Hypomagnesaemia

Hypomagnesaemia, characterised by low magnesium levels in the blood, is often associated with other electrolyte imbalances such as hypocalcaemia and hypokalaemia. Magnesium plays a key role in numerous biochemical processes, including muscle and nerve function, and its deficiency can lead to neuromuscular and cardiovascular symptoms.

Clinical Features of Hypomagnesaemia

Hypomagnesaemia presents with a variety of symptoms and signs, many of which resemble hypocalcaemia due to magnesium’s role in calcium metabolism. Common clinical manifestations include:

• Neuromuscular Symptoms:
  • Tremors, muscle cramps, twitching.
  • Tetany: Similar to hypocalcaemia, with positive Trousseau’s and Chvostek’s signs.
  • Seizures: Especially in severe cases.
  • Weakness and paraesthesia.
• Cardiovascular Symptoms:
  • Palpitations, chest pain, and dizziness.
  • ECG changes: Widened QRS complexes, prolonged PR intervals, and increased risk of arrhythmias such as ventricular tachycardia and fibrillation.
• Gastrointestinal Symptoms:
  • Diarrhoea is both a symptom and a cause of magnesium loss.
• Other Symptoms:
  • Confusion, lethargy, and generalised weakness.

Causes of Hypomagnesaemia

The causes of hypomagnesaemia can be divided into several categories, including gastrointestinal loss, renal loss, and drug-induced depletion.

Diagnostic Approach

To diagnose hypomagnesaemia, clinical suspicion must be confirmed by laboratory investigations:

1. Serum Magnesium Level: Hypomagnesaemia is defined as a serum magnesium level below 0.7 mmol/L.
   • Mild: 0.5–0.7 mmol/L.
   • Severe: <0.5 mmol/L.
2. Associated Electrolyte Abnormalities:
   • Hypocalcaemia: Often accompanies hypomagnesaemia, partly due to impaired PTH release.
   • Hypokalaemia: Magnesium depletion often causes refractory hypokalaemia.
3. ECG: Look for changes associated with magnesium deficiency, such as widened QRS, prolonged PR interval, and risk of arrhythmias.
4. Blood Tests:
   • Calcium and phosphate levels.
   • Parathyroid hormone (PTH) to assess for associated hypocalcaemia.
   • Urea and electrolytes (U&Es) to assess for renal dysfunction.

Management

The management of hypomagnesaemia depends on its severity and the presence of symptoms. Treatment focuses on magnesium replacement and addressing the underlying cause.

1. Oral Magnesium Replacement:
   • For patients with mild hypomagnesaemia (≥0.5 mmol/L) and no significant symptoms or ECG changes.
   • Magnesium glycerophosphate 4 mmol PO, 2 tablets up to three times daily (TDS).
2. Intravenous Magnesium Replacement:
   • For patients with severe hypomagnesaemia (<0.5 mmol/L), significant symptoms, or ECG changes.
   • IV Magnesium sulphate 16–20 mmol in 100 mL 5% dextrose or 0.9% saline, administered over 4–6 hours.
   • Careful monitoring of serum magnesium is required, particularly in patients with renal impairment.
3. Address Underlying Causes:
   • Drug Review: Stop or adjust doses of diuretics, proton pump inhibitors, or other drugs contributing to magnesium loss.
   • Fluid Management: Review fluid balance charts to ensure adequate hydration and electrolyte balance.
   • Correct Other Electrolyte Disturbances: Hypokalaemia and hypocalcaemia should be corrected alongside magnesium replacement.

Key Takeaways

• Common Symptoms: Neuromuscular symptoms such as tremors, muscle twitching, tetany, and seizures are common. Cardiovascular complications, including arrhythmias, are also a concern.
• Causes: Hypomagnesaemia is typically caused by gastrointestinal losses, renal wasting, or drug effects (particularly diuretics).
• Associated Conditions: Hypocalcaemia and hypokalaemia frequently coexist with hypomagnesaemia.
• Management: Mild cases can be managed with oral supplementation, while severe cases require intravenous magnesium replacement.