The thorax is commonly affected in polymyositis or dermatomyositis generally in one or more of three forms:

1. hypo-ventilation and respiratory failure as a result of involvement of respiratory muscles
2. interstitial pneumonitis, usually with a histologic pattern of UIP or NSIP
3. aspiration pneumonia secondary to pharyngeal muscle weakness (probably the most common pulmonary complication)

The case demonstrates interstitial pneumonia in the setting of polymyositis (path proven by deltoid muscle biopsy). The pattern is not very specific but we favor fibrotic NSIP over UIP in the absence of definitive honeycombing. Nonetheless, another pattern of interstitial pneumonia has been described in the setting of polymyositis-dermatomyositis including COP and DAD.

Clinically, interstitial lung disease in polymyositis usually presents in a manner similar to acute or subacute community-acquired pneumonia. Chest radiographs and CT scans typically demonstrate bilateral irregular linear opacities involving the lung bases. Jo-1 antibody is present in about 38% of patients. There is no significant survival difference between Jo-1 positive or negative groups. Histologically, polymyositis related UIP, NSIP, DAD and organizing pneumonia is similar to their idiopathic counterparts. however, most cases of polymyositis related interstitial lung disease are also associated with mild to moderate pleural fibrosis.

Pulmonary vasculitis and pulmonary hypertension have been reported rarely in patients with dermatomyositis-polymyositis.