Hypocalcaemia

Hypocalcaemia is a medical condition characterised by low calcium levels in the blood serum. The normal range of blood calcium is typically between 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L), while levels less than 2.1 mmol/L are defined as hypocalcaemic. Mildly low levels that develop slowly often have no symptoms. Otherwise, symptoms may include numbness, muscle spasms, seizures, confusion, or in extreme cases, cardiac arrest.

Causes

The most common cause of hypocalcaemia is iatrogenic hypoparathyroidism. Other causes include various forms of hypoparathyroidism, vitamin D deficiency, kidney failure, pancreatitis, calcium channel blocker overdose, rhabdomyolysis, tumour lysis syndrome, and medications such as bisphosphonates or denosumab. Hypoparathyroidism is commonly due to surgical destruction of the parathyroid glands but may also be due to autoimmune problems. Calcium is tightly regulated by parathyroid hormone (PTH), and in the absence of effective PTH, hypocalcaemia ensues.

Signs and Symptoms

The neuromuscular symptoms of hypocalcaemia are caused by a positive bathmotropic effect due to the decreased interaction of calcium with sodium channels. Since calcium blocks sodium channels and inhibits depolarisation of nerve and muscle fibres, reduced calcium lowers the threshold for depolarisation. The symptoms can be recalled by the mnemonic "CATs go numb" - convulsions, arrhythmias, tetany, and numbness in the hands and feet and around the mouth.

Additional symptoms include:

• Petechiae and purpura.
• Oral, perioral, and acral paresthesias.
• Carpopedal and generalised tetany.
• Latent tetany, such as Trousseau sign and Chvostek's sign.
• Hyperactive tendon reflexes.
• Life-threatening complications like cardiac arrhythmias.
• ECG changes, including intermittent QT prolongation, which predisposes to torsades de pointes, a type of ventricular tachycardia.

Diagnosis

Diagnosis should generally be confirmed by determining the corrected calcium or ionised calcium level. Specific changes may also be seen on an electrocardiogram (ECG), such as intermittent QT prolongation.

Because a significant portion of calcium is bound to albumin, any alteration in the level of albumin will affect the measured level of calcium. A corrected calcium level based on the albumin level is calculated as: Corrected calcium (mg/dL) = measured total Ca (mg/dL) + 0.8 * (4.0 - serum albumin [g/dL]).

Treatment

Management of hypocalcaemia includes:

• Intravenous calcium gluconate 10% or, if severe, calcium chloride. This is appropriate if the hypocalcaemia is acute and has occurred over a short time frame. Chronic severe hypocalcaemia requires careful management to avoid overcorrection and resulting hypercalcaemia.
• Maintenance doses of both calcium and vitamin D (often as 1,25-(OH)2-D3, i.e. calcitriol) are often necessary.
• In cases due to hypoparathyroidism, additional treatments may include hydrochlorothiazide, phosphate binders, and a low salt diet.

About 18% of people being treated in hospital have hypocalcaemia, indicating its relative commonality in clinical settings.

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Self-assessment MCQs (single best answer)

What is the normal range of blood calcium levels in mmol/L?

Which of the following is NOT a symptom of hypocalcaemia?

The most common cause of hypocalcaemia is:

The mnemonic "CATs go numb" for hypocalcaemia symptoms includes all EXCEPT:

What role does calcium play in neuromuscular function?

Which ECG change is associated with hypocalcaemia?

How is corrected calcium level calculated in the context of altered albumin levels?

Which vitamin is often necessary to maintain calcium levels in hypocalcaemia?

What is a severe treatment option for acute hypocalcaemia?

What percentage of people treated in hospitals have hypocalcaemia?