Intraocular lens power calculation in eyes with previous excimer laser surgery for myopia

Author: Pantanelli SM, Lin CC, Al-Mohtaseb Z, Rose-Nussbaumer JR, Santhiago MR, Steigleman WA, Schallhorn JM.

Geographical coverage: Not reported

Sector: Biomedical

Sub-sector: Treatment

Equity focus: None

Study population: People with eyes with a history of LASIK or photorefractive keratectomy (PRK) for myopia.

Review type: Effectiveness review

Quantitative synthesis method: Systematic review

Qualitative synthesis method: Not applicable

Background:

A key outcome of cataract surgery is achieving a refractive target, which necessitates accurate prediction of postoperative refractive error. With advancements in technology and the development of various intraocular lens (IOL) power prediction formulas, postoperative spherical equivalent predictions have improved significantly. However, achieving satisfactory results in eyes with a history of corneal refractive surgery remains challenging due to the lower prediction accuracy of formulas for this population. The existence of over 30 different methods to calculate IOL power underscores the complexity of the issue and contributes to the confusion among ophthalmologists about the best method to use.

Objectives: To review the literature to evaluate the outcomes of intraocular lens (IOL) power calculation in eyes with a history of myopic LASIK or photorefractive keratectomy (PRK).

Main findings:

The clinical history method, once recognised as the gold standard for IOL power calculations in this population, produces suboptimal outcomes compared with the best-known methods available today, which include the Barrett TrueK, OCT (RTVue) and Potvin-Hill methods.

A total of 11 studies met the inclusion criteria and were included in the review. All studies relied exclusively on retrospective data on which to perform refractive outcomes analysis, except for one, in which some of the eyes included were prospectively enrolled. All included studies were assigned a level III rating of evidence by the panel methodologist.

Authors found that when automated keratometry was applied with a theoretical formula intended for eyes without prior laser vision correction, the mean prediction error was consistently positive (hyperopic), and the mean and median absolute errors were relatively high. The review observed that only a small proportion of eyes were within 0.5 dioptres of the target spherical equivalent. Formulas specifically developed for this population, which require both pre-refractive surgery keratometry and manifest refraction, resulted in a higher proportion of eyes within 0.5 dioptres of the target. Formulas requiring only preoperative keratometry or no history at all had lower mean and median absolute errors and a higher proportion of eyes within the target range. Strategies averaging several methods yielded the lowest reported median absolute errors and the highest proportions within the target range. However, even with the best-known methods, refractive outcomes were less accurate in eyes that had undergone previous excimer laser surgery for myopia compared to those that had not. Overall, authors state that conclusions are limited by the small sample sizes and retrospective nature of nearly all existing literature in this domain – and the fact that they test a range of different calculation methods.

Review authors report that calculation methods requiring both pre refractive surgery keratectomy and manifest refraction are no longer considered the gold standard. Refractive outcomes of cataract surgery in eyes that had previous excimer laser surgery are less accurate than in eyes that did not. Patients should be advised of this refractive limitation when considering cataract surgery in the setting of previous corneal refractive surgery.

Methodology:

Inclusion criteria consisted of: (1) the purpose was to report refractive outcomes in eyes with a history of LASIK or PRK; (2) the sample size or subgroup of myopic eye was equal to or greater than 40; (3) postoperative manifest refractions were performed one month or more after surgery; and (4) only monofocal IOLs were used. Meta-analyses of primary literature were excluded. Minimum sample size determination was based on the desire to detect a difference in mean prediction error (MPE), defined as the mean difference between actual and predicted postoperative SE, between formulas that are and are not designed to be used in postmyopic LASIK/PRK eyes.

A literature search of the PubMed database was conducted in January of 2020. The panel methodologist assigned a level of evidence to each of the 11 articles that met the inclusion criteria based on the rating scale developed by the Centre for Evidence-Based Medicine and adopted by the American Academy of Ophthalmology. A level I rating is reserved for well-designed randomised clinical trials; a level II rating for well-designed case-control and cohort studies and lower-quality randomised clinical trials; and a level III rating for case series, case reports and lower-quality cohort and case-control studies.

Included studies were subject to systematic review – although few methodological details are included to indicate how this was undertaken in practice.

Applicability/external validity: Authors indicate that small sample sizes and the retrospective nature of included studies are weaknesses of this review. In addition, different levels of data are available in relation to the different calculation methods under review.

Geographic focus: Search not limited by geography, but there is no indication as to whether data from low- and middle-income countries was included in the review.

Summary of quality assessment:

There are a number of limitations in the approaches to identify, include and critically appraise studies used for this review. There is no evidence that unpublished or non-English material was included, or indication that screening or data extraction was undertaken by more than one reviewer. In addition, information on the assessments of quality for individual studies is not provided.  While the data was subject to systematic review, there is no description of how this methodology was implemented in practice. In addition, there was little consideration of the quality of studies in the synthesis of the data, although the quality ratings provided suggest that this was relatively low. For these reasons, we have low confidence in the findings of this review.