Evaluation of ambulatory BP in patients with glaucoma can provide insight into the presence of associated modifiable vascular risk factors and ischemic risk. In the current study and consistent with previous research,7,32,41,42 no significant differences in the average diurnal and nocturnal BP variables or the nocturnal dipping status were found between the POAG, NTG, and control groups following 24-hour BP assessment. There is, however, a lack of consistency in results reported by various groups regarding the relationship between systemic BP and glaucoma, with other studies40,43- 55 finding associations between abnormal systemic BP and the presence of GON, particularly with regard to hypotension and nocturnal BP dipping. The inclusion/exclusion criteria for these studies are variable, making direct comparison difficult, and it is possible that the strict selection of patients in our study could have contributed to the findings herein. Interestingly, an increased variability in nocturnal SBP, as well as a reduced OPP at the time of testing, was identified in both glaucoma groups, compared with the control group in this study. Increased variability in BP is widely recognized in cardiovascular and hypertension research as a signal of increased risk for end-organ damage56 and, during recent years, its occurrence has been increasingly considered with regard to other vascular diseases, including glaucoma.57- 59 The mechanism by which such increased variability, during the nocturnal period in particular, may relate to the development of GON is unclear. It could be hypothesized that, because of the close relationship between BP and OPP, an increased variability in BP may subsequently lead to increased variability or fluctuation in OPP, which may have a particular effect nocturnally when OPP is already physiologically reduced and the optic nerve head is more vulnerable.60- 63 To validate this hypothesis, calculation of 24-hour OPP would be ideal; however, in this study and in line with current clinical practice in England, this information is not available since 24-hour IOP was not measured. At the time of assessment, however, OPP was significantly reduced in POAG and NTG patients compared with controls, suggesting that perfusion-related vascular alterations are likely to be playing a part in the pathogenesis of both conditions. However, the relative contribution of IOP to this reduced OPP is likely to vary between our glaucoma groups. Because of this, consideration of the 24-hour IOP and its fluctuations, which themselves have been linked to GON development,48,64 would also be beneficial.