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Editors Selection IGR 8-4

Surgical treatment: Irodotomy in pigmentary glaucoma

Comment by Graham Trope on:

12454 The influence of peripheral iridotomy on the intraocular pressure course in patients with pigmentary glaucoma, Reistad CE; Shields MB; Campbell DG et al., Journal of Glaucoma, 2005; 14: 255-259

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This paper by Reistad et al. (573) reports lack of support for laser iridotomy in pigmentary glaucoma (PG). If you are an 'iridotomist', should you now discontinue your practice? Should you stop doing laser iridotomy in patients with pigmentary glaucoma? Unfortunately this paper does not clarify the situation for you.

What do we know about laser iridotomy in pigmentary glaucoma? We know that laser iridotomy straightens the iris in patients with posterior bowing and pigment dispersion. It is presumed this straightening reduces pigment shedding ultimately leading to better intraocular pressure (IOP) control. In 1996 Gandolfi reported that iridotomy prevents IOP increase in PG. However, to my knowledge no one has ever confirmed this finding. Some ophthalmologists routinely perform iridotomy in PG.

The purpose of scientific study is to support or reject hypotheses. The question one must ask is does this paper provide enough evidence to reject the interesting hypothesis that iridotomy leads to improved IOP control in PG? No, it does not. The authors clearly admit this.

This paper has major weaknesses. It is a retrospective study with all of the problems associated with retrospective studies, including selection bias. In fact the mean baseline IOP was higher in the eyes undergoing iridotomy suggesting that eyes with higher IOPs were likely the ones selected and subjected to iridotomy. The follow-up time was relatively short for glaucoma. The patients were relatively older than expected for this condition with the 11 women participants having a mean age of 43 years. Also it is not mentioned if a sample size calculation was performed. Interestingly the authors found that iridotomy does statistically lower IOP when they used a t test but on recognizing their selection bias they performed linear testing models which did not confirm this difference when the baseline IOPs were taken into account.

Unfortunately this paper does little to help with our dilemma regarding the practice of laser iridotomy in PG. This paper neither supports nor rejects the hypothesis that straightening the iris with laser iridotomy leads to better pressure control. The authors are to be congratulated on clearly stressing the studies strengths and weaknesses and for recommending a prospective study to address the question.

In conclusion, if you are an 'iridotomist', based on this paper there is little reason for you to stop your current practice. However, the evidence for a clinical effect on IOP with iridotomy remains very weak and indeed is not supported by this paper. I for one will continue with my practice of not performing iridotomy until such time as I see some convincing evidence from a randomized prospective study to support such an intervention.

See also Editorial in Journal of Glaucoma 2005; 14: 253-254: "The issue at hand is that, by 2005, most clinicians base their clinical decisions about new treatment modalities, not on the theories and anecdotes of others but upon the results from prospective randomized trials. Shield's study was long overdue, recommended in this journal over ten years ago, but in the meantime, the unsupported theory made its way into routine of clinical glaucoma practice such that it is no longer even discussed to any degree. This is not the only instance that theory crept into contemporary glaucoma practice without scientific supporting evidence."

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