Deep brain stimulation is an effective treatment for motor symptoms in patients with movement disorders such as Parkinson's disease. While ultimately regarded as safe, DBS can yield noticeable declines in verbal fluency, which affects how fast words can be retrieved and generated.

Researchers at the University of Alabama at Birmingham studied the impact of unilateral DBS on verbal fluency in Parkinson's patients. Findings showed that electrode implants in the left hemisphere of the brain appeared to be the primary cause of verbal fluency declines in patients who had undergone DBS surgery.

The study was part of the National Institutes of Health's Brain Research through Advancing Innovative Neurotechnologies Initiative, or The BRAIN Initiative, and the results were published in the Annals of Neurology.

"Patients who received right-sided DBS implants actually showed mild improvements in verbal fluency function over the time interval of the study," said Victor Del Bene, Ph.D., a neuropsychologist in the UAB Department of Neurology and lead author.

Traditionally, DBS involves placing tiny wire electrodes bilaterally, on both sides of the brain, in patients with Parkinson's disease. However, motor symptoms from Parkinson's disease are asymmetric at onset, which means they affect one side of the brain more than the other.

Surgeons at UAB Medicine have used unilateral stimulation on the most affected side of the brain as a less invasive, alternative option for DBS therapy.

"We found that unilateral intervention substantially improves motor symptoms for years, with the option to proceed with DBS on the opposite side of the brain if and when it is needed," said Harrison Walker, M.D., professor in the UAB Department of Neurology. "Despite this, little is known about how unilateral DBS impacts verbal fluency and other domains of cognitive function in these patients."

The study results suggest that rather than stimulating both sides of the brain using DBS, unilateral right DBS may prevent DBS-related declines in verbal fluency and response inhibition in patients with movement disorders like Parkinson's disease. Additionally, the left hemisphere stimulation was associated with greater declines in response inhibition, though there appeared to be no DBS-related hemisphere differences in terms of immediate or delayed recall of verbal information.

"We hope these results may convince neurologists and neurosurgeons at other medical centers that unilateral DBS on the most affected side of the brain is a viable therapeutic option, with potential advantages over the more invasive bilateral option for therapy," Walker said. "In particular, unilateral right hemisphere stimulation might spare occasional declines in some aspects of cognitive function that accompany the bilateral DBS intervention."