HYPOMAGNESAEMIA

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

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

• 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

• 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:

(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

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. Minimum dilution is 100 mL but more concentrated infusions should ideally be given centrally

Life-threatening features

• Cardiac monitoring with resuscitation facilities available
• Give a bolus of 2–4 g over 20 min

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

• Leave at least 2 hr after end of infusion before checking serum Mg²⁺

• if still <0.5 mmol/L, repeat dose
• 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)