Dr.ANKITA GUPTA, Dr.Parmar Gautam Singh,Dr.Ashok Kumar Meena
Author -: Dr.Gautam Singh Parmar, Dr. Ashok Kumar, Dr.Ankita Gupta
AIM:To compare accuracy of IOL power calculation methods postmyopic LASIK surgery patients without previous refractive surgery data using American Society of Cataract and Refractive Surgery IOL Power Calculator
METHODS: Retrospectiveanalysis of 10 eyesundergone cataract surgery postmyopicLASIK.4 methods used to calculate IOL power: Shammas-PL, Haigis-L,Potvin-Hill Pentacam and Barrett True-K No History method.Mean arithmetic and absolute refractive prediction errorand %of eyes within ±0.5D,±1D of refractive prediction errors calculated
RESULTS:Mean arithmetic refractive prediction error was -0.52,-0.41,-0.34 and 0.21D forShammas, Haigis-L,Barrett True-K and Potvin-Hill respectively.Least mean absolute prediction error of 0.52D was in Barrett True K, with prediction accuracy of 50% within ±0.5 D and 90% within ±1.0 D of target refraction
CONCLUSION: All methods are comparable. Potvin Hill method gives a hyperopic shift in contrast with other 3 methodswhich gave a myopic shift Financial Disclosure: Neither author has a financial or proprietary interest in any material or method mentioned.
Introduction
In the 1990s, with the introduction of refractive procedures usingexcimer laser treatment like LASIK and PRK,many patients underwent the procedure to be spectacle independent.
Today, many of the post-LASIK surgery patients are reaching an age at which they will develop senile cataractous changes .In the coming years,Intraocular lens (IOL) power calculation in these patients is emerging as an challenge to most surgeons.
IOL power calculation in these patients using standard formulae like SRK/T, Hoffer Q, or Holladay frequently results in undercorrection,thus yielding significant hyperopic error(1).
This error arises due because after LASIK surgery the anterior surface of cornea becomes flatter and conventional keratometer or topography estimate ‘K’value incorrectly and tend to overestimate the value(2). Also there is inaccurate ELP estimation arising from flatter cornea post LASIK. In the last decade,to improve the accuracy of IOLpower calculation in eyes with previous refractivesurgery,many methods(1,2)have been formulated. These methods are of 2 types:
- those that require the previous refractive surgerydata and (B) those that use current biometryonly.(3).
Earlier, the clinical history method(4) hadbeen considered the gold standard of determiningcorneal power after laser refractive surgery. However,the clinical history method requires pre-refractivekeratometry (K) readings and pre-refractive and finalstable post-refractive manifest refraction, which are often not available or are of questionable accuracy due tothe use of older imaging technologies at the time of therefractive surgery.Furthermore,potential nuclear sclerosis and/or progressiveaxial myopia at the time of refractive surgery can resultin a biased calculation.
Because of limitations of methods that require clinical history data(5)severalnewer methods that require only current measurements withoutprevious refractive data have been developed.Nevertheless, IOL power predictionfor these eyes remains problematic.(6,7)The use of multipleformulas or calculations for these eyes can bedifficult to execute and time consuming. Recently,AmericanSociety of Cataract and Refractive Surgery (ASCRS)online calculatorwas developed to facilitate this process.
However, despite the availability of thesesoftware programs,surgeons are sometimes in a dubious state when trying todecide which method would generate the most accurateIOL power.
The purpose of this study was to compare theaccuracy of these methods using no previous dataavailable on updatedASCRS IOL calculation software. Theseinclude the Shammas No-History, Haigis-L, andPotvin-Hill Pentacam and Barrett True-K No History method
PATIENTS AND METHODS
Study Groups and Protocol
This study was a retrospective case series. The case records of patients who had previously undergone LASIK for myopiaand subsequently had cataract surgery between January 2015 and May 2018 at SadguruNetraChikitsalyawere reviewed.All caseswere operated by same surgeon.All patients had an uneventful surgery.
4 methods used to calculate IOL power: Shammas-PL, Haigis-L, Potvin-Hill Pentacam and Barrett True-K No History method were evaluated.
Using the ASCRS IOL power calculation software, with an optimizedlens constant taken from the type of IOL implantedand the target refraction after cataractsurgery, the predicted IOL power for each method was calculated.
The surgeon had selected the IOL power to be implanteddepending on his judgment.Cataract surgeries were performed using a temporal clear corneal incision and phacoemulsification with implantation of Alcon IQ lens. The stablemanifest refraction at the spectacle plane after cataractsurgery was obtained at 1month postoperatively. TheIOL power prediction error was computed as the differencebetween the predicted IOL power using a particular methodand the back-calculated ideal IOL power based on the target refraction. Forback-calculation, as in the method described by Feiz et al(8)and later adopted by other authors,(9)the assumption wasthat 1.00 diopter (D) of IOL prediction error produces0.70 D of refractive error at the spectacle plane.
