Hypomagnesaemia

Overview

Magnesium is an essential constituent of many enzyme systems, particularly those involved in energy generation; the largest stores are in the skeleton.

Hypomagnesaemia is usually defined as a magnesium level of <0.7 mmol/l. It may also cause secondary hypocalcaemia and hypokalaemia.

Symptoms and Signs - more likely if levels <0.4 mmol/L

Most patients with hypomagnesaemia are asymptomatic. Symptoms usually do not manifest until the serum magnesium is <0.4 mmol/L. Clinical manifestation may depend more on the rate of development of deficiency or the total body deficit rather than serum magnesium concentration.

Symptoms and signs may include anorexia, nausea, confusion, weakness, ataxia, paraesthesia, tetany, tremor, muscle fasciculations. Cardiac arrhythmias may occur. Digitalis toxicity may be exacerbated. With very low levels seizures, drowsiness and coma (1,2)

Causes

Hypomagnesaemia can happen for a number of reasons (3) , including if a person:

Investigations in primary care

In most cases, the cause of the hypomagnesaemia can be obtained from the history. If no cause is apparent, further tests may be required.

Bloods

It is important to check bloods as hypomagnesaemia can cause hypokalaemia and hypocalcaemia:

• U and E
• Calcium

Urine

If a cause is not clear then a 24 hour urine magnesium excretion can be undertaken. This should be collected in an HCl acid bottle, which can be requested from the lab.

This is a useful indicator of renal magnesium loss. In the presence of hypomagnesaemia, urine magnesium excretion <0.5 mmol/24 hours is consistent with an intact renal response. Magnesium excretion >1mmol/24 hours indicate abnormal renal wasting.

Management

See the BNSSG Formulary - Oral Magnesium Supplementation guide (4) for details on recommended formulations, doses and treatment schedules.

Management should be based around treatment of the underlying cause and supplementation according to magnesium level and presence of any symptoms.

Oral supplements are usually sufficient for patients with Mg levels between 0.4 and 0.7 mmol/l. If Mg is <0.4 mmol/l or a patient is symptomatic then admission for initial parenteral treatment may be required.

Oral magnesium salts cause diarrhoea (or increased output in patients with stomas) so should be given with food if possible.

Monitoring

Normalisation of magnesium may take 6-8 weeks and Mg level should be monitored weekly.

Maintenance

Long term maintenance treatment may be required if a reversible cause is not identified.

Referral

Consider emergency admission for parenteral magnesium supplements if Mg <0.4 mmol/l or patient is symptomatic.

See Medical Assessment/Admission and Weekday IUC Professional Line page.

If admission is not required then manage underlying causes in primary care if possible and prescribe oral supplements as advised in the management section above.

If further secondary care advice is required consider the following options:

• Biochemistry advice - see Clinical Biochemistry in Primary Care.
• Gastroenterology advice - see Gastroenterology advice and guidance - available via eRS
• Renal advice - see Nephrology advice page or discuss with on call renal team

Causes

Drugs

• PPIs
• Diuretics (loop & thiazide)
• Antimicrobials (aminoglycosides, amphotericin B, forscarnet)
• Chemotherapy (cisplatin, carboplatin, epidermal growth factor receptor (EGFR) inhibitors, IL-2, pegylated liposomal doxorubicin
• Immunosupressants (ciclosporin, tacrolimus)
• Theophylline

 Inadequate intake

• Malnutrition
• Alcoholism
• Anorexia nervosa
• Terminal cancer
• Total parenteral nutrition

 Redistribution

• Treatment of DKA
• Refeeding syndrome
• Hungry bone syndrome
• Correction of metabolic acidosis
• Acute pancreatitis

Increased losses - Gastrointestinal

• Diarrhoea
• Malabsorption & steatorrhoea
• Small bowel bypass surgery
• Vomiting
• GI fistulas
• Hypomagnesaemia with secondary hypocalcaemia
• Laxative abuse
• Proton-pump inhibitors

 Increased losses - Renal Tubular Disorders

• Gitelman syndrome
• Classic Bartter syndrome (Type III)

Resources

(1) Hypomagnesaemia_jcPg0oV.pdf (gloshospitals.nhs.uk)

(2) PATH-021_Hypomagnesaemia_Guideline.pdf (ruh.nhs.uk)

(3) About oral magnesium glycerophosphate | Information for the public | Preventing recurrent hypomagnesaemia: oral magnesium glycerophosphate | Advice | NICE

(4) BNSSG Formulary - Oral Magnesium Supplementation 

(5) How is acute hypomagnesaemia treated in adults? – SPS - Specialist Pharmacy Service – The first stop for professional medicines advice

Efforts are made to ensure the accuracy and agreement of these guidelines, including any content uploaded, referred to or linked to from the system. However, BNSSG ICB cannot guarantee this. This guidance does not override the individual responsibility of healthcare professionals to make decisions appropriate to the circumstances of the individual patient, in consultation with the patient and/or guardian or carer, in accordance with the mental capacity act, and informed by the summary of product characteristics of any drugs they are considering. Practitioners are required to perform their duties in accordance with the law and their regulators and nothing in this guidance should be interpreted in a way that would be inconsistent with compliance with those duties.

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