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).
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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.
