Cause

Low levels of dopamine, a brain chemical (neurotransmitter) involved in controlling movement, cause symptoms of Parkinson's disease. The shortage of this brain chemical occurs when nerve cells in a part of the brain (substantia nigra) that produces dopamine fail and deteriorate. The exact cause of this deterioration is not known.

The links between Parkinson's disease and factors such as genetics, aging, toxins in the environment, and free radicals are all under investigation. Although these studies are beginning to provide some answers, experts do not know the exact cause of the disease.

Studies are ongoing to determine whether there is a genetic cause of Parkinson's disease. Only a small percentage of people with Parkinson's disease have a parent, brother, or sister who has the disease. But abnormal genes do seem to be a factor in a few families where early-onset Parkinson's disease is common.

Symptoms

The type and severity of symptoms experienced by a person with Parkinson's disease vary with each individual and the stage of Parkinson's disease. Symptoms that develop in the early stages of the disease in one person may not develop until later—or not at all—in another person.

• Symptoms of Parkinson's disease typically begin appearing between the ages 50 and 60. They develop slowly and often go unnoticed by family, friends, and even the person who has them.
• A small number of people have symptoms on only one side of the body that never progress to the other side.

The most common symptoms include:

• Tremor, or shaking, often in a hand, arm, or leg. Tremor caused by Parkinson's disease occurs when the person is awake and sitting or standing still (resting tremor) and subsides when the person moves the affected body part.
• Stiff muscles (rigidity) and aching muscles. One of the most common early signs of Parkinson's disease is a reduced arm swing on one side when the person is walking that is caused by rigid muscles. Rigidity can also affect the muscles of the legs, face, neck, or other parts of the body and may cause muscles to feel tired and achy.
• Slow, limited movement (bradykinesia), especially when the person tries to move from a resting position. For instance, it may be difficult to get out of a chair or turn over in bed.
• Weakness of face and throat muscles. Talking and swallowing may become more difficult, and the person may choke, cough, or drool. Speech becomes softer and monotonous. Loss of movement in the muscles in the face can cause a fixed, vacant facial expression, often called the "Parkinson's mask."
• Difficulty with walking (gait disturbance) and balance (postural instability). A person with Parkinson's disease is likely to take small steps and shuffle with his or her feet close together, bend forward slightly at the waist (stooped posture), and have trouble turning around. Balance and posture problems may result in frequent falls. But these problems usually do not develop until later in the course of the disease.

Tremor is often the first symptom that people with Parkinson's disease or their family members notice. Initially, the tremor may appear in just one arm or leg or only on one side of the body. The tremor also may affect the chin, lips, and tongue. As the disease progresses, the tremor may spread to both sides of the body. But in some cases the tremor remains on just one side.

Emotional and physical stress tend to make the tremor more noticeable. Sleep, complete relaxation, and intentional movement or action usually reduce or stop the tremor.

Although tremor is one of the most common signs of Parkinson's disease, not everyone with tremor has Parkinson's disease. Unlike tremor caused by Parkinson's disease, tremor caused by other conditions gets better when your arm or hand is not moving and gets worse when you try to move it. The most common cause of non-Parkinson's tremor is essential tremor, a treatable condition that is often wrongly diagnosed as Parkinson's disease.

Other symptoms

Parkinson's disease can cause many other symptoms. These can be disabling and may include:

• Decreased dexterity and coordination. Changes in handwriting are common, with writing becoming smaller. Athletic abilities decline, and daily activities such as dressing and eating become harder.
• Cramps in the muscles and joints.
• Oily skin or increased dandruff.
• Digestive and urinary problems. Constipation is common. Controlling urination (incontinence) may be difficult, and urination may be frequent and at times urgent. Drugs used to treat Parkinson's disease may help or may sometimes make these symptoms worse.
• Problems with involuntary or automatic body functions, such as increased sweating, low blood pressure when the person stands up (orthostatic hypotension), and problems with sexual function. These symptoms may also be caused by Parkinson's-plus conditions or drugs used to treat Parkinson's disease.
• Freezing, a sudden, brief inability to move. It most often affects walking.

Problems with sleep, mood, and thought also are common in people who have Parkinson's disease.

• Problems falling asleep or staying asleep (insomnia) can result from anxiety, depression, or physical restlessness. People with Parkinson's disease may not be able to sleep well because they cannot easily turn over or change position in bed.
• A person with Parkinson's disease may slowly become more dependent, fearful, indecisive, and passive. The person may talk less often than he or she used to, withdraw from family and friends, and remain inactive unless encouraged to move about. Depression is very common in people with this disease and can be caused by chemical changes in the brain or can be a reaction to having a disabling disease. Depression often improves with proper treatment.
• Up to one-third of people with Parkinson's disease may develop dementia and confusion, similar to Alzheimer's disease, late in the course of the disease. Depression can further contribute to memory loss and confusion. Memory loss, hallucinations (seeing or hearing things that aren't really there), and vivid dreams may sometimes be caused by drugs taken to treat Parkinson's disease.

There are many other conditions with similar symptoms. Some of these may be reversible.