Write a short note on hypocalcaemia.

Definition

Hypocalcaemia is a corrected serum calcium below 8.5 mg/dL (2.1 mmol/L). As with hypercalcaemia, correction for albumin is essential (add 0.8 mg/dL for every 1 g/dL fall in albumin below 4 g/dL), and ionised calcium should be measured where available, particularly in critical illness.

Causes

Clinical Features

Manifestations reflect increased neuromuscular excitability, and depend on the rapidity of onset — acute falls are far more symptomatic.

Neuromuscular:

• Perioral and acral paraesthesiae — the earliest symptom.
• Carpopedal spasm and tetany; laryngospasm with stridor (life-threatening); bronchospasm.
• Chvostek's sign — tapping over the facial nerve anterior to the ear produces twitching of the ipsilateral facial muscles. Present in up to 10–25% of normal people, so it is suggestive but not specific.
• Trousseau's sign — inflating a sphygmomanometer cuff above systolic pressure for 3 minutes produces carpal spasm (flexion at wrist and metacarpophalangeal joints, extended interphalangeal joints, adducted thumb). More specific than Chvostek's sign.
• Seizures — hypocalcaemia is a recognised and reversible cause of seizures, easily missed.

Cardiac: prolonged QT interval, arrhythmias, hypotension and, in severe chronic deficiency, cardiomyopathy with heart failure.

Chronic features: cataract, basal ganglia calcification with extrapyramidal features, dry skin, brittle nails, coarse hair, dental hypoplasia, papilloedema, and psychiatric symptoms including depression and psychosis.

Investigations

• Corrected calcium and ionised calcium
• magnesium (always — hypomagnesaemia is a common and correctable cause)
• phosphate (high in hypoparathyroidism and CKD, low in vitamin D deficiency — a useful discriminator)
• intact PTH
• 25-OH vitamin D
• alkaline phosphatase
• renal function
• amylase or lipase if pancreatitis is suspected
• creatine kinase in rhabdomyolysis
• ECG for QT interval
• arterial blood gas where alkalosis is suspected.

Management

Acute symptomatic hypocalcaemia (tetany, seizures, laryngospasm, prolonged QT) is an emergency:

• Intravenous calcium gluconate, diluted and given slowly with cardiac monitoring, followed by a continuous infusion until symptoms resolve and oral therapy takes effect. Calcium gluconate is preferred peripherally, as calcium chloride is more irritant.
• Correct hypomagnesaemia simultaneously — calcium replacement will fail while magnesium remains low.
• Secure the airway if there is laryngospasm.

Chronic or asymptomatic hypocalcaemia:

• Oral calcium supplements with vitamin D — cholecalciferol for simple deficiency; active analogues (calcitriol or alfacalcidol) in hypoparathyroidism and renal failure, where 1-alpha-hydroxylation is impaired or PTH is absent.
• Treat the underlying cause — thyroid or parathyroid surgery follow-up, malabsorption, renal disease, drug withdrawal.
• Monitor calcium, phosphate and renal function; in hypoparathyroidism aim for a low-normal calcium to avoid hypercalciuria and nephrocalcinosis, since the calcium-conserving action of PTH is absent.