Hypomagnesaemia
Summary
Hypomagnesaemia is defined as serum magnesium <0.7 mmol/L. It is a frequently underdiagnosed electrolyte disturbance that can cause serious cardiac arrhythmias (especially Torsades de Pointes), neuromuscular dysfunction, and refractory hypokalaemia and hypocalcaemia. Common causes include diuretics (loop and thiazide), proton pump inhibitors (long-term), alcohol excess, and GI losses. Importantly, magnesium is required for both potassium retention and PTH secretion - so hypokalaemia and hypocalcaemia will NOT correct until magnesium is repleted. Treatment is with oral or IV magnesium supplementation.
Key Facts
- Definition: Serum Mg <0.7 mmol/L
- Major Causes: Diuretics (loop/thiazide), PPIs (long-term), Alcoholism, Diarrhoea
- Critical Effect: Causes refractory hypokalaemia and hypocalcaemia
- Cardiac Risk: Torsades de Pointes, prolonged QTc
- Treatment: IV Magnesium Sulphate (severe); Oral Mg (mild)
- Key Point: Always check Mg in unexplained hypoK or hypoCa
Clinical Pearls
"You Can't Fix K Without Mg": Magnesium is required for the Na-K-ATPase pump. If Mg is low, potassium replacement will fail. Always check and correct Mg first.
"PPIs Are a Common Culprit": Long-term PPI use (especially >1 year) causes hypomagnesaemia through impaired intestinal absorption. Think of this in any patient on long-term PPIs.
"Mg for Torsades": Even if serum Mg is normal, IV magnesium sulphate is the first-line treatment for Torsades de Pointes.
"Alcohol = Low Everything": Chronic alcohol excess causes hypomagnesaemia via GI losses and renal wasting. Check K, Mg, Ca, and phosphate in alcoholics.
Incidence
- 10-20% of hospitalised patients
- Up to 65% in ICU patients
- Often undiagnosed (not routinely measured)
Demographics
- All ages
- Higher in elderly (polypharmacy, diuretics, malnutrition)
- Higher in alcoholics
Causes
| Mechanism | Causes |
|---|---|
| Renal losses | Loop diuretics, Thiazides, Aminoglycosides, Cisplatin, Amphotericin B, CNIs |
| GI losses | Diarrhoea, Vomiting, Malabsorption, PPI (long-term), NG suction |
| Redistribution | Refeeding syndrome, Insulin, Pancreatitis |
| Poor intake | Alcoholism, Malnutrition |
| Genetic | Gitelman, Bartter syndrome |
Magnesium Homeostasis
- Total body Mg: ~24g (50% bone, 48% intracellular, 2% extracellular)
- Absorbed in small intestine
- Excreted by kidneys (filtered and reabsorbed in loop of Henle)
Why Does Low Mg Cause Low K?
- Mg required for Na-K-ATPase (maintains K gradient)
- Mg blocks ROMK channel in distal nephron
- Low Mg → Increased K excretion in urine
- K replacement fails until Mg corrected
Why Does Low Mg Cause Low Ca?
- Mg required for PTH secretion from parathyroid glands
- Low Mg → Impaired PTH release → Low Ca
- Low Mg also causes PTH resistance
Cardiac Effects
- Prolonged QT interval
- Torsades de Pointes
- Digoxin toxicity potentiated
Symptoms
Many overlap with hypokalaemia and hypocalcaemia.
