Abstract
Purpose: :
To quantify oxygen saturation (O2SAT) and O2SAT responses in the optic nerve head (ONH) during an acute increase of intraocular pressure (IOP) in a primate model.
Methods: :
Four cynomolgus monkeys were anesthetized, and IOP was set with a saline reservoir connected to a needle inserted into the anterior chamber. Reflectance light spectroscopy of ONH structures (artery, vein, nasal, temporal, cup, superior, and inferior areas) was performed by hyperspectral imaging. Twelve images were obtained using constant illumination while IOP was held at 10 mmHg (n=3), 30 mmHg (n=3), 45 mmHg (n=3), and 55 mmHg (n=3) for 30 minutes each. Five sessions were conducted for each eye of each animal. Blood pressure was measured periodically throughout the procedure. O2SAT of separate structures was found using linear least–squares curve fits.
Results: :
O2SAT of artery, vein, and ONH (nasal, temporal, superior, inferior, temporal cup and nasal cup) were 80.2±0.9, 42.2±0.6, 65.9±1.2, 70.0±1.2, 63.1±1.1, 61.2±0.8, 73.4±1.1, and 66.2±1.0(Mean±SE) at IOP=10 mmHg, respectively. O2SAT dropped to 39.5, 44.8, 48.5, 44.2, 42.4, 50.2, 13.1, and 33.3 percents, respectively, when IOP was raised to 55 mmHg. In the retinal arteries overlying the ONH, there was no change in O2SAT up to 45 mmHg, but significant reduction occurred at 55 mmHg (P<0.0001). In the retinal veins, the O2SAT level was roughly parallel that of the artery up to 45 mmHg; at 55 mmHg there was a reduction in O2SAT, but it was markedly less. In the ONH, the relationship between O2SAT and IOP was similar to that in the arteries, showing significant changes only at the higher IOP (55 mm Hg). The exception was the ONH cup, where the relationship was more similar to that of the retinal vein.
Conclusions: :
Elevation of IOP reduces O2SAT of the retinal vessels and areas of the ONH. Deep anesthesia caused oxygen depletion of retinal vessels and ONH tissue at normal IOP. The decrease in the artery/vein saturation difference at 55 mmHg may indicate an autoregulatory response to high IOP. The ONH temporal cup shows the highest O2SAT and the most effective autoregulation compared to other areas. Our findings suggest that the nerve fiber layer (NFL) could become ischemic during low perfusion pressure as a passive occlusion of the central retinal artery at sustained high IOP, while the anterior ONH regions underlying the NFL could be protected from ischemia.
Keywords: intraocular pressure • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • optic disc