Researchers find drug increases visual sensitivity, eye circulation
In a pilot study, Health Science Center researchers have found that the drug sildenafil (Viagra) increases contrast sensitivity in the central part of vision used for reading. The finding is reported in a letter to the editor in the June 1 issue of The New England Journal of Medicine.
Ten men and two women with no eye diseases participated in the study, which was headed by Dr. William Sponsel, ophthalmology. The subjects were tested before and after taking Viagra to determine how little contrast is necessary to see different shades of light. This minimal contrast is used to calculate contrast sensitivity.
"We asked the participants to look at TV displays of stripes that were parallel to one another and ranged from light gray to dark gray. We then measured the individuals' contrast sensitivity using different methods," Dr. Sponsel said. The light and dark gray stripes exchanged places at a rate that stimulates an important group of axons carrying visual signals through the optic nerve to the back of the brain. The optic nerve, like a cable consisting of bundled wires, contains an estimated 1.2 million axons.
The researchers noted significant increases in contrast sensitivity in all subjects after Viagra administration. "It's rather exciting," Dr. Sponsel said.
Co-authors are Drs. Gianmarco Paris, Sal Sandoval, Donald Sanford and Joseph Harrison, and W. Rowe Elliott and Yolanda Trigo, all from the Health Science Center. Dr. Harrison designed the contrast sensitivity measurements used in the experiment.
A major article in the same issue of the New England Journal addresses Viagra's effect on blood flow in the heart and lung.
Dr. Sponsel and his colleagues also measured blood flow to tissue called the choroid, situated behind the retina in the back of the eyes. Study participants showed a one-third increase in blood flow after taking Viagra. "The result was remarkable for what we originally intended to be a pilot study," Dr. Sponsel said. "We saw circulation increase in all the blood vessels in the back of the eye. Increasing ocular perfusion might be of therapeutic value for various eye diseases."
One patient with severe eye disease accompanied by vision loss showed dramatic improvement just an hour and 10 minutes after taking Viagra, one of the lead researchers said. However, that patient was not part of the pilot study.
"Several Health Science Center experts, although not listed as co-authors on the study, were indispensable to its outcome," Dr. Sponsel said. They included Dr. Jeffrey Kiel, one of the world's experts in understanding eye blood vessels, and Dr. Wichard A.J. van Heuven, chairman of the Department of Ophthalmology.