What changes can a muscle biopsy show in polymyositis?

In the context of polymyositis, a muscle biopsy is a key diagnostic tool used to confirm the condition through microscopic examination of affected tissue.1

A biopsy typically reveals the following pathological changes:

• Infiltration of White Blood Cells: The striated muscle tissue shows an infiltration of white blood cells, primarily lymphocytes. These cells, specifically T cells, are believed to attack the muscle tissue as part of an autoimmune response.23
• Distribution of Infiltrates: These inflammatory cells collect between the muscle fibres and around small blood vessels.2
• Muscle Fibre Damage: The presence of these cells is associated with visible damage to the muscle fibres. As the disease progresses, the affected muscles undergo atrophy and hardening.23

While vascular damage is also a feature of inflammatory myopathies, it is particularly noted in the childhood form of dermatomyositis. The exact trigger for this autoimmune reaction remains unknown.23