Respiratory failure in polymyositis and dermatomyositis: differential diagnosis between pulmonary infection and interstitial pneumonitis

Pulmonary diseases are important cause of morbidity and mortality in patients with PM/DM. Thirteen (27%) out of 49 PM/DM patients in the study had developed respiratory failure. Respiratory failure resulted form interstitial pneumonitis (i.p.) in 6, pulmonary infection in 2 and both in 5 patients with PM/DM. Respiratory failure was fatal in PM/DM patients with pulmonary diseases and eleven of the 13 patients expired. More importantly, 2 PM/DM patients with respiratory failure had responded to chemotherapy, if it was due to pulmonary infection. Accordingly, it is almost important to distinguish i.p. and infection for the cause of respiratory failure. However, plain chest X-ray as well as standard laboratory tests failed to differentiate i.p. and pulmonary infection. On the other hand, high resolution CT of the lungs, serum endotoxin and serum beta-D-glucan were found to be useful for the differentiation of these conditions associated with respiratory failure in PM/DM patients. And additionally low serum level of IgG and lymphopenia at the onset of respiratory failure may suggest that the patients may have pulmonary infection rather than i.p.