The effect of trabeculectomy on refraction, keratometry and corneal topography

After successful trabeculectomy patients often complain of reduction in vision even after several months. Amongst other factors, corneal astigmatism appears to be altered. A pilot study measuring, pre- and post-operative corneal topography indicated three types of astigmatic change: some patients develop a relative superior corneal steepening, others a superior flattening and yet others complex regional changes that do not conform to either of these patterns. The present study was designed to evaluate further these patterns of variation in corneal curvature and to look for corresponding refractive and keratometry changes. Twenty-nine patients admitted for trabeculectomy had pre-operative assessment of subjective and automated refraction, manual keratometry and corneal topography from which simulated keratometry values were calculated. A standard trabeculectomy procedure was performed and post-operative measurements of the same parameters were taken at 1 and 3 months after surgery. Similar patterns of corneal topographic change to those found in our pilot study were noted. In both the superior steepening and superior flattening groups there was an increase in vertical keratometry and a shift towards 'with-the-rule' astigmatism. Furthermore a 1 year analysis of 13 patients from our pilot study indicated that the topographic changes lasted for at least 12 months after surgery. We conclude that computer-assisted corneal topography reveals complex regional changes in corneal curvature that are not readily detected from alterations in refraction or keratometry. These changes are sufficiently great to have a significant effect on visual function in some patients.