This study set out to compare biomechanical properties between normal and keratoconic eyes. It was a retrospective study evaluating 89 eyes (47 normal, 42 keratoconic) and a validation arm of 72 eyes (33 normal, 39 keratoconic) using a high speed Scheimpflug Corvis ST camera. Keratoconus was diagnosed by clinical findings confirmed by topography and tomography. Dynamic corneal response parameters collected by the Corvis ST (A1 velocity, deformation amplitude, deformation amplitude ratio max 1mm, and max inverse radius) and a stiffness parameter at first applanation (SP-A1) were incorporated into a logistic regression equation (DCR index). Area under the receiver operating curve was used to assess the sensitivity and specificity of the DCR index. DA, DA Ratio Max 1mm, Max Inverse Radius, and SP-A1 were each found to be statistically significantly different between normal and keratoconic eyes (Mann-Whitney test [independent samples]; P=.0077, <.0001, <.0001 and <.0001, respectively; significance level: P<.05). DCR index demonstrated high sensitivity, specificity, and overall correct detection rate (92.9%, 95.7%, and 94.4%, respectively; AUC = 98.5). The sensitivity and overall correct detection rate improved when eyes with Topographical Keratoconus Classification grades (TKC) greater than 0 were re-evaluated (from 92.9% to 96.6% and from 94.4% to 96.1%, respectively). Combining multiple biomechanical parameters (A1 velocity, DA, DA Ratio Max 1mm, Max Inverse Radius, and SP-A1) into a logistic regression equation allowed for high sensitivity and specificity for distinguishing keratoconic from normal eyes. This study does not address eyes with subclinical keratoconus, however it does address eyes at varying stages of severity of keratoconus. It identifies biomechanical parameters that differed between normal and keratoconic eyes however it is a pilot study and further studies are needed to determine how well the dynamic corneal response index performs at distinguishing subclinical keratoconic eyes.

Comparison of corneal deformation parameters in keratoconic and normal eyes using a non-contact tonometer with a dynamic ultra-high-speed Scheimpflug camera.
Mercer RN, Waring GO 4th, Roberts CJ, et al.
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Inderpaul S Sian

MB, ChB, BOptom (Hons), MCOptom, Corneal & External Eye Disease Fellow, Moorfields Eye Hospital.

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