Second, we found a significant positive correlation between peripapillary CT and retinal vessel SO
2 in young healthy individuals but not in glaucoma patients. To explain these findings, one must note that the relative contribution of the retinal and the choroidal circulation to the retinal oxygen supply is not fixed, but rather a dynamic interplay.
38 As mentioned before, the retina relies primarily upon autoregulation of the retinal vasculature in cases of increased metabolic demand or reduced ocular perfusion pressure. The choroidal blood vessels are highly innervated by the autonomic nervous system, but are separated too far from the retina to respond to the local accumulation of vasoactive metabolites. It could therefore be hypothesized that conditions interfering with either the autoregulation or the autonomic nervous regulation could render the retina at a higher risk of ischemic damage. Since glaucoma has been associated with disturbances in both types of regulation, we chose to compare the correlation between CT and SO
2 in young healthy individuals and glaucoma patients.
19,39 In the healthy group, we observed a positive correlation between CT and both arterial and venous SO
2. Studies have demonstrated that when oxygen is abundant, the relative contribution of the retinal circulation in the retinal oxygen supply diminishes, resulting in a higher saturation in the retinal arteries and veins.
40 With the retinal oxygen consumption and blood flow remaining unchanged, this shift toward the choroidal circulation would also result in a lower AV difference in the retinal circulation. Although not all studies are consistent on the direct correlation between CT and choroidal blood flow, a plausible explanation of our findings could therefore be that in patients with a thicker choroid, proportionally more oxygen is delivered to the retina via the choroidal circulation.
41–44 In the healthy individuals, both arterial and venous SO
2 were positively correlated with CT. Interestingly, AV difference showed a negative correlation in multivariate analysis, supporting the hypothesis that the relative contribution of the retinal circulation is less in patients with a thicker choroid. In the OAG patients, no such relationship was observed. This lack of correlation could reflect a disturbance in blood flow regulation in glaucoma patients. To further investigate this hypothesis, age-matched studies measuring also retinal, choroidal, and retrobulbar blood flow are needed.