Myositis with endomysial cell invasion indicates inclusion body myositis even if other criteria are not fulfilled.

The objective of this study was to investigate if patients with endomysial mononuclear cell infiltrates invading non-necrotic fibers have a disease course consistent with inclusion body myositis (IBM), irrespective of other histopathological and clinical characteristics. All patients with a muscle biopsy showing endomysial inflammation with invasion of non-necrotic muscle fibers during the period 1979-2006 in two tertiary neuromuscular referral centers were classified into three groups: 1) patients whose biopsies also showed rimmed vacuoles; 2) patients whose biopsies showed no vacuoles but fulfilled clinical criteria for IBM, and 3) patients whose biopsies showed no vacuoles, and also did not fulfill clinical criteria for IBM (unclassified patients). These groups were compared with regard to age, gender, clinical features, and disease course including response to immunosuppressive treatment. Eighty-one individuals (41 men) were included. Rimmed vacuoles were found in 49 patients (60.5%). Fourteen patients (17.3%) fulfilled clinical criteria for IBM and 18 patients (22.2%) were unclassified at presentation. At follow up (mean duration 9 years) three women remained unclassified (4%). There were no differences in disease course or effect of treatment between the three groups. Men had more often rimmed vacuoles than women (73% vs 48%; p = 0.018), and women more often than men were unclassified. Women tended to show more often temporary improvement if treated (p = 0.07), but none had sustained improvement. In conclusion, patients with a muscle biopsy showing endomysial cell infiltration with invasion of non-necrotic muscle fibers most probably have IBM, regardless of clinical and other pathological features. Women lack typical features more often than men.