An enlarged optic nerve sheath diameter and low cerebral spinal fluid pressure, the contradiction? The role of trans-laminar cribrosa pressure difference in the pathogenesis of primary open-angle glaucoma aroused great interests in glaucoma field by the recent retrospective studies done by Berdahl et al.1,2 and prospective studies done by Ren and Wang et al.3,4 However, the trans-laminar cribrosa pressure difference in these studies were solely based on lumbar CSF-P measurements, but not the retrobulbar space orbital CSF-P, since measuring of the trans-laminar cribrosa pressure difference is invasive and cannot be done clinically.
Jaggi et al. (790) employed CT scan to assess the optic nerve sheath diameter (ONSD) in patients with normal-tension glaucoma (NTG), to compare with controls without known optic nerve (ON) or intracranial disease, and some intriguing results were provided. Although the NTG patients in this study had a normal or rather low lumbar CSF-P, which is consistent with previous studies described,1-4 the ONSD in NTG patients is enlarged and conflict with the CSF-P result. The result of this study is thought provoking. Why do NTG patients with a normal or rather low lumbar CSF-P had an enlarged ONSD? Besides the three explanations provided by the authors, we suggest that several detail things of this study may lead to the result itself and hope it may help the readers to get a more thorough understanding of this interesting work. Firstly, the authors only mentioned that NTG patients in this study were scanned in prone position, but not the control group. We may infer that control group should be scanned in a normal supine position, since it was selected in a retrospective way. Due to hydrostatic reasons and in considering of gravity, different body positions during the scanning procedure may have led to differences in the perioptic CSF-P and it may influence the measurement of ONSD. Secondly, authors only provided 11 lumbar CSF-P results in 18 NTG patients with unknown reason. Since the other seven NTG patients were only known with a lumbar CSF-P less than 20 mm H2O and the CSF-P of the control subjects were unknown, one may consider the CSF-P in control subjects were greater than the NTG patients. Thirdly, only the widest part of the ONSD was measured in this study. In considering the uncertainty of the scanning layer of the round optic nerve sheath with CT and with the fact of the tortuous property of optic nerve in the orbit, only measuring of the widest part of ONSD may be misleading. Last but not the least, the NTG patients underwent lumbar puncture and cisternography just before the computer tomography was performed. Injection of 10 ml iopamidol may disturb the circulation of CSF and affect the CSF-P. In considering of this effect, the result of an enlarged ONSD may be a consequence of artificially increased CSF-P.
In conclusion, this work is really thought provoking and is initiative in considering of measuring ONSD as a surrogate for orbital CSF-P. Further prospective study of POAG patients with normal intraocular pressure or high intraocular pressure and measuring of the optic nerve sub-arachnoid CSF space with other noninvasive techniques would be interesting and significant. Nevertheless, we thanks for the great initial work done by the authors and the thoughts they give us.