Microphthalmia (or microphthalmos) means small eyes. In mammals the failure of expression of a transcription factor, MITF (microphthalmia-associated transcription factor), in the pigmented retina prevents this structure from fully differentiating. This in turn causes a malformation of the choroid fissure of the eye, resulting in the drainage of vitreous humor fluid. Without this fluid, the eye fails to enlarge, thus the name microphthalmia.
The gene encoding the microphthalmia-associated transcription factor (Mitf) is a member of the basic helix-loop-helix-leucine zipper (bHLH-ZIP) family. Waardenburg syndrome type 2 (WS type 2) in humans is also a type of microphthalmia syndrome. Mutations in MITF gene are thought to be responsible for this syndrome. The human MITF gene is homologous to the mouse MITF gene (aka mouse mi or microphthalmia gene); mouse with mutations in this gene are hypopigmented in their fur. The identification of the genetics of WS type 2 owes a lot to observations of phenotypes of MITF mutant mice. Microphthalmia in newborns is also associated with infections during pregnancy, particularly rubella and cytomegalovirus (CMV), and trisomy 13 ( Patau syndrome). In addition, microphthalmia may also be a result of Fetal alcohol syndrome.