Differential diagnosis of inflammatory myopathies
Which conditions should be considered in the differential diagnosis of inflammatory myopathies
It is important to not miss treatable causes of muscle weakness that can be confused with an IIM. Therefore, all patients need a thyroid-stimulating hormone, 25-hydroxyvitamin D, potassium, calcium, magnesium, and screen for drugs and infections that affect muscles. Patients are more likely to have a cause of weakness other than IIM if they have a slow onset (i.e., years) or periodic episodes of weakness, severe muscle pain with cramps associated with exercise, facial or ocular/early-onset respiratory/distal weakness, early muscle atrophy, neurologic symptoms, family history of muscle disease, no rashes, no abnormal serologies, and/or a nondiagnostic EMG. Additionally, an elevated CPK (<1000 U/L) with a normal aldolase in an asymptomatic African-American individual is usually a normal variant.
Differential Diagnosis of Myopathies
Drug- and toxin-induced myopathies (especially statins, interferon-α, d-penicillamine, colchicine, amiodarone, antimalarials, ZDV/AZT (zidovudine), alcohol, glucocorticoids, cocaine, antifungals, anti-tumor necrosis factor inhibitors)
| Neuromuscular disorders | Infectious myositis |
| Muscular dystrophies (e.g., Duchenne’s) Neuromuscular junction disorders (e.g., myasthenia gravis, Eaton–Lambert syndrome) Denervating conditions (e.g., amyotrophic lateral sclerosis) | Bacterial ( Staphylococcus, Streptococcus, Borrelia burgdorferi ) Viral (e.g., HIV, adenovirus, influenza) Parasitic (e.g., Toxoplasma, Trichinella, Taenia ) |
| Endocrine disorders Hypothyroidism (may see CK as high as 3000) Hyperthyroidism Acromegaly Cushing’s disease Addison’s disease Miscellaneous Sarcoidosis Other rheumatic disorders (e.g., polymyalgia rheumatica, fibromyalgia syndrome, inflammatory arthritides, vasculitis) Carcinomatous neuromyopathy Organ failure (uremia, liver failure) Amyloidosis Acute rhabdomyolysis | Metabolic myopathies Glycogen storage diseases (e.g., McArdle’s or myophosphorylase deficiency, acid maltase deficiency) Abnormalities of lipid metabolism (e.g., carnitine deficiency, carnitine palmitoyl transferase deficiency) Mitochondrial myopathies Nutritional disorders (malabsorption, vitamin D and E deficiencies) Electrolyte disorders (hypocalcemia and hypercalcemia, hypokalemia, hypophosphatemia) |
