This condition presents at puberty and has been found in association with systemic disorders like Marfan's syndrome, Down's syndrome [52, 88], neurofibromatosis, Ehlers-Danlos syndrome [56, 94, 129] and atopic dermatitis [93, 109, 119]. It can also be seen in combination with ocular disorders like aniridia, cataract and retinitis pigmentosa [11, 33, 53]. A progressive, non-inflamma tory, bilateral thinning of the central corneal stroma leads to severe astigmatism. Aetiology and pathogenesis are unclear. At histological examination the epithelium can be either atrophic or hyperplastic. The most striking finding is interruption of Bowman's membrane, with downgrowths of epithelium or upgrowths of corneal stroma in the breaking spot. The breaks may be narrow and the pathology is often restricted to a narrow 1-2 mm zone, sometimes serial sections are required to find the lesion. At the edge of the conus, iron can be found in the epithelium (Fleischer's ring). The axial stroma shows mucoid degeneration, the peripheral stroma is of normal appearance. In severe cases, rupture of Descemet's membrane and the endothelium can occur, resulting in an inflow of water and the appearance of cystic spaces. Keratoconus can be treated by surgery with use of stro-mal cornea grafts.

Corneal dystrophies are inherited, bilateral disorders that can be divided into epithelial, stromal and endo-thelial abnormalities. Routine stains for suspected corneal dystrophy must include PAS, Masson, Alcian blue, Congo red and Trichrome stains. In end-stage dystrophies a keratopathy can develop, in which all layers of the cornea are involved. Many patients with corneal dystrophies have a point mutation in a gene on chromosome 5q31 [54, 77, 113].

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