LAUSR

Dear Editor, Nd:YAG laser goniopuncture (LGP) is an important intervention for the postoperative management of eyes with nonpenetrating glaucoma surgery. Application of the laser to the trabeculo-Descemet’s membrane (TDM) increases filtering capacity and quickly reduces intraocular pressure (IOP). A recent study by Volkova et al.1 retrospectively compared the efficacy of early (<1 month) versus delayed (>1 month) LGP in patients with primary open angle glaucoma who were treated with deep sclerectomy (DS). Eyes were assigned to receive LGP based on ultrasound biomicroscopy (UBM) findings. Eyes with TDM thickening within 1-month post DS were assigned to the early LGP group and eyes with TDM thickening that occurred later were assigned to the delayed LGP group. The rationale for this approach to LGP is a tendency toward a smaller intrascleral lake, decreased bleb filtering capacity, and increased IOP in eyes with thickened TDM. Long term survival analysis demonstrated almost double the complete success rate (42.6% early LGP vs 23.2% delayed LGP, for IOP ⩽ 18 mmHg) 5 years after LGP. This is in contrast to other reports on LGP timing after nonpenetrating filtering surgery.2 These results initially appear convincing, but there is clear systematic bias which was not discussed in the paper that skewed the results to falsely increase the success rate in the early LGP group. All eyes that received DS had their TDM evaluated by UBM three times: (1) 3 days after DS, then (2) before, and (3) after LGP. Eyes were assigned to group based on TDM thickening and decreased intrascleral canal height at Scan 2. The UBM parameters presented in the article’s Table 3 clearly demonstrate that eyes assigned to early LGP had significantly different baseline characteristics than eyes in the delayed LGP group. Eyes in the early LGP group had less thick, less dense TDM with larger intrascleral canal height, all characteristics of a more efficient bleb filtering system. Therefore, eyes assigned to the early LGP group were more likely to have efficient bleb filtering systems before LGP was done and thus would have demonstrated increased survival regardless of how the LGP affected the drainage system. This also explains the greater number of post-operative interventions in the delayed LGP group. It therefore appears that the early laser group was systematically biased to include eyes which were already more successful. When considered along with the lack of control group (DS without LGP), the results of this study become significantly less meaningful in the context of LGP timing after DS. There does, however, appear to be some prognostic significance to the UBM evaluation of DS bleb characteristics – an interesting and potentially useful result. It is entirely possible that LGP timing is important, but balanced, well-powered prospective randomized studies are required to elucidate any effect.