Close to 1 million people in the U.S. have Parkinson's disease, a progressive neurological disease marked by tremors and difficulty with movement and walking.
The researchers analyzed 12 studies of people with both Parkinson's disease and melanoma. These studies were conducted from 1965 and 2010, and most had fewer than 10 people with both conditions.
When compared to those without Parkinson's disease, men with Parkinson's were twice as likely to develop melanoma. Women with Parkinson's disease were 1.5 times as likely to be diagnosed with this form of skin cancer. Parkinson's disease was not associated with an increased risk of other types of skin cancer.
Exactly how the two conditions are linked is not fully understood. Initially, there was some suspicion that a Parkinson's medication called levodopa may be responsible for this increased risk, but this has not been substantiated. There may be some genetic of environmental risk factor that serves as the common denominator between the two conditions.
"Further research is needed to examine the nature and mechanisms of this relationship in order to advance our understanding about the [causes] of both diseases," conclude researchers who were led by Honglei Chen, MD, PhD, with the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.
This increased risk needs to be placed in its proper perspective, says Roy Alcalay, MD, an assistant professor of neurology at Columbia University Medical Center in New York City and an advisor for the Parkinson's Disease Foundation.
"Melanoma is an extremely rare cancer, and Parkinson's disease doubles the risk of what is still a very rare cancer," he says. "Get your skin checked by a dermatologist each year."
This advice is important for everyone -- not just people with Parkinson's disease. Other cancer screening tests are also important for people with Parkinson's disease.
Andrew Feigin, MD, an associate investigator at the Feinstein Institute for Medical Research in Manhasset, N.Y, says the new report stresses the importance of regular skin checks. He conducts studies of Parkinson's disease, and says that annual dematologic screening exams are often part of the protocol.
"This study doesn't suggest people with Parkinson's disease need screening tests more often, but people with Parkinson's disease should be more careful about sticking to the recommended screening schedule," he says.
"The message for anyone, especially for those who may have an increased risk for melanoma, is to go and get skin exams," says Michelle Greene, MD, a dermatologist at Lenox Hill Hospital in New York City
"Know the signs of melanoma," she says. Suspicious moles may be asymmetrical, have ragged, notched or blurred borders, and changes in color distribution, size, or shape.