How is polymyositis primarily diagnosed?

Polymyositis is primarily diagnosed through a muscle biopsy, which allows for the examination of muscle tissue under a microscope. This method is crucial because it can reveal the presence of specific inflammatory changes and damage characteristic of polymyositis.

A muscle biopsy typically shows variations in muscle fiber size, inflammatory cell infiltration, and muscle fiber necrosis, providing definitive evidence for the diagnosis. While blood tests can support the diagnosis by identifying elevated muscle enzymes and specific autoantibodies, they cannot confirm the diagnosis on their own.

Magnetic resonance imaging (MRI) can aid in visualizing muscle inflammation and damage, and electromyography (EMG) can assess the electrical activity of muscles, helping to rule out other neuromuscular disorders. However, neither MRI nor EMG provides a definitive diagnosis as effectively as a muscle biopsy does. Thus, the muscle biopsy remains the gold standard for diagnosing polymyositis.