Retinal vessel oximetry could provide valuable information

Indianapolis-Measurement of retinal oxygen utilization could provide vital clinical information about the metabolic state of the retina, according an Indiana glaucoma researcher.

Alon Harris, PhD, the Letzter Professor of Ophthalmology at Indiana University School of Medicine, Indianapolis, reported that in the future, mapping of retinal blood oxygen saturation will be as commonplace as fundus photography.

"There are no current treatment interventions in glaucoma that can elicit immediate improvements in visual function," he said. "Relief of vascular dysfunction could one day prove to be a novel therapeutic strategy of immediate benefit in the treatment of glaucoma."

Over the past decade, significant strides have been made in the technology and methods to quantify ocular blood perfusion. These advances have profoundly changed the knowledge base and understanding of how blood flow relates to the pathophysiology of glaucoma.

"In glaucoma, numerous blood flow deficits have been identified compared with normal, healthy eyes," Dr. Harris explained. "Nevertheless, despite accumulating evidence that glaucoma patients suffer from inadequate ocular blood flow, current clinical treatment of the illness involves neither documentation of, nor treatment for, these deficits."

He said that recent evidence suggests that glaucoma characteristically damages the photoreceptors and the horizontal cells, as well as the ganglion cells.

"The retinal ganglion cells are nourished by the retinal circulation, while the photoreceptors receive their blood supply from the underlying choroid," he continued. "Therefore, to define how enhanced blood flow improves visual function, it is essential to evaluate blood flow to the retina and to the choroid, for the retinal ganglion cells and photoreceptor cells, respectively."

Dr. Harris, who is also professor of physiology and biophysics, and director of Indiana University's Glaucoma Research and Diagnostic Center, explained that if visual function deterioration in glaucoma is indeed caused by specific damage to retinal ganglion cells, improving retinal flow should improve this deficit. Blood flow deficits suggest ischemia within the retina; however, the measurement of blood flow is merely a surrogate for metabolic measurements.

"This is where the measurement of retinal oxygen utilization could help give us a better picture of the metabolic state of the retina," he emphasized. "If the total blood flow and oxygen saturations in the retinal arterioles and venules are calculated, the difference between the oxygen delivered to and away from the retina can be examined."

Dr. Harris noted that in vivo measurements of oxygen consumption began in the 1930s, when oxygen consumption in the hand was measured in patients following circulatory occlusion. Soon afterward, transillumination of the ear was used to measure the saturation of human blood continuously.

"The first attempts to quantify the oxygen saturation of blood noninvasively within the retinal vasculature dates back to the 1950s," he said. "Some 30 years later, a photographic means of measuring oxygen saturation in the human retinal vessels was developed."

Complications in the eye Oximetry measurements in the retinal vessels utilize reflection, which may be influenced by fundus pigmentation. In the range of 450 to 540 nm, the reflection for the retinal surface varies in intensity based on fundus pigmentation.

"The problem is, the distribution of pigments in the retina and choroid isn't uniform, and this nonuniformity is particularly pronounced in the choroid," Dr. Harris explained. "This has complicated the assessment of oxygen saturation within retinal blood, and has impeded the creation of a marketable device to date."