December 2012

 

RESIDENTS

 

EyeWorld journal club

Review of "Contribution of posterior corneal astigmatism to total corneal astigmatism"


by Ken Downes, M.D., Edie Deniro, M.D., Geoff Wilkes, M.D., Alan Lowinger, M.D., Patrick Coady, M.D., Michael Clamp, M.D., Ako Takakura, M.D., Maya Ling, M.D., Adrian Dokey, M.D., Sally Lin, M.D., and Kevin Denny, M.D., service chief, Cataract and Anterior Segment Surgery, California Pacific Medical Center, San Francisco

 

Kevin Denny, M.D., service chief, Cataract and Anterior Segment Surgery, California Pacific Medical Center (CPMC)

 

This month's JCRS includes the paper that Doug Koch delivered for his Kelman Innovator Lecture at the 2012 ASCRS•ASOA Symposium & Congress. I asked the CPMC residents to review it for this month's journal club.

David F. Chang, M.D., EyeWorld chief medical editor

Top, left to right: Ken Downes, Edie Deniro, Geoff Wilkes, Alan Lowinger, and Patrick Coady. Bottom, left to right: Michael Clamp, Ako Takakura, Maya Ling, and Adrian Dokey Source: Geoff Wilkes, M.D.

Over the years, cataract surgery has evolved to fall squarely within the realm of refractive surgery. Ophthalmologists are continuously looking for ways to further improve the predictability of post-cataract surgery refractive outcomes when choosing intraocular lenses and planning their surgeries. Correcting pre-op astigmatism with toric intraocular lenses and limbal relaxing incisions is now commonplace in ophthalmology, and understanding how to best correct astigmatism is important in the march toward more accurate and predictable surgical outcomes. In their recent article, Douglas Koch, M.D., and his colleagues at the Cullen Eye Institute, Houston, address posterior corneal astigmatism, which may become an important player when planning to correct pre-op astigmatism.

In this large, single-center, retrospective case series, the authors analyze measurements gathered from a Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems, Port, Switzerland) from 715 corneas of 435 consecutive patients presenting to their clinic for cataract and refractive surgery assessment from January 2008 to March 2011. The study's intended purpose was to determine both the contribution of posterior corneal astigmatism to overall corneal astigmatism and the error introduced when estimating total corneal astigmatism from anterior corneal measurements alone. They included an age-stratified group of patients all with "good-quality Scheimpflug analyzer scans," an absence of previous ocular trauma, ocular surgery, corneal or ocular disease, and a lack of contact lens wear for 2 weeks prior to measurements. Results included reporting magnitude of posterior corneal astigmatism, correlations of anterior and posterior astigmatism, age-related changes in anterior and posterior astigmatism, and the error from estimating corneal astigmatism from anterior corneal astigmatism alone.

Regarding the study's methods, we felt further explanation of why an eye might not have a "good quality" scan may prove useful for most ophthalmologists who presumably do not have access to a Scheimpflug analyzer in their own offices. Did these eyes have any common characteristics that will allow an ophthalmologist to predict poor quality Scheimpflug analysis? Can anything be gathered from the data of the poor quality scans? Are there any trends in posterior corneal astigmatism in these eyes despite the poor scans? When considering the magnitude of posterior corneal astigmatism found in this study, it is encouraging to note that the authors' findings are quite complimentary to several previous studies, including the largest by Ho et al.,1 which used an alternate Scheimpflug analyzer (Pentacam, Oculus, Lynnwood, Wash.). While the authors do note that a weakness of this study is their inability to validate the accuracy of posterior corneal measurements, the similar findings between studies certainly seem to lend support to each other's findings. When discussing the magnitude of posterior corneal astigmatism, the authors report it exceeding 0.50 D in 9% of eyes in their study. We think it might be interesting to know which type of eyes these outliers represent. Do these 9% of eyes have anything in common? Would sub-analysis yield any clues to better determine which patients might benefit more from Scheimpflug analysis prior to astigmatic surgical interventions? For example: Do eyes with long axial lengths have a greater propensity to have a high degree of posterior corneal astigmatism? Does race or gender make any difference? When considering the importance of an analysis that many ophthalmologists can not presently obtain in their own office, we think it will be important to attempt to tease out which patients would benefit most from Scheimpflug analysis. One posterior corneal astigmatism magnitude sub-analysis undertaken by the authors was that of the relationship between anterior corneal astigmatism and posterior corneal astigmatism. Even though they found moderate correlation between the two when the steep anterior corneal meridian was aligned vertically, we agree with the authors' stated conclusion that one cannot confidently predict the amount of posterior astigmatism from anterior corneal measurements alone. With each type of anterior corneal astigmatism (WTR, ATR, or oblique), there were a number of corneas with posterior astigmatism >0.5 D. If the type of anterior corneal astigmatism isn't predictive of these outliers, what factors might be? Perhaps the most enthusiastic portion of our resident journal club centered on the authors' conclusion drawn from their measured simulated keratometry anterior corneal astigmatism estimation error. They found on average that determining corneal astigmatism by anterior corneal measurements alone produced an estimation error of 0.22 @ 180 when compared to measurements produced by the Scheimpflug analyzer. While statistically significant, we wondered if this error will prove to be clinically significant. For what percentage of patients will it be significant? Will this justify increased costs for new equipment? Given the inherent errors associated with current toric IOL implantation techniques and limbal relaxing incisions, our group felt that such a small average gain in accuracy must be considered carefully when determining the significance of using posterior corneal measurements to determine total astigmatic corneal power. That being said, incremental improvements in accuracy coupled with ever-evolving intraocular lens technology and surgical technique is certainly a path we support traveling down. Future clinical trials comparing refractive outcomes in patients undergoing astigmatic surgical interventions with and without prior Scheimpflug analysis should be helpful in determining the significance of Scheimpflug corneal analysis.

In summary, we applaud the authors' well-designed and analyzed study. We find this study by Dr. Koch and his colleagues to be an excellent stepping stone toward improved understanding of corneal astigmatism and hopefully improved astigmatic surgical intervention outcomes. The study suggests that posterior corneal astigmatism plays an important role in overall astigmatism in a significant number of corneas, is not yet easy to predict without direct measurement, and may become another important measurement when considering work-up of astigmatic surgical interventions. We look forward to the authors' reporting on their continued investigation of this important topic.

Reference

1. Ho J-D, Tsai C-Y, Liou S-W. Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement. Am J Ophthalmol 2009; 147:788-798.

Contact information

Denny: kjdenny1@aol.com

Review of "Contribution of posterior corneal astigmatism to total corneal astigmatism" Review of "Contribution of posterior corneal astigmatism to total corneal astigmatism"
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