Iris Thickness Increases With Age, Pigmentation

Iris thickness increases with age, and is thicker in more heavily pigmented irises, according to research presented at the Association for Research in Vision and Ophthalmology (ARVO) April 23-27, 2023, in New Orleans. The differences are attributable to differences in the thickness of the stromal anterior border (SAB) layer of the iris.

Researchers conducted a cross-sectional study to determine the development of iris structure from birth to adulthood using handheld anterior segment optical coherence tomography (HHAS-OCT) and to investigate the effect of pigmentation on iris thickness.

They collected HHAS-OCT images from the eyes of 123 participants who were younger than 18 years of age and 83 participants between 18 and 40 years of age at the outset of the study. Eyes were classified as lightly (blue and green eyes) and heavily (all brown eyes) pigmented. The researchers calculated the thickness of the SAB layer and posterior epithelial layer (PEL) as well as total thickness (TT) of the iris based on nasal and temporal surfaces/borders from the HHAS-OCT data.

The study showed that the mean total thickness increases from 237 µm (95% confidence interval [CI], 208-268) at birth to 385 µm (95% CI, 293-477) at 18-40 years of age and plateaued at approximately 10 years of age. The researchers found that these changes were a result of thickening of the SAB layer with age, while the thickness of the PEL remained relatively unchanged over time. 

They also demonstrated that heavily pigmented irises had thicker SAB layers (P =.005) and TTs (P =.043) and thinner PELs (P =.020) than lightly pigmented eyes. 

“The iris thickness increases rapidly in the first few years of life plateauing at around 10 years,” the researchers report. “The change in iris thickness with age is mainly due to the changes in SAB layer rather than the changes in the PEL.”