G.P.67: Myositis with invasion of endomysial cell infiltrate but without rimmed vacuoles: Separate phenotype or variant of sporadic inclusion body myositis?

Abstract

To investigate the hypothesis that patients with inclusion body myositis (IBM) are underdiagnosed if strict histopathological and/or clinical criteria are applied. Muscle biopsies and clinical data of all adult patients diagnosed with an inflammatory myopathy during the years 1979–2006 in two tertiary neuromuscular referral centers were re-evaluated. Patients with endomysial inflammation with invasion of healthy looking muscle fibres were divided into three groups: (1) patients whose biopsies showed rimmed vacuoles; (2) patients whose biopsies showed no vacuoles but fulfilled clinical criteria for IBM, and (3) patients with no vacuoles, and also did not fulfill clinical criteria (unclassified). These groups were compared as regards gender, disease course including response to immunosuppressive treatment and clinical features at follow up. There were 81 patients (40 women). Rimmed vacuoles were found in 49 patients (60%; significantly more often in men (61%) than in women (39%; p =0.018), 14 (17%) fulfilled clinical criteria for IBM and 18 (22%, 78% women) were unclassified. At follow up (mean duration 9years, SD 5years) three women remained unclassified (4 %) and 96 % were classified as IBM. All patients showed stable or progressive disease and none experienced sustained improvement. There were no differences in disease course or effect of treatment between the three groups. At onset there were more unclassified women ( p =0,018), but not at follow up because then they fulfilled clinical criteria for IBM. Women more often had a concomitant autoimmune disease ( p =0,012). Men and women did not differ with respect to age at presentation, time to biopsy, or duration of follow up. (1) Endomysial mononuclear cell infiltrates with invasion of non-necrotic fibres indicates a diagnosis of IBM even if clinical criteria are not fulfilled. (2) There are gender differences at onset, and especially women with IBM are at risk for underdiagnosis.