Hypomagnesaemia

Last updated 21st March 2019 - Tom Heaton

This is defined as a plasma level below 0.7 mmol/L.
As it is not predominantly an extracellular ion, this may not clearly reflect total body levels.
In the case of plasma hypomagnesaemia, total body levels will often also be low.  
It is a common abnormality in hospital, particularly in critical care where it may be a high as 60-65% of patients.

Hypomagnesaemia primarily arises from increased GI or renal losses.
As there is limited ability to mobilise the body stores (in the bone) this can lead to plasma levels dropping fairly quickly.
Other mechanisms may include impaired absorption and saponification.

Aetiology

GI Losses/Impaired absorption

• Diarrhoea (acute and chronic)
  
• Malabsorption syndromes
  
• Vomiting (less common than with diarrhoea)
  

• PPIs
  
• Familial
  
• Small bowel bypass surgery
  

Renal Losses

• Diuretics
  
  • Loop
    
  • Thiazide
    
• Other drugs
  
  • Aminoglycosides
    
  • Digoxin
    
  • Cisplatin
    
  • Amphotericin
    
• Alcohol
  
• Hypervolaemia (reduced sodium reuptake)
  
• Poorly controlled diabetes
  
• Post transplant
  
• Tubular dysfunction pathology
  
• Hypercalcaemia
  
• Familial
  

Presentation

It will usually be alongside another problem e.g. critical illness, although may be a primary problem.
However, it often is accompanied by other biochemical derangement:

• Hypokalaemia
  
• Hypocalcaemia
  
• Metabolic alkalosis
  

The depletion can affect multiple systems.
CNS:

• Weakness
  
• Tremor
  
• Tetany
  
• Seizures
  
• Movement disorder
  
• Delirium
  
• Depressed GCS
  

CVS

• Widening QRS (mod)
  
• Peaked T waves (mod)
  
• Wide PR (severe)
  
• Atrial/ventricular arrhythmias (severe)
  
• Promotes digoxin toxicity
  

Metabolic

• Hypokalaemia
  
• Hypocalcaemia
  
• Hypoparathyroidism/PTH resistance
  
• Insulin resistance
  

Management

This essentially involves replacement of the deficit.
The urgency will impact on the rapidity of replacement:

• Emergency - 10-20 mmol IV immediately, then 40 mmol IV over 4h
  
• Urgent - 40 mmol initially IV. Can step down
  
• Non-urgent - 15 mmol/day - usually enterally
  

Caution should be taken in cases of IV administration (especially when rapid) as may cause hypotension and bradycardia.
Monitoring of levels is also essential given the potential for toxicity.
The development of absence of deep tendon reflexes is often used as a rapid clinical assessment of toxicity in cases where it may develop (e.g. preeclampsia treatment protocols).

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Links & References

1. Watson, V. Vaughan, R. Magnesium and the anaesthetist. CEPD Review. 2001.
   
2. Parikh, M. Webb, S. Cations: Potassium, calcium and magnesium. CEACCP. 2012. 12(4):195-198. https://academic.oup.com/bjaed/article/12/4/195/275340
   
3. Nickson, C. Magnesium. LITFL. 2012. https://lifeinthefastlane.com/ccc/magnesium/
   
4. Yu, A. Evaluation and treatment of hypomagnesemia. UpToDate. 2017.
   
5. Yu, A. Regulation of magnesium balance. UpToDate. 2017.
   
6. Lyness, D. Magnesium. Propofology. 2016. https://www.propofology.com/infographs/magnesium-uses-pharmacology-doses-cautions
   
7. Yartsev, A. Hypermagnesemia. DerangedPhysiology. 2016. https://derangedphysiology.com/main/required-reading/electrolytes-and-fluids/Chapter%208.4.2/hypermagnesemia
   
8. Yu, A. Gupta, A. Causes and treatment of hypermagnesemia. UpToDate. 2019.
   
9. Yu, A. Gupta, A. Symptoms of hypermagnesemia. UpToDate. 2017. ​