Eyeworld

63 EW RESIDENTS November 2015 published earlier this year, which suggested no significant difference in the incidence of CME between laser-assisted and standard cata- ract extraction. 7 Lastly, the authors were very frank about the need for further investigation, as they could not definitively conclude that LCS causes subclinical retinal damage and increases the risk of postopera- tive CME. In order to build on the com- pelling goals and framework of this study, we would include a power analysis to aid interpretation of the results. For example, the power needed to detect a statistically sig- nificant difference can be calculated from the magnitude of the differ- ence between outcome incidences of the two groups (i.e., the incidence of postoperative CME in the LCS group minus that in the PCS group, or 0.8% – 0.2% = 0.6%). We used a power calculator available online (clincalc.com/Stats/SampleSize.aspx) with a standard alpha of p=0.05 to detect a difference at 80% sensitiv- ity. Using the authors' empirical data, we found that the sample size required to detect a difference would be 4,336 patients, or 2,168 patients per group. This analysis demonstrates the critical need for a power analysis in clinical studies. While the number of patients in the related to treatment modality. For example, the PCS group includes patients who had prior surgery for glaucoma. It has been reported previously 6 that ocular hypertension reduces the risk of CME by an un- known mechanism. This could bias the subjects in the PCS group toward less CME. The strengths of this study center around its prospective design utilizing consecutive patients with resultant limitation of confirmation bias. The study was also meant to be clinically significant and reflects decision making in a real-world setting. The investigators diagnosed CME using a clinical algorithm rely- ing upon outcome measures relevant to patient satisfaction (i.e., poor visual acuity) to drive further evalua- tion of unexpected decreased vision. In the study, SD-OCT was performed to confirm suspicion of CME on an as-needed basis rather than on each study participant. This represents real-world use of SD-OCT in the postoperative setting, and may make the study findings more applicable to daily practice. In addition, the au- thors are to be commended for their continued non-industry-supported efforts to characterize femtosecond laser technology in a practice setting with human research and ethics committee approval. Their re- sults are consistent with a study 1 PCS eye. Assuming the numbers of patients in the LCS and PCS groups after the upgrade were reported correctly in the table, this would change the values to 5/570 (0.88%) in the LCS group and 1/315 (0.32%) in the PCS group. Clearly, this would diminish the difference between groups and lessen the reported trend. Moreover, this post-hoc subgroup analysis includes only 885 patients (66% of the study patients). Another potential confounder was the inclusion of patients in the PCS group that were not offered an option of LCS. This introduces potential bias in the outcomes not reported that sub-analysis of all patients after the December 2012 software speed upgrade included CME events in 6/570 cases (1.05%) in the LCS group versus 0/315 (0%) in the PCS group (p=0.07). Although this post-hoc subgroup analysis still did not achieve statistical signifi- cance, they felt this was a strong trend toward higher CME in the LCS group. In fact, scrutinization of the data suggests a different conclusion. Looking at the table below, the two CME events prior to the December 2012 upgrade were listed under the LCS group. The CME events after that upgrade include 5 LCS eyes and femtosecond laser-assisted versus prospective comparative cohort case series" Thomas Shute, MD, Susan Culican, MD, PhD, Grace Paley, MD, PhD, Angela Jiang, MD, Kisha Piggott, MD, PhD, and Wesley Green, MD Source: Washington University School of Medicine Patient Age Sex Group CME risk factors/co- morbidities Eye VA CME Presentation VA Final CMT (µm) CME Presentation CMT (µm) Final 1 50 M LCS NIDDM R 6/9.5 6/6 447 318 2 52 M LCS Nil L 6/15 6/6 479 274 Catalys Software (Speed) upgrade 2.15 – Dec 2012 2 52 M LCS RRD 2007 R 6/30 6/7.5 481 287 3 74 F LCS Nil L 6/24 6/9.5 577 300 Catalys Software upgrade 2.15.13 – May 2013 4 83 F PCS Nil L 6/24 6/9.5 349 266 5 61 M LCS Nil R 6/24 6/9.5 443 362 6 70 F LCS Nil R 6/9.5 6/6 302 274 6 70 F LCS Nil L 6/12 6/6 359 283 continued on page 64

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