LOS ANGELES - US researchers have found the bidirectional association between Parkinson's disease and melanoma, according to a new study to be published in the upcoming issue of Neurology.

Not only are people with Parkinson's disease more likely to develop melanoma, but people with melanoma are more likely to develop Parkinson's disease, said researchers from the US National Institute of Environmental Health Sciences.

Study findings were published on Monday by HealthDay News.

For the study, the researchers analyzed 12 epidemiologic studies dating from 1965 through 2010 that looked at a possible association between Parkinson's disease and melanoma. This is a process called a meta-analysis, in which data from various studies are pooled together to try to find a consistent statistical pattern.

While most of these studies reported fewer than 10 cases where both conditions occurred, the researchers found that men with Parkinson's disease were twice as likely to develop melanoma as men without Parkinson's disease.

For women, the risk of melanoma was one-and-a-half times greater if they had Parkinson's disease than if they didn't, the researchers found.

Not only Parkinson's disease is more likely to raise the risk of melanoma, but melanoma is also more likely to raise the risk of Parkinson's disease, the researchers said.

"It appears the association is bidirectional," said lead researcher Dr Honglei Chen, head of the Aging & Neuroepidemiology Group at institute.

However, since both conditions are relatively rare, the odds of having Parkinson's disease and melanoma are only about 4 percent, Chen said. "The absolute risk is not that high," he added.

Still, "it is prudent for Parkinson's disease patients to be a little cautious about their skin health," Chen said. "Be prudent, but do not be alarmed."

Scientists have theories about why these conditions seem to co-occur, but no firm answers. They know it's not the drugs Parkinson' s patients take, Chen said.

"Both conditions may have common genetic pathways or environmental factors or both," he said. "We understand very little about this possible relationship."

Parkinson's disease is a progressive neurological disorder that causes tremors, stiffness and difficulty with movement, coordination and walking, while Melanoma is the most dangerous type of skin cancer.