The researchers also evaluated the disease in patients who were taking their normal medication as well as receiving stimulation at 36 months. They found that the patients with GPi stimulation, unlike the STN group, were still improved compared to their baseline score.
"You would expect some decline because the disease progresses over time, but what we saw was that that didn't seem to happen with GPi," Weaver said.
"It was a surprise, at least to me, that the GPi group did as well as the STN group," said Stacy, who generally targets the STN site in his practice. "This further suggests that there is more reason to look into the GPi site on the long term."
The rate of side effects was similar between the two groups, and most of them occurred within six months of surgery, Weaver said.
In addition, the researchers found that GPi stimulation was associated with improvements in mental ability, based on measures such as verbal learning, after three years while mental scores for the STN group were lower than baseline.
The difference could be because targeting GPi stimulates a larger area of the brain than STN or has some off-site effects, Weaver said.
However, the researchers did not see improvements in aspects like emotional well-being or social support in either group after three years.
Quality of life often does not improve in this type of study, Stacy said. "[But] I think if you asked a patient, many would say, 'Yes, my quality of life is better after surgery,'" he noted.
Weaver and her colleagues are now following patients with Parkinson's for seven or nine years after DBS surgery. "Ideally they ought to have that stimulator for the rest of their lives if it continues to be helpful to them, so we want to look longer to see what happens," she said.
To learn more about Parkinson's disease, visit the Parkinson's Disease Foundation.
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