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Editors Selection IGR 17-4

Surgical Treatment: What does SLT do to Schlemm?

Comment by Murray Johnstone on:

71263 Microarchitecture of Schlemm Canal Before and After Selective Laser Trabeculoplasty in Enhanced Depth Imaging Optical Coherence Tomography, Skaat A; Rosman MS; Chien JL et al., Journal of Glaucoma, 2017; 26: 361-366

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This study uses OCT to explore the effects of SLT on Schlemm's canal (SC) dimensions in 13 eyes. Imaging was done with commercially available SD-OCT image acquisition and 3D volumetric analysis software. Mean IOP was significantly reduced 4 weeks after OCT; (From 19.8 ± 7.6 to 14.4 ± 3.8 mm Hg; P = 0.003). Mean SC cross-sectional area (CSA) increased by 8% reaching statistical significance: (P = 0.029) and SC CSA was significantly positively correlated with post-SLT SC expansion (P = 0.023, R = 0.622).

The authors use the above results as a warrant to conclude that SLT results in SC expansion in eyes of patients with POAG. The authors point out that SC imaging by OCT is challenging because of tissue depth, shadowing by superficial vessels and intrinsic structural variations in SC dimensions. Other limitations involve inclusion of only the nasal quadrant of the eyes which may not be representative of the entire SC circumference.

In view of SC anatomic variability even small changes in positioning of the OCT beam before and after laser Rx may result in sampling quite different regions. Additional limitations include the need for manually delineating both the hyporeflective SC lumen and alignment of B-scans. It is reassuring that despite the above limitations significant SC CSA changes could be identified.

Physical expansion of SC is suggested by the authors to be directly related to increased aqueous outflow and IOP reduction. They conclude that in vivo OCT imaging techniques may be useful to evaluate the action of laser procedures, pharmacologic agents, surgical techniques, and devices. While the current study requires laborious techniques that may not be clinically practical, the study does a great service in pointing to a future likely to have further improvements in imaging acquisition and automated analysis algorithms. Such evaluative capabilities may well provide major advances in glaucoma management.

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