Major muscle biopsy findings
| Pathological feature | Dermatomyositis | Polymyositis | Inclusion body myositis |
|---|---|---|---|
| Particularly important distinguishing features are emphasised either in bold or with ++. | |||
| Inflammatory infiltrates | Perivascular | Endomysial | Endomysial |
| T cells > B cells | − | + | + |
| B cells > T cells | + | − | – |
| Partial invasion of fibres | − | ++ | ++ |
| Microinfarcts | ++ | − | − |
| Scattered necrotic fibres | − | + | + |
| Perifascicular atrophy | ++ | − | − |
| Zonal myofibrillar loss | + | − | − |
| Rimmed vacuoles | − | − | ++ |
| 1° capillary loss | + | − | − |
| 15 nm filaments | − | − | ++ |
| MHC-1 expression | + | ++ | ++ |