Laser eye surgery, which was performed on almost one million patients worldwide last year, may do long-term damage to the eye's ability to see at night, a British study has found.

Of patients who had undergone the two most common types of surgery - LASIK and PRK -58 per cent failed a night vision test, said ophthalmologist Dr. William Jory of the London Centre for Refractive Surgery in London, England. Jory, who presented his findings at the May meeting of the American Society of Cataract and Refractive Surgeons in Boston, said even patients who responded well overall showed a decrease in night vision. "What really concerned me was that 41 per cent had had gained daytime vision, but some of those had lost nighttime vision. In Germany, they would have been illegal for driving on the roads." Jory tested 38 people who underwent surgery to seven years ago. He said his study was inconclusive because it looked at patients with moderate to severe corrections, a group which may he more prune to developing night vision problems after surgery.

But Dr. Evanne Casson, a researcher at the University of Ottawa Eye Institute, said Jory's figures were consistent with tests she did on photorefractive keratectomy (PRK) patients between 1996 and 1998. Sixty percent showed reduced contrast sensitivity - a leading indicator night vision - in tests spanning from just before surgery to two years later.

Early results of another study show fewer patients - 30 per cent - are affected, Casson said. But even that figure is alarming because doctors don't know how surgery will affect the eye as patients age and night vision naturally decreases, she said. "What happens if 30 percent of the population s myopic, and half of them get laser surgery?" That means 15 per cent of people attempting to drive on the road at night (have had laser surgery) - that's pretty scary. Jory said an independent, international study of LASIK patients should he launched immediately.

The LASIK procedure, which overtook PRK as the surgery of choice about five years ago, is now used by up to 95 per cent of patients. Already, more than 100,000 Canadians have had the procedure, which costs between S 1,000 and $3,000, and involves cutting a small flap in the cornea to remove underlying tissue.

Patients who have had the surgery frequently complain of double images or -`ghosting," halo images around lights and trouble seeing at night. said Ron Link, executive director of Surgical Eyes Foundation in New York. a non-profit support group for patients. "People say, I almost hit a kid (while driving),' or `I went blind in a tunnel,' he said. "There is mounting and alarming evidence this is an unexplored, major emerging issue." Some experts said Jory's numbers were too high, and said the loss of night vision had not reduced patients' quality of life. "I sometimes tell patients to drive with the overhead light on at night, or I give them drops to make the pupils smaller.' said Dr. Harold Stein, director of the Bochner Eye Institute in Toronto, who said the problem is more common in patients with large pupils.

Dr. Michel Pop, who performs the surgery at his clinic in Montreal. said he published a study of 1,300 patients which found 50 percent had night vision problems in the first month after surgery, but only five per cent had problems one year later. The problem is now being addressed by a new generation of lasers. Pop said. "It's not as big an issue as it was five years ago", he said.