Idiopathic inflammatory myopathies, such as polymyositis (PM), may present with general symptoms such as fever and fatigue and only minimal muscle weakness, making it difficult to make a definite diagnosis and provide adequate treatment. Here a case is described in which interstitial lung disease was the first and most prominent manifestation of PM. Later, when muscle weakness became apparent and inflammatory muscle disease was suspected the first muscle biopsy was non-diagnostic. However, magnetic resonance imaging (MRI) scans of the clinically weak thigh muscles showed high signal on T(2) weighted images, suggesting muscle inflammation more proximal to the first biopsy site. A second biopsy at this site disclosed typical histopathological findings for myositis. After treatment with prednisolone in combination with cyclophosphamide both pulmonary and muscle function improved.
MRI scans of muscles may be helpful in selection of a site for muscle biopsy in patients with suspected inflammatory myopathy when a first muscle biopsy turns out to be negative. Additionally, patients with interstitial lung disease of unknown cause should be tested for muscular function to exclude an associated inflammatory muscle disorder.