A study published this week in Mayo Clinic Proceedings confirms that Parkinson's disease can, perhaps surprisingly, increase the risk of melanoma. The researchers also show that the link is a two-way relationship, with melanoma also suspected to increase the risk of Parkinson's.
Studies carried out over recent decades have clearly demonstrated this interaction. For instance, a study published in JAMA Neurology in 2010 concluded that, "Melanoma prevalence appears to be higher in patients with PD [Parkinson's disease] than in the general population. [...] the study supports increased melanoma screening in patients with PD."
Similarly, a paper published in Translational Neurodegeneration in 2015 came to a similar conclusion, stating, "An association between PD and melanoma was confirmed. Most of the evidences were of high quality, and the conclusion was robust."
Although the link seems clear, there has been debate regarding the reasons for this interaction. Some researchers have theorized that a frequently used Parkinson's drug called levodopa might be the reason for the increase in melanoma incidence in people with the neurodegenerative condition.
In fact, the first research to point the finger at levodopa was an article called "Multiple primary melanoma following administration of levodopa," which was published in the journal Archives of Pathology in June 1972.
Recently, researchers from the Mayo Clinic set out to investigate the relationship between Parkinson's disease and melanoma in more detail. Firstly, they wanted to understand whether or not levodopa was, in fact, the pivotal factor. They also wanted to work out whether the interaction could go both ways.
Probing Parkinson's and melanoma links
The team took data from the Rochester Epidemiology Project medical records database. They collated all neurologist-confirmed Parkinson's disease cases in Olmsted County, MN, from January 1976 to December 2013. Within this 974 subject-strong group, they checked the prevalence of melanoma and compared it with a control group of 2,922 people without Parkinson's.
Next, the team identified 1,544 melanoma cases over the same period of time to determine the subjects' 35-year risk of developing Parkinson's, and they compared that with 1,544 people without melanoma.
The analysis showed that individuals with Parkinson's disease were four times more likely to have a history of melanoma. Additionally, people with melanoma had four times the risk of developing Parkinson's.
The study not only reconfirms the links between the conditions, but it also argues against levodopa being the cause, as do some of the other more recent studies to investigate this phenomenon.
However, aside from being able to discount Parkinson's drugs as the cause, the reasons behind the relationship are no clearer. The authors believe that the causal factors could potentially include shared genetic, environmental, or immune system irregularities. More work is needed.
First author Dr. Lauren Dalvin, Mayo Foundation Scholar in ocular oncology, says, "Future research should focus on identifying common genes, immune responses, and environmental exposures that may link these two diseases."
"If we can pinpoint the cause of the association between Parkinson's disease and melanoma, we will be better able to counsel patients and families about their risk of developing one disease in the setting of the other."
So, regardless of the causal factors, the take-home message centers on awareness. According to the authors, a patient with either condition should be closely monitored for the other. Further research will be needed to ultimately tease out the factors involved in this unexpected and intriguing link.