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
Inheritance
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.
Pedigree
Autosomal dominant
Autosomal dominant disorders require only one mutation for the disease to be expressed. Since an affected parent has two chromosomes, only one of which has the mutant gene, parents can expect that half (50%) of their children will receive that one and inherit the disease. It is common for individuals that inherit the mutation, however, to not have evidence of the disease (nonpenetrance).
Autosomal dominant inheritance leads to a vertical pattern of transmission
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.