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Defective proteoglycans present on the cell surface and in the extracellular matric are responsible for cornea plana.
Defective proteoglycans present on the cell surface and in the extracellular matric are responsible for cornea plana.

Cornea Plana

Clinical Characteristics

Ocular Features

Enlargement of the cornea with flattening is characteristic of cornea plana although corneal diameters vary widely.  Corneal thinning may be present.  The mean corneal refraction value at the horizontal median has been measured at 37.8 D for the dominant form (CNA 1) of the disease, compared with 29.9 D for the recessive form (CNA 2) and 43.4 D for controls accounting for the hyperopia found among many patients.  The limbal margin may be widened with blurring of the corneolimbal junction.  Recessive cases can often be distinguished from the dominant ones by the presence of a central 5 mm area of thickening and clouding.  Recessively inherited cases are also more likely to have anterior synechiae and other iris anomalies.  Early onset arcus has been reported.

Vision in mild cases may be as good as 20/25 or 20/30 but considerably worse in recessive cases with central opacification.  Glaucoma may occur in older individuals.

Systemic Features

None reported.

Genetics

Multiple families in Finland have been reported with inheritance patterns suggesting autosomal recessive inheritance (CNA2).  The gene has been mapped to chromosome 12 (12q21) in a region containing the KERA gene.  A Cuban family with autosomal dominant cornea plana (CDA1) also yielded linkage to 12q where the recessive gene is located.  However, this locus could be excluded in two Finnish families suggesting that at least 3 autosomal mutations may be responsible.

Treatment Options

Correction of the hyperopia may be helpful.  Patients need to be followed and treated for glaucoma if it develops.  Outcomes of penetrating keratoplasty are not available but the procedure carries increased risk since the stroma is often thinner than normal. 
 

References

Tahvanainen, E.; Villanueva, A. S.; Forsius, H.; Salo, P.; de la Chapelle, A. : Dominantly and recessively inherited cornea plana congenita map to the same small region of chromosome 12. Genome Res. 6: 249-254, 1996.

PubMed ID: 
8723718

Tahvanainen, E.; Forsius, H.; Kolehmainen, J.; Damsten, M.; Fellman, J.; de la Chapelle, A. :  The genetics of cornea plana congenita. J. Med. Genet. 33: 116-119, 1996.

PubMed ID: 
8929947

Pellegata, N. S.; Dieguez-Lucena, J. L.; Joensuu, T.; Lau, S.; Montgomery, K. T.; Krahe, R.; Kivela, T.; Kucherlapati, R.; Forsius, H.; de la Chapelle, A. :  Mutations in KERA, encoding keratocan, cause cornea plana. Nature Genet. 25: 91-95, 2000.

PubMed ID: 
10802664