Is now the time to abandon Kmax?

Modern corneal tomography measurements, including anterior and posterior elevation and pachymetric data, can help diagnose and detect progression in keratoconus earlier and more reliably than can changes in maximum anterior sagittal curvature (Kmax), Michael W Belin MD, told the 10th EuCornea Congress in Paris. He outlined several reasons why Kmax is a poor parameter for assessing ectasia severity. Kmax often correlates poorly with clinical measures. For example, a Kmax cone of 55D that overlaps the visual axis may reduce visual acuity much more than an inferior 62D cone that does not, observed Dr Belin, who is professor of ophthalmology and vision science at the University of Arizona, Tucson, USA. “Patients don’t complain of increasing Kmax, they complain of poor vision, glare, night-driving difficulty and distortion.” Nor does Kmax accurately describe morphologic change. In fact, progression from a central nipple cone to a more inferior globular cone is often associated with a decrease in Kmax, Dr Belin noted. Kmax is neither specific nor sensitive enough to reliably diagnose keratoconus. Using 46.1D Kmax as a cut-off would include about 95% of keratoconus patients – but nearly one-third included patients who would not have keratoconus, Dr Belin said. Moving the cut-off to 45.3D would capture 97.5% of keratoconus patients, but more than half the patients identified would not have the condition. BETTER MEASURES FOR EARLY TREATMENT Kmax does not identify early disease, Dr Belin said. He showed several cases of extreme posterior ectasia with normal anterior surface curvature. Indeed, by the time ectasia progresses enough to significantly alter Kmax, significant vision loss has occurred in most cases, he noted. “We don’t treat glaucoma only after patients have visual field loss, and we should be doing the same with keratoconus.” So why is Kmax used? It is an artefact of the initial cross-linking study from 2003 in which Kmax was the only parameter statistically associated with a positive effect of cross-linking, Dr Belin explained. It was then adopted by insurers for determining payment for corneal cross-linking and other treatments. Dr Belin and colleagues have developed other measures that can detect ectasia earlier, and are more specific and sensitive in characterising and tracking progress. These include the anterior and posterior radius of curvature taken from a 3.0mm optical zone centered on the thinnest point, minimal corneal thickness and best spectacle-corrected distance visual acuity. These have been combined into an easy-to-interpret Belin ABCD progression display, currently available on the Oculus Pentacam (Oculus GmbH, Wetzlar, Germany). Early detection may allow early treatment, preventing irreversible vision loss, Dr Belin said. Therefore, he advocates abandoning Kmax in favour of more predictive measures. Dr Belin is a consultant to Oculus. He receives no compensation for any of the mentioned products/displays Michael Belin: mwbelin@eyes.arizona.edu