ESCRS - FPS02.06 - The Impact Of Coma On Manifest Refractive Astigmatism In Keratoconus

The Impact Of Coma On Manifest Refractive Astigmatism In Keratoconus

Published 2022 - 40th Congress of the ESCRS

Reference: FPS02.06 | Type: Free paper

Authors: Max Davidson 1 , Elizabeth Law* 2 , Mohamed Elkadim 3 , James Myerscough 2

1Sheffield Teaching Hospitals NHS Trust,Sheffield,United Kingdom, 2Southend University Hospital,Southend-on-Sea,United Kingdom, 3Ophthalmology,Tanta University,Tanta,Egypt

Purpose

To determine factors associated with disparity between manifest refractive cylinder (MRC) and keratometric astigmatism (KA) and the effect of coma on the MRC in keratoconic eyes.

Setting

Retrospective study of patients from two hospitals within one UK National Health Service Trust.

Methods

145 keratoconic eyes were identified. Pentacam and corresponding subjective refraction values were obtained. MRC power and topographic variables were correlated using univariate and multivariate regression analysis. Vector difference between KA and MRC was also correlated to tomographic variables to detect factors associated with disparity. Axis of MRC was compared and correlated to the axis of vertical coma.

Results

There was a disparity found between MRC (mean = 2.72 + 2.04) and KA (mean = 3.37 + 1.98), with KA being significantly greater (p<0.01). 51% of eyes had with-the-rule corneal astigmatism (WTR), yet only 14.7% of eyes had MRC considered WTR. KA and coma values showed significant correlation in univariate and multivariate analysis with the MRC power. The mean vector difference between KA and MRC was 2.81 ± 2.25 and correlated with the KA (p<0.01), posterior keratometric astigmatism (KApost)(p=0.05) and coma (p=0.04). MRC axis showed a moderate but significant correlation with the axis of coma (r = 0.54, p<0.01). The disparity between these axes decreased as the value of coma increased.

Conclusions

Vertical coma is present in keratoconic eyes and is a significant contributor to the disparity found between KA and MRC. Its refractive effect is similar to that of against-the-rule (ATR) astigmatism and as such reduces or cancels the effect of WTR corneal astigmatism while augmenting ATR corneal astigmatism. Thus, when planning visual rehabilitation procedures in keratoconic eyes, the effects of coma on MRC must be fully considered with particular regard to the axis.