Polymyositis May Increase Risk of Amyotrophic Lateral Sclerosis

Researchers have found that polymyositis (PM) may increase the risk of developing amyotrophic lateral sclerosis (ALS), independent of age, sex, and comorbid autoimmune diseases. The findings were published in Arthritis Care & Research.

While the mechanism linking the 2 disorders is not well understood, evidence suggests a common pathophysiology, according to study author Chia-Chun Tseng, MD, of the Department of Internal Medicine at Kaohsiung Municipal Ta-Tung Hospital in Kaohsiung, Taiwan and colleagues.

Some of this evidence includes an association between ALS and histocompatibility types and autoantibody production, the immunological alterations patients with ALS exhibit, and an association found between ALS and a higher risk of autoimmune diseases in families.

“Similar immunological responses [are] observed in PM,” the authors noted. “The accumulated data suggests a common autoimmune pathway in both PM and ALS, leading us to believe that PM can be linked to the increased incidence of ALS.”

In the current study, the researchers identified all newly diagnosed cases of PM in Taiwan’s Registry of Catastrophic Illness Database between January 1, 1998 and December 31, 2011 and matched each case to 5 controls.

A total of 1778 patients with PM and 8124 control patients were enrolled in the study. The mean age of men in the PM  and control groups were 51.09 ± 14.86 years and  50.55 ± 14.21 years, respectively, and the mean age of women in the PM and controls groups were was 49.82 ± 13.75 years and  49.26 ± 13.20 years, respectively.

During the follow-up period, 6 cases of ALS were found in the PM group, compared with 1 case in the control group. The incidence rate of ALS in the PM group was 0.65 cases per 1000 person-years (95% CI, 0.24-1.42), compared with 0.02 cases per 1000 person-years in the control group (95% CI, 0.00-0.14).

The association remained significant after adjusting for sex, age, and co-occurring autoimmune diseases (hazard ratio: 25.72; 95% CI, 2.95-224.58, P= .003).

Summary and Clinical Applicability

In the first population-based nationwide retrospective cohort study examining the rate of ALS in patients with PM, researchers found that ALS incidence was higher in those with PM compared with controls. The association remained significant after adjusting for sex, age, and comorbid autoimmune diseases.

“Findings in this study potentially have some important clinical implications… ALS associated with PM might suggest a higher mortality than with either disease entity alone,” the authors noted.

Previous research has also indicated that dyspnea and proximal muscle weakness, both of which are common symptoms found in patients with PM, both predict lower ALS survivability,^2-3 as do dysphagia⁴ and decreased forced vital capacity.⁵

The researchers also noted that while the goal of the study was not to determine explanations for the apparent association between PM and ALS, “common cytokines shared between PM and ALS, including interferon (IFN)-γ, interleukin (IL)-6, and IL-7, may underpin the link.”

Future studies are needed to identify the meaning of, predictors of, and new therapeutic options for PM in patients with ALS. Case-control study designs may allow better prognostic information to be collected. “The relationship between PM and ALS diseases could indicate a common pathophysiology in these 2 diseases,” the authors wrote.

Limitations and Disclosures

• Potential misclassification in the administrative databases can’t be completely ruled out
• The relatively small amount of ALS cases allowed only a gross assessment of risk and resulted in relatively wide confidence intervals
• Confounding factors such as physical activity may have influenced the observed association between PM and ALS
• The researchers used administrative data and therefore had a limited ability to assess how clinical phenotypes or serologic characteristics might affect ALS susceptibility

References 

1. Tseng C, Chang S, Tsai W, et al. Increased incidence of amyotrophic lateral sclerosis in polymyositis: a nationwide cohort study. Arthritis Care Res (Hoboken). 2016 Oct 4. doi:10.1002/acr.23119. [Epub ahead of print]
2. Devine MS, Ballard E, O’Rourke P, Kiernan MC, Mccombe PA, Henderson RD. Targeted assessment of lower motor neuron burden is associated with survival in amyotrophic lateral sclerosis. Amyotroph Lateral Scler Frontotemporal Degener. 2016;17:184-90.
3. Watanabe H, Atsuta N, Nakamura R, Hirakawa A, Watanabe H, Ito M, et al. Factors affecting longitudinal functional decline and survival in amyotrophic lateral sclerosis patients. Amyotroph Lateral Scler Frontotemporal Degener. 2015;16:230-6.
4. Oliver D. The quality of care and symptom control—the effects on the terminal phase of ALS/MND. J Neurol Sci. 1996;139 Suppl:134-6.
5. Kim DG, Hong YH, Shin JY, Lee KW, Park KS, Seong SY, et al. Pattern of respiratory deterioration in sporadic amyotrophic lateral sclerosis according to onset lesion by using respiratory function tests. Exp Neurobiol. 2015;24:351-7.

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