Hypocalcemia

Essentials of Diagnosis

• Often mistaken as a neurologic disorder

• Decreased serum parathyroid hormone (PTH), vitamin D, or magnesium levels

• Despite a low total serum calcium, calcium metabolism is likely normal if ionized calcium level is normal

General Considerations

• Increased excitation of nerve and muscle cells, primarily affects the neuromuscular and cardiovascular systems

• Hypoalbuminemia is most common cause of low total serum calcium

• When serum albumin concentration is < 4 g/dL (40 g/L), serum Ca²⁺ concentration is reduced by 0.8–1 mg/dL (0.20–0.25 mmol/L) for every 1 g/dL (10 g/L) of albumin

• The most accurate measurement of serum calcium is the ionized calcium concentration

• True hypocalcemia (decreased ionized calcium) implies insufficient action of PTH or active vitamin D

• Important causes of hypocalcemia are listed in Table 21–6

• The most common cause of hypocalcemia is advanced CKD

• Magnesium depletion reduces both PTH release and tissue responsiveness to PTH, causing hypocalcemia

• Hypocalcemia in pancreatitis is a marker of severe disease

• Elderly hospitalized patients with hypocalcemia and hypophosphatemia, with or without an elevated PTH level, are likely vitamin D deficient

Symptoms and Signs

• Spasm of skeletal muscle causes cramps and tetany

• Laryngospasm with stridor can obstruct the airway

• Convulsions, paresthesias of lips and extremities, and abdominal pain

• Chvostek sign (contraction of the facial muscle in response to tapping the facial nerve)

• Trousseau sign (carpal spasm occurring after occlusion of the brachial artery by a blood pressure cuff) is usually readily elicited

• In chronic hypoparathyroidism, cataracts and calcification of basal ganglia may appear

• QT prolongation predisposes to ventricular arrhythmias

Laboratory Tests

• Serum calcium concentration is low (< 8.5 mg/dL or < 2.21 mmol/L)

• In true hypocalcemia, the ionized serum calcium concentration is low (< 4.6 mg/dL or < 1.15 mmol/L)

• Serum phosphate is usually elevated in hypoparathyroidism or advanced CKD, whereas it is suppressed in early CKD or vitamin D deficiency

• Serum magnesium concentration is commonly low

• In respiratory alkalosis, total serum calcium is normal but ionized calcium is low

Diagnostic Procedure

• ECG can show prolongation of the QT interval

Medications

Severe, symptomatic hypocalcemia

• In the presence of tetany, arrhythmias, or seizures, intravenous calcium gluconate is indicated

• Because of the short duration of action, calcium infusion is usually required; add 10–15 mg of calcium per kilogram body weight, or six to eight 10-mL vials of 10% calcium gluconate (558–744 mg of calcium), to 1 L of D5W and infuse over 4–6 h

• Adjust the infusion rate to maintain the serum calcium level at 7.0–8.5 mg/dL

Asymptomatic hypocalcemia

• Oral calcium (1–2 g of elemental calcium) and vitamin D preparations (Table 26–8)

• Calcium carbonate is well tolerated and less expensive than many other calcium tablets

• Check urinary calcium excretion after initiation of therapy because hypercalciuria (urine calcium excretion > 300 mg/day [or 7.5 mmol/day] or urine calcium:creatinine ratio > 0.3) may impair kidney function in these patients

• The low serum Ca²⁺ associated with hypoalbuminemia does not require replacement therapy

• If serum Mg²⁺ is low, therapy must include magnesium replacement, which by itself usually will correct hypocalcemia

Follow-Up

• Monitor the serum calcium level frequently (every 4–6 hours) during calcium infusions for severe hypocalcemia

Prognosis

• Depends on the underlying cause (eg, CKD)

When to Refer

• Patients with complicated hypocalcemia from hypoparathyroidism, familial hypocalcemia, or CKD require referral from an endocrinologist or nephrologist

When to Admit

• Patients with tetany, arrhythmias, seizures, or other symptoms of hypocalcemia require immediate therapy