Correlation of muscle biopsy, clinical course, and outcome in PM and sporadic IBM

Objective: To correlate muscle biopsy findings with prebiopsy and postbiopsy clinical course and response to therapy in polymyositis (PM) and sporadic inclusion body myositis (IBM). Background: Existence of pure PM has recently been questioned; subsequently, the definition and criteria for diagnosing PM were debated. Methods: Patient records, follow-up information, and muscle biopsies were analyzed in 107 patients whose biopsies were initially read as PM and IBM. Results: The patients fell into three groups by combined biopsy and clinical criteria: PM, 27 patients; IBM, 64 patients; PM/IBM, 16 patients with biopsy diagnosis of PM but clinical features of IBM. For the three groups, the respective mean periods from disease onset to end of follow-up were 5.9, 8.5, and 9.6 years. Another autoimmune disease was present in 4 of 27 PM, 8 of 64 IBM, and 1 of 16 PM/IBM cases. An autoimmune serologic marker occurred in one-third of each group. Nineteen PM patients had no associated autoimmune disease or marker. Nonnecrotic fiber invasion by mononuclear cells appeared in all IBM, 17 of 27 PM, and 13 of 16 PM/IBM patients. The density of both invaded fibers and cytochrome- c oxidase–negative fibers was higher in IBM and PM/IBM than in PM. Immunotherapy improved 22 of 27 PM patients but had only transient beneficial effects in 2 of 32 IBM and 1 of 14 PM/IBM patients. Conclusions: 1) Sixteen of 43 patients (37%) with biopsy features of polymyositis (PM) had clinical features of inclusion body myositis (IBM). 2) Absence of canonical biopsy features of IBM from clinically affected muscles of IBM patients challenges biopsy criteria for IBM, or the IBM markers appear late in some patients, or their distribution in muscle is patchy and restricted compared with that of the inflammatory exudate. 3) The muscle biopsy is a reliable instrument in the diagnosis of PM and IBM in close to 85% of the patients. Errors of diagnosis in the remaining 15% can be avoided or reduced by combined evaluation of the clinical and pathologic findings.