Macular Dystrophy of the Corneal Stroma
Definition: an inherited corneal stromal dystrophy characterized by deposition of acid mucopolysaccharides in the corneal stroma.
Incidence/Prevalence: Macular dystrophy is much less common than BIGH3 dystrophies
Etiology: Macular dystrophy exhibits an autosomal recessive pattern of inheritance. The gene involved in macular dystrophy is a carbohydrate sulfotransferase gene (CHST6) on chromosome 16q22. Two types of macular dystrophy have been described depending on the absence in the production of keratan sulfate (type I) and presence (albeit reduced) of keratan sulfate and dermatan sulfate-proteoglycan (Type II). In type I, homozygous mutations of the missense, deletion and frameshift type occur that result in the absence of production. In Type II, mutations are found upstream from CHST6 suggesting a regulatory pathway is involved. Both types of mutations may be seen in the same family and the result is deposition of “unsulfated keratan sulfate”.
Clinical Findings: Macular stromal dystrophy eventually involves the entire cornea to the limbus in a diffuse pattern of poorly defined stromal white gray areas with intervening hazy stroma.
The entire stromal thickness may be affected. In the figure notice that the grey white deposits extend all the way to the limbus (arrow 1). Corneal erosions and stromal thinning may occur.
Histopathology: Acid mucopolysaccharide material is deposited both intracellularly and extracellularly in the corneal stroma.
The deposits stain blue with the alcian blue and colloidal iron stains. The blue color in the figure is Alcian blue positive glycoaminoglycans acid mucopolysaccharides (unsulfated keratan sulfate).
Treatment: Penetrating keratoplasty is advocated as the main treatment although recurrences have been described.
References:Akama TO et al. Macular corneal dystrophy type I and type II are caused by distinct mutations in a new sulphotransferase gene.Nat Genet. 2000 Oct;26(2):237-41.
