COMMENTARY

Posterior Corneal Astigmatism: Not to Be Ignored!

William W. Culbertson, MD

Disclosures

February 19, 2013

Contribution of Posterior Corneal Astigmatism to Total Corneal Astigmatism

Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L
J Cataract Refract Surg. 2012;38:2080-2087

Residual Refractive Astigmatism

Residual refractive astigmatism has a deleterious effect on postoperative visual outcomes, often disappointing patients' expectations. This is especially important in multifocal intraocular lens (IOL) surgery where as little as 0.50 diopters of postoperative astigmatism may be the difference between subjective success and failure. Often, despite meticulous attention to ascertainment of the measured astigmatism power and axis and subsequent placement of the correction (relaxing incision or toric IOL), both the surgeon and the patient are frequently confronted with unexpected and unexplained postoperative residual vision affecting astigmatism. Many obvious factors contribute to this shortfall, including inaccurate keratometry and/or axis placement of correcting toric IOLs or incisions. In this study, Koch and colleagues have now discovered and quantified another potentially correctable variable that should be accounted for: posterior corneal astigmatism.

Study Summary

The GALILEI™ analyzer (Zeimer Group; Alton, Illinois) uses a Placido disc in combination with a dual-channel Scheimpflug camera to use ray tracing with proprietary software to calculate total corneal power as well as anterior and posterior curvature. Koch and colleagues evaluated these parameters in 715 eyes from patients aged 20-89 years. Their goals were to determine the "contribution of posterior corneal astigmatism to overall corneal astigmatism" and the "error introduced by estimating total corneal astigmatism from only anterior corneal measurements."

Results confirmed earlier reports that with increasing age, with-the-rule anterior corneal astigmatism tended to decline and shift toward against-the-rule astigmatism. In contrast, posterior corneal astigmatism remained stable in both axis and magnitude regardless of age. The average posterior corneal astigmatism was measured to be -0.30 diopters (ranging up to 1.10 diopters) with 9% of eyes having more than 0.50 diopters. The steep axis was oriented vertically (at approximately 90˚) in about 90% of eyes. Steep posterior astigmatism subtracts from anterior astigmatism in the same axis and adds to it in the opposite axis. Thus, in most eyes, posterior corneal astigmatism reduces the total corneal astigmatism in with-the-rule anterior astigmatism and may augment against-the-rule astigmatism.

Viewpoint

This study alerts surgeons to the potential latent effects of posterior astigmatism on total corneal astigmatism and the optimal power and axis for astigmatism that requires correction. Preoperative Scheimpflug imaging, particularly when coupled with Placido topography, can enhance the surgeon's plan for correcting the refractive effect of corneal astigmatism. Alternatively, intraoperative aphakic aberrometry can supply the magnitude and axis of optimal correction and calculate the proper toric IOL power and axis placement. As a rule of thumb, small (0.30 diopters) reductions in with-the-rule corrections and increases in against-the-rule corrections can optimize postoperative results, although pre- or intraoperative calculation is obviously best.

It is interesting to speculate why posterior corneal astigmatism is stable in orientation and magnitude throughout life, whereas anterior astigmatism shifts from predominant with-the-rule astigmatism to an against-the-rule axis. Perhaps this observation can be better explained on the basis of gradual corneal epithelial remodeling than from corneal stromal changes (the "everything goes south in later life" theory). In addition, correction of astigmatism in a patient's younger years -- with LASIK, for instance -- should take into account this gradual age-dependent shift in power and axis.

Abstract

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