are exquisitely tender to palpation Myoglobinuria (positive urinary dipstick for heme, in the absence of red blood cells) and rhabdomyolysis (markedly elevated serum CK) sometimes complicate viral myositis Viral myositis resolves with rest, adequate hydration, and analgesics Rhabdomyolysis and myoglobinuria may also occur from excessive physical exertion, prolonged seizures, toxic exposures, and envenomations Along with weakness and muscle tenderness, patients present with the characteristic dark or tea-colored urine of myoglobinuria, which may lead to renal insufficiency or failure Uremia itself may also lead to muscle weakness TABLE 82.4 COMMON CAUSES OF ACUTE WEAKNESS IN CHILDREN Viral myositis Guillain–Barré syndrome Medications/toxins Tumors Seizures Trichinosis is the most common parasitic disease of skeletal muscle It occurs when Trichinella spiralis is ingested in inadequately cooked meat (usually pork) Most patients are asymptomatic, but some develop constitutional symptoms (fever, headache, abdominal pain, and diarrhea) along with myalgias and generalized weakness Cysticercosis and toxoplasmosis may have similar presentations Myopathies may also be caused by various metabolic abnormalities Hyponatremia, hypocalcemia, and hypochloremia are frequently associated with weakness High, low, or normal potassium levels have been described with distinct familial syndromes that present with weakness These autosomaldominant periodic paralysis syndromes may present from infancy to adolescence Attacks of weakness may be precipitated by rest, shortly after exercise Mild attacks last for less than an hour, whereas severe attacks can cause flaccid paralysis for many hours During attacks, the serum potassium is high or low, and electrocardiogram changes sometimes occur Inborn errors of metabolism, such as defects in glycogen metabolism (acid maltase deficiency, or Pompe disease), and disorders of lipid or mitochondrial metabolism may also cause weakness Hyperthyroidism/hypothyroidism, hyperparathyroidism/hypoparathyroidism, and hyperadrenalism/hypoadrenalism