EyeWorld India December 2013 Issue

17 December 2013 EWAP FEAturE Views from Asia-Pacific Mahipal SACHDEV, MD Chairman & Medical Director, Centre For Sight B-5/24 Safdarjung Enclave, New Delhi, India Tel. no. +91-9810046017 Fax no. 01141651744 drmahipal@gmail.com I t is interesting to note 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. Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. Selecting toric intraocular lenses based on anterior corneal measurements could lead to overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes that have against-the-rule astigmatism. Dr. Koch, Wang and their colleagues have published their results in the Journal of Cataract & Refractive Surgery , where they have evaluated 715 corneas of 435 consecutive patients, calculating total corneal astigmatism using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism. 1 They found that toric IOL selection based on anterior corneal measurements only could lead to unpredictable outcomes. Patients who have anterior with-the-rule astigmatism where the cornea is steep at 90 degrees anteriorly tend to have, on average, 0.5 D of steepness vertically along the posterior cornea. This steepness vertically translates into power horizontally or against-the-rule effect refractive power at 180. So a patient may be having 2 D on the anterior cornea, but once you take the posterior corneal astigmatism into account the patient may only have 1.3 or 1.4 D on the total corneal power as the posterior cornea contributes to about 0.5 or 0.6 D in the opposite direction. Preoperative Scheimpflug imaging, particularly when coupled with Placido topography, can enhance the surgeon’s plan for correcting the refractive effect of corneal astigmatism. Reference 1. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg . 2012;38:2080-2087. Editors’ note: Dr. Sachdev has no financial interests related to his comments. inaccuracies in measurements can occur, which could lead to a decreased efficacy of toric intraocular lens implants. IOL calculators and PCA Dr. Hamilton said many of the toric IOL calculators do not take posterior corneal astigmatism into account. The toric calculator from Alcon (Fort Worth, Texas, USA/ Hünenberg, Switzerland) does not have this ability. Although the Abbott Medical Optics IOL calculator (AMO, Santa Ana, Calif., USA) is more sophisticated, it does not account for posterior corneal astigmatism. The Alcon calculator gives one option for the astigmatic IOL power a surgeon should use, while the AMO calculator and the Holladay IOL Consultant (Holladay Consulting, Bellaire, Texas, USA) give multiple options for the astigmatic IOL power, indicating what the residual will be in all of these cases. However, Dr. Hamilton said none of these IOL calculators are able to factor in the back of the cornea at this time. Age and other factors Age is one of the factors that needs to be considered when determining the measurement of the cornea. Dr. Weikert said a study on 700 corneas recently published in the Journal of Cataract & Refractive Surgery looked at this and measuring both the front and back surface of the cornea. “We found that as people age, the astigmatism on the front surface of the cornea tended to change from with-the-rule to against-the-rule, which is generally accepted,” he said. However, he said that the back of the cornea adds effective against-the- rule astigmatism that changes minimally with age. This is important to know because it can cause an error in measurement. Dr. Weikert said this could cause an overestimation of with-the-rule astigmatism in a younger person if the against-the- rule astigmatism from the back of the eye is not being measured. Similarly, the astigmatism could be underestimated in an older person if only measuring the front of the cornea. Although some of these differences may only be less than a diopter, Dr. Weikert said it’s important to have accurate results, especially for toric lenses. “Basically, against-the-rule becomes more common with age,” Dr. Hamilton said. In 80% of eyes, the back surface of the cornea has a with-the-rule shape, which subtracts from with-the-rule astigmatism on the anterior surface and does not change with age, he said. Leaving room for change or differences in measurements Despite wanting to have accurate and exact measurements for total astigmatism, surgeons sometimes have to adjust and leave room for change. Dr. Weikert said he will aim for a bit of with-the-rule astigmatism after cataract surgery to account for the against-the-rule shift that happens to many over time. Toric IOLs will generally be chosen to correct a specific amount of astigmatism, but this can change as patients age. One of the reasons that IOL calculators and measurement of posterior corneal astigmatism are so important is because they help a surgeon be even more accurate and know exactly the amount of astigmatism that needs to be factored in. Knowing surgically induced astigmatism and posterior corneal astigmatism can help obtain an even more accurate measurement. “I think industry is motivated to continually improve the accuracy of those measurements,” Dr. Weikert said. Similarly, Dr. Hamilton said you could end up with very little expected residual astigmatism when using these toric IOL calculators, but in order to know for sure if the measurements are correct or if you will be overcorrecting or undercorrecting the astigmatism, the posterior corneal astigmatism needs to be considered. EWAP Editors’ note: Dr. Hamilton has financial interests with Ziemer, Alcon, and AMO. Dr. Weikert has financial interests with Ziemer. Contact information Hamilton: Hamilton@jsei.ucla.edu Weikert: mweikert@bcm.tmc.edu