Makindo Medical Notes

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

Hypomagnesaemia may be a symptom of refeeding syndrome. Low Mg may be caused by prolonged use of PPIs

Definition: Deficiency (4 mmol Mg = 1g MgSO4)

• Mild: 0.5-0.7 mmol/L: only require oral replacement if symptomatic
• Moderate: 0.4-0.5 mmol/L: advise oral replacement if asymptomatic, IV replacement if symptomatic
• Severe:<0.4 mmol/L: Give IV replacement. Risks seizure, tremor, arrhythmias, tetany

Associations with severe hypomagnesaemia

• Hypocalcaemia (Mg²⁺ required for PTH secretion): replace Mg
• Hypokalaemia (Mg²⁺ required for Na⁺/K⁺ transmembrane ATPase).

Causes

Aetiology

• Mainly intracellular and stabilises cell membranes though no one is very sure how it works
• Magnesium probably competes with calcium at the level of the plasma membrane voltage-gated channels.
• Many uses - preeclampsia, asthma, torsade de pointes etc.

Clinical: Symptoms typically occur when Mg <0.5 mmol/L)

• Nausea, vomiting, Lethargy, muscle weakness, drowsiness
• Tetany, tremor, twitching, agitation
• Vertigo, Confusion, Cardiac arrhythmias, Seizures

Cautions

• Magnesium excreted kidneys and is retained in renal failure
• Cautions with severe renal failure and IV magnesium

Management of Hypomagnesaemia

• For asymptomatic or mild hypomagnesaemia review the patient for the underlying cause. Most commonly this will be due to recent losses i.e. diarrhoea or medications. If appropriate, stop medications which may cause hypomagnesaemia
• Generally a maximum daily dose of 50 mmol is recommended in 24 hours; a total of up to 160mmol may be required over 5 days to correct the deficiency
• Magnesium Level < 0.5 mmol/L: Consider IV Supplement and then oral as needed
  • Avoid or significantly reduce doses of Mg salts in patients with reduced renal function (eGFR <30 mL/minute/1.732) as there is a risk of hypermagnesaemia
  • Severe: 8mmol (2g) magnesium in 100ml compatible fluid IV over 15 mins
  • Prescribe 20 mmol magnesium in at least 100ml of sodium chloride 0.9% or glucose 5% IV over at least 1 hour. A blood sample should be taken 2 hours later. Oral supplements may also be started.
  • If having IV fluids can add 20 mmol magnesium to 500ml or 1 L bag of compatible fluid and give IV over 6 to 24 hours. Depends on urgency.
  • If the patient also has hypokalaemia generally give magnesium first; do not combine magnesium and potassium in the same bag.
• Magnesium Level 0.5 -0.7 mmol/L: Generally oral replacement is reasonable unless symptoms
  • If Symptoms of hypomagnesaemia (e.g. paraesthesia, fits, tetany, arrhythmia) then treat as level < 0.5 above and prescribe 8mmol magnesium in 100ml compatible fluid IV over 2 hours
  • Otherwise start standard dose of oral magnesium for hypomagnesaemia is around 24 mmol/day
    • Magnesium aspartate (Magnaspartate®) 1 sachet (10 mmol) BD
    • Magnesium glycerophosphate (Neomag®) 2x4 mmol chewable tablets TDS

References

• Hypomagnesaemia