Hypocalcemia

A post-thyroidectomy patient reports tingling around their mouth and fingertips. The vitals look stable — but neuromuscular crisis is minutes away if you miss the calcium connection.

Normal total serum calcium is 9.0–10.5 mg/dL (ionized: 4.5–5.5 mg/dL). Hypocalcemia occurs below these thresholds and most commonly follows thyroid/parathyroid surgery, vitamin D deficiency, hypoparathyroidism, or massive blood transfusions (citrate binds calcium). Calcium stabilizes nerve and muscle cell membranes. When calcium drops, sodium channels open more easily, making neurons hyperexcitable — this is why the hallmark presentation is neuromuscular irritability: perioral numbness, tingling in fingers and toes, muscle cramps, and carpopedal spasm. Untreated, it progresses to laryngospasm, seizures, and prolonged QT interval leading to cardiac arrest. Two bedside assessment signs are critical: Trousseau's sign (inflate a BP cuff above systolic for 3 minutes → carpal spasm indicates positive) and Chvostek's sign (tap facial nerve anterior to the ear → ipsilateral facial twitching). IV calcium gluconate is the standard replacement for symptomatic hypocalcemia — administered slowly via pump, never pushed rapidly, because fast infusion can cause cardiac arrest. Monitor the ECG continuously during infusion. Alkalosis worsens hypocalcemia by increasing calcium binding to albumin, reducing the ionized (active) fraction.

Key Distinctions

Don't confuse Trousseau's (BP cuff → hand spasm) with Chvostek's (facial tap → twitch) — Trousseau's is more reliable and specific. Students mix up hypocalcemia (increased neuromuscular excitability, prolonged QT) with hyperkalemia (also cardiac but shows peaked T waves, wide QRS). Remember that low albumin gives a falsely low total calcium — always check ionized calcium or correct for albumin before interpreting.

Clinical Pearl

Think "CATs go numb": Convulsions, Arrhythmias (prolonged QT), Tetany, Spasms and numbness. Trousseau's = BP Trouser on the arm; Chvostek's = Cheek tap.