[Preliminary study on the measurement of optic nerve subarachnoid space in patients with normal tension glaucoma]

Objective: To investigate the ultrasonographic measurements of the optic nerve subarachnoid space (ONSAS) in patients with normal tension glaucoma (NTG). Methods: Cross-sectional study. Patients with NTG or primary open-angle glaucoma (POAG) but without any anti-glaucoma treatment and the control group were collected from June 2016 to March 2017 at Beijing Tongren Hospital affiliated to Capital Medical University. Measurements of 24-h intraocular pressure, central corneal thickness, mean visual field damage, visual axis, blood pressure and body mass index and ocular ultrasound scans were performed. The differences in the ONSAS of the two-dimensional ultrasound images of the three groups of subjects and their correlation with various clinical variables were evaluated. Qualitative data were analyzed by the chi-square test for comparison between groups; quantitative data were analyzed by the one-way ANOVA for comparison between groups, and the LSD-t test was used for comparison between groups; the Pearson correlation analysis was used for correlation analysis of measurement data. The intra-group correlation coefficient was used to evaluate the reproducibility and repeatability of two measurements by different measurers or the same measurer. Results: A total of 35 patients (35 eyes) with NTG were enrolled in the NTG group, including 14 males and 21 females, aged (50±8) years; 32 patients (32 eyes) with POAG were enrolled in the POAG group, including 14 males and 18 females, aged (52±10) years; 37 healthy people (37 eyes) were enrolled in the control group, including 20 males and 17 females, aged (49±9) years. The ONSAS was (5.07±0.83) mm(2) in the retrobulbar 3- to 7-mm range in the NTG group, significantly smaller than the control group (6.57±1.43) mm(2) and the POAG group (6.19±0.90) mm(2) (t=1.17, 1.29; P=0.03, 0.01). There was no significant difference in the statistical results between the control group and the POAG group (t=1.31, P=0.75). Between the ONSAS and mean intraocular pressure and maximum intraocular pressure, there was a statistically positive correlation in the NTG group (r=0.66, 0.48; both P<0.01), but there was no linear correlation; there was no statistical correlation in the control group or the POAG group (all P>0.05). There was no statistically significant association between the ONSAS and age, mean arterial pressure, body mass index, central corneal thickness, visual axis length, and mean visual field loss in any group (all P>0.05). Conclusion: Compared with the POAG group and the control group, the ONSAS is smaller in the NTG group, indicating lower retrobulbar cerebrospinal fluid pressure, and it is positively correlated with the mean intraocular pressure and the highest intraocular pressure. (Chin J Ophthalmol, 2018, 54: 827-832).