Study identifies flap striae risk influenced by climatic conditions

Take Home

A retrospective study including more than 614,000 eyes undergoing femto-LASIK at a single practice over a 5-year period identified a flap striae incidence of 0.85% overall, however a climate conditions influence was also noted.

Watch video of the 5,000-plus flap-lifting procedures from the Japan study. (Video courtesy of Minoru Tomita, MD)

By Cheryl Guttman Krader; Reviewed by Ronald R. Krueger, MD

Dr. Krueger

Cleveland, OH-Results of a large retrospective study show that the incidence of flap macrostriae after LASIK varies by season and correlates with outdoor temperature and humidity.

These findings add another dimension to the understanding that ocular surface and corneal hydration conditions affect the mechanical microenvironment of the LASIK flap, and should be optimized to achieve the best surgical outcomes, according to Ronald R. Krueger, MD.

Dr. Krueger-medical director of refractive surgery, Cleveland Clinic Cole Eye Institute, Cleveland, OH-presented the study, which included data for 614,340 consecutive LASIK cases performed between June 2007 and April 2012 at the Shinagawa LASIK Center, Tokyo, Japan. It used information from the Japan Meteorological Agency to investigate the correlation between monthly and seasonal flap striae incidence and climatic measures.

A full, peer reviewed manuscript, outlining the details of this retrospective study, and including his co-authors Neema Hashemi, MD and Minoru Tomita, MD, has been submitted for publication and is under review but not published yet. 

Understanding the study

Watch an additional video of the 5,000-plus flap-lifting procedures from the Japan study. (Video courtesy of Minoru Tomita, MD)

Over the entire series, the incidence of flap striae requiring re-alignment was 0.85%, and the incidence was fairly constant from year to year. However, the average monthly incidence ranged from 0.657% to 1.006%, and when the monthly data were collapsed to calculate seasonal rates, the analyses showed flap striae requiring re-alignment were significantly less likely to occur during the summer months (June through August; 0.671%) than in the fall (September through November; 0.881%), winter (December through February; 0.963%), or spring (March through May; 0.875%).

Analyses using Pearson’s correlation coefficient showed average monthly humidity and temperature were strongly correlated (R=0.9) and also identified strong, inverse correlations between the average monthly flap striae incidence and both average monthly humidity (R = -.902) and temperature (R = -.888).

“At first glance, the findings from this study might lead to the conclusion that in regions where there are seasonal climate variations and with respect to minimizing the risk of flap striae, it is safer to perform LASIK during the summer than in the winter. However, the occurrence of flap striae requiring a flap lift is relatively rare and these events do not carry a high morbidity,” Dr. Krueger said. “Therefore, these data should not prevent us from recommending LASIK during any season. Instead, the real clinical implication of this study is that it reinforces the desirability of having a moister surface when performing LASIK. We already know this to be true in terms of optimizing refractive and visual outcomes, which is why we screen for and manage pre-existing dry eye syndrome and other ocular surface disease.

“However, this study goes a step further by shedding light on the effect of seasonal environmental changes on presumed normal eyes undergoing LASIK,” he continued. “Hence, it stresses the importance of having adequate corneal surface moisture in those eyes as well.”

Further throughts

Dr. Krueger also observed that with its very large sample size, this study had the power to investigate how factors as subtle as environmental humidity and temperature might affect the risk of a rare complication (i.e., flap striae.) However, its size is also a limitation as the individual eyes and patients could not be assessed for other factors potentially affecting the risk of flap striae.

“All of the procedures were performed according to the same standards and by expert surgeons, and so it is assumed that the ocular surface was normal in the eyes at the time of surgery,” Dr. Krueger said. “However, it is possible that some of the eyes developing flap striae underwent surgery with pre-existing ocular surface disease.

“Nevertheless, the high statistical significance and consistency of the seasonal incidence during each year of the 5-year study period show that among all eyes undergoing LASIK and presumed to be normal, higher humidity and temperature make a difference in creating a more favorable environment for the LASIK flap,” he added.

In addition to theorizing that the strong inverse correlation between flap striae incidence and monthly/seasonal temperature and humidity suggests air moisture provides a protective effect in the early postoperative period, other factors contributing to the higher incidence of striae in colder drier months and seasons may be due to increased flap contraction, as well as increased eye rubbing for dryness-related discomfort.

Ronald R. Krueger, MD

E: krueger@ccf.org

Dr. Krueger did not indicate any financial interest in the subject matter.