The topographical map indicates any distortions or scarring in the cornea, with keratoconus revealed by a characteristic steepening of curvature that is usually below the centerline of the eye. The technique can record a snapshot of the degree and extent of the deformation as a benchmark for assessing its rate of progression. It is of particular value in detecting the disorder in its early stages when other signs have not yet presented. Stages
Once keratoconus has been diagnosed, its degree may be classified by several metrics:
The steepness of greatest curvature from mild (< 45 D), advanced (up to 52 D) or severe (> 52 D);
The morphology of the cone: nipple (small: 5 mm and near-central), oval (larger, below-center and often sagging), or globus (more than 75% of cornea affected);
The corneal thickness from mild (> 506 μm) to advanced (< 446 μm).Increasing use of corneal topography has led to a decline in use of these terms. Treatment
Lenses
In early stages of keratoconus, glasses or soft contact lenses can suffice to correct for the mild astigmatism. As the condition progresses, these may no longer provide the person with a satisfactory degree of visual acuity, and most practitioners will move to manage the condition with rigid contact lenses, known as rigid, gas-permeable, (RGP) lenses.
Scleral lenses
Scleral lenses are sometimes prescribed for cases of advanced or very irregular keratoconus; these lenses cover a greater proportion of the surface of the eye and hence can offer improved stability. Easier handling can find favor with people with reduced dexterity, such as the elderly. Piggybacking
Some people find good vision correction and comfort with a "piggyback" lens combination, in which RGP lenses are worn over soft lenses, both providing a degree of vision correction. One form of piggyback lens makes use of a soft lens with a countersunk central area to accept the rigid lens. Fitting a piggyback lens combination requires experience on the part of the lens fitter, and tolerance on the part of the person with keratoconus. Surgery
Corneal transplant
Between 11% and 27% of cases of keratoconus will progress to a point where vision correction is no longer possible, thinning of the cornea becomes excessive, or scarring as a result of contact lens wear causes problems of its own, and a corneal transplantation or penetrating keratoplasty becomes required. Keratoconus is the most common grounds for conducting a penetrating keratoplasty, generally accounting for around a quarter of such procedures. The corneal transplant surgeon trephines a lenticule of corneal tissue and then grafts the donor cornea to the existing eye tissue, usually using a combination of running and individual sutures. The cornea does not have a direct blood supply, so the donor tissue is not required to be blood type matched. Eye banks check the donor corneas for any disease or cellular irregularities.