Myalgia with the presence of pathologic EMG correlates with perimysial inflammatory infiltrates

Objective We aimed to define normal numbers of inflammatory cells in muscle biopsies and to identify the predictive value of isolated muscle pain and increased creatine kinase regarding the diagnosis of myositis. Methods We analyzed muscle biopsies of 71 patients using immunostains for CD3 + , CD4 + , CD8 + , CD68 + , major histocompatibility complex class I, perforin, and myeloid-related protein (MRP) 8. Patients were categorized as follows—group 1: myalgia without further clinical or laboratory abnormalities (n = 24); group 2: asymptomatic elevation of creatine kinase (hyperCKemia, n = 26); group 3: myalgia and pathologic EMG findings (n = 9); and group 4: otherwise healthy controls who had malignant hyperthermia susceptibility testing (n = 12). Results In the normal muscle biopsy specimens from group 4, mean endomysial macrophage (CD68 + ) density was 21.7 ± 5.6/mm 2 , and perimysial density was 13.0 ± 5.6/mm 2 . Numbers of T-lymphocytes (CD3 + ) were 5 ± 3.5 endomysially and 2.2 ± 3.9/mm 2 perimysially. This was not different from groups 1 and 2. Only group 3 patients had increased mean numbers of perimysial macrophages (24.1 ± 6.3/mm 2 ; p = 0.0005), CD3 + (7.6 ± 4.9/mm 2 ; p = 0.0056), and CD8 + T-lymphocytes (5.4 ± 3.1/mm 2 ; p = 0.0008) and displayed the activation marker MRP8 in all cases. Although inflammatory cells were increased in the perimysium in group 3, histology did not fulfill the criteria for dermatomyositis, polymyositis, or inclusion body myositis. Conclusions Normal muscle contains a considerable number of macrophages and T-lymphocytes. Muscle biopsy is likely to detect inflammatory changes in patients with myalgia or hyperCKemia only if pathologic EMG findings are present.