Cornea plana is an extremely rare, congenital hereditary malformation of the corneoscleral shape, characterized by a flattened corneal surface associated with hyperopia and various anterior segment abnormalities. Autosomal cornea plana 2 is distinguished from dominant cornea plana 2 by a round and opaque thickening, -5 mm in width located centrally. This thickening occurs in most cases of the CAN2 recessive form, but never in the dominant form. Additional anomalies, such as malformations of the iris, a slit-like pupil, and adhesions between the iris and cornea, are more prevalent in the recessive CAN2 form. Also the phenotype of recessive cornea plana 2 is generally non-progressive.
Autosomal recessive cornea plana 2 is associated with mutations in KERA gene. KERA encodes keratan sulfate proteoglycan, which is involved in the maintenance of corneal transparency.
Khan et al. (2004) described an Arab family of first cousin parents with six siblings, five of whom were affected with autosomal recessive cornea plana. All affected children had high hyperopia, small flat corneas, arcus senilis, short axial lengths, and asymmetric anterior chamber depth. The parents and the unaffected brother had normal ophthalmic features. Mutation analysis revealed a novel homozygous point mutation in the KERA gene in all affected members, the parents were carriers, and the unaffected brother had no KERA mutation. In a second case, Khan et al. (2005) described 12 individuals from a Saudi nuclear family with autosomal recessive cornea plana. Only one family member had a history of progressive visual difficulty over the last several years, and this was due to an increasing astigmatic refractive error. A novel homozygous mutation was detected in five siblings with clinically evident cornea plana and the sister with clinical findings of pellucid marginal degeneration and cornea plana. All other family members were heterozygous for the mutation and clinically unaffected.