Non-contact in vivo oximetry of episcleral and bulbar conjunctival microvasculature using snapshot multispectral imaging

To investigate the use of Snapshot Multispectral Imaging (SMSI) for non-invasive in vivo oximetry of the superficial ocular microvasculature. Additionally to characterise changes in blood oxygen saturation (sO2) due to systemic acute mild hypoxia and the diffusion of oxygen from surrounding air. Multispectral images of conjunctival and episcleral vasculature were acquired for 10 young healthy subjects under normoxic and hypoxic conditions using a retinal fundus camera modified with an Image Replicating Imaging Spectrometer (IRIS). Diffusion of oxygen was controlled by eyelid closure. Acute-mild hypoxia was induced by breathing reduced oxygen content air. Images were processed posthoc to calculate average Optical Density Ratio (ODR) of vessels. ODR is directly and inversely proportional to sO2. Average ODR of bulbar conjunctival vasculature was lower than the average ODR of episcleral vasculature, indicating that episcleral vessels had lower sO2 than conjunctival vessels. At normoxia all bulbar conjunctival vessels shared the same approximate ODR, indicating that they are equally oxygenated. Acute mild hypoxia resulted in an increase in ODR (i.e. a decrease in sO2) of both episcleral and bulbar conjunctival vessels. Oxygen-deficient bulbar conjunctival vasculature was observed to rapidly re-oxygenate when exposed to air: typically taking less than 20 seconds to return to normoxic levels. SMSI can be used to measure changes in sO2 of conjunctival and episcleral vessels due to acute mild hypoxia. Measurements must be carefully controlled because the superficial bulbar conjunctival vessels undergo rapid diffusion of oxygen when exposed to air. Oximetry of these vascular beds may provide insights into oxygen delivery and consumption in conditions such as diabetes; dry-eye syndrome; chemical burns; scleritis, and scarring of the anterior segment after surgery.