CHAPTER 10

Corneal Dystrophies and Ectasias

Historically, corneal dystrophies are defined as bilateral, symmetric, inherited conditions that appear to have little or no relationship to environmental or systemic factors. Dystrophies begin early in life but may not become clinically apparent until later. They tend to be slowly progressive and more pronounced with age. Corneal dystrophies can be classified according to genetic pattern, severity, histologic features, biochemical characteristics, or anatomical location. The anatomical scheme that classifies the dystrophies according to the levels of the cornea that are involved is the one that has been used most often.

However, there are exceptions to each part of the dystrophy definition, as some dystrophies are unilateral and/or asymmetric, have no obvious heredity, and have associated systemic findings. In addition, dystrophies that appear the same phenotypically may map to different chromosomes, and dystrophies that map to the same gene may have different phenotypes. For example, mutations in the transforming growth factor β–induced gene (TGFBI) lead to the majority of stromal corneal dystrophies. In order to more accurately reflect the genetic, clinical, and histologic characteristics of the dystrophies, the International Committee for the Classification of Corneal Dystrophies (IC3D) has revised the dystrophy nomenclature. In this system, each dystrophy is still organized according to the anatomical level affected, with a template summarizing genetic, clinical, and pathologic information. The system is upgradable and can be retrieved at www.corneasociety.org.

Category 1: A well-defined corneal dystrophy in which the gene has been mapped and identified and specific mutations are known.

Category 2: A well-defined corneal dystrophy that has been mapped to one or more specific chromosomal loci, but the gene(s) remains to be identified.

Category 3: A well-defined corneal dystrophy in which the disorder has not yet been mapped to a chromosomal locus.

Category 4: Reserved for a suspected new, or previously documented, corneal dystrophy, although the evidence for its being a distinct entity is not yet convincing.

The category assigned to a corneal dystrophy may change with time as more information about it is obtained. Eventually, all valid corneal dystrophies should attain category 1 status. The IC3D classification is summarized in Table 10-1. The genetics of major corneal dystrophies is summarized

in Table 10-2.

Weiss JS, Møller H, Lisch W, et al. The IC3D classification of the corneal dystrophies. Cornea. 2008;27(10:Suppl 2):S1–S42.

Table 10-1

Table 10-2