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Editors Selection IGR 21-1
Surgical treatment: Irodotomy in pigmentary glaucoma
Comment by Stefano Gandolfi 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
See also comment(s) by Graham Trope •This is a multi-center retrospective study designed to explore the role of YAG-laser iridotomy in pigmentary glaucoma. The data were obtained from the clinical records of patients treated for bilateral pigmentary glaucoma and exposed to iridotomy in one eye only. Reistad et al. (573) succeeded in gathering data from n = 46 patients who were observed for two years or more. Mean baseline IOP was 22.5 mmHg and 21.2 mmHg for the study and control eyes respectively. The mean IOP reduction was 4 &plm; 5.4 mmHg in the treated eyes and 1.9 &plm; 3.8 mmHg in the untreated eyes, and the difference was 2.07 mmHg (p = 0.005). This effect becomes not significant upon application of statistical multivariate models who take into account the difference in the baseline IOP. The authors, then, suggest that their retrospective study "…does not support the benefit of laser peripheral iridotomy in the longterm pressure course of patients with pigmentary glaucoma…"
This paper leaves the reader with several doubts and concerns: According to Campbell's theory, iridotomy should reduce the liberation of pigment from the posterior iris in eyes showing a concave iris root. Accumulation of pigment in the trabecular meshwork should lead to (a) disfunction and then (b) further irreversible damage with subsequent impaired outflow and increased IOP. Eyes with active pigment dispersion but showing still an IOP in the 'normal range' should represent the best model to test the efficacy of iridotomy. In fact, we designed our longterm trial on those very patients (Gandolfi and Vecchi, Ophthalmology 1996; 103: 1693-1695; Ungaro et al., ARVO 2003). One might argue if eyes, showing already an increased IOP and a pigment dispersion, are the best candidate for an iridotomy. In fact, pigment-induced TM damage has already occurred in such eyes. The potential benefit of iridotomy is then greatly reduced. Besides, the authors do not provide data on the pre-laser configuration of the iris root. It is well known that not every eye suffering for pigmentary glaucoma does necessarily show a concave iris root. Performing a laser iridotomy in the absence of an iris concavity does not make sense. According to what detailed above, one might ask if the patients' selection was appropriate for the study aims.
The difference in the baseline IOP is suggestive for a surgeon-induced selection bias. One might argue that the treated eyes beared a more damaged TM; scheduling the worse eye for laser iridotomy introduced a major bias.
The issue of laser iridotomy in pigmentary glaucoma is still unresolvedThe statistical analysis is rather difficult to understand. Analysis of the co-variance helps in evaluating between-group differences when the groups starts from baseline values consistent with the observed difference (i.e. the group with the lower final IOP starts from a lower baseline and vice-versa). Here we see the opposite: in fact, the group showing the lower final IOP was starting from the higher baseline. Therefore, clinically speaking, the authors say that decreasing IOP from 22.5 to 18.5 (laser treated eye) is equal as decreasing IOP from 21.2 to 19.3 mmHg (untreated eye).
The application of the three general linear models was clearly a post-hoc analysis. In fact, the authors presented at ARVO a poster with the same data analyzed by a parametric statistics (Pigmentary Glaucoma Iridotomy Study - G.L.C. Reistad, D.G. Campbell, R. Ritch, M.B. Shields. Presentation Number: 292. Poster Board Number: B267) and their conclusion stated that "… this study suggests that laser iridotomy has a beneficial effect on the course of IOP in patients with pigmentary glaucoma". Post-hoc analysis are sometimes dangerous and may be potentially misleading.
Therefore, the issue of laser iridotomy in pigmentary glaucoma is still unresolved. Until prospective, well designed, randomized and properly controlled clinical trials will be available, we will not be allowed to deny (or trace) any role for laser iridotomy in eyes showing a pigmentary glaucoma.
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