DO NOT USE FOR CLINICAL PRACTICE
Please use current guidelines available on the UHNM intranet for patient treatment
Please use current guidelines available on the UHNM intranet for patient treatment
DEFINITION
Severe deficit
- Serum Mg2+ <0.5 mmol/L
Moderate deficit
- Serum Mg2+ 0.5-0.7 mmol/L
Mild deficit
- Magnesium is largely intracellular so mild deficiency can occur with a normal serum concentration, but urine excretion will be reduced:
- urine Mg2+/urine creatinine <0.1 = deficiency; <0.05 = severe deficiency, except if secondary to renal loss - see Investigations
COMMON CAUSES
Gastrointestinal loss
- Diarrhoea
- Stoma
- Fistula
- Malabsorption states
- Proton pump inhibitors (PPIs)
Renal loss
- Tubular damage
- Genetic syndromes (e.g. Gitelman's syndrome)
- Chronic acidosis
- Phosphate or potassium depletion
- Hypoparathyroidism
- Drug-induced (e.g. loop and thiazide diuretics, aminoglycosides, ciclosporin, cisplatin)
Other
- Alcoholism
- Insulin administration
- Critical illness
SYMPTOMS AND SIGNS
- Non-specific and often attributed to hypocalcaemia or hypokalaemia
Musculoskeletal
- Muscle twitching
- Tremor
- Tetany
- Cramps
CNS
- Apathy
- Depression
- Hallucinations
- Agitation
- Confusion
- Fits
Cardiovascular
- Tachycardia
- Hypertension
- Arrhythmias (e.g. torsade de pointes)
- Digoxin toxicity
INVESTIGATIONS
- Cause usually apparent from clinical picture - investigation necessary only if not obvious
- Check U&E, bone profile and PTH as Mg2+ deficiency associated with hypocalcaemia and hypokalaemia
- Calculate fractional excretion of Mg2+ in a random urine sample from:
Urine Mg2+x serum creatinine x 100 |
Serum Mg2+ x urine creatinine x 0.7 |
(units for each of urine and serum must be the same)
- fractional excretion of Mg2+ >3% indicates renal loss. See above for causes
- If hypocalcaemia or hyperphosphataemia present, check plasma parathyroid hormone
IMMEDIATE TREATMENT
Severe deficiency (Serum Mg2+ <0.5 mmol/L), intractable loss or symptoms of hypocalcaemia or hypokalaemia
- IV route: Magnesium sulphate 5 g (20 mmol in 10 mL) into 250 mL glucose 5% (or sodium chloride 0.9%) over 4 hr
- if given peripherally, monitor insertion site closely for phlebitis using a recognised infusion phlebitis scoring tool
- through a central line in critical care only, minimum dilution is 100 mL
Life-threatening features
- Cardiac monitoring with resuscitation facilities available
- Give a bolus of 4g (16mmol) in 20mL sodium chloride 0.9% over 30 min
Mild to Moderate deficiency (serum Mg2+ >0.5 mmol/L)
- Oral magnesium aspartate 243 mg powder for oral solution. Dose: 1-2 sachets (equivalent to 243-486 mg magnesium or 10-20 mmol magnesium) dissolved in 50-200 mL water, tea or orange juice, daily
- if tolerance to oral intake limited by diarrhoea, reduce dose to maximum tolerated
- Stop PPIs if possible, substituting H2 antagonists if necessary
MONITORING
- Leave at least 2 hr after end of infusion before checking serum Mg2+
- if still <0.5 mmol/L, repeat dose as required by repeated serum Mg2+ level and symptoms
- otherwise, check again after 24 hr
- Toxicity rare if renal function normal
- Clinical signs of overdose:
- loss of tendon reflexes (>5 mmol/L)
- hypotension
- bradycardia
- respiratory depression (>7.5 mmol/L)