DEFINING KERATOCONUS

An improved definition of the different stages of keratoconus will aid in the assessment of candidates for corneal refractive surgery and research into potential treatments for the condition, said Stephen Tuft FRCS, Moorfields Eye Hospital, London UK.

“The drive in investigation and research should be towards early diagnosis and better ways of intercepting patients at a stage when treatment and intervention is more appropriate,” Dr Tuft told the 19th ESCRS Winter Meeting in Istanbul.

The basic definition of keratoconus is a bilateral asymmetric thinning of the cornea that causes keratometric myopia and irregular astigmatism. Its onset generally occurs when patients are in their teens, but it can occur later. Most patients have a stabilisation of the condition by their late 30s, possibly as a result of the general tendency of the body’s collagen to stiffen with age.

Among European populations the prevalence of keratoconus is about one per 1,200 individuals. The prevalence is about four times higher among South Asian populations, with a tendency towards an earlier age of onset and more rapid progression. In developed countries, keratoconus accounts for about one third of patients requiring a corneal graft procedure.

There are four main phenotypes of keratoconus. They are apex keratoconus, inferior or slipped cone keratoconus, pellucid marginal degeneration, and keratoglobus. The phenotype tends to be the same in families with the condition and, as the disease progresses, the phenotype does not shift into a different phenotype.

The cause of keratoconus appears to be a combination of genetic and environmental factors. Allergy and consequent eye rubbing has a role in some cases, although the exact pathological mechanism whereby this occurs has yet to be elucidated. Genetic studies have identified gene variants that may have a role in the disease, but none that are exclusive to keratoconus patients.

Dr Tuft said that when assessing patients for corneal refractive surgery, a surgeon should begin with the assumption that all patients under the age of 40 are keratoconus suspects in the broadest sense. The most reliable way to identify patients in an early stage of the disease is to see if there is a lack of concordance between the area of maximum curvature and the area of maximum elevation, and between the position of the corneal apex and the thinnest point.

“Overall, I think that measurements such as topography, refractive error and astigmatism should be combined and used towards developing better algorithms to improve discrimination in the diagnosis of keratoconus,” Dr Tuft added.

Stephen Tuft: s.tuft@ucl.ac.uk