The same kind of deep brain stimulation used to treat some patients for Parkinson's disease also helped a few people suffering from obsessive-compulsive disorder, French scientists reported.

Their study involved only 16 patients, but in four of them, symptoms nearly disappeared. However, many patients had serious side effects, including one case of bleeding in the brain.

The treatment involved an experimental brain pacemaker, and it reduced repetitive thoughts and behaviors in some of the patients — just as it blocks tremors for some Parkinson's sufferers.

The researchers came up with the approach after noticing that two Parkinson's patients who got the treatment also saw an improvement to their obsessive-compulsive disorders. Other small studies have targeted a different part of the brain for that disorder and depression.

In the French study, symptoms were reduced more than 25 percent, the researchers said.

The results are "very encouraging," said the study's lead author, Dr. Luc Mallet of Pitie-Salpetriere Hospital in Paris. In an e-mail, he said the procedure should be used only in medical studies at the moment because of the possible side effects.

The findings are reported in Thursday's New England Journal of Medicine.

About 2.2 million American adults have obsessive-compulsive disorder. It involves recurring, unwanted thoughts, such as a fear of germs, and people who have it engage in rituals such as repeatedly washing their hands or checking on something again and again.

Standard treatment, antidepressants and psychotherapy, doesn't work in everyone. The patients in the French study were severe cases who didn't respond well to treatment.

All had surgery to have the pacemaker — similar to a heart pacemaker — implanted in their chest and connected to electrodes inserted into their brains. Each patient had the pacemaker turned on for three months and turned off for three months. Neither the patients nor their doctors knew when the device was on or off.

The researchers used different tests to measure changes in symptoms. In one evaluation, after three months of stimulation, the severity of symptoms overall had dipped to 19 on a 40-point scale, compared to a score of 28 after three months of no treatment.

Eleven patients had serious side effects; one had bleeding in the brain and two had infections from the surgery. For some patients, the stimulation resulted in a mild form of mania and other problems that went away when adjustments were made.

Mallet said the area of the brain they targeted — the subthalamic nucleus — deals with motion, thinking and emotion. Previous studies for obsessive-compulsive disorder, or OCD, focused on regions involving mood and anxiety, he said.

"We're still not exactly sure where the sweet spot is in the brain to reduce the symptoms of OCD," said Dr. Wayne Goodman, a psychiatrist at the National Institute of Mental Health. "Even if you think you're in the right neighborhood, you may be one block off. And one block off in the brain may be just 1 millimeter."

Goodman said he was initially alarmed by the serious side effects but noted that many were temporary and others were not unexpected. He said the challenge will be deciding whether the risks are worth it for individual patients.

Another French researcher, Dr. Antoine Pelissolo, said the patients in the study, who now all have their pacemakers turned on, are still being followed. Researchers are also testing stimulating two areas of the brain at the same time, he said.

The pacemakers used in the study were bought from Medtronic Inc., which had no role but paid for the researchers' meetings. Some of the scientists have received consulting fees and grants from Medtronic.