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Editors Selection IGR 20-3

IOP-Related: CCT in NTG

Comment by Jost Jonas on:

22623 Correlation between corneal and scleral thickness in glaucoma, Mohamed-Noor J; Bochmann F; Siddiqui MA et al., Journal of Glaucoma, 2009; 18: 32-36

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Previous hospital-based studies and population-based investigations have agreed that a thin cornea is a diagnostic risk factor for glaucoma, if the dependence of the intraocular pressure measurements on the corneal thickness is not taken into account. The present study by Mohamed-Noor et al. (27) addresses the clinically important question, whether a thin cornea is additionally a structural risk for glaucomatous optic neuropathy. The hypothesis has been that eyes with a thin cornea may have a thin lamina cribrosa and a thin peripapillary sclera. Due to biomechanical reasons, a thin cornea and a thin peripapillary sclera have been recognized as structural risk factors for an increased susceptibility of the optic nerve fibers for glaucomatous damage. Mohamed-Noor and colleagues measured the central corneal thickness (CCT) and the thickness of the anterior sclera as surrogate for the thickness of the posterior sclera and (potentially) of the lamina cribrosa. They found that the sclera was thinner in patients with normal-pressure glaucoma than in ocular hypertensive subjects, while the differences to all other study groups were statistically not significant. Only in the normalpressure glaucoma group, the scleral thickness was significantly correlated with the CCT. The other glaucoma groups did not show a significant correlation. It remained open for the authors whether a thin sclera and a thin cornea may be markers for an increased glaucoma susceptibility.Although the study was carefully performed, there may be some limitations. Firstly, the refractive error was not mentioned in the results. It may be that the normal-pressure glaucoma group was more myopic (even within the range of up to -6 diopters) than the other groups. Myopic eyes tend to have thinner sclera's, so that the observed difference between the study groups in anterior scleral thickness might potentially have been due just to a difference in myopia. Secondly, the statistical significance of the relationship between scleral thickness and CCT was relatively low (P = 0.013). Carrying out a Bonferoni correction increases the p-value to values higher than 0.05, so that the correlation may no longer be statistically significant. Thirdly, in view of clinically marginal differences between high-pressure and normal-pressure glaucoma, one would like to see whether all subjects included together into the study showed a significant relationship between CCT and anterior scleral thickness or not. In conclusion, the study addresses an important topic and was carefully performed. As pointed out by the authors, additional studies may be necessary to achieve a final result.

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