General Principles

• Myopathies: Rapidly progressive proximal muscle weakness can be caused by many drugs including but not limited to ethanol, steroids, colchicine, cyclosporine, and cholesterol-lowering drugs (particularly in combination). Other common causes include HIV or HIV therapies, particularly zidovudine, and hypothyroidism.
  • Critical illness myopathy is increasingly recognized in patients with critical illness. Myosin loss myopathy accounts for a percentage of patients with critical illness myopathy and is commonly associated with clinical features of respiratory failure (e.g., difficulties weaning from the ventilator), severe weakness, and classic risk factors including exposure to high-dose corticosteroids and/or neuromuscular blocking agents. The diagnosis of myosin loss myopathy requires pathologic confirmation (i.e., a muscle biopsy must be performed).
  • Polymyositis (PM) and dermatomyositis (DM) fall into a class of diseases now referred to as the idiopathic immune and inflammatory myopathies. Most forms respond well to immunomodulatory therapy with a notable exception being the inclusion body myopathies. DM and PM can also be a component of a syndrome affecting multiple different organ systems. Perhaps the best examples are antisynthetase syndromes (e.g., Jo-1 myositis), which involve skin, joint, lung, and muscle. Patients suspected of having DM or PM should have myositis-specific and myositis-associated autoantibodies checked and should be screened for interstitial lung disease, which has a high degree of morbidity if left untreated (see Chapter 25, Arthritis and Rheumatologic Diseases).
• Rhabdomyolysis may produce rapid muscle weakness, leading to hyperkalemia, myoglobinuria (by definition true rhabdomyolysis causes myoglobinuria), and renal failure (for management, see Chapter 12, Fluid and Electrolyte Management, and Chapter 13, Renal Diseases). The potential etiologies include Metabolic (deficits of lipid or carbohydrate metabolism), Excessive exercise/exertion (including seizures/dystonia), Drugs (abuse and prescribed), Ischemic, Compression/crush (trauma), Infection/Inflammatory, Noxious (toxins), and Electrolyte abnormalities (diabetic ketoacidosis, hyperosmolar hyperglycemic nonketotic syndrome, hypokalemia) (“MEDICINE”).
• Botulism is a disorder of the neuromuscular junction caused by ingestion of an exotoxin produced by Clostridium botulinum, acquired through a wound, or via an iatrogenic route.
  • The exotoxin interferes with release of acetylcholine from presynaptic terminals at the neuromuscular junction.
    • Classically, it is associated with ingestion of raw honey in infants, but inhalation/ingestion of soil-based spores and wound botulism from “skin popping” (i.e., injection of drugs of abuse underneath the skin) are likely more common means of absorption.
  • Symptoms begin within 12–36 hours of ingestion in food-borne botulism and within 10 days in wound botulism.
  • Symptoms include autonomic dysfunction (xerostomia, blurred vision, urinary retention, and constipation), followed by cranial nerve palsies, descending weakness, and possibly respiratory distress.
  • Serum assays for botulinum toxin may aid diagnosis in adults.
  • Management includes removing nonabsorbed toxin with cathartics, supportive care, and neutralizing absorbed toxin with equine trivalent (A, B, E) antitoxin (more immunogenic because it contains both the Fab and Fc portions) or heptavalent (A, B, C, D, E, F, G) antitoxin (less immunogenic as Fc portion is cleaved off and has F(ab)₂ portions).
  • Recovery is slow and occurs spontaneously, but with appropriate ventilatory and supportive care, most make a full recovery.