light reactivity. In some cases, the pupil appears fixed or sluggish, suggesting compromised neural or muscular function. These characteristics help determine whether the anomaly is isolated or part of a broader ocular or systemic condition.

Potential Connection to Iridocorneal Endothelial Changes

Structural abnormalities affecting a triangular pupil may extend to the iridocorneal endothelial interface, which includes the corneal endothelium, trabecular meshwork, and peripheral iris. Disruptions in this region can impact aqueous humor dynamics, corneal transparency, and pupil morphology. Conditions such as iridocorneal endothelial (ICE) syndrome, posterior polymorphous corneal dystrophy (PPCD), and Axenfeld-Rieger syndrome illustrate how endothelial dysfunction can influence iris shape.

ICE syndrome involves abnormal endothelial proliferation that extends onto the iris, causing atrophy and distortion. This overgrowth can lead to peripheral anterior synechiae, where adhesions form between the iris and cornea, further altering pupil shape. The endothelial cells in ICE syndrome exhibit an atypical phenotype, disrupting aqueous humor outflow and potentially leading to secondary glaucoma. Patients with ICE syndrome often present with eccentric, oval, or polygonal pupils, reinforcing the link between endothelial abnormalities and iris contour changes.

PPCD, a genetically linked corneal dystrophy, demonstrates another way endothelial dysfunction affects anterior segment morphology. Mutations in genes such as ZEB1 and OVOL2 alter endothelial cell behavior, leading to adhesions or irregular traction forces that distort the pupil. Clinical observations suggest that individuals with PPCD may exhibit atypical iris configurations, further supporting the connection between endothelial pathology and pupil irregularities.

Axenfeld-Rieger syndrome, a developmental disorder affecting neural crest-derived anterior segment structures, provides additional insight. Patients with this condition often display posterior embryotoxon, iridocorneal adhesions, and corectopia—where the pupil is displaced or misshapen. Genetic mutations in transcription factors such as PITX2 and FOXC1 disrupt normal mesenchymal differentiation, leading to iris and corneal endothelium defects. These abnormalities may contribute to a triangular pupil, particularly when segmental iris hypoplasia or synechiae create asymmetric tension on the pupillary margin.

Differential Shape Variations

Pupillary anomalies extend beyond the triangular form, encompassing a range of irregularities that offer insight into underlying conditions. While a round pupil results from balanced sphincter and dilator muscle action, disruptions in neural signaling, mechanical integrity, or development can lead to oval, teardrop, or colobomatous pupils. Each variation arises from distinct mechanisms, from congenital malformations to acquired structural changes.

An oval pupil is often linked to mechanical or neurological dysfunctions affecting the iris musculature. It may appear transiently during pharmacologic dilation or persist due to conditions such as Adie’s tonic pupil, where parasympathetic denervation leads to segmental sphincter paralysis. Teardrop-shaped pupils frequently indicate trauma, particularly when an iris sphincter rupture or penetrating injury distorts the normal contour. These distortions can be exacerbated by post-surgical scarring or synechiae formation, further altering the pupil’s shape.

Colobomatous pupils, characterized by a keyhole or notched appearance, result from incomplete closure of the embryonic optic fissure. This congenital defect can extend to deeper ocular structures such as the choroid and retina, affecting visual acuity and field perception. Unlike other shape anomalies that may fluctuate with pupillary response, colobomatous pupils remain fixed due to the absence of normal iris tissue in the affected region.

Observations During Routine Eye Examinations

During an eye examination, an irregular pupil shape is often first noticed under slit-lamp biomicroscopy. A triangular pupil may prompt further assessment to determine whether the anomaly is congenital or acquired. The examiner evaluates its symmetry, reactivity to light, and any associated ocular abnormalities. A sluggish or asymmetric pupillary response could indicate dysfunction in the iris musculature or underlying neurological involvement, necessitating additional diagnostic testing such as infrared pupillography or anterior segment optical coherence tomography (OCT).

Pupillary abnormalities are examined under varying lighting conditions to assess dynamic changes in shape. Dim illumination may exaggerate distortions due to asymmetric dilator muscle activity, while bright light can highlight structural irregularities along the pupillary margin. Gonioscopy is often used to inspect the iridocorneal angle, particularly if synechiae or adhesions are suspected. If endothelial involvement is a concern, specular microscopy can evaluate corneal cell morphology and density, helping differentiate primary iris abnormalities from broader anterior segment pathologies.