An analysis of the astigmatic changes induced by accelerated orthokeratology

The change in corneal astigmatism induced by reverse geometry lenses for orthokeratology has not been described previously. This study examines the efficacy of accelerated orthokeratology for reducing astigmatism and whether this varies with the degree of pre-existing astigmatism.

Twenty-three randomly chosen eyes exhibiting 0.50 D to 1.75 D pre-fitting with-the-rule astigmatism were retrospectively analysed. Astigmatism was measured by simulated keratometry and corneal topography before and at the completion of a course of orthokeratology. The change in astigmatism measured by keratometry was calculated by two vector analysis techniques: the Bailey-Carney method, which was designed for contact lens-induced corneal shape changes, and the Alpins method, which was designed for surgically-induced corneal shape changes. The change in astigmatism measured by corneal topography was calculated by the EyeSys Version 3.2 software.

Most patients (20/23) had some reduction of astigmatism but orthokeratology is incapable of a total elimination of pre-fit astigmatism. Alpins vector analysis showed that an increased efficacy of 60 to 80 per cent would be required to eliminate astigmatism. All three methods found a 50 per cent mean reduction in astigmatism from the pre-fit level. Topographical analysis indicates that the reduction in astigmatism occurs mainly over the central 2.00 mm chord. There is a very poor correlation between the pre-and post-wear corneal astigmatism at the 2.00 mm chord (R(2) = 0.11, p = 0.04) and the predictability of the final astigmatic axis is also poor (angle of error = 1.22 +/- 27.35).

Accelerated orthokeratology seems more successful than conventional orthokeratology at reducing with-the-rule astigmatism. However, it reduces pre-existing astigmatism by an average of only 50 per cent and it does not do so reliably either for magnitude or direction. These results provide two useful patient selection criteria for orthokeratology. They are: assuming 0.50 D to 0.75 D of astigmatism is a satisfactory outcome, orthokeratology can be expected to be successful for pre-fitting astigmatism of up to 1.00 D to 1.50 D; and the greater the pre-existing astigmatism, the less likely orthokeratology is to be successful.