Central discoid corneal dystrophy

IC3D classification of corneal dystrophies--edition 2

PURPOSE

To update the 2008 International Classification of Corneal Dystrophies (IC3D) incorporating new clinical, histopathologic, and genetic information.

METHODS

The IC3D reviewed worldwide peer-reviewed articles for new information on corneal dystrophies published between 2008 and 2014. Using this information, corneal dystrophy templates and anatomic classification were updated. New clinical, histopathologic, and confocal photographs were added.

RESULTS

On the basis of revisiting the cellular origin of corneal dystrophy, a modified anatomic classification is proposed consisting of (1) epithelial and subepithelial dystrophies, (2) epithelial-stromal TGFBI dystrophies, (3) stromal dystrophies, and (4) endothelial dystrophies. Most of the dystrophy templates are updated. The entity "Epithelial recurrent erosion dystrophies" actually includes a number of potentially distinct epithelial dystrophies (Franceschetti corneal dystrophy, Dystrophia Smolandiensis, and Dystrophia Helsinglandica) but must be differentiated from dystrophies such as TGFBI-induced dystrophies, which are also often associated with recurrent epithelial erosions. The chromosome locus of Thiel-Behnke corneal dystrophy is only located on 5q31. The entity previously designated as a variant of Thiel-Behnke corneal dystrophy on chromosome 10q24 may represent a novel corneal dystrophy. Congenital hereditary endothelial dystrophy (CHED, formerly CHED2) is most likely only an autosomal recessive disorder. The so-called autosomal dominant inherited CHED (formerly CHED1) is insufficiently distinct to continue to be considered a unique corneal dystrophy. On review of almost all of the published cases, the description appeared most similar to a type of posterior polymorphous corneal dystrophy linked to the same chromosome 20 locus (PPCD1). Confocal microscopy also has emerged as a helpful tool to reveal in vivo features of several corneal dystrophies that previously required histopathologic examination to definitively diagnose.

CONCLUSIONS

This revision of the IC3D classification includes an updated anatomic classification of corneal dystrophies more accurately classifying TGFBI dystrophies that affect multiple layers rather than are confined to one corneal layer. Typical histopathologic and confocal images have been added to the corneal dystrophy templates.

Newly reported p.Asp240Asn mutation in UBIAD1 suggests central discoid corneal dystrophy is a variant of Schnyder corneal dystrophy

PURPOSE

To determine whether central discoid corneal dystrophy (CDCD), previously reported as a novel corneal dystrophy, is actually Schnyder corneal dystrophy (SCD) through screening of the UBIAD1 gene in the members of the family in which CDCD was reported.

METHODS

Genetic analysis was performed in 3 affected members and 1 unaffected member of a pedigree with CDCD including the affected 31-year-old proband.

RESULTS

All 4 affected members of the described pedigree demonstrated discoid central corneal clouding, with subtle, superficial, crystalline deposits noted in one of the affected individuals. Screening of UBIAD1 in the affected individuals demonstrated a previously unreported missense mutation, p.Asp240Asn, which was not identified in an unaffected family member or in 100 control individuals.

CONCLUSIONS

The clinical findings of the family reported to have CDCD were indistinguishable from those found in SCD. However, CDCD was originally thought to be distinct from SCD because of the absence of positive lipid staining and the presence of alcian blue staining consistent with glycosaminoglycans in the proband's cornea. Our recent investigation has revealed that corneal specimens from other patients with SCD have also demonstrated staining for glycosaminoglycans. Discovery that mutations in UBIAD1 caused SCD allowed genetic testing of this CDCD family. Our newly reported UBIAD1 mutation suggests that CDCD is actually a variant of SCD. This report underscores the utility of genetic testing in determining whether newly described corneal dystrophies are in fact unique entities or just variants of well-known diseases.

The IC3D classification of the corneal dystrophies

BACKGROUND

The recent availability of genetic analyses has demonstrated the shortcomings of the current phenotypic method of corneal dystrophy classification. Abnormalities in different genes can cause a single phenotype, whereas different defects in a single gene can cause different phenotypes. Some disorders termed corneal dystrophies do not appear to have a genetic basis.

PURPOSE

The purpose of this study was to develop a new classification system for corneal dystrophies, integrating up-to-date information on phenotypic description, pathologic examination, and genetic analysis.

METHODS

The International Committee for Classification of Corneal Dystrophies (IC3D) was created to devise a current and accurate nomenclature.

RESULTS

This anatomic classification continues to organize dystrophies according to the level chiefly affected. Each dystrophy has a template summarizing genetic, clinical, and pathologic information. A category number from 1 through 4 is assigned, reflecting the level of evidence supporting the existence of a given dystrophy. The most defined dystrophies belong to category 1 (a well-defined corneal dystrophy in which a gene has been mapped and identified and specific mutations are known) and the least defined belong to category 4 (a suspected dystrophy where the clinical and genetic evidence is not yet convincing). The nomenclature may be updated over time as new information regarding the dystrophies becomes available.

CONCLUSIONS

The IC3D Classification of Corneal Dystrophies is a new classification system that incorporates many aspects of the traditional definitions of corneal dystrophies with new genetic, clinical, and pathologic information. Standardized templates provide key information that includes a level of evidence for there being a corneal dystrophy. The system is user-friendly and upgradeable and can be retrieved on the website www.corneasociety.org/ic3d.