written by: Emjay Annavi Baclay•edited by: lrohner•updated: 10/21/2010
Cornea plana congenita is an abnormality in the cornea of the eyes, associated with mutations in the KERA gene, with subtypes that vary based on the severity and pattern of inheritance.
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Cornea plana congenita is a rare developmental and congenital abnormality of the cornea, characterized by a flattening of the cornea’s anterior contour, and a radius of curvature tantamount to less than 43 dioptres. Apart from the marked astigmatism and flattened shape, the cornea may also be small, and the disorder also leads to the patient’s moderately impaired visual capacity. The disease is further clustered into two types— type 1 or CNA1, and type 2 or CNA2. CNA 2 is more highly prevalent in the Finnish population as compared to other parts of the world, although the rare disorder has a worldwide distribution.
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The Genetic Aspect
The subtypes of cornea plana congenita arise from the difference in the gravity of symptoms and the inheritance pattern. CNA 1 is a milder form of the disease that is autosomal dominant or dominantly inherited. On the other hand, CNA 2 is a rare, autosomal recessive or recessively inherited, and more severe form of the disease. Effects on the cornea’s development in cornea plana have been association with abnormalities and gene mutations of the KERA, which encodes the Keratocan on chromosome 12q22. Keratocan is a leucine-rich protein component of a small proteoglycan that is essential for the cornea’s transparency, wherein the small leucine-rich proteoglycans or SLRPs interact with specialized collagens.
According to "Harley’s Pediatric Ophthalmology," although Keratocan mutations have been associated with the two subtypes of the disorder, no mutations were found in the original families of CNA1. A research study on Finnish patients, conducted by a team from the Ohio State University, also shows that the reported mutations in the KERA are mainly found in CNA2 patients. The evidence yielded through the said study establishes the role of KERA mutation in the development of CNA2, while the case of CNA 1 patients was apparently not characterized by any mutation in the proteoglycan genes. As such, there seems to be a stronger link between CNA 2 and KERA mutation as compared to the milder form of the disease.
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Clinical features and treatment methods
According to the clinical and genetic study on Finnish patients conducted by members of the Department of Medical Genetics of the University of Helsinki and The Folkhlslsan Institute of Genetics, the following are some of the symptoms and features associated with the two forms:
CNA 1 – normal visual acuity, abnormally broad limbus zone, corneal refraction is weaker compared to normal refraction (43 dioptres) by just 3-7 dioptres, and clear corneal parenchyma
CNA 2 – greatly reduced corneal refraction to around 25 -35 dioptres, limbus zone is widened, strong hyperopia, the anterior chamber is shallow
Opthalmological examination must be carefully carried out to provide the proper diagnosis of the disease, and a gene test may be employed to detect the founder mutation. As stated in Harley’s Pediatric Ophthalmology, treatment methods for corneal plana congenita include efforts to manage glaucoma and to neutralize the refractive errors. However, for cases wherein the central clarity is lost, health care providers may consider penetrating keratoplasty.