Focal myositis: New insights on diagnosis and pathology.

OBJECTIVE: To better define in a cohort study the clinical and pathologic features of focal myositis (FM).

METHODS: With the use of the usual clinicopathologic definition, each confirmed case of FM in the Lyon University Hospital's myopathologic database between 2000 and 2016 was retrieved. Clinical, pathologic, imaging, serologic, and therapeutic data were collected. When data were missing but feasible, appropriate pathologic analyses were performed.

RESULTS: Of the 924 patients included in the database, 37 (4%) had confirmed FM (14 female, 23 male patients). The main symptoms were pain (n = 30, 81%), muscular mass (n = 16, 43%), erythema at the site of FM (n = 12, 32%), and fever (n = 9, 24%). Serum creatine kinase was normal in most patients (81%); serum immune abnormalities were frequent (inflammatory syndrome in sera [39%], dysglobulinemia [52%], and anti-nuclear antibody positivity [29%]). In addition to confirming previously reported findings, pathologic analyses found significant rates of vasculitis (68%) and fasciitis (73%). Here, FM appeared frequently to be associated with other diseases such as immune-mediated inflammatory disease (IMID; 32%), neoplasia (24%), and radiculopathy (11%). Regarding outcomes, 64% of the cases had received immunosuppressive drugs, and the relapse rate was 41%.

CONCLUSION: The present study suggests that FM is not as innocuous as previously believed, particularly considering the associated disorders. Notably, patients with FM should receive careful IMID and neoplasia screening.