Tetany (Guyton and Hall, 4th South Asia Edition)

• Calcium ions normally stabilize neuronal membranes by reducing their permeability to sodium.
• When extracellular fluid calcium concentration falls below normal, neuronal membrane permeability to Na⁺ increases, making it much easier to generate an action potential.
• When plasma calcium falls to about 50% below normal, peripheral nerve fibers become so excitable that they discharge spontaneously, sending trains of impulses to skeletal muscle and producing tetanic contraction.
• Hypocalcemia can also increase excitability of the brain, occasionally precipitating seizures.

• Normal plasma calcium: ~9.4 mg/dL.
• Tetany develops when calcium falls to about 6 mg/dL (only ~35% below normal).
• Levels around 4 mg/dL are usually lethal (tetanic spasm of respiratory muscles causing death).

• The earliest and most characteristic sign is tetany of the hand, called carpopedal spasm - flexion at the wrist and metacarpophalangeal joints with extension of the fingers - which typically appears before tetany develops elsewhere in the body.
• In children, severe hypocalcemia (blood calcium below 7 mg/dL) can cause fatal tetanic respiratory spasm unless calcium is administered promptly.

Figure 80.2 - Hypocalcemic tetany in the hand (carpopedal spasm) - Guyton and Hall Textbook of Medical Physiology, p. 3260 (block ref)

• Common causes: hypoparathyroidism, vitamin D deficiency (rickets/osteomalacia), acute pancreatitis, respiratory alkalosis (hyperventilation lowers ionized calcium).
• Treatment of acute tetany: IV calcium gluconate to rapidly raise ionized calcium and relieve neuromuscular excitability.
• Guyton and Hall Textbook of Medical Physiology, "Nonbone Physiological Effects of Altered Calcium and Phosphate Concentrations in the Body Fluids," p. 3258-3262
• Guyton and Hall Textbook of Medical Physiology, "Rickets Caused by Vitamin D Deficiency," p. 3782-3787