The mean numerical error,mean absolute error, and percentages of eyes within ±0.50 diopter (D) and ±1.00 D from the target refraction werecalculated for each formula or method
Statistical Analysis
Assessment of absolute refractive prediction errorwere performed with the Shapiro-Wilk normalitytest (alpha Z 0.05). Then a paired t test and 1-way analysisof variance were used to analyze the differences betweenmethods in refractive predictionerror.
Thepercentages of eyes within certain refractive prediction errorswere compared using the chi-square test and Fisher exacttest. A P value less than 0.05 was considered statistically significant.
RESULTS
Patients Demographics
10eyes of 9 patients were retrospectively analyzed. Table 1 shows the patients’ demographics.
The mean interval between refractive surgery and cataract surgery was 13.6 years .IOL implanted was Acrysof SN60WF (Alcon Laboratories,Inc.) with A constant of 118.7.
Mean axial length was 28.27mm.
Patient Demographics 1
Refractive prediction error in formula of Shammas ranged from -1.83 to 0.39D with aMean arithmetic refractive prediction error(MARE) of -0.52±0.62D.Using Haigis-L formula, the Refractive prediction error ranged from-1.46 to 1.40D with a MARE of -0.41±0.82D.
MARE was -0.34 ±0.61Dand 0.21D ±0.72D for Barrett True-K and Potvin-Hill respectively.
Least Mean Absolute Prediction Error(MAE) of 0.52D was in Barrett True K while MAE was 0.60,0.62 and 0.58 for Shammas, Haigis-L, and Potvin-Hill respectively(p=0.089).
Barrett True K (no history) demonstrated prediction accuracy of 50% within ±0.5 D and 90% within ±1.0 D of target refraction
IOL power predicted by ASCRS formulae
Refractive prediction error
| Mean | SD | Min – Max | Mean Absolute Error | |
| Shamma REF Error | -0.52 | 0.62 | -1.83 to 0.39 | 0.60 |
| Haigis REF Error | -0.41 | 0.82 | -1.46 to 1.40 | 0.62 |
| Potwin REF Error | 0.21 | 0.72 | -1.35 to 1.30 | 0.58 |
| Barrett REF Error | -0.34 | 0.61 | -1.63 to 0.68 | 0.52 |
Table 3. Percentage of eyes within ±0.50 D and ±1.00 D from the target refraction
| Within ±0.50 D | Within ±1.00 D | |
| Shamma REF Error | 50% | 80% |
| Haigis REF Error | 40% | 80% |
| Potwin REF Error | 50% | 80% |
| Barrett REF Error | 50% | 90% |
DISCUSSION
It is a challenging task for the surgeons to decide which would be the most ideal IOL power for eyesthat had undergoneLASIK surgery earlier .Taking a review of literature, it is observed that many methods have been proposed toaddress the relative lack of predictability in post-LASIK myopic eyes.(1,2,4,6–8).Further to it,in majority of the cases, previous refractive data is often unavailable .ASCRS has presently provided a net-based IOL power calculator for such eyes to eliminate the need to perform multiple calculations for each case.
The online calculator has recently been updated,with less predictable methods having been eliminated and several new methods of calculationincluded. The purpose of the current study was toevaluate those methods,which don’t use previous refractive surgery data as included in the online calculator
From our study ,we observed that the refractive prediction error in all the 4 methods was not statistically significant . In a Abu lafia(10) in 2016,there was no difference in varience in barett true K,Shammas and Haigis-L methods.However,it was observed that shammas and haigis gave a significant negative mean numerical error while barret gave a value of mean .Yang et al reported negative values for median arithematic refractive error for Shammas and haigis-L ,thus indicating a slight postoperative myopic shift .Potvin et al (11)reported that error in refractive prediction error ranged from +1.38D to +1.77D using Potvin Hill Pentacam method; thus indicating a postoperative hyperopic shift.This is in consistence with our results . However, as per our knowledge there is no other study which compares all these ASCRS formulas (using no prior data)from one dataset.This is probably due to recent individual addition of potvin and barrett true K (no history) formulae to the ASCRS site.
Limitation of the present study is small sample size.This is due to the fact that those candidates who had undergone LASIK surgery 15 years ago,are now entering the age of cataract development .Hence,in near future this population is going to constitute a major proportion of those undergoing cataract surgery.Hopefully,this study serves as a useful reference to the surgeons to make the desired choices as per patient refractive needs.More clinical studies areneeded to investigate the best of the combinedformulas.
Conclusion
All 4 ASCRS methods are comparable. Potvin Hill method gives a hyperopic shift in contrast with other 3 methods which gave a myopic shift.
REFERENCES
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