Dr.Tamilarasi,Dr.Ramamurthy D
ABSTRACT
PURPOSE: To analyze the results of ASCRS post refractive IOL calculator in post myopic LASIK patients undergoing cataract surgery.
METHODS: Retrospective observational study. Accuracy of ASCRS post refractive IOL calculator was evaluated in 58 eyes of patients undergoing cataract surgery. All eyes had undergone myopic Lasik in the past. Mean spherical equivalent of eyes at 3 months post operative period within 0.50 D, 0.50-1.00D, > 1.00 D and > 2.00 D were calculated.
RESULTS: Out of the 58 eyes, the spherical equivalent was <+/- 0.5D in 51% of eyes , 81 % of eyes within <+/-1.00 D .
CONCLUSION: ASCRS post refractive IOL calculator provided better IOL predictability in post myopic LASIK eyes undergoing cataract surgery.
INTRODUCTION:
The best available techniques for calculating IOL power in patients who have undergone previous corneal refractive surgery can achieve the required minimal standards for predictive accuracy, but they will not match the predictability which normal patients and their surgeons expect of modern cataract surgery. The sources of error in IOL calculations in eyes that have undergone previous corneal refractive surgery include flawed assumptions in keratometric index of the conventional eye model, the changed asphericity of cornea, the changed relationship between the cornea’s anterior and posterior curvature and difficulties in calculating effective lens position. There are more than 20 published ways of performing IOL power calculation in eyes with a history of corneal refractive surgery. This likely means that none of them is truly correct and certainly that none is perfectly accurate. Methods are usually grouped according to the data that they require like those that use both pre corneal refractive keratometry reading and change in refraction, those that use the change in refraction and those that only use current corneal data. ASCRS post refractive IOL calculator is open access web based tool, which has assembled a large variety of these methods into single, easy to use location. We did a retrospective analysis of 58 eyes that had undergone corneal laser refractive surgery for myopia in the past and the IOL power calculation was done using ASCRS post refractive IOL calculator during cataract surgery. The mean spherical equivalent and the percentage of eyes within 0.50D, 0.50-1.00D, > 1.00D and > 2.00 D at 3 months post cataract surgery were analyzed.
MATERIALS AND METHODS:
This retrospective observational study was done at The Eye Foundation, Coimbatore. It included eyes that had undergone myopic LASIK in the past and subsequently had cataract surgery from January 2013 to March 2018 and had IOL power calculated using the ASCRS post refractive IOL calculator and with minimum of 3 months follow up. Exclusion criteria was eyes with any coexisting ocular pathologies like myopic maculopathy, retinitis pigmentosa, retinal detachment and glaucoma; any other ocular surgeries; eyes with best corrected visual acuity <0.2 log MAR due to co existent ocular pathology ,ocular trauma , intraoperative complications during cataract surgery /LASIK, and eyes with astigmatism of > 1.5D prior to cataract surgery. After meeting the inclusion and exclusion criteria we included 58 eyes for analysis.
The preoperative examination included history and details of previous refractive surgery, manifest refraction and best corrected visual acuity, detailed anterior and posterior segment examination. Keratometry (Manual and Optical), Rotating Schiemflug imaging (Pentacam, Oculus , Inc), Optical biometry were done. Preoperative parameters were entered into the ASCRS post refractive IOL calculator (available on ASCRS website www.ascrs.org). This is an open access, web based tool that has assembled a large variety of post LASIK IOL power calculation formulas into a single easy to use location. After entering all of the biometric, topographic, tomographic and IOL data, the ASCRS calculator provides the recommended IOL power for each method as well as the minimum, maximum and the average IOL power. The surgeon selects the IOL power based on his judgment in terms of the target refraction and the IOL powers predicted by these methods. All surgeries were done by experienced surgeons by temporal clear corneal 2.2 mm incision and by standard phacoemulsification followed by implantation of aspheric single piece IOL in the bag. Postoperatively the manifest refraction and best corrected visual acuity was documented followed by thorough ocular examination.
RESULTS:
| Number of eyes | 58 |
| OD:OS | 31:27 |
| MRSE | -0.56±0.94 |
| BCVA | 0.12±0.18 logMar |
| SPHERICAL EQUIVALENT AT 3 MONTHS POST OP | |
| <-0.5 D | 24(41%) |
| -0.51 TO -1.00 D | 14(24%) |
| -1.01 TO -2.00 D | 9(15%) |
| >-2.00 D | 0 |
| <0.50 D | 6(10%) |
| 0.51 TO 1.00 D | 4(6%) |
| >1.00 D | 1(1%) |
| IOL power implanted | 19.07±2.5 |
| Refractive error for which LASIK was done | -7.7±4.79 |
DISCUSSION:
Many well-established methods have been proposed to calculate IOL power after corneal refractive surgery. Using the IOL calculator on the ASCRS website for eyes with prior myopic LASIK/PRK, the data were classified into three groups. (1) The methods using pre-LASIK/PRK Ks and ∆MR were the clinical history method, the Feiz Mannis method, and corneal bypass. These methods depended on the information obtained before refractive surgery, (2)The methods using only ∆MR, the recalled prior laser correction in spherical equivalent, included the Masket method, the Modified Masket method, adjusted average central corneal power, adjusted effective refractive power (EffRP) ,adjusted atlas 0-3and Barrett true K.(3) Methods that did not involve the use of existing data included the Wang Koch Maloney method, the Shammas method, the Haigis-L method, OCT and Barrett True K no history method.
The purpose of this study is to share our results using ASCRS post refractive IOL calculator. The spherical equivalent at 3 months post op was <+/- 0.50 D in 51% of eyes and <+/- 1.00 D in 81% of eyes. In normal eyes, benchmark standards for refractive outcomes after cataract surgery have been established in National Health Service of the United Kingdom. These standards are 55 % of eyes achieving refraction within +/- 0.5D of the predicted refraction and 85 % of eyes achieving refraction within +/- 1.00 D of predicted refraction. Our results are very close to these results in spite of post refractive status.
Without modern equipment such as the Scheimpflug camera and the IOL master, other accurate methods can be used for IOL power calculation n after myopic laser refractive surgery. ASCRS offers the use of a post refractive IOL calculator online, free of charge.
At present, however, there is no perfect way of determining IOL power for all eyes and this is particularly true for eyes that have undergone corneal refractive surgery. Until newer methods become widely available, the best advice is to use all available data with multiple formulas, choose the higher power to err on the side of residual myopia and set realistic expectation for patients.
CONCLUSION:
Understanding the reasons leading to refractive errors in these eyes is highly recommended and helps avoiding errors in many cases. When selecting the IOL power, look at the results of the most accurate formulas in any specific patient and patient should always be advised that a prediction error in refraction can occur in these cases, even with the most advanced technologies. ASCRS post refractive IOL calculator provided better IOL predictability in post myopic LASIK eyes undergoing cataract surgery
References
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