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Editors Selection IGR 11-2

Surgical Therapy: Does bleb morphology influence positional IOP change?

Comment by Tony Wells on:

27561 Bleb morphology characteristics and effect on positional intraocular pressure variation, Weizer JS; Goyal A; Ple-Plakon P et al., Ophthalmic Surgery Lasers and Imaging, 2010; 41: 532-537

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It will not surprise readers of IGR that bleb morphology after glaucoma filtering surgery is associated with the success of the procedure, since the bleb is responsible for controlling IOP in the vast majority of post-trabeculectomy patients. Bleb morphology, which correlates very well with bleb function, has always been recognised as a determinant of surgical success overall. Weizer et al. (1986) have explored the relationship between morphology of the bleb and IOP control a little further; they asked if bleb morphology might influence positional IOP change, which they used as an easily measured surrogate for diurnal IOP fluctuation. This is an interesting topic: IOP fluctuation has generated discussion recently, following work by Asrani¹ suggesting that it might be an independent risk factor for glaucoma progression, albeit confounded by several factors including the magnitude of fluctuation in mmHg being larger at higher IOP. More recently, a publication from the EMGT² did not find a relationship between IOP fluctuation and risk of glaucoma progression.

Ninety-five eyes of 68 quite heterogeneous subjects with previous trabeculectomy had bleb grading and measurement of the difference between supine and sitting IOPs measured using pneumotonometry. Twenty of these subjects had trabeculectomy in one eye and medical management alone in the other, allowing a comparison between medically treated and surgically treated eyes. The features typically associated with blebs that tend to generate a lower IOP (higher elevation, microcysts, decreased vascularity) had less positional IOP change, and that surgically treated eyes also had less positional IOP change. These findings would seem to support the concept that successfully filtered eyes have a greater outflow facility.

The limitations imposed by the study methods and data collection are well covered in the discussion. It was not clear in the manuscript that the analysis was controlled for the possibly significant confounding factor of baseline IOP, which were different in the medically versus surgically treated groups and likely also in those with varying bleb morphology; for the reasons mentioned above this could have significantly influence the results. The average number of medications in the previously trabeculectomized eyes included in this study was 1.0 ± 1.3, with implications for bleb function and outflow facility in these eyes, the results might have been more dramatic had only unqualified success eyes been included. Also, while attempting ‐ possibly unsuccessfully ‐ to ignore my personal bias in bleb grading, I think that it might have been better to use a more comprehensive and robust bleb grading system (for example the Moorfields Bleb Grading System at www.blebs.net^3,4).

This is an interesting paper, and an enjoyable read, but it could only begin to address the issues that it raised. As the authors conclude, further investigation should help to clarify these.

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