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People with Parkinsonís with Tremors Show Different Brain Activity Than Those Without Tremors

People with Parkinson’s disease (PD) who experience tremor of the hand or leg show different brain activity than people without tremors, according to a report in the January 2013 issue of the Journal of the American Medical Association Neurology. These differences, revealed by brain scans with functional magnetic resonance imaging (fMRI), point to pathologically distinct forms of Parkinson’s disease.

Currently, clinicians diagnose PD based upon visible symptoms, including tremors and slow movement. However, up to a third of people with PD have little or no signs of tremors, and researchers have suspected that these individuals represent a neurologically distinct subcategory of PD. Indeed, previous studies suggest that PD may progress more rapidly in people without tremors.

To compare these two sub-types of PD, researchers led by Janey Prodoehl, Ph.D., at the University of Illinois at Chicago, working in the lab of David E. Vaillancourt, Ph.D., investigated the issue, working with 10 people with tremor-dominant PD, 10 with nontremor-dominant PD, and 20 people without Parkinson’s disease. None of the participants had yet taken drugs to treat PD. Each individual underwent fMRI brain scans focusing on two brain regions (the cortical and sub-cortical brain) as they gripped a rod, which requires activation of those brain regions. The research team also observed the volume of white and gray matter in each individual’s brain to assess whether any variation in brain matter might contribute to the findings.

Results

  • People with PD without tremors experienced less brain activity in the brain region that plays a role in executive functions, such as working memory and problem solving, compared with those who have tremors.
  • People with PD without tremors showed less brain activity in a component of the basal ganglia, the globus pallidus, compared to those without tremors and healthy controls. Neurologists have previously linked basal ganglia dysfunction to motor control problems.
  • The differences between these groups were not due to differences in brain anatomy (white matter versus grey matter).
  • Because the PD participants had not yet started medication, there was a possibility that they had been misdiagnosed. To check for accuracy, the research team found that two years after the study, 17 of the 20 PD participants had started and responded well to dopamine therapy, suggesting the original PD diagnoses were correct.

What Does It Mean?

Epidemiological studies have previously shown that people affected by PD can be divided into three groups based on the presentation of their motor symptoms: 1) those with tremor-dominant PD, 2) those without tremor who have difficulty standing and walking, and 3) those who fall in between the two.

While those with tremor dominant PD have tended to show best prognosis, i.e., slower disease progression and less cognitive impairment, this study marks the first time that scientists have been able to use brain activity in living people as a means of distinguishing between subtypes of PD and shedding light on the clinical differences between the groups.  Previously, these differences could only be observed in the postmortem analysis of brain tissue. 

Some scientists believe that tremor in PD is a compensatory mechanism, and not necessarily pathological.  Better understanding of the differences between the brain activity in those affected by tremor to those who are not may help researchers better understand the biology of PD.

Reference:
Prodoehl J, Planetta PJ, Kurani AS, Comella CL, Corcos DM, Vaillancourt DE (2013) Differences in brain activation between tremor- and nontremor-dominant Parkinson disease. JAMA Neurol 70:100–106. 

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Source Date: Feb 26 2013