Congenital microcoria is a rare eye disorder affecting the development of the irises. It is characterized by absence or incomplete development of the iris dilator muscles (also called the pupil dilator muscles), making the pupils unable to dilate (widen). People with Congenital microcoria have small pupils (with a diameter less than 2 millimeters) that dilate poorly or not at all, even when pupil-dilating medicines (eye drops) are put on the eye. The disorder is also often associated with nearsightedness, and glaucoma in childhood or early adulthood. Congenital microcoria that is not associated with an underlying syndrome is caused by a small, missing piece of genetic material (deletion) around a region of chromosome 13 designated as 13q32. This region contains several genes, although it appears the exact gene or genes in the region responsible for Congenital microcoria are still under investigation. Inheritance is autosomal dominant. Congenital microcoria is also a feature of autosomal recessive Pierson syndrome, caused by genetic changes in the LAMB2 gene.