| System | Features |
|---|---|
| Neuromuscular | Tremor, muscle cramps, weakness, tetany, Chvostek/Trousseau signs |
| Cardiac | Palpitations, arrhythmias |
| CNS | Confusion, seizures (severe), apathy |
| GI | Nausea, vomiting, anorexia |
Severity vs Symptoms
| Level | Symptoms |
|---|---|
| Mild (0.5-0.7) | Often asymptomatic |
| Moderate (0.3-0.5) | Neuromuscular, GI symptoms |
| Severe (<0.3) | Arrhythmias, tetany, seizures |
Neuromuscular Signs
- Chvostek's sign: Facial twitching on tapping facial nerve (hypocalcaemia proxy)
- Trousseau's sign: Carpopedal spasm with BP cuff inflated (hypocalcaemia proxy)
- Hyper-reflexia
- Tremor
Cardiac
- Irregular pulse (arrhythmia)
First-Line
| Test | Purpose |
|---|---|
| Serum Magnesium | Confirm diagnosis (<0.7 mmol/L) |
| Potassium | Often low (same causes; related physiology) |
| Calcium (corrected) | Often low (PTH impairment) |
| U&E | Renal function, Na |
| ECG | Prolonged QT, U waves, arrhythmias |
Additional
| Test | When |
|---|---|
| 24h urinary Mg | Distinguish renal from GI losses (FEMg) |
| Phosphate | Often low in refeeding, alcoholism |
| PTH | If hypocalcaemia (will be low or inappropriately normal) |
| Drug history | PPIs, diuretics, aminoglycosides |
Interpretation
- Low urinary Mg → GI losses (appropriate renal conservation)
- High urinary Mg (FEMg >4%) → Renal wasting (diuretics, drugs)
Replacement Principles
┌──────────────────────────────────────────────────────────┐
│ HYPOMAGNESAEMIA MANAGEMENT │
├──────────────────────────────────────────────────────────┤
│ │
│ ASSESS SEVERITY: │
│ • Mild (0.5-0.7): Oral replacement │
│ • Moderate (0.3-0.5): Oral or IV depending on symptoms │
│ • Severe (<0.3) OR Symptomatic: IV replacement │
│ │
│ ORAL REPLACEMENT: │
│ • Magnesium Glycerophosphate 4-8mmol BD-TDS │
│ • Or Magnesium Aspartate │
│ • Side effect: Diarrhoea (common, limits dose) │
│ │
│ IV REPLACEMENT: │
│ • Magnesium Sulphate 8mmol (2g) IV over 2-4 hours │
│ • Can repeat if needed │
│ • Max rate: 8mmol/hour (faster causes flushing) │
│ │
│ TORSADES DE POINTES (EMERGENCY): │
│ • Magnesium Sulphate 8mmol (2g) IV over 10-15 minutes │
│ • Even if serum Mg is normal │
│ │
│ CORRECT UNDERLYING CAUSE: │
│ • Stop causative drug if possible (PPI, diuretic) │
│ • Treat GI losses │
│ • Alcohol cessation │
│ │
│ RECHECK: │
│ • Mg, K, Ca after replacement │
│ │
└──────────────────────────────────────────────────────────┘
Key Points
- Correct Mg BEFORE giving K (otherwise K won't hold)
- Check Ca (may need correction after Mg repleted)
- Long-term oral supplementation if chronic cause (e.g., diuretics)
Of Hypomagnesaemia
- Cardiac arrhythmias (Torsades de Pointes, AF)
- Digoxin toxicity (potentiated)
- Refractory hypokalaemia
- Refractory hypocalcaemia
- Seizures (severe)
- Death (arrhythmias)
Of Treatment
- IV Mg: Flushing, hypotension (if too fast), respiratory depression (overdose)
- Oral Mg: Diarrhoea (common)
With Treatment
- Correction usually straightforward if cause identified
- Chronic low Mg can recur if underlying cause not addressed
Monitoring
- Recheck Mg, K, Ca 1-2 days after starting replacement
- May need ongoing oral maintenance
Key Guidelines
- Clinical Biochemistry Textbooks
- NICE - Electrolyte Replacement
- Resuscitation Council UK (Torsades de Pointes)
Key Evidence
Mg and K Relationship
- Well-documented: Mg deficiency causes renal K wasting
- K replacement fails without Mg correction
PPI-associated Hypomagnesaemia
- FDA warning (2011): Long-term PPI use causes low Mg
- May take weeks to months to resolve after stopping PPI
What is Hypomagnesaemia?
Hypomagnesaemia means you have low magnesium levels in your blood. Magnesium is an important mineral that helps your muscles, nerves, and heart work properly.
What Causes It?
Common causes include:
- Certain medications (water tablets, acid-reducing tablets like omeprazole)
- Heavy alcohol use
- Chronic diarrhoea or vomiting
- Poor diet
What Are the Symptoms?
Many people have no symptoms, but low magnesium can cause:
- Muscle twitches, cramps, or weakness
- Tiredness
- Heart palpitations
- Confusion (if severe)
Why Does It Matter?
Low magnesium can also cause low potassium and low calcium levels. In severe cases, it can cause dangerous heart rhythm problems.
How is it Treated?
- Mild cases: Magnesium tablets by mouth
- Severe cases: Magnesium given through a drip (IV)
- You'll also need blood tests to check other minerals (potassium, calcium)
What Should I Do?
If you've been told you have low magnesium:
- Take your supplements as prescribed
- Tell your doctor about all your medications
- Get blood tests as recommended
Primary Guidelines
- NICE Clinical Knowledge Summaries. Electrolyte Disturbances.
- Resuscitation Council UK. Adult Advanced Life Support Guidelines (Torsades de Pointes).
Key Studies
- Whang R, et al. Magnesium deficiency as a cause of acute refractory potassium repletion. Arch Intern Med. 1992;152(1):40-45. PMID: 1728927
- Hoorn EJ, et al. Hypomagnesaemia and the kidney. Nat Rev Nephrol. 2015;11(8):432-448. PMID: 25963588