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Editors Selection IGR 8-4

Optic Nerve Head: Change ONH by IOP lowering

Comment by Claude Burgoyne on:

19555 The acute morphologic changes that occur at the optic nerve head induced by medical reduction of intraocular pressure, Meredith SP; Swift L; Eke T et al., Journal of Glaucoma, 2007; 16: 556-561

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The relationship between lamina cribrosa compliance and optic nerve head neural and connective tissue susceptibility to glaucomatous damage is under active study by clinicians, basic scientists and biomedical engineers. In this report, Meredith et al. (872) report the pooled magnitudes of optic nerve head surface change following one hour of acute, medical IOP lowering in 38 eyes of 19 human patients representing a range of age (48 to 85), corneal thickness (465-610) and severity of damage (ocular hypertension with normal ONH to eyes with early visual field loss and substantial clinical cupping). At baseline, at least one eye of all 19 patients had an IOP of 28 mmHg or greater. Baseline examinations was performed including pachymetry, both eyes were imaged with Heidelberg Retinal tomography (HRT), Iopidine and Diamox (250 Mg tablet) were administered and after 60 minutes a second set of HRT images were obtained. Data were pooled for all 38 eyes and parametric and non-parametric tests for significance were performed on eight HRT parameters including a separate random effects model that assumed normality and assessed for dependence within subjects. By both parametric and non-parametric analysis, mean cup volume and depth decreased and mean rim area increased following acute IOP lowering. This study was rigorously performed and the data were well analyzed and presented. The authors appropriately acknowledge that changes in the ONH surface and lamina cribrosa may not be well correlated. Future studies performed with 3D OCT imaging that resolves the internal limiting membrane, Bruch's Membrane

Opening and the anterior laminar surface will powerfully address this problem. However, this study should have reported individual eye detection of change before data were pooled to summarize overall results. Ideally, an untreated control group should have had sham eye drop and placebo pill administration. Finally, the authors' hypothesized that corneal thickness and age would correlate to the magnitude of surface change but found no correlation. Future studies, which incorporate individual-eye detection of ONH surface and laminar change in larger series of control and study patients, should include the effects of not only corneal thickness and age but severity of glaucomatous damage on their results.

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