Standing up quickly can leave a person slightly light-headed. Physicians have long known that the feeling and the lack of blood pressure response that causes it occur more often in people with Parkinson’s disease than in healthy individuals. A report in the Sept. 5 Annals of Internal Medicine indicates that damage to nerve endings in the heart may be at fault.
Blood pressure and heart rate jump when a person stands up because the sympathetic nervous system—which controls some involuntary body functions—directs nerve endings in the heart to release norepinephrine. This compound causes blood vessels to constrict, forcing blood to flow faster. If nerves that carry signals to the heart are damaged, the vessels don’t get the message.
Norepinephrine is closely related to another neurotransmitter, dopamine. Parkinson’s patients typically have less than half as much dopamine in their systems as healthy people do, a shortage that results in the movement problems that characterize the disease.
Researchers conducted positron emission tomography scans of the hearts of 29 Parkinson’s patients to determine whether they also had a shortage of norepinephrine there. Nine of the Parkinson’s patients had complained of dizziness when standing quickly. All nine had considerable loss of heart nerve endings that release norepinephrine, says study coauthor David S. Goldstein of the National Institute of Neurological Disorders and Stroke in Bethesda, Md. In six of these patients, samples of blood leaving the heart-muscle tissue confirmed a lack of norepinephrine.
Among the other 20 patients, 11 showed as much nerve destruction in the heart as those who reported dizziness did.
No significant nerve damage appeared in heart scans of 52 people without Parkinson’s disease, some of whom had a history of fainting or light-headedness. Use of levadopa, a drug often prescribed for Parkinson’s, can cause dizziness. In the new study, levadopa therapy didn’t correlate with heart-nerve damage. “This is the first study that shows [sympathetic nerve damage in the heart] convincingly and beyond a shadow of a doubt,” says Horacio C. Kaufmann of Mount Sinai School of Medicine in New York.
The cause of most Parkinson’s disease remains unknown. “It may well be that the patients identified in this study [with nerve damage] have one particular form of Parkinson’s disease in which the cause is also affecting certain peripheral components of the nervous system,” says Donald B. Calne of the University of British Columbia in Vancouver.
Finding a common feature that threatens both dopamine-releasing cells in the brain and norepinephrine-releasing cells in the heart could help reveal the origin of the disease, Goldstein says. Unstable compounds called free radicals are known to destroy dopamine-releasing brain cells. Goldstein and his team are currently investigating the effects of the compounds on nerve cells.