Makindo Medical Notes.com Comprehensive Medical Encyclopedia | Education for Healthcare Professionals

Makindo Medical Notes.com
Download all this content in the Apps now Android App and Apple iPhone/Pad App

MEDICAL DISCLAIMER:The contents are under continuing development and improvements and despite all efforts may contain errors of omission or fact. This is not to be used for the assessment, diagnosis or management of patients. It should not be regarded as medical advice by healthcare workers or laypeople. It is for educational purposes only. Please adhere to your local protocols. Use the BNF for drug information. If you are unwell please seek urgent healthcare advice. If you do not accept this then please do not use the website. Makindo Ltd

Hypomagnesaemia

About

Anaesthetics and Critical Care

Obstetrics Gynaecology

Oncology

Ophthalmology

Oral Medicine and Dentistry

Statistics and Research

Stroke

Surgery

Toxicology

Trauma and Orthopaedics

Twitter

Urology

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

Management Summary: Mg²⁺ < 0.5 mmol/L or < 0.7 mmol/L and severe symptoms
Low Mg²⁺ prevents PTH release with low Ca²⁺ and Arrhythmias or Seizures Give Mg²⁺ 8 mmol/L (2g) in 100 mls of 5% Dextrose or Normal saline over 15-30 mins and then additional slower loading 20 mmols (5g) over 6 hrs. Monitor ECG and deep tendon reflexes which are lost with low Mg. Treat any coexisting hypokalaemia or other electrolytes issues Contraindicated AV block and severe bradycardia and caution with dose in renal failure

Management Summary: Mg²⁺ < 0.5 mmol/L or < 0.7 mmol/L and severe symptoms

Low Mg²⁺ prevents PTH release with low Ca²⁺ and Arrhythmias or Seizures
Give Mg²⁺ 8 mmol/L (2g) in 100 mls of 5% Dextrose or Normal saline over 15-30 mins and then additional slower loading 20 mmols (5g) over 6 hrs.
Monitor ECG and deep tendon reflexes which are lost with low Mg.
Treat any coexisting hypokalaemia or other electrolytes issues
Contraindicated AV block and severe bradycardia and caution with dose in renal failure

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

Cause	Clinical Features	Investigations	Management
Chronic Alcoholism	Muscle cramps, tremors, weakness, seizures, irritability, symptoms of hypocalcemia (tetany, Chvostek's and Trousseau's signs).	Serum magnesium, calcium, and potassium levels, renal function tests, liver function tests.	Oral or intravenous magnesium supplementation, dietary counseling, treatment of alcohol dependence, correction of associated electrolyte imbalances.
Gastrointestinal Losses (e.g., Diarrhea, Malabsorption)	Muscle weakness, cramps, paresthesias, fatigue, symptoms of concurrent hypokalemia and hypocalcemia.	Serum magnesium, calcium, potassium, and phosphate levels, stool analysis, tests for malabsorption (e.g., celiac serology, stool fat analysis).	Oral magnesium supplements, address underlying cause (e.g., treatment of diarrhea or malabsorption), electrolyte monitoring.
Renal Losses (e.g., Diuretics, Gitelman Syndrome)	Symptoms of hypomagnesemia, often with concurrent hypokalemia and metabolic alkalosis, muscle cramps, fatigue.	Serum magnesium, urine magnesium, calcium, and potassium levels, renal function tests, genetic testing for Gitelman syndrome if suspected.	Discontinue or adjust diuretic use, oral magnesium supplementation, potassium-sparing diuretics (e.g., amiloride) in some cases, genetic counseling if hereditary condition.
Proton Pump Inhibitors (PPIs)	Chronic use can lead to fatigue, muscle cramps, weakness, and tetany, especially in the elderly.	Serum magnesium, calcium, and potassium levels, review of medication history.	Discontinue or reduce PPI use if possible, oral magnesium supplementation, monitor magnesium levels regularly if PPI use is necessary.
Diabetes Mellitus (Poor Glycemic Control)	Polyuria, polydipsia, fatigue, muscle cramps, paresthesias, may present with symptoms of diabetic ketoacidosis (DKA) if severe.	Serum magnesium, glucose, HbA1c, urine magnesium, and potassium levels.	Optimize glycemic control, oral or IV magnesium supplementation, regular monitoring of electrolyte levels, management of DKA if present.
Acute Pancreatitis	Severe abdominal pain, nausea, vomiting, signs of hypocalcemia, hypokalemia, and hypomagnesemia.	Serum magnesium, calcium, potassium, and amylase/lipase levels, imaging (e.g., abdominal CT) to assess pancreatic inflammation.	Supportive care (IV fluids, pain control), IV magnesium supplementation, monitor and correct other electrolyte imbalances, treat underlying pancreatitis.
Refeeding Syndrome	Muscle weakness, fatigue, arrhythmias, respiratory failure, hypokalemia, hypophosphatemia, hypocalcemia after initiating nutrition in malnourished patients.	Serum magnesium, phosphate, calcium, and potassium levels, close monitoring during refeeding.	Slow reintroduction of nutrition, oral or IV magnesium, phosphate, and potassium supplementation, careful monitoring of electrolytes.
Medications (e.g., Aminoglycosides, Amphotericin B)	Weakness, tremors, seizures, arrhythmias, especially with concurrent hypokalemia and hypocalcemia.	Serum magnesium, calcium, and potassium levels, renal function tests, review of medication history.	Discontinue or switch medications if possible, magnesium supplementation, monitor renal function and electrolytes closely.

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