DERMATOMYOSITIS AND POLYMYOSITIS
Polymyositis and dermatomyositis are two of the idiopathic inﬂammatory myopathies, a group of rare systemic autoinﬂammatory disorders of unknown cause. They are characterized by proximal muscle weakness (most patients present with the subacute onset of weakness and myalgias), increased serum skeletal muscle enzymes, characteristic electromyography abnormalities, and the presence of inﬂammatory cell inﬁltrates in muscle tissue. Patients with dermatomyositis are deﬁned by the additional presence of an exanthem, most commonly a purple discoloration of the eyelids (heliotrope rash) or a symmetric, palpable, erythematous rash over the extensor surfaces of the metacarpophalangeal and proximal interphalangeal joints of the ﬁngers (Gottron papules). Extramuscular organ involvement is common, particularly the skin, joints, and lungs. Pulmonary complications are a major cause of morbidity and mortality. These can be either primarily associated with the underlying autoinﬂammatory disorder or secondary to the muscle weakness. As with all autoimmune disorders, drug-induced disease and infection should always be an early consideration. Myositis speciﬁc ﬁndings include hypoventilation, aspiration pneumonia, and interstitial lung disease (ILD).
Hypoventilation and respiratory failure as a result of respiratory muscle weakness has been thought of as uncommon but has been reported in up to 20% of patients. There is often an associated reduction in the cough reﬂex with resulting basilar atelectasis and the inability to clear airway secretions. Both occur in patients with severe generalized weakness of the inspiratory and expiratory respiratory muscles. Chest imaging reveals small lung volumes, bibasilar inﬁltrates, and elevation of the diaphragm. Pulmonary physiology demonstrates reduced total lung capacity (TLC) with an increased residual volume, reduced forced vital capacity (FVC), and a preserved forced expiratory volume in 1 second (FEV1) and FEV1/FVC ratio. Maximal inspiratory and expiratory pressures are reduced. Aspiration pneumonia is also described in up to 20% of patients and is more common in patients with extensive muscle and skin disease. It is caused by pharyngeal and upper esophageal dysfunction of striated muscle with a loss of the normal swallowing mechanism and regurgitation. Not surprisingly, half of these patients have symptomatic dysphagia.
ILD has been reported in up to two-thirds of patients, depending on patient selection and the chosen diagnostic methods, and not all patients with identiﬁed abnormalities are symptomatic. Antisynthetase antibodies are found in most patients, and the presence of positive antiaminoacyl tRNA synthetase antibodies, of which the antihistidyl tRNA synthetase antibody (anti-Jo-1) is the most common, is frequently found. Similar to the other autoimmune disorders, the ILD may precede, appear simultaneously, or develop after the onset the muscle disease. Its presentation may be acute, chronic, and progressive or asymptomatic with only chest imaging abnormalities. Cough and dyspnea are the typical presenting symptoms. A restrictive ventilatory impairment with decreased TLC, functional residual capacity, residual volume, FEV1, and FVC and reduced DLCO (diffusing capacity for carbon monoxide) are generally seen. Chest imaging with high-resolution computed tomography (HRCT) is the most sensitive test for the detection of ILD and provides a description of the pattern and extent of the disease. The chest imaging patterns are identical to those found in idiopathic interstitial pneumonias with nonspeciﬁc interstitial pneumonia and organizing pneumonia patterns the most common. The natural history of ILD in myositis is not well understood but is considered to be a major risk factor for premature death. The pulmonary abnormalities associated with antisynthetase antibodies appear to show a clinically relevant response to immunosuppressive therapy.