Patients with multiple sclerosis (MS) who smoke marijuana regularly have more cognitive deficits than patients with MS who do not smoke marijuana, according to research published May 27 in Neurology. Marijuana use may be associated with impairments in information processing speed, visual memory, and working memory among these individuals.
In a cross-sectional study, cannabis users had more diffuse cerebral activation during a test of working memory, compared with nonusers. The cannabis users also had increased activation in parietal and anterior cingulate brain regions implicated in working memory, relative to nonusers.
But investigators found no differences in brain structure between the study groups, and this finding is consistent with the results of cannabis imaging studies in healthy subjects.
Subjects Underwent Imaging and Neuropsychologic Tests
Bennis Pavisian, research assistant at Sunnybrook Hospital in Toronto, and colleagues recruited 39 patients (ages 18 to 60) with a confirmed diagnosis of MS for their study. They enrolled 20 participants who regularly used cannabis and whose urine tested positive for cannabis metabolites. The investigators asked participants not to use cannabis for 12 hours before the trial. Nineteen patients with MS who had never used cannabis were matched to the other participants using demographic and disease-related variables.
All participants underwent the Brief Repeatable Battery of Neuropsychological Tests in MS, which includes measures of verbal and visual memory, information processing speed, and attention.
The researchers assessed patients’ premorbid IQ with the Wechsler Test of Adult Reading, measured anxiety and depression with the Hospital Anxiety and Depression Scale, and gauged fatigue with the modified Fatigue Impact Scale.
All subjects also underwent the n-Back test of working memory, which had been modified to avoid verbal responses.
The investigators performed fMRI while the participants were taking the n-Back test. They also collected resting-state fMRI and structural MRI data, including lesion and normal-appearing brain tissue volumes and diffusion tensor imaging metrics.
Brain Activation Was More Diffuse Among Marijuana Users
Patients who used cannabis performed worse on the two-second Paced Auditory Serial Addition Test and the 10/36 Spatial Recall Test. Both groups had similar scores on the tests of fatigue, anxiety, and depression. Global cognitive impairment did not correlate with frequency of cannabis use or urine concentration of metabolites.
The investigators found no statistically significant between-group differences on fMRI for the 0- and 1-Back tests. The cannabis group gave fewer correct responses on the 2-Back trial, compared with controls, but had no difference in reaction times.
The researchers found no significant differences in resting state network activation between the two patient groups. During the n-Back test, all patients activated prefrontal, anterior cingulate, and posterior parietal circuits. Several secondary regions activated as well on the 0-Back and 2-Back tests, and the activation pattern was more diffuse in the cannabis group.
Parietal and anterior cingulate activations were only present in the cannabis group for both tasks, according to the investigators. Frontal activation was more prominent in the cannabis group across tasks.
—Erik Greb