Treatment of Hypomagnesemia
For mild hypomagnesemia, start with oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium), preferably administered at night when intestinal transit is slowest; for severe or symptomatic hypomagnesemia (serum Mg <1.2 mg/dL or <0.5 mmol/L), use intravenous magnesium sulfate 1-2 g over 15 minutes for acute cases, or 4-5 g added to one liter of IV fluid infused over 3 hours for less urgent situations. 1, 2
Initial Assessment and Correction of Underlying Factors
Before initiating magnesium replacement, first correct water and sodium depletion if present, as secondary hyperaldosteronism from volume depletion increases renal magnesium wasting and will render supplementation ineffective. 1 This is particularly critical in patients with diarrhea, high-output stomas, or short bowel syndrome where hyperaldosteronism drives ongoing renal magnesium losses despite total body depletion. 1
Check renal function before any magnesium supplementation - avoid magnesium in patients with creatinine clearance <20 mL/min due to risk of life-threatening hypermagnesemia. 1, 3
Treatment Algorithm by Severity
Mild Hypomagnesemia (Serum Mg 1.2-1.8 mg/dL or 0.5-0.74 mmol/L)
Oral magnesium oxide 12-24 mmol daily (480-960 mg elemental magnesium) is first-line treatment, given preferably at night when intestinal transit is slowest to maximize absorption. 1 The dose can be divided throughout the day if gastrointestinal side effects occur. 1
For patients with malabsorption or short bowel syndrome, higher doses or parenteral supplementation may be required as most oral magnesium salts are poorly absorbed and may worsen diarrhea. 1
Severe or Symptomatic Hypomagnesemia (Serum Mg <1.2 mg/dL or <0.5 mmol/L)
Parenteral magnesium sulfate is indicated for symptomatic patients or those with severe deficiency. 2, 3 The FDA-approved dosing includes:
- For acute severe hypomagnesemia: 1-2 g IV magnesium sulfate over 15 minutes 2
- For less urgent cases: 4-5 g (approximately 40 mEq) added to one liter of IV fluid infused over 3 hours 2
- For moderate deficiency: 1 g IM every 6 hours for 4 doses (total 32.5 mEq per 24 hours) 2
- Maximum rate of IV injection should not exceed 150 mg/minute except in life-threatening situations 2
Life-Threatening Presentations
For torsades de pointes with prolonged QT interval, administer 1-2 g magnesium sulfate as IV bolus over 5 minutes, regardless of baseline magnesium level. 1 For patients with QTc >500 ms, replete magnesium to >2 mg/dL as an anti-torsadogenic countermeasure. 1
For severe hypomagnesemia with tetany or seizures, up to 250 mg/kg (approximately 2 mEq/kg) may be given IM within 4 hours if necessary. 2
Special Clinical Considerations
Refractory Hypokalemia
Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment until magnesium is corrected. 1 Always check and normalize magnesium levels before expecting potassium supplementation to be effective. 1
Hypocalcemia
For hypomagnesemia-induced hypocalcemia, magnesium replacement must precede calcium supplementation, as magnesium is required for parathyroid hormone secretion and action. 1
Cancer Patients on Chemotherapy
Magnesium replacement is recommended for managing hypomagnesemia in cancer patients, particularly those receiving cisplatin or cetuximab which cause significant magnesium wasting. 4 Intravenous magnesium sulfate may reverse neurological symptoms including confusion, hallucinations, irritability, nystagmus, and seizures. 4
Patients on Nilotinib or Other QT-Prolonging Drugs
Hypomagnesemia must be corrected before initiating nilotinib and monitored periodically, as the drug prolongs QT interval and hypomagnesemia increases risk of sudden death. 4
Refractory Cases
If oral supplementation fails to normalize levels despite adequate dosing and correction of volume status, consider oral 1-alpha hydroxy-cholecalciferol in gradually increasing doses (0.25-9.00 μg daily) to improve magnesium balance, while monitoring serum calcium regularly to avoid hypercalcemia. 1
For patients with short bowel syndrome or severe malabsorption, subcutaneous administration with 4 mmol magnesium sulfate added to saline may be necessary. 1
Monitoring and Follow-up
Observe for resolution of clinical symptoms and monitor for secondary electrolyte abnormalities, particularly potassium and calcium, which often accompany hypomagnesemia. 1
In patients undergoing continuous renal replacement therapy, use dialysis solutions containing magnesium to prevent ongoing electrolyte derangements, as hypomagnesemia occurs in up to 65% of these patients. 1
Common Pitfalls
Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders - this is a critical consideration when selecting oral formulations. 1
Attempting to correct magnesium without first addressing volume depletion and hyperaldosteronism will fail, as ongoing renal losses will exceed supplementation. 1
Do not exceed 30-40 g total daily dose of magnesium sulfate, and in severe renal insufficiency