Congenital iris deficiency: complete absence of the iris, with direct visualization of the equatorial rim of the lens, suspensory ligament and ciliary process. There may be photophobia and various ocular abnormalities that cause low vision, and more patients become blind due to progressive corneal and lens clouding or glaucoma. To reduce photophobia, tinted glasses or corneal contact lenses may be worn. So, what are the causes of congenital iris deficiency? The following is a brief description of the causes: Congenital iris deficiency is mostly associated with incomplete closure of the embryonic fissure during the development of the early embryonic eye. There is more evidence to support that the neural ectoderm and mesoderm are impaired during embryonic development, resulting in abnormal development of multiple structures in the eye. There are two main theories, the ectodermal theory and the mesodermal theory. In addition, environmental factors should be taken into account as teratogenic factors. The ectodermal theory suggests that the neuroectodermal development is impaired between 12 and 14 weeks of gestation, resulting in the failure of the optic cup rim to develop and other ectodermal defects, such as abnormal development of the retina and macula, and pupillary muscle defects. The mesodermal theory suggests that the mesoderm is impaired in the second month of embryonic life and that the lens capsule-pupillary vessels of the lens vascular membrane are not atrophied but remain to obstruct the growth of the optic cup rim, and that when the mesoderm extends forward into the ectoderm and optic cup rim, it appears as a narrow band around the anterior chamber, blocking the forward growth of the ectoderm, resulting in iris hypoplasia. However, this theory cannot explain other neuroectodermal developmental abnormalities.