Cause	Weakness	Age of onset/diagnosis	Systemic symptoms and findings	Laboratory abnormalities	Creatine kinase	Electromyogram	Muscle biopsy
Drugs
Alcohol	Proximal (may be distal)	Variable	Change in mental status; telangiectasia; peripheral neuropathy	Elevated transaminase and GGT levels; anemia; decreased vitamin B12	Normal to elevated	Normal	Myopathic changes*; selected atrophy of type II muscle fibers
Endocrine
Adrenal insufficiency	Generalized	Variable	Hypotension; hypoglycemia; bronzing of the skin	Hyponatremia; hyperkalemia; ACTH assay; ACTH stimulation test	Normal	Myotonic discharges†	Diminished glycogen content
Glucocorticoid excess	Proximal	Variable	Buffalo hump; striae; osteoporosis	Elevated urine-free cortisol, dexamethasone suppression, or corticotropin-releasing hormone stimulation tests	Normal	Myopathic MUAPs‡	Selective atrophy of type II muscle fibers
Parathyroid hormone (secondary hyperparathyroidism§)	Proximal, lower extremity more than upper extremity	Variable, older adult	Usually has associated comorbidities (cardiovascular disease, diabetes)	Hypocalcemia; uremia	Normal	Myopathic MUAPs‡	Atrophy of type II muscle fibers; increased lipofuscin beneath cell membrane; calcium deposits in muscle
Thyroid hormone (hyperthyroidism)	Proximal, bulbar	40 to 49 years	Weight loss; tachycardia; increased perspiration; tremor	Elevated T4 and T3; TSH variable, depending on cause	Normal or elevated	Myopathic MUAPs‡ with or without fibrillation potentials∥	Usually normal
Thyroid hormone (hypothyroidism)	Proximal	30 to 49 years	Menorrhagia; bradycardia; goiter; delayed relaxation of deep tendon reflexes	TSH	Elevated	With or without myopathic MUAPs‡ and fibrillation potentials∥	Myopathic changes*; glycogen accumulation
Inflammatory
Dermatomyositis	Proximal	Variable, increased incidence with age	Gottron papules; heliotrope rash; calcinosis; interstitial lung disease; disordered GI motility	Elevated myoglobin; ANA positive; myositis autoantibodies may be present	Greater than 10 times normal elevations	Myopathic MUAPs‡ with fibrillation potentials∥	Inflammatory infiltrate with myopathic changes* and replacement by adipose and collagen
Inclusion body myositis	Distal, especially forearm and hand	At least 50 years (younger than 50 years: rare)	Dysphagia; extramuscular involvement not as common	Elevated myoglobin; positive ANA less common; myositis autoantibodies may be present	Elevated	Myopathic MUAPs‡ with fibrillation potentials∥	Inflammatory infiltrate with vacuoles containing eosinophilic inclusions
Polymyositis	Proximal	Variable, increased incidence with age	Interstitial lung disease, disordered GI motility; overlap with rheumatologic diseases more common	Elevated myoglobin; ANA positive; myositis autoantibodies may be present	Greater than 10 times normal elevations	Myopathic MUAPs‡ with fibrillation potentials∥	Inflammatory infiltrate with myopathic changes* and replacement by adipose and collagen
Rheumatologic
Rheumatoid arthritis	Focal, periarticular, or diffuse	Adult	Symmetric joint inflammation (especially MCP, PIP joints); dry eyes and mouth	Elevated rheumatoid factor	Normal or elevated	No data	Atrophy of type II muscle fibers; may have overlap syndrome with polymyositis
Systemic lupus erythematosus	Proximal	Adult	Malar rash; nephritis; arthritis	ANA, anti-DNA antibodies, depressed C3 and C4	Normal to elevated	No data	Type II fiber atrophy; lymphocytic vasculitis; myositis
Genetic
Becker muscular dystrophy	Hip; proximal leg and arm	Late childhood to adulthood	Mental retardation; cardiomyopathy	None	Elevated	Myopathic MUAPs‡ with fibrillation potentials∥	Myopathic changes*; decreased and patchy staining of dystrophin
Limb-girdle muscular dystrophies**	Variable, usually proximal limb, pelvic, and shoulder girdle muscles	Variable	Variable, may have cardiac abnormalities	None	Variable, normal, or elevated	Myopathic MUAPs‡ +/– fibrillation potentials∥	Myopathic changes*; may demonstrate absence of specific protein on immunohistochemical staining
Myotonic dystrophy type 1	Distal greater than proximal; foot drop; temporal and masseter wasting	Adolescence to adulthood	Conduction abnormalities; mental retardation; cataracts; insulin resistance	None	Normal to minimally elevated	Myopathic MUAPs‡; myotonic discharges∥	Less necrosis and remodeling than in muscular dystrophies; atrophy of type I muscle fibers; ring fibers
Metabolic
Glycogen and lipid storage diseases; mitochondrial dis ease	Proximal	Variable	Variable; exercise intolerance and cardiomyopathy more common	Some glycogenoses associated with abnormal FIET††	Variable, may increase with exercise	Normal or myopathic MUAPs‡ +/– fibrillation potentials∥	Myopathic changes* with glycogen deposits, lipid deposits, or ragged red fibers (for glycogen, lipid, or mitochondrial disease, respectively)