|Classification and external resources|
Microphthalmia also referred to as microphthalmos, nanophthalmia or nanophthalmos, is a developmental disorder of the eye that literally means small eye (micros = small; ophthalmos = eye). One (Unilateral Microphthalmia) or both (Bilateral Microphthalmia) eyes may be involved 
The presence of a small eye within the orbit can be a normal incidental finding but in most cases it is abnormal and results in blindness. The incidence is 14 per 100,000 and the condition affects 3-11% of blind children.
Microphthalmia in newborns is sometimes associated with fetal alcohol syndrome or infections during pregnancy, particularly herpes simplex virus, rubella and cytomegalovirus (CMV), but the evidence is inconclusive. Genetic causes of microphthalmia include chromosomal abnormalities (trisomy 13 (Patau syndrome), Triploid Syndrome, and Wolf-Hirschhorn Syndrome) or monogenetic Mendelian disorders. The latter may be autosomal dominant, autosomal recessive or X linked. Genes that have been implicated in microphthamia include many transcription and regulatory factors. Those identified from family studies include the following:
|CRYBA4||crystallin, beta A4|
|FOXE3||forkhead box E3|
|GDF6||growth differentiation factor 6|
|MITF||microphthalmia-associated transcription factor|
|OTX2||orthodenticle homeobox 2|
|PAX6||paired box 6|
|PITX3||Paired-like homeodomain transcription factor 3|
|RAX||retina and anterior neural fold homeobox|
|SHH||sonic hedgehog homolog|
|SIX6||SIX homeobox 6|
|SOX2||SRY (sex determining region Y)-box 2||206900||MCOPS3|
|VSX1||visual system homeobox 1 VSX1||visual system homeobox 1|
|RAB18||Ras-related protein 18|
|VSX2 (CHX10)||visual system homeobox 2|
How these genes result in the eye disorder is unknown but it has been postulated that interference with the process of eye growth after birth may be involved in contrast to anophthalmia (absence of eyeball) which originates much earlier during foetal development. SOX2 has been implicated in a substantial number (10-15%) of cases and in many other cases failure to develop the ocular lens often results in microphthamia. Microphthalmia-associated transcription factor (MITF) located on chromosome 14q32 is associated with one form of isolated microphthalmia (MCOP1. In mammals the failure of expression of the 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.
- "Definition of Micropthalmia". MedicineNet. Retrieved 2009-01-01.
- GeneReviews/NCBI/NIH/UW entry on Anophthalmia / Microphthalmia Overview
- Parent Support group for parents with Anophthalmic and Microphthalmic children MAPS
- Anophthalmia and Microphthalmia Resource Guide from the National Eye Institute (NEI).
- GeneReviews/NCBI/NIH/UW entry on Microphthalmia with Linear Skin Defects Syndrome
- MACS The Micro and Anophthalmic Children's Society - Offers support and Information to families in the UK and around the world
- HUGO Gene Nomenclature Committee
- OMIM-Online Mendelian Inheritance in Man