Background/Purpose: Seasonal patterns of disease onset and severity in idiopathic inflammatory myopathies (IIMs) as a whole are conflicting. In recent years, over 10 myositis-specific antibodies (MSAs) have been identified. They are able to divide patients into homogenous subgroups and inform on prognosis. We conducted a large multi-centred study of a homogeneous disease group, based on serology and ethnicity, aiming to investigate the seasonal variation of IIMs.

Methods: This was a retrospective observational study. Consecutive Chinese patients with IIMs admitted to the rheumatology wards of the participating major regional hospitals in Beijing and Hong Kong from July 2013 to June 2018 were recruited. The diagnosis of IIMs was based on the Bohan and Peter’s criteria with definite or probable cases being included. Patients with clinically amyopathic disease must have the typical Gottron’s papules or heliotrope rash as determined by rheumatologists or dermatologists, and with no symptoms or signs of muscle involvement according to Sontheimer. Patients with juvenile myositis, inclusion body myositis, cancer-associated myositis and myositis associated with a previously diagnosed connective tissue disease were excluded. A commercial line blot immunoassay kit (EUROIMMUN) was used to detect the MSAs. The primary objective of the study was to investigate the seasonal variation of onset of IIMs characterised serologically. The secondary objective was to examine the seasonal patterns of the  various clinical manifestations.

Results: All together 416 patients were studied. The mean age of the patients at disease onset was 48.1 years (S.D. 13.3). There was a female predominance (68.5%). The subgroups of IIMs were: dermatomyositis (63.9%), polymyositis (22.4%), clinically amyopathic dermatomyositis (10.1%), immune mediated necrotising myopathy (3.1%) and nonspecific myositis (0.5%). No particular seasonal pattern in disease onset was observed in IIM patients as a whole or in any classical subgroups. However, significantly more patients with any one MSA had their disease started in the first half of the year (p=0.013). The same pattern was also seen in the hospitalisation rate in patients with either anti-synthetase or anti-MDA5 antibodies (p=0.029). It was also found that rapidly progressive interstitial lung disease (RPILD) which was a well known severe complication associated with the anti-MDA5 antibody occurred more in the first half of the year (p=0.044). At the same time, most of the mortalities in our patients happened in the first quarter of the year (January to March) (p=0.013). Further analyses showed that infection was significantly associated with anti-MDA5 antibody, RPILD and mortality (p< 0.001).

Conclusion: Apparent seasonal patterns were noticed in our ethno-serologically defined IIM patients. Certain environmental factors, particularly infection, could be potential triggers. Our findings could shed light on the identification of etiologic factors and enhance our understanding of disease pathogenesis. They could also increase our awareness of the disease and its complications during the peak seasons.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/seasonal-variation-in-idiopathic-inflammatory-myopathies-incidence-and-presentation-a-retrospective-study-in-beijing-and-hong-kong/