BOSTON—Full-scale IQ and list learning are the best predictors of cognitive change following bilateral subthalamic nucleus deep brain stimulation (DBS), investigators reported at the 2012 Annual Meeting of the American Neurological Association. Preoperative full-scale IQ may account for 53% of postoperative variance in cognition. Full-scale IQ and list learning combined may account for 67% of postoperative cognitive variance.
Delayed story recall, single-word retrieval, and verbal fluency had significant positive correlations with cognitive change but did not contribute significantly to variance, said Angela Costello, D ClinPsych. Executive function had a significant negative correlation with postoperative cognitive change.
Evaluating Cognition Before and After DBS
To improve patient selection for subthalamic nucleus DBS, as well as outcomes and patient satisfaction, Dr. Costello, Consultant in Clinical Psychiatry and Neuropsychology at King’s College Hospital in London, and colleagues sought to identify preoperative cognitive factors that could predict postoperative cognitive change. The researchers studied 30 patients with Parkinson’s disease who underwent DBS. A multidisciplinary team concluded that the patients were nondemented, and no patients had reported cognitive difficulties in everyday life.
The investigators conducted comprehensive neuropsychologic assessment of all patients at a mean of 7.1 months before surgery and again at a mean of 9.4 months after surgery. The group assessed intellect, memory, language, perception, and executive functions. Patients who scored two or more standard deviations (SD) below the normative mean of a particular test were considered impaired on that test and, therefore, to have had mild cognitive impairment. Study participants were classified into four subtypes of mild cognitive impairment, depending on the cognitive domains in which they were impaired.
Memory Was Most Affected by DBS
Immediate story recall, delayed story recall, and list learning all showed statistically significant decline following DBS. The decline was moderate for delayed story recall and list learning and large for immediate story recall, which was the function most affected by DBS. Pearson correlation coefficients and stepwise regression analyses indicated that preoperative full-scale IQ and list learning accounted for the majority of postoperative variance in immediate story recall.
Verbal IQ and full-scale IQ showed a statistically significant decline in all participants. Patients’ mean preoperative verbal IQ was 103.2, compared with a mean postoperative verbal IQ of 100.3. The group’s mean preoperative full-scale IQ was 99.9, compared with 97.6 after surgery.
Verbal fluency was the only executive function to show a statistically significant postoperative decline.Verbal and visual recognition memory did not change postoperatively. Neither did language and perceptual domains.
“List learning and full-scale IQ are the best predictors of change in immediate story recall, as defined by our mild cognitive impairment criteria,” said Dr. Costello. “Pre-existing frontal executive dysfunction, as measured by the Brixton test primarily and, to a much lesser extent, the Hayling test, seems more robust to subthalamic nucleus DBS. One possible explanation could be that temporal lobe functions are perhaps more sensitive to decline following subthalamic nucleus DBS than are frontostriatally based functions,” she added.
The study analysis did not identify the cause of the association between the preoperative cognitive factors and postoperative cognitive change. “One possibility is that it’s due to either DBS or the surgical procedure,” explained Dr. Costello. Patients’ rate of disease progression also could be responsible for the association, she added.
—Erik Greb
Senior Associate Editor