MONDAY, May 12 (HealthDay News) -- A few millimeters can make a huge difference for Parkinson's patients treated with electrodes surgically implanted in the brain, a new study finds.

Implanting electrodes to stimulate the subthalamic nucleus, a part of the brain potentially related to impulsivity, has helped people with Parkinson's disease reduce their medication intake by up to 65 percent and lessen characteristic motor impairment by as much as 70 percent, according to background information for the study published in the May issue of Archives of Neurology.

But a study of seven Parkinson's patients who did not respond well to the procedure shows that additional surgery to correct the electrode placement greatly improved the patients' condition and dependence on medication.

"The principal cause of these poor results arises from imprecision of electrode placement, leading to non-stimulation of the target as required," the authors wrote. "Misplacement of the electrode by only a few millimeters may have occurred."

All but one of the patients who received the second surgery showed improvement afterward. When not on medication, their motor scores increased by 26.7 percent following the first surgery and 59.4 percent after the second. Their dose of levodopa, a Parkinson's medication, dropped by more than half from 1,202 milligrams to 534 milligrams.

The average distance between the electrodes and the target point of stimulation -- which was determined by evaluating electrode placement in patients whose surgery was successful the first time -- decreased from 5.4 to 2 millimeters. The shorter the distance, the greater the patients' improvement in motor scores.

The patients had the re-implantation surgery 12 to 23 months after the original procedure. Motor scores and medication doses were assessed a year after the second procedure.

"Although appropriate patient selection is important for the desired surgical outcome, the key to marked improvement following subthalamic nucleus stimulation is optimal surgical technique for precise implantation of stimulation electrodes in the target," the authors wrote.