About 40 to 50 percent of people with Parkinson's develop depression. Usually it's mild to moderate in intensity, but roughly 5 to 10 percent of cases are the more severe form known as major depression. Evidence suggests that depression results partly from neurochemical changes in the brain caused by the Parkinson's disease process -- it's not just a psychological reaction to having the illness.

Diagnosing depression in a person with Parkinson's disease can be complicated, because the two disorders share many symptoms: fatigue, loss of pleasure or interest in normal activities, problems with sleep, slowness of movement, lack of facial animation. Parkinson's patients may look sad even when they're not, because on top of having stiff facial muscles, they speak softly and don't respond quickly.

To correctly identify the mood disorder, doctors look for distinguishing clues such as sustained feelings of sadness, worthlessness, guilt, or irritability. Depression can be treated with psychotherapy and drugs, such as selective serotonin reuptake inhibitor (SSRI) drugs or tricyclic antidepressants. In severe cases, electroconvulsive therapy may be considered.