WEAKNESS, like fever or pain, accompanies so many conditions that the scope of its differential diagnosis is impossibly large. But when the answers to a few questions narrow the field to a complaint of muscle weakness, the differential shrinks (Table). In the illnesses called idiopathic inflammatory myositis—dermatomyositis, polymyositis, inclusion body myositis, and several rarer disorders—the patients are usually middleaged and have noticed the onset of weakness over weeks, months, or even years. The weakness most often affects proximal muscles so that climbing stairs, getting into or out of a car, lifting packages, combing the hair, or lifting the head off the pillow may be difficult. Muscle damage is shown by elevated serum levels of the muscle-associated enzymes, creatine kinase and aldolase; there are abnormalities on the electromyogram (EMG); and the muscle biopsy in most cases shows dying and regenerating muscle cells and an infiltrate of chronic inflammatory cells, predominantly lymphocytes.
Plotz PH. Not MyositisA Series of Chance Encounters. JAMA. 1992;268(15):2074-2077. doi:10.1001/jama.1992.03490150126037