Ask about truncal and proximal-muscle weakness.
Ask about medications (e.g., corticosteroids, statins) and excessive alcohol use that might cause a myopathy.
Look for a pattern of bilateral proximal-muscle weakness and note absence of muscle fasciculations, oculomotor and lower facial muscle weakness, sensory symptoms, spastic reflexes, and muscle tenderness.
Examine the patient for the characteristic rashes of dermatomyositis, including the heliotrope of the upper eyelids and periorbital tissues; scaly purple plaques over metacarpal and interphalangeal joints; cracked horizontal lines on palmar and lateral aspects of fingers (mechanic's hands); erythematous, papular eruptions on sun exposed areas.
Obtain CK (or aldolase, ALT, AST) and exclude other causes of CK elevation, such as drugs, alcohol, and hypothyroidism.
Obtain a muscle biopsy as the most definitive test to diagnose polymyositis, a muscular dystrophy, inclusion body myositis, or some other uncommon disease; MRI can help localize the site of muscle biopsy, if needed.
Obtain age- and sex-appropriate cancer screening in all patients; some experts obtain endoscopic and imaging studies to exclude pharyngeal, GI, pulmonary, intra-abdominal, and pelvic and ovarian malignancies.