Hutnik and colleagues have conducted a prospective, randomized trial in which medically- uncontrolled eyes with POAG previously treated with 360° selective laser trabeculoplasty (SLT) underwent subsequent repeat SLT or argon laser trabeculoplasty (ALT) (each 180°) to determine whether there were differences in intraocular pressure (IOP) reduction between these two modalities in this setting. The results of the study demonstrated that both laser platforms delivered approximately 3 mmHg IOP reduction from baseline at Month 12. The authors note that this magnitude of effect is approximately 50% of the reported effect for first SLT in prior studies, and they conclude that this observation 'implies strongly that although initial SLT may produce no histologic changes to the TM, there is likely some change nevertheless because the second SLT is not as effective as the first.' In fact, this conclusion may be entirely wrong, as their findings can be easily explained by a methodological error in their study design. They utilize change from baseline in IOP as their measure of repeat SLT efficacy. The problem with this approach is that the baseline for repeat SLT is not the same as the baseline for first SLT. A simple clinical scenario makes this clear.
Consider a patient with newly diagnosed POAG and IOP of 24 mmHg whose target IOP is a 25% reduction (18 mmHg) and who undergoes initial SLT and achieves IOP of 16 mmHg (an 8-mmHg, 33% reduction). Over time, the SLT effect begins to wear off and IOP drifts up to 20 mmHg, which is above target. SLT is repeated, and IOP of 16 mmHg is regained. Virtually everyone would consider that repeat SLT was as effective as first SLT in that both achieved the same post-laser IOP level, even though repeat SLT only lowered IOP 4 mmHg, which is half the change from baseline observed with initial SLT. This is the problem with using change from baseline for evaluation of repeat SLT: because we do not let first SLT wear off completely (i.e., we do not let IOP return to pretreatment levels) before retreating, second SLT's baseline is necessarily lower than initial SLT's baseline, so the magnitude of change from baseline will always be smaller with repeat SLT. Future studies of repeat SLT should consider re-attainment of IOP delivered by initial SLT as the optimal metric of repeat SLT success.