BEIJING, Mar. 31 (Xinhuanet) -- Ophthalmologists at University of Minnesota researchers Wednesday said a condition that causes permanent vision loss has been diagnosed in a small group of men who took the erectile dysfunction drug Viagra. Combined with past reports, this study brings the total number of sildenafil-related NAION cases to 14.

    The condition, nonarteritic ischemic optic neuropathy (NAION), described as "stroke of the eye," occurs when blood flow is cut off to the optic nerve, which injures the nerve and results in permanent vision loss, according to Ophthalmologists at the University of Minnesota.

    Seven patients, aged between 50 and 69 years, had typical features of NAION within 36 hours after ingestion of Viagra for erectile dysfunction. Seven similar cases have been previously reported. These cases were published in the March 2005 issue of the Journal of Neuro-ophthalmology.

    All of the patients had at least one arteriosclerotic risk factor, including hypertension, diabetes, hypercholesterolemia, or hyperlipidemia. All of the patients also had a low cup to disk ratio, which is a way doctors measure the small circular indentation where the optic nerve connects to the eyeball. The low cup to disk ratio means that the blood vessels and nerves are tightly bundled together into the small space in the back of the eye.

    "For years, we've known that some men who take Viagra will experience temporary color changes in their vision and see things as blue or green," study co-author Dr. Howard D. Pomeranz, from the University of Minnesota in Minneapolis, said in a statement. "NAION is a much more serious condition because it can lead to permanent vision loss."

    "Viagra regulates a chemical in the body to constrict the arteries. This constriction may cut off the blood flow to the optic nerve, especially in people with a low cup to disk ratio, where the blood vessels and nerves are tightly bundled provoking NAION," says Pomeranz.

    A representative from Pfizer, the maker of Viagra, was not immediately available for comment.

    (Agencies)