By Cari Nierenberg, Live Science Contributor

Parkinson's disease is a neurodegenerative disease that primarily affects the part of the brain responsible for normal movement. People with the disease have a deficiency of dopamine, a brain chemical that helps control movement, according to Dr. Danny Bega, a neurologist at Northwestern University Feinberg School of Medicine in Chicago.

In Parkinson's, nerve cells in the substantia nigra, an area of the brain that produces dopamine, become impaired or die off. This results in the loss of dopamine-producing nerve cells in the brain and leads to symptoms such as tremors, slowed movements and muscle stiffness.

About 60,000 Americans are diagnosed with Parkinson's disease each year, according to the Parkinson's Foundation.

Symptoms

Parkinson's disease may cause the following motor symptoms, or those that generally affect a person's movement:

• Tremors (a slight trembling or shaking), usually in a hand, finger, foot or leg, or the chin, typically while at rest. Tremors may also be an early sign of Parkinson's.
• Muscle stiffness and rigidity of the arms, legs or body. For example, arms may not swing freely when the person is walking or feet may seem stuck when the person is walking or turning.
• Slowed movement, including being slow to initiate movements, like getting out of a chair; slow involuntary movements, such as blinking; or slow at performing routine movements like buttoning a shirt. Facial muscles can also be affected, causing a lack of expression known as "facial masking."
• Balance, gait and posture problems. A shuffling gait, with short steps and a stooped posture, is characteristic of people with Parkinson's and may throw off balance and increase the risk of falls.

Symptoms typically develop slowly over time, making them hard to detect in the early stages of the disease. In addition, the progression of symptoms and their intensity can vary from one person to another.

Parkinson's causes more than motor problems; there can also be nonmotor symptoms that are unrelated to movement, Bega said. These symptoms can affect a person's quality of life and everyday function, and they may include:

• Mood disorders. Depression and anxiety are common in people with Parkinson's.
• Cognitive changes affecting memory, thinking, judgment and the ability to think of words. These usually occur in the later stages of Parkinson's.
• Smell disturbances. A reduced sensitivity to smell or a loss of smell is an early symptom of Parkinson's.
• Swallowing difficulties. The ability to swallow slows down as the disease progresses. Saliva may accumulate in the mouth and cause drooling.
• Chewing and eating problems. Late-stage Parkinson's may affect muscles in a person's mouth, making it harder to chew. This can lead to choking and weight loss.
• Speech changes. A person may speak more softly or sound monotone.
• Writing changes. Handwriting may look smaller, and words may be crowded together.
• Sleep problems. Insomnia, daytime fatigue and vivid dreams may all be linked with Parkinson's.
• Constipation. Food moves more slowly through the digestive tract, making regular bowel movements difficult.
• Dizziness. Lightheadedness when standing up after sitting or lying down, caused by a sudden drop in blood pressure in people with Parkinson's.

Causes and risk factors

The cause of Parkinson's disease is not yet known, according to the Mayo Clinic. But researchers suspect that PD is caused by a combination of genetic and environmental factors.

Parkinson's is a disease of aging, and getting older is the most common risk factor for it, Bega told Live Science. PD is more likely to develop in people around age 60, and the risk increases with every decade after 60, he noted.

The disorder can also be diagnosed in younger people, but it's rare. Only 5 to 10 percent of people have "early-onset" disease, meaning people are diagnosed before age 50.

Besides age, other risk factors for Parkinson's disease include:

• Being male: Men are 1.2 to 1.5 times more likely than women to develop Parkinson's, for reasons that are not yet known, Bega said.
• Heredity: Some genetic mutations may contribute to the development of Parkinson's and can slightly increase a person's risk. But most cases of the disease are not caused by inheriting genes linked to it. Only about 10 percent of people with Parkinson's are genetically predisposed to the condition, according to the American Parkinson Disease Association.
• Exposure to toxins: Studies have shown that environmental factors — such as exposure to pesticides, herbicides (like Agent Orange) and drinking well water — may be tied to an increased risk of Parkinson's, but that risk is relatively small, Bega said.
• Repeated head injuries: When these injuries trigger a loss of consciousness, they have been linked with an increased risk of Parkinson's.

Diagnosis

There is no specific test, such as a blood test or MRI, that can be used to diagnose Parkinson's. Instead, the diagnosis is based on a constellation of findings from a thorough examination, Bega said.

For example, diagnosis may partially come from identifying symptoms during a physical exam, such as stiffness and slow movement, he said. Doctors may also perform a thorough neurological exam, which can help exclude other disorders that may cause symptoms. For example, a stroke may trigger balance problems, or a tremor could be a side effect of taking certain medications.

Parkinson's is a gradually progressive disease, so a physician must also assess whether symptoms seem to worsen over time, Bega said. Because this disease can be tricky to diagnose, it's best for patients to work with a neurologist or movement-disorder specialist who sees these problems daily, Bega said.

Treatment

Many medications are available to treat the symptoms of Parkinson's disease, but none of them can slow the disease's progression, Bega said.

Levodopa, a dopamine-promoter drug, in combination with the drug carbidopa, is the most commonly prescribed treatment for controlling Parkinson's motor symptoms. Carbidopa helps prevent nausea and vomiting associated with taking levodopa by itself.

Although it's an effective medication for Parkinson's, levodopa's benefits may wax and wane, with the drug unexpectedly stopping and starting working the longer a person takes it. In addition, levodopa may produce undesirable side effects, such as nausea, dizziness and a disorder of uncontrolled jerky movements known as dyskinesia.

Some people with Parkinson's may be concerned about starting treatment with levodopa too early in the disease's progression or fear the potential side effects. But those fears may be overblown, and the treatment's benefits far outweigh its risks, Bega said.

Studies have suggested that being physically active may slow the progression of Parkinson's symptoms. Bega said he encourages regular exercise — stationary biking, swimming, strength training or doing tai chi — to improve mobility, balance and mood for people with the disease.

Deep brain stimulation may also provide symptom relief. In this surgical procedure, electrodes are implanted in the brain to reduce the movement-related symptoms of Parkinson's.

Research

One of the hottest areas of Parkinson's research involves the protein alpha-synuclein. In autopsies, many brain cells of people who'd had Parkinson's disease have been found to contain Lewy bodies, which are unusual clumps of alpha-synuclein.

These clumps of protein in the brain are the pathological hallmark for Parkinson's and may be one reason the brain is not working properly in those with the condition, Bega said. If researchers can prevent the protein from clumping into Lewy bodies, by either clearing them out or stopping their spread within brain cells, then that may lead to a method of stopping the disease's progression, said the Michael J. Fox Foundation for Parkinson's Research.