Effects of intraocular pressure on oxygen saturation of optic nerve head in cynomolgus monkey
Description
Assessment of oxygen saturation (O2Sat) in the optic nerve head (ONH) under controlled intraocular pressure (IOP) elevations may facilitate the detection of early phase glaucoma. Objective of this research is to quantify O2Sat in response to acute IOP elevations in the ONH of cynomolgus monkey Hyperspectral imaging (HSI) was used to test the hypothesis that O 2Sat distribution resulting from graded IOP elevations in retinal vessels and the ONH can be detected by HSI technique. Assessment of percent O 2Sat in retinal vessels and the ONH was carried out using least-squares curve fit of experimental spectral curves calibrated with reference curves. A previous algorithm for relative O2Sat calculation was improved by correction for blood volume which allowed comparison of O 2Sat in vessels and in the ONU, and tested on a multi-spectral imaging (MSI) system to reduce the time required to collect imagery from human subjects. Linear relationship was found between relative and calculated percent O 2Sat. The effects of IOP on the O2Sat were visualized in saturation maps. Retinal arteries showed nonsignificant reduction in O 2Sat between IOPs of 10 and 30 mm Hg (P = 0.805) and significant reductions at 45 and 55 mm Hg (P = 0.01, P < 0.0001 respectively); veins showed incremental reduction in O2Sat at each elevation of IOP. The responses in the rim of the ONH were similar to those in the artery, with the exception of the temporal cup, which only showed significant reduction in O2Sat at 55 min Hg. Models of oxygen diffusion out of the retinal artery, and of oxygen diffusion between paired arterioles and venules suggest that a further measurement of retinal arterial blood velocity is necessary Results are consistent with early studies that optic nerve fiber layer and prelaminar neural tissues may become ischemic at low level of perfusion pressure. Findings of O2Sat at graded IOP elevations in experimental glaucoma and direct measurements of blood flow will be necessary to further assess the importance of these findings. The HSI and MSI techniques are useful in diagnosing conditions, monitoring treatments, and studying the mechanisms of the diseases