A Comparative Study on Accuracy of SRK-T And Haigis Formulas in IOL Power Calculation in Axial Myopes Undergoing Cataract Surgery.

Background and Objective: It has been a significant challenge since the advent of intraocular lens to give the best postoperative visual acuity and prevent refractive surprises due to biometry error. Among myopic eyes, it has been a debate among the various formulas introduced and their efficacy to prevent postoperative refractive surprises. Hence, the need for an accurate formula in high myopic eyes is obligatory. Objectives of the Study: To compare the accuracy of SRK-T and Haigis formulas in IOL power calculation in axial myopic eyes undergoing cataract surgery.
Methods: A total of 50 cases with axial length >24 mm were taken up for the study and were examined in detail and error between both formulas were assessed.
Results: The mean age of the subjects in the study was 53.50 ± 16.12 years; 27 were males, and 23 were females. The majority of patients had PSC + NS II. Seven out of 50 cases had posterior staphyloma. Most of the patients had average K value in the 44-46-D range. AL of most of the patients (66%) was between 24 and 26 mm. The majority of patients had IOL power >15 D, and 82% (41 eyes) were found to have no post-op complications. Four eyes had severe iritis, and five eyes had striate keratopathy. At the follow-up at 6th week postoperatively, 82% were found to have 6/6-6/9 vision on Snellen's chart. Four eyes had 6/12-6/24 post-op vision, mainly attributed to primary PCO. Five eyes (10%) had <6/24 post op vision at the end of 1 week due to the presence of posterior staphyloma. A higher percentage of eyes in the SRK/T group had a mean error >0.5 D. Upon comparing the mean error between the two groups, P was 0.005; hence, the results are statistically very significant, showing that Haigis formula is better than SRK/T formula in achieving target refraction (-1) in myopic eyes undergoing phacoemulsification. Interpretation and
Conclusion: Our study shows that Haigis formula was better than SRK/T formula for achieving the target postoperative refraction in high axial myopes. Copyright:

INTRODUCTION

Cataract is the leading cause of blindness next to refractive error worldwide. Among the refractive errors, myopic eyes are associated with high incidence of cataract, and progression is noted to be faster possibly due to the inflammatory markers in myopic eyes.[12] Even after good phaco surgery in myopic eyes, unexpected postoperative refractive surprises are high. Many are due to inaccurate IOL calculations. Hence, calculation of IOL in high myopic eyes is pivotal.[345678910]

One of the important factors for post-op refractive surprise in myopic eyes is due to the use of inaccurate IOL formula. Many studies have shown that various formulas are accurate for myopic eyes. Hence, this study was done to check the accuracy of the two best IOL formulas (Haigis and SRK/T) for myopic eyes.

METHODOLOGY

The study was a prospective interventional study for 2 years conducted in a tertiary care center after taking ethical clearance. A sample size of 50 cases was taken as part of convenience sampling. Inclusion criteria included cataract eyes grading PSC NS1-3 with axial length >24 mm and mature cataract. Exclusion criteria included corneal and retinal pathologies other than myopic degenerations and eyes that had undergone VitreoRetina surgeries. Written/informed consent was obtained from each patient in the local vernacular. Patients were subjected to detailed ophthalmic evaluation.

Corneal power was measured with Bausch and Lomb keratometry. Axial length was performed using Axis Nano ultrasonic A-scan using a probe frequency of 11 MHz. An average of ten readings were taken, and A and B-scans were used in case of staphylomas.

The powers of IOL to be implanted were calculated using Haigis or SRK/T formulas. The planned postoperative refraction was considered as a slight tendency toward low myopia (predicted postoperative refraction of −1.00 D). The eyes with AL >24 mm were subdivided into two groups, each consisting of 25 eyes (total: 50 patients) according to the formula used for IOL power calculation.

All patients were subjected to phacoemulsification surgery with a 2.8-mm temporal clear corneal incision. Only those patients in which foldable PCIOL was implanted in the capsular bag were considered for the study.

Patients were followed up regularly at 1, 3, and 6 weeks intervals. At each visit, the patient underwent a complete examination of the anterior segment and cycloplegic refraction with retinoscopy. The postoperative refraction value in our study was taken as refraction obtained at 6th week.

With each formula, the mean error was calculated as follows.

Mean error = Actual postoperative refraction – Planned targeted postoperative refraction

RESULTS

Fifty eyes of 50 patients were included in this study, which was conducted over a duration of 1 year. The mean age of subjects in our study was 53.50 ± 16.12 years. Of the 50 patients, 27 (54%) were males and 23 (46%) were females. Six patients (12%) were <30 years of age, seven patients (14%) were 30–44 years of age, 12 patients (24%) were 45–59 years of age, and 25 patients (50%) were more than 60 years of age. Males and patients above the age of 60 years showed greater frequency. The majority of patients had PSC + NS II. Four patients had mature cataract, but all four patients were very young; thus, the cataract was soft and ideal for phacoemulsification. Seven (14%) out of 50 subjects had posterior staphyloma in the study group.

