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Editors Selection IGR 18-4

Clinical Forms of Glaucoma: Optic nerve sheath diameter in NTG

Comment by Hanspeter Killer on:

48780 Intraocular pressure correlates with optic nerve sheath diameter in patients with normal tension glaucoma, Abegã,o Pinto L; Vandewalle E; Pronk A et al., Graefe's Archive for Clinical and Experimental Ophthalmology, 2012; 250: 1075-1080

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Pinto et al. measured the diameter of the optic nerve sheath with ultrasound in patients with normal tension glaucoma (n = 46; 6.03 ± 0.69) and found similar optic nerve sheath diameters (ONSD) as in healthy controls (n = 42; 6.09 ± 0.78). There were smaller diameters in patients with POAG (n = 61; 5.71 ± 0.83). The authors conclude from this study that indirect ICP measurements by ultrasound may provide insight into the retrolaminar pressure component in glaucoma. This approach is interesting and promising but this study and the interpretation of the authors also raise some important questions. Compared to other studies with ultrasound, the normal controls in the study by Pinto measured markedly larger ONSD.1,2 Unfortunately, the authors do not comment on this obvious discrepancy between their findings and the literature. As the ONSD gains more interest in glaucoma studies, and different imaging modalities (MRI, CT, ultrasound) are going to be involved, this difference within the same method (ultrasound) is therefore of high interest. The measurements for the normal controls in the study by Pinto are close to the measurements in the study by Jaggi et al.³ that used CT imaging ‐ and not MRI as stated by Pinto. In order to explain the difference of the ONSD in this CT based study compared with the present study, Pinto et al. argue that the application of 10 ml iopamidol as well as the prone position during cisternography (CT measurement) could have influenced the diameter. However, in the study by Jaggi et al. some CSF was taken prior to the injection of iopamidol and the total volume of CSF did therefore not markedly change. Due to the stiffness of the sheath, the prone position seems unlikely to have influenced the ONSD. The difference in these two groups needs further explanation. For a better understanding of the interesting associations between IOP and ONSD in patients with POAG, it would be helpful to know more about the statistical power of the data.

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