Corneal dystrophies

Chapter 34 Corneal dystrophies

Hans Ulrik Møller

Chapter contents

CLASSIFICATION

MUTATION RATE

Corneal dystrophies are rare Mendelian inherited conditions which exhibit bilateral and usually symmetrical corneal changes.

Nomenclature has been difficult because of controversies about the phenotype definitions. Many authors published different phenotypes under the same headings.

In this chapter, I will focus on a few classic dystrophies, highlighting the clinical presentation in children and emphasizing the differences from the adult. Slit-lamp pictures of the often very subtle changes of corneal dystrophies in children are difficult to take. The clinical pictures in this chapter look almost alike. However, when examining children for a corneal dystrophy, do not look for the well-known adult slit-lamp appearance. You need to look for subtle opacities, similar to the adult clinical picture, but the final diagnosis is made after examining the whole family. Most of the dystrophies are progressive, and the figures illustrate how different the clinical picture may be in youngsters.

Definition

The term dystrophy is from the Greek words dys (wrong or difficult) and trophe (nourishment). There is no universally accepted definition of the term dystrophy. The word was first used 150 years ago for a group of entities clearly not of traumatic or infectious origin. They were believed to be due to poor nerve supply or nourishment. Most later proved to be genetic in origin, but the word stuck to this group of inherited diseases.

Inherited opacification of the cornea in systemic conditions such as cystinosis are not included here.

Degenerations are secondary, non-genetic processes resulting from aging or previous corneal inflammation.

Classification

Recently an international group of cornea specialists proposed a new classification of 25 entities considered to be dystrophies.¹ The committee developed a series of descriptive categories as shown in Box 34.1.

Box 34.1 The International Committee for Classification of Corneal Dystrophies (IC3D)

the four levels of corneal dystrophies

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 clinical corneal dystrophy in which the disorder has not yet been mapped to a chromosomal locus

Category 4. This category is reserved for a suspected new, or previously documented, corneal dystrophy, although the evidence for it being a distinct entity is not yet convincing

The category of a specific corneal dystrophy can be expected to change over time as knowledge advances. Eventually, all valid corneal dystrophies should attain category 1

The classification comprising an extensive description of each condition, clinical pictures, key references, and a few words on onset in childhood can be retrieved at http://www.corneasociety.org/ic3d.

Mutation rate

The prevalence and clinical significance of the conditions vary. The founder effect (i.e., an increase in the number of cases of a certain genetic trait due to the introduction of a new mutation in an isolated population) has, in some countries, given rise to publication of large pedigrees of conditions that may be almost non-existent elsewhere.

As the mutation rates for many of the corneal dystrophies are low, it is important to be cautious when diagnosing apparently sporadic cases. A family history and examination of family members are mandatory.

Dystrophies related to mutations in the TGFBI gene

New genetic information describes the different allelic mutations within the TGFBI (transforming growth factor-beta induced) gene on chromosome 5q31. They account for many of the classic corneal dystrophies, which are allelic variations of dominant entities of the same gene. Thus, genetic knowledge is bringing order to this field of ophthalmology. Although naming the diseases according to mutation number has not gained universal recognition, most ophthalmologists rely on a genetic analysis to distinguish between the rarer varieties.

Tags: Pediatric Ophthalmology and Strabismus

Jun 4, 2016 | Posted by admin in OPHTHALMOLOGY | Comments Off

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