Amsler–Krumeich classification
Stage I	Eccentric corneal steppening Myopia and/or astigmatism <5.00D Mean central K readings <48.00D
Stage II	Myopia and/or astigmatism 5.00–8.00D Mean central K readings <53.00D Absence of scarring Minimal corneal thickness >400 µm
Stage III	Myopia and/or astigmatism 8.00–10.00D Mean central K readings >53.00D Absence of scarring Minimal corneal thickness 300–400 µm
Stage IV	Refraction not measurable Mean central K readings >55.00D Central corneal scarring Minimal corneal thickness 200 µm