Parkinson’s disease is second to Alzheimer’s as the most common neurodegenerative ailment in the United States, affecting at least 1 million Americans. This disabling condition, marked by tremors, muscular rigidity, and slow, imprecise movements, primarily affects middle-age and elderly people.

The precise cause of Parkinson’s disease is unknown, but many of its symptoms are caused by a loss of neurons that produce a chemical messenger called dopamine. When dopamine levels decrease, it causes abnormal brain activity that leads to impaired movement and other symptoms.

Parkinson’s disease is also known to affect the heart. In fact, people with Parkinson’s disease are not often listed as dying from Parkinson’s itself, but from associated disorders with which it is linked, including heart disease.

By the time Parkinson’s patients are diagnosed — usually based on the appearance of tremors and motor-control symptoms associated with the disease — about 60 percent are also suffering from serious damage to the heart’s connections with the sympathetic nervous system — which, when healthy, spurs the heart to accelerate its pumping to match changes in activity and blood pressure.

As nerves that govern heart function degenerate, the body is less prepared to react to stress. Simple changes such as standing up can result in fatigue, fainting, and falling — all events that are related to Parkinson’s disease.

Other studies have noted that Parkinson’s sufferers are twice as likely to develop heart disease, and they have a 50 percent higher risk of dying from the condition. The website ParkinsonsDisease.net reports: “People with Parkinson’s disease have a similarly increased risk of developing congestive heart failure.”

Congestive heart failure (also often called simply “heart failure”) indicates that the heart has become weakened to the point that it can no longer pump a sufficient supply of oxygenated blood to the body.

Although the reason isn’t yet known, one study found that the missing link between the Parkinson’s brain and the heart might lie within a person’s cells — specifically in the mitochondria, which are the cells’ power plants. In that study, the researchers described similar mutations found in both inherited Parkinson’s disease and familial congestive heart failure, the type that runs in families.