Environmental factors and LASIK

The recent publication by Walter and Stevenson1 on the effect of environmental factors on laser in situ keratomileusis (LASIK) has received tremendous attention with reports throughout the media including Time magazine, the Chicago Tribune, the Los Angeles Times, and several television reports. These reports have caused concern in potential LASIK patients as they suggest that the results of LASIK depend on environmental factors such as outdoor humidity. This has been interpreted by some journalists to mean that LASIK is better performed in months with moderate humidity. While this paper has provided the best data to date demonstrating the association of LASIK results with procedure-room humidity,2 the statistical methods used have not been clearly reported and the results have not been clearly presented. As a result, the other associations described in the paper and reported in the media have been interpreted incorrectly. The 2 analyses used in this study are univariable and multivariable linear regression models (less correctly referred to in the paper as univariate and multivariate analyses). A univariable linear regression analysis determines the association between a single independent variable and a single dependent variable. In a study that has multiple independent variables, the results for 1 variable can be influenced by other “confounding” variables. In a study with multiple variables, multivariable linear regression analysis is used to determine the associations of independent variables while using various statistical methods to control for the other variables. As an example of the use of the 2 methods of linear regression, we can consider a study of the effect of 2 medications on blood pressure. To determine the effect of 2 drugs on blood pressure, regression analyses considering both age and the drug effects are performed. Univariable linear regression analysis would determine whether there was an association between the drugs and the blood pressure. However, the association could be explained by adjusting for the effect of patient age. A multivariable linear regression analysis would compare the 2 drugs, while making the statistical adjustment for age. In the paper by Walter and Stevenson, multivariable regression analysis is required to determine the significance of each multiple variable studied. Laser in situ keratomileusis enhancement rates were found to be significantly associated with age, procedure-room humidity, outdoor temperature, and 2-week preoperative outdoor humidity using univariable analysis. However, some of these variables could confound the others; ie, outdoor humidity could affect indoor humidity if there were not adequate indoor humidity control systems. In the multivariable model, the authors state that procedure-room humidity is associated with LASIK enhancement rates, but no mention is made of the associations of the other variables using multivariable analysis. Since the authors do not present the other results of the multivariable model, no comment can be made about the association of other factors with enhancement rates. Presumably the authors controlled for all other variables in the study and found them to be not significant. Once room humidity has been considered in the multivariable model, the effects of all other variables, including outdoor humidity, do not contribute significantly to explaining LASIK enhancement rates. For percentage of correction, the paper indicates that procedure-room humidity, outdoor temperature, 2-week preoperative mean outdoor humidity, and room temperature are associated with percentage of correction using the univariable linear regression analysis. In this case, the authors acknowledge that the multivariable model found that only room humidity was associated with percentage of correction, while outdoor temperature, procedure-room temperature, 2-week preoperative mean outdoor humidity, and room temperature did not have significant associations while controlling for the other confounding variables. The univariable association of these variables with percentage of correction disappears when other variables are “adjusted for” or “controlled for” with the multivariable analysis. Therefore, using multivariable linear regression analysis, this paper demonstrates that only procedure-room humidity is associated with LASIK enhancement rates and the percentage of correction. The authors suggest the development of a nomogram that considers indoor room humidity “since controlling indoor humidity is difficult.” While LASIK nomogram refinements are certainly 1 method to address the issue of humidity, the most obvious solution would be to control the indoor humidity. The Liebert Mini-Mate2® by the Liebert Corp. and CeilAiR® by Stultz Air Technology Systems provide excellent procedure humidity and temperature control with 2 degrees of variation of temperature and 5 units of variation of humidity through the year, even in the extreme temperature fluctuations of the Midwest. While these systems can cost between $20000 and $60000 to install, the expense would be justified considering the desire to further improve LASIK outcomes. Larry Stitt, MSc, Biostatistical Support Unit, Department of Epidemiology & Biostatistics, University of Western Ontario, London, Ontario, Canada, assisted with the statistical analysis. Louis Probst MD Chicago, Illinois, USA

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