Evaluation of Relative Oxygen Saturation in a Primate Model of Glaucoma Using Hyperspectral Imaging

Abstract

Abstract: : Purpose: To evaluate oxygen saturation responses of the optic nerve head (ONH) and overlying retinal vessels in a primate model of glaucoma. Methods: Normal intraocular pressure (IOP) (15 mmHg / 3 minutes) and high sustained IOP (60 mmHg / 4 minutes causing very low perfusion pressure) were controlled in cynomolgus monkeys by elevation of a fluid reservoir connected to the anterior chamber. Oxygen–dependent changes in hemoglobin reflectance spectra were evaluated from the ONH and overlying retinal vessels at one minute intervals. Spectral information was obtained using a hyperspectral imaging system that viewed the ONH through a fundus camera. A relative saturation index (RSI), derived from amplitudes of oxy– and deoxyhemoglobin spectra, was employed to map relative saturation over the ONH and vessels, and to follow relative saturation changes that resulted from high IOP. The value of RSI at 100% saturation was 0.312 ± 0.002 (5 time points) by calibration with pure oxygen breathing. Results: At normal IOP, the oxyhemoglobin spectrum (absorption maxima: 541 nm, 577 nm) was present in retinal vessels (3 time points, RSI_artery = 0.21 ± 0.008, RSI_vein = 0.139 ± 0.005) and in the averaged ONH response (RSI_ONH = 0.101 ± 0.001). For durations greater than 3 minutes at high IOP, the deoxyhemoglobin spectrum (absorption maximum: 555 nm) was present in both vessel types. RSI was significantly (p<0.05) reduced in vessels (2 time points, RSI_artery = 0.030 ± 0.010, RSI_vein = 0.029 ± 0.010) and in the averaged ONH response (RSI_ONH = 0.041 ± 0.002). This same duration of high IOP resulted in the return of the oxyhemoglobin spectrum with low amplitude in the temporal ONH, and an increase in RSI above the average ONH value (2 time points, RSI_{Temporal ONH} = 0.054 ± 0.0007). Conclusions: Desaturation to the same end point in the retinal artery and vein at 60 mm Hg IOP is consistent with blockage of blood flow through the inner retinal circulation. A smaller desaturation in the ONH may indicate that the blood supply to this tissue is capable of autoregulatory response to ischemia during sustained high IOP, whereas autoregulatory responses in the retina are prevented. This is supported by evidence of re–saturation in the temporal ONH after sustained high IOP without re–saturation in the retinal venous blood. Deeper layers of ONH circulation may be responsible for this regulatory response in glaucoma.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • ischemia • optic disc