Clinicopathological Features of Mixed Connective Tissue Disease-Related Myositis: A Case Series

Introduction: Mixed connective tissue disease (MCTD) patients often have myositis, however, myopathological and clinical data for MCTD are limited. Recent reports have shown that the pathology of MCTD myositis resembles that of immune-mediated necrotizing myopathy (IMNM), whereas earlier reports described perifascicular atrophy or inflammatory infiltrates predominantly in the perivascular region in MCTD myositis. We aim to describe the clinical and myopathological features of MCTD myositis. Methods: We analyzed the clinical and myopathological findings of nine myositis patients with U1-RNP antibodies who fulfilled the diagnostic criteria for MCTD. Results: Eight patients had muscle weakness in the proximal extremities, and overall, six patients had atypical weakness in the face, neck, wrist, or fingers. Four of those patients required additional intensive treatment (intravenous immunoglobulin or methylprednisolone). Therapeutic responses were consistently favorable overall, and there were no deaths during the observation period. In biopsied muscle specimens, common findings were mild myogenic change, increased necrotic and regenerating fibers, and inflammatory infiltrates predominating in the perivascular region. Two specimens were classified into the spectrum of dermatomyositis (DM); the remaining seven specimens, which had a smaller number of necrotic fibers and nonspecific infiltration, were unclassifiable. Discussion: Patients with MCTD myositis often exhibit an axial or atypical distribution of muscle weakness, which may require intensive therapy. Histological study demonstrates the heterogeneity of myopathology of MCTD myositis and suggests that DM and underlying vasculopathy might be present in these patients.