The brain-signaling chemical dopamine is one of many neurotransmitters in the brain. It is made by cells in the brain stem. These cells release dopamine, which helps control muscle movement. The shortage of dopamine is caused by the breakdown of neurons in the area of the brain that controls movement. The breakdown of the neurons does not stop and gets worse.

About 60,000 new cases are reported each year, according to the National Institute of Neurological Disorders and Stroke (NINDS).

It is rare among people younger than 40, and the incidence rises with age. It is slightly more common in men than in women, according to the NINDS.

About 7 in 10 people with Parkinson first have a slight tremor in a hand or foot, according to the Parkinson Foundation. The tremor usually occurs when the muscles are at rest and relaxed. It usually appears on one side of the body. Later it may spread to the other side of the body, but to a lesser degree. Common symptoms as the disease gets worse include:

• Slowness of movement
• Rigid or stiff muscles
• A shuffling gait
• A stooped posture

Secondary symptoms include:

• Changes in speech
• Loss of facial expression (called "flat effect")
• Difficulty swallowing
• Drooling
• Dementia
• Sleep problems
• Depression
• Memory difficulties and slowed thinking
• Sexual dysfunction
• Fatigue and aching
• Urinary problems

It's difficult to diagnose in the early stages. No test currently exists that can diagnose Parkinson. Usually, a neurologist makes the diagnosis by evaluating the symptoms and how serious they are, as well as the results of a neurological exam. The person should also see a specialist in movement disorders to make sure the correct diagnosis is made. A brain scan can help find out if the person has Parkinson or a disease with similar symptoms. Sometimes medicines used to treat Parkinson are given to the person to see if the symptoms improve.

Parkinson has no cure, but the symptoms often can be improved with medicine and sometimes with surgery. Treatment varies greatly from person to person. It depends on whether the symptoms are mild, moderate, or severe. Levodopa is a medicine that is often prescribed. It is converted to dopamine in the brain. It helps replace the dopamine that the brain no longer makes. Other medicines act like dopamine in the brain, and others that keep dopamine from being destroyed so more of it is available. It is common for a person with Parkinson to take more than one medicine. Possible surgeries include implanting neurostimulators, pallidotomy, and transplanting stem cells.