Heavy methamphetamine users have a higher than normal risk of developing Parkinson’s disease (PD), a new study suggests.

Compared with patients hospitalized for another medical diagnosis, appendicitis, those hospitalized for methamphetamine abuse or dependence had a greater than 2-fold increase in PD risk during follow-up.

“We strongly emphasize the preliminary nature of the findings,” lead study author Russell C. Callaghan, PhD, from the Centre for Addiction and Mental Health, Toronto, Ontario, Canada, and colleagues note in their report. Nevertheless, they say, the data, “provide some evidence that exposure to methamphetamine of a severity sufficient to contribute to hospital admission might be associated with increased risk of Parkinson’s disease.”

The study findings were published online August 24 in the journal Movement Disorders.

Public Health Concern

Worldwide, it is estimated that 16 million people use methamphetamine, the study authors note. Findings in animals that exposure to the illicit drug can damage brain dopamine neurons “raise the public health concern” that methamphetamine might also damage dopamine neurons in humans, leading to parkinsonism, they write.

To investigate, Dr. Callaghan and colleagues identified 1863 methamphetamine users, 50 years or older, who were hospitalized in California for a methamphetamine-related condition from July 1, 1990, to June 30, 2000, and were followed up for up to 10 years after discharge.

Roughly 87% of these individuals received a single International Classification of Diseases, Ninth Revision (ICD-9) diagnostic code for either methamphetamine abuse or methamphetamine dependence. The methamphetamine users were matched on demographic variables and follow-up time to 9315 patients hospitalized for primary appendicitis conditions.

In Cox regression analysis, compared with the appendicitis group, the methamphetamine group was at significantly greater risk of being hospitalized with PD during follow-up. The adjusted hazard ratio was 2.65 (95% confidence interval, 1.17 – 5.98, P = .019).

Reached for comment, Tomas R. Guilarte, PhD, who was not involved in the study, said that “it’s important to look at this issue because methamphetamine targets the dopaminergic presynaptic neurons and there is clearly injury to these neurons.

“It is certainly possible that this could modify progression of a disease in a person that has a propensity to develop Parkinson’s disease,” added Dr. Guilarte, who is a professor of neurotoxicology and molecular imaging in the Department of Environmental Health Sciences at Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland.

Limitations, Caveats

In their article, Dr. Callaghan and colleagues point to several limitations of the study, chief among them the small sample size limited to hospital admission cases and the uncertainty of whether the hospital diagnosis of PD was confirmed by a neurologist. They also did not have neuropathological data to confirm the diagnosis of PD, and ICD-9 codes do not distinguish between methamphetamine and other amphetamines.

On the other hand, for a variety of reasons, the current study, Dr. Callaghan and colleagues say, could have underestimated the actual association between methamphetamine use and incident PD.

For instance, methamphetamine users may have been more likely to die during follow-up and less likely to have health insurance, resulting in less access to medical care, compared with the appendicitis group — 2 factors that may have attenuated the incidence rate of PD among methamphetamine users.

In addition, studies have consistently shown that tobacco smoking is associated with a lower incidence of PD. Given that methamphetamine users typically have higher rates of tobacco use relative to the general population, it’s possible that the link between methamphetamine use and PD is stronger than that observed in the current study, Dr. Callaghan and colleagues note.

More Study Warranted

Dr. Guilarte told Medscape Medical News, “This is a good start. It is not a neuropathologic study, but it certainly suggests there may be a connection” between methamphetamine abuse and parkinsonism.

Dr. Guilarte thinks it would be important to look at the incidence of movement abnormalities in younger methamphetamine abusers.

“In this study, they started looking at people at age 50 or above, when idiopathic Parkinson’s typically presents. Typically, methamphetamine abuse starts in the teen years or early 20s, and it would be interesting to see whether you can express movement abnormalities before 50 years of age. If so, then it might suggest that methamphetamine use is a risk factor,” he explained.

The study was supported by grants from the Ontario Ministry of Health and Long-Term Care to the Centre for Addiction and Mental Health. Dr. Callaghan has received funding from the Canadian Institutes of Health Research, Public Health Agency of Canada, and Ontario Tobacco Research Unit. A complete list of author disclosures (none of which are drug company related) appears in the original article. Dr. Guilarte has disclosed no relevant financial relationships.

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Megan Brooks
Mov Disord. Published online From Medscape Medical News