Eyeworld

FEB 2011

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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EW REFRACTIVE SURGERY 56 February 2011 by Matt Young EyeWorld Contributing Editor Statistics explain LASIK post-op acuity I t's not exactly an old wives' tale: Pre-op acuity predicts post-op acuity in wavefront- guided LASIK, according to new research by Michelle T. Aaron, O.D., Ph.D., U.S. Air Force School of Aerospace Medicine, Brooks City Base, Texas. But there's a statistical reason for this—a reason that can't be at- tributed to surgery, Dr. Aaron and colleagues found. Their study was published online in September 2010 in Optometry & Vision Science. "Dividing the [LASIK popula- tion] sample into two subsamples based on preoperative acuity con- firmed the common clinical observa- tion that eyes with better-than- average acuity tend to remain the same or lose acuity, whereas eyes with worse-than-average acuity tend to gain acuity," Dr. Aaron reported. "However, when only one acuity measurement is taken at a single time point and the sample is sub- sampled nonrandomly, this clinical observation is due to a statistical ar- tifact (regression to the mean) and is not attributable to the surgery." Understanding the phenomenon In one eye of 79 patients, Dr. Aaron analyzed high-contrast best cor- rected visual acuity (BCVA) once be- fore and also 3 months after custom LASIK. Before surgery, patient spher- ical equivalent ranged between – 1.00 D and –10.38 D. "The sample was divided into one of two subsamples: eyes that had better-than-average preoperative BCVA (<0.11 logMAR) and eyes that had average or worse-than-average preoperative BCVA (≥0.11 logMAR)," Dr. Aaron reported. "Controls were implemented for retinal magnifica- tion and for the statistical phenome- non of regression to the mean of the preoperative acuity measurement." Dr. Aaron and colleagues ob- served that pre-op, visually worse eyes fared better post-op (compared to better eyes), but they determined this was not related to surgical rea- sons. "Although the 'clinical wis- dom' was confirmed in our study, our analysis demonstrates that the majority of the observed effect is ac- counted for by the statistical phe- nomenon called regression to the mean," Dr. Aaron reported. Dr. Aaron explained that regres- sion to the mean occurs with non- random sampling in a given population. "Creating a subsample of patients with worse-than-average BCVA is an example of nonrandom sampling of the greater population of patients who underwent corneal refractive surgery," Dr. Aaron re- ported. "Even if the surgery had no effect on BCVA, the mean BCVA of this subsample would tend to in- crease after treatment because of random fluctuations in BCVA for any given individual. One way to avoid regression to the mean is to take multiple measurements of BCVA for each individual, but this is rarely done in clinical practice." Dr. Aaron cites knowledge of statistics to explain these conclu- sions. "If such a surgical population is subsampled based on whether their initial acuity is better or worse than the total sample mean, it is ex- pected (on statistical grounds) that the subsample of better eyes will have a mean acuity on retest that is slightly worse and the subsample of worse eyes will have a mean acuity on retest that is slightly better," Dr. Aaron reported. "That outcome is called regression to the mean and is independent of any treatment effect that can occur for the entire test population as a whole." Retinal magnification also likely affected BCVA changes "due to mov- ing the refractive correction from the spectacle plane to the cornea," Dr. Aaron reported. "Retinal magnification effects are a well-known consequence of moving a spectacle correction from the spectacle plane to the corneal plane as happens with contact lens corrections," Dr. Aaron noted. "A change in retinal image size could be partly responsible for the clinical and experimental observations of changes in BCVA after refractive sur- gery, because moving a myopic cor- rection from the spectacle plane to the corneal plane magnifies the reti- nal image. Consequently, measuring acuity of myopic eyes with spectacle corrections before refractive surgery and without spectacles after surgery is equivalent to moving the acuity chart closer to the patient after sur- gery for some subjects." The study's conclusions may have important implications for fu- ture clinical trials for the FDA. "Clin- ical trials in the Food and Drug Administration approval process used to assess safety and efficacy could be biased by selecting patients based on their preoperative BCVA," Dr. Aaron reported. "If a clinical study selected patients with preoper- ative BCVAs that were worse-than- average and measured BCVA only once, the study could have an in- creased probability of indicating bet- ter safety and efficacy than if patients with a full range of preoper- ative BCVAs were included in the study." Many clinicians support the no- tion that LASIK can cause great in- creases in BCVA. Mohan Rajan, M.D., medical director, Rajan Eye Care Hospital, Chennai, India, for instance, believes that improve- ments in BCVA can occur after Alle- gretto Wave (Alcon, Fort Worth, Texas) treatment. "BCVA even in amblyopia eyes becomes better," Dr. Rajan said."After a treatment operation, it's common for patients to have 6/24 or 6/18 vision. BCVA or UCVA [uncorrected visual acuity] improves after LASIK generally." EW Editors' note: Dr. Aaron has no finan- cial interests related to this study. Dr. Rajan has no financial interests related to his comments. Contact information Aaron: michkoe7@yahoo.com Rajan: rajaneye@vsnl.com Pre-op acuity may predict post-op acuity after LASIK, but for a reason not attributable to surgery. A femtosecond LASIK flap is being created here Source: Daniel S. Durrie, M.D. BE PART OF THE AUDIENCE! SPACE IS LIMITED. REGISTER NOW AT www.eyeworld.org/meetings/alcon The deadline to register is March 21st. Saturday, March 26, 2011 5:30 – 6:00 pm Registration 6:00 – 7:00 pm Symposium with Reception to Follow Symposium | Spreckels Theatre 121 Broadway San Diego, CA 92101 Dr. Stephen Slade Dr. Robert Cionni Dr. Eric Donnenfeld Dr. Richard Mackool Get a sneak peek at the Alcon LenSx ® Laser, a brilliant innovation in laser refractive cataract surgery! Register now at http://www.eyeworld.org/meetings/alcon by March 21st. Space is limited. 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