Most of the patients (64%) had vision >6/60 in the operating eye. Four patients (8%) had <3/60 vision in the operating eye. AL of many patients (66%) was <26 mm. Fourteen out of 50 patients had AL in the range of 26–28 mm. Only three out of 50 patients (6%) had AL >28 mm. IOL power of 33 patients (66%) was between 15 and 19 D. Twelve out of 50 patients had IOL power in the range of 10.5–14.5 D. Only five out of 50 patients (10%) had IOL power <10 D. Following phacoemulsification surgery in myopic eyes, 82% (41 eyes) were found to have no post-op complications. Four eyes that had severe iritis were treated with intensive topical steroids and cycloplegics. Five out of 50 eyes that had striate keratopathy were also treated with intensive topical steroids and responded well postoperatively. At the follow-up at 1 week postoperatively, 70% (35 eyes) were found to have 6/6–6/9 vision on Snellen's chart. Nine eyes (18%) had 6/12–6/24 post-op vision. Six out of 50 eyes had <6/24 post-op vision at the end of 1 week. At the follow-up at 3rd and 6th week postoperatively, 82% (41 eyes) were found to have 6/6–6/9 vision on Snellen's chart. Four eyes (8%) had 6/12–6/24 post-op vision, mainly attributed to primary PCO. Five eyes (10%) had <6/24 post-op vision at the end of 1 week due to the presence of posterior staphyloma Follow-up refraction (spherical equivalent) at 6th week postoperatively was noted. Further, 80% (20 out of 25 eyes) in the Haigis group and only 56% (14 out of 25 eyes) in SRK/T group were found to have refraction within −1.5 to −0.5 D. A higher percentage of eyes in the Haigis group had refraction closer to the targeted refraction. The mean error at 6th week postoperatively was noted; 80% (20 out of 25 eyes) in the Haigis group and 56% (14 out of 25 eyes) in the SRK/T group were found to have a mean error of 0–0.5 D. A higher percentage of eyes in the SRK/T group had a mean error of >0.5 D. Comparing the mean error between the two groups, P was 0.005; hence, the results are statistically very significant, showing that Haigis formula is better than SRK/T formula in achieving target refraction (−1) in myopic eyes undergoing phacoemulsification.

DISCUSSION

Out of 50 eyes in our study, 27 (54%) were males and 23 (46%) were females. Six patients were less than 30 years of age, seven patients were 30–44 years of age, 12 patients were 45–59 years of age, and 25 patients were more than 60 years of age. The mean age was 53.50 ± 16.12 years. It is in accordance with the study conducted by Saif et al.,[11] which included a total of 215 eyes of myopic patients with a mean age of 53.977 ± 10.1 years.

The majority of patients had PSC + NS II. Four patients had mature cataract, but all four patients were very young; hence, the cataract was soft and ideal for phacoemulsification. This was comparable with the study conducted by Chen et al.[8] in China on 148 myopic eyes Posterior staphyloma was present in seven out of 50 eyes in our study.

Most of the patients (64%) had preoperative vision better than 6/60 in the operating eye. In our study, the mean keratometric value was 44.132 ± 1.275 D, and the mean axial length was 25.812 ± 1.228 mm. This was similar to the findings reported in the study conducted by Saif et al.[11] Follow-up refraction (spherical equivalent) at 6th week postoperatively was noted. The majority (~68%; 34 eyes) were found to have mean error between −1.5 and −0.5 D. A higher percentage of eyes in the Haigis group had refraction closer to the targeted refraction. A study by et al.[11] had a mean post-op SE of 0.54 D, which was also in accordance with our study.

The mean error at 6th week postoperatively was noted; 80% (20 out of 25 eyes) in the Haigis group and 56% (14 out of 25 eyes) in the SRK/T group were found to have a mean error of 0–0.5 D. The mean error in the Haigis group was 0.3300 and in SRK/T was 0.5950, and P was 0.005; hence, the results are statistically very significant, showing that Haigis formula is better than SRK/T formula in achieving target refraction (−1) in myopic eyes undergoing phacoemulsification. A study done by Wang et al.[9] showed that the lowest mean error was obtained in the Haigis formula, which gave good results, consistent with our study. According to the study by Petermeier, both Haigis and SRKT are suitable formulas for long eyes.[4]

Ghanem et al.[12] study showed that Haigis formula showed the least variation, while SRK/T and other formulas showed a greater tendency toward hyperopia. This was similar to our study. Roessler et al.[13] showed Haigis formula to be superior to Holladay 1 formula.

A study conducted by Kapamajian and Miller reported that SRK-T formula was the most accurate for highly myopic eyes, which is in contrast with our study,[14] El Nafees R et al.[15] showed no statically significant difference between Haigis and SRK/T.

CONCLUSION

In conclusion, our results were statistically very significant, thus showing that Haigis formula is better than SRK/T formula in achieving target refraction in myopic eyes undergoing phacoemulsification.

Cases with posterior staphyloma will require accurate axial length measurement and should be explained about postoperative refractive surprises and poor postoperative visual outcome.

There are certain limitations present in our study. All patients underwent clear corneal temporal phacoemulsification irrespective of the K reading. Incision was not based on preoperative keratometry. For better results, a larger study group and longer study duration were needed, which was also a limitation in our study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.