Idiopathic myopathies include polymyositis, dermatomyositis, non-specific myositis, and necrotizing myopathy.1 These are very rare conditions and may be missed if one is unfamiliar with the presenting symptoms. The combined incidence of polymyositis and dermatomyositis is 4 to 10 cases per million population per year.2 The idiopathic inflammatory myopathies share common features, including proximal muscle weakness, elevated muscle enzymes (creatinine phosphokinase, aldolase) and potentially elevated liver functions (aspartate transaminase [AST]/alanine transaminase [ALT]), resulting from damaged muscle and not actual liver damage. The onset is usually insidious and painless and is characterized by progressive symmetric proximal muscle weakness during the course of 3 to 6 months.3 Patients frequently complain of difficulty walking upstairs, getting up and down from a chair, getting in and out of a car, falling, and difficulty raising their arms over their heads to dress, shower, or comb their hair. Muscles of the throat and upper airway may be affected, resulting in dysphagia, nasal regurgitation of fluids, aspiration, and hoarseness.3-5
History gathering should solicit the patient’s abilities or limitations with these activities of daily living (ADLs). The patient may have mild arthralgias. Lungs are the most common extra muscular organ involved in idiopathic inflammatory myopathies.3 Patients may complain of dyspnea, and pulmonary function tests (PFTs) may demonstrate a restrictive pattern with decreased lung volumes and reduced diffusing capacity of the lungs for carbon monoxide (DLCO).3 A high resolution CT of the chest typically demonstrates “ground glass opacities” consistent with alveolitis or honeycombing consistent with fibrosis.3 Given dysphagia, patients may develop aspiration pneumonia. Patients may have cardiac involvement most often in the form of rhythm disturbances.2 More rarely, they may experience conduction abnormalities, pericarditis, myocarditis, cardiac arrest, and congestive heart failure.2,3 All of the preceding are often accompanied by fatigue, fever, and weight loss.
Dermatomyositis presents similarly to polymyositis, as previously described, but also includes characteristic rashes or skin involvement. A heliotrope rash consists of a purplish discoloration, almost resembling eye shadow, above the eyes with associated periorbital edema. A “shawl sign” consists of an erythema around the neck and upper chest. Gottron’s papules, which are hypertrophied calloused-looking plaques, may occur over the metacarpophalangeal joints (MCPs) and proximal interphalangeal joints (PIPs). Mechanics hands refers to skin cracking and fissuring of the fingers and hands. Patients may also have periungual erythema.2,3
Immune-mediated necrotizing myopathies present with symptoms similar to those of other idiopathic inflammatory myopathies, but the muscle biopsy demonstrates necrosis of muscle fibers, as opposed to inflammatory infiltrates found on muscle biopsy of those with polymyositis or dermatomyositis.4,6 Statin exposure has been associated with necrotizing myopathy. It is thought that the statin may induce an autoimmune process that persists despite withdrawal of the statin.2 This is in contrast to a statin-induced myopathy that improves over weeks to months when the statin is discontinued.5 Specific autoantibodies have been noted in those with necrotizing myopathy, including anti-HMGCR antibody and signal recognition particle (SRP).4,6
The etiology of idiopathic inflammatory myopathies is unknown but may involve a genetic/familial component.7 It is thought that environmental triggers (viruses, bacteria, ultraviolet light, physical exertion, psychological stress, medications, and vaccines) may initiate an autoimmune response in those with an underlying genetic susceptibility.7
Accurate diagnosis is a process that involves a comprehensive history, baseline lab work identifying elevated muscle enzymes, and unilateral electromyogram/nerve conduction study (EMG/NCS) assessing for characteristic changes. It is important to recognize that EMG/NCS may cause changesthat interfere with interpretation of muscle tissue when biopsied. Conducting a unilateral EMG/NCS preserves a contralateral muscle biopsy site for study.3 Imaging, MRI in particular, is the most sensitive in detecting muscle edema associated with myositis.2 A muscle biopsy is the final piece of the diagnostic puzzle.
Potential differential diagnoses include metabolic muscle disorders, polymyalgia rheumatica, drug-related myopathy, hypothyroidism, neuromuscular disorders, and paraneoplastic syndrome. Clues to these other etiologies are often apparent in the history. Patients with metabolic muscle disorders (ie, McArdle disease) often complain of muscle fatigue after initiating exercise or activity but have normal strength during an examination.2,8 The possibility of drug-related myopathies has been noted with statins, colchicine, steroids, ipecac, cocaine, alcohol, zidovudine, and D-penicillamine.2