No need for non-necrotic fiber inflammatory invasion for diagnosis of polymyositis
Posted Aug 25 2008 6:42pm
In the February 5, 2008 issue of Neurology (“The Green Journal”, link below), Nizar Chahin and Andrew Engel of the Mayo Clinic reported on a study of the correlation between muscle biopsy results, clinical course, and outcome in polymyositis (PM) and sporadic inclusion body myositis (IBM). The authors looked at the records of 107 patients with biopsy-diagnosed PM or IBM. The most common diagnosis in this group was IBM (64 patients). The remainder (43 patients) had PM on biopsy, but 16 of those had a clinical presentation that would suggest IBM (this last subgroup are designated as PM/IBM patients).
Non-necrotic fiber invasion by mononuclear cells appeared in all IBM cases, 17 of 27 PM, and 13 of 16 PM/IBM. So only 30 of 43 patients with a pathologic diagnosis of PM actually had non-necrotic fiber invasion by mononuclear cells. That begs the question of what histologic criteria were used to diagnose the other 13 patients with PM when mononuclear fiber invasion was absent?
Well, I emailed this question to one of the authors, Andrew Engel, who was kind enough to promptly reply. He writes that the pathologic criteria for PM used in this study was the presence of an inflammatory exudate present in endomysium, and the absence of features of dermatomyositis, necrotizing vasculitis, muscular dystrophy, or overlap syndromes with myositis but without endomysial inflammation. “In other words,” he writes, “we do not consider invasion of nonnecrotic fibers by cytotoxic T cells and macrophages a necessary criterion for the diagnosis of PM.”
The authors conclude that “although the muscle biopsy is not infallible for the diagnosis of IBM, it remains important for excluding other possible diagnoses, such as distal muscular dystrophies, myofibrillar myopathies, and neurogenic disorders.”