Makindo Medical Notes

Related Subjects: |Calcium Physiology |Magnesium Physiology |Calcitonin |Hypocalcaemia |Hypercalcaemia |Hypomagnesaemia |Hypermagnesaemia |Primary Hyperparathyroidism |Familial hypocalciuric hypercalcaemia (FHH) |Sarcoidosis

⚠️ Hypomagnesaemia: May occur in refeeding syndrome and is a recognised complication of prolonged PPI therapy. Magnesium is vital for PTH release, potassium balance, and cardiac stability — deficiency can cause seizures, arrhythmias, and refractory hypocalcaemia or hypokalaemia.

🧾 Definition (4 mmol Mg = 1 g MgSO₄)

• 🙂 Mild: 0.5–0.7 mmol/L → oral replacement if symptomatic.
• 😟 Moderate: 0.4–0.5 mmol/L → oral if asymptomatic, IV if symptomatic.
• 🚨 Severe: <0.4 mmol/L → IV replacement; risks = seizure, tremor, arrhythmias, tetany.

🧩 Associations

• ⬇️ Hypocalcaemia → Mg²⁺ needed for PTH secretion.
• ⬇️ Hypokalaemia → Mg²⁺ needed for Na⁺/K⁺ ATPase activity.

📊 Causes of Hypomagnesaemia

🧬 Aetiology & Physiology

• Mostly intracellular, stabilises cell membranes, competes with calcium at voltage-gated channels.
• Clinical uses: 💊 pre-eclampsia, severe asthma, torsades de pointes.

🧠 Symptoms (usually if Mg <0.5 mmol/L)

• Nausea, vomiting, lethargy, drowsiness.
• Tetany, tremor, twitching, agitation.
• Vertigo, confusion, seizures, cardiac arrhythmias.

⚠️ Cautions

• Renal excretion → risk of hypermagnesaemia in renal failure.
• Use reduced doses if eGFR <30.

💊 Management

• 🔍 Review underlying cause (stop culprit drugs e.g. PPIs, diuretics).
• 🔄 Max 50 mmol/day (up to 160 mmol/5 days).
• 📉 Severe (<0.5 mmol/L):
  • 8 mmol (2 g) MgSO₄ in 100 ml IV fluid over 15 mins.
  • + 20 mmol Mg in 100–500 ml fluid IV over 6–24 hrs.
  • Check Mg 2 hrs post infusion.
  • Give Mg before K⁺ if coexisting hypokalaemia (never mix in same bag).
• 🙂 Mild (0.5–0.7 mmol/L):
  • Oral Mg unless symptomatic.
  • If symptomatic → IV regimen as above.
  • Oral options:
    • Magnesium aspartate (Magnaspartate®) 1 sachet (10 mmol) BD.
    • Magnesium glycerophosphate (Neomag®) 2 × 4 mmol chewable tablets TDS.

📚 Reference

• Royal Cornwall NHS – Hypomagnesaemia Guideline

🩺 Case 1 — GI Losses

A 60-year-old man with chronic diarrhoea due to inflammatory bowel disease presents with muscle cramps, tremor, and palpitations. ECG shows prolonged QT interval with frequent ventricular ectopics. Labs reveal Mg²⁺ 0.35 mmol/L (low), K⁺ 3.0 mmol/L, Ca²⁺ 2.05 mmol/L. Management: 💉 IV magnesium sulphate replacement if symptomatic/ECG changes, otherwise oral magnesium supplements. Correct associated hypokalaemia and hypocalcaemia. Avoid: ❌ Using digoxin until magnesium is corrected (risk of arrhythmia). Avoid rapid IV infusion which can cause hypotension.

🩺 Case 2 — Drug-Induced

A 72-year-old woman with atrial fibrillation and hypertension presents with weakness and dizziness. She has been on a loop diuretic and a PPI for several months. Labs: Mg²⁺ 0.40 mmol/L, K⁺ normal, Ca²⁺ low-normal. ECG shows flat T waves. Management: 💊 Stop/review offending drugs (loop diuretic, long-term PPI). Give oral or IV magnesium replacement depending on severity. Monitor electrolytes closely. Avoid: ❌ Ignoring mild hypomagnesaemia — it predisposes to arrhythmias and potentiates digoxin toxicity. Avoid ongoing PPI use without indication.