(USA TODAY) -- Depression takes a bigger toll on Parkinson's patients than physical problems linked to the neurological disease, and often goes undiagnosed, according to early findings from an international study out Wednesday.

"Nearly everyone thinks of the disease as a mobility disorder, but the No. 1 problem turns out to be depression," says Joyce Oberdorf, president of the National Parkinson Foundation, an advocacy group.

Its long-term study is the largest ever undertaken on the degenerative disease, she says. Parkinson's affects about 1 million people in the USA and 5 million worldwide, and is characterized by tremors, stiffness, slowness of movement and speech difficulties.

Trying to pinpoint which treatments enable some patients to thrive while others decline led the foundation to launch the research three years ago. There is no cure.

"Some patients stay active and can live at home rather than go to a nursing home," says physician Michael Okun, co-director of the Center for Movement Disorders and Neurorestoration at the University of Florida. "There's a wide disparity of treatments. We wanted to know what treatments were improving quality of life and to set guidelines for good outcomes."

The project involves 20 research centers and 5,557 patients. Each year, patients fill out a health survey; responses are entered into a database. The early findings on depression show the "magnitude of the problem," says physician Laura Marsh. Among the 61% who reported depression, 21% had minor symptoms, 22% had mild depression, and 18% reported severe depressive disorders.

"It's not because they're sad they have the disease, which they may very well be, but this depression is related to underlying changes in the brain, and for many, it will occur before diagnosis of Parkinson's," says Marsh, director of mental health services at Michael E. DeBakey Veterans Affairs Medical Center in Houston.

"The more aggressively it is treated, the better the outcome," she says. "There's a real problem with under- recognition and under-treatment."

Oberdorf says early results suggest "divergence in outcomes. We're finding the jewels of treatment."

Patients who get both medication and supportive therapy for depression do the best, says Marsh. Until depression is addressed, she adds, patients might not want to exercise, an important therapy for the disease.

"The more exercise you get, the more it helps with stiffness. It can also help prevent falls as the disease progresses," says Okun. "Exercise might actually modify the disease."

Exercise can also elevate mood, he adds.

Profile of Parkinson's

WHO HAS IT: About 1 million people in the USA and 5 million worldwide.

WHAT IT IS: The neurological disease is characterized by tremors, stiffness, slowness of movement and speech difficulties. There is no cure.

DEPRESSION: 61% of 5,557 patients in a large new survey report symptoms of depression; almost a third of those were severe. Doctors note that it is related to brain changes from the disease, not sadness about the diagnosis.

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