Lindén et al.1 presented a prospective study on intracranial pressure (ICP) and intraocular pressure (IOP) in different body positions, and demonstrated that there was no evidence of reduced ICP in normal tension glaucoma (NTG) patients as compared with healthy controls, either in supine or in upright position. The conclusion of their study differed from that of the previous studies by Berdahl et al.2 and our team.3 The discrepancy could be the result of synergy of several possible causes:
In prior studies by Berdahl et al.2 and Ren et al.3, all NTG patients underwent 24-hour IOP monitoring to make sure their IOP &se; 21mmHg. However, some NTG patients in Lindén et al.'s study had occasional IOP up to 24mmHg, and they did not mention whether the 24-hour IOP monitoring was performed. There is possibility that hypertension glaucoma patients were included in the NTG group, which may influence the final results.
Moreover, previous study has already revealed an average ICP of 12.5mmHg among healthy population.4 However, ICP in the control group reported in this study is only 11.3mmHg. The lower ICP in the control group may have contributed to the lower TLCPD in this study.
Additionally, it has been reported that blood pressure and body mass index (BMI) have positive correlation with ICP4, hence, the higher ICP in the NTG group could be well-explained by the higher blood pressure and BMI in this study. In comparison, the blood pressure and BMI of the NTG patients in Chinese population were relatively low3, which may be attributable to the racial difference.
Considering that current studies in this field have small sample sizes, differ in the methodology and were conducted among population of different ethnic groups, an international cooperative study among different ethnic groups with a standard protocol should be carried out to clarify the contribution of each factors to the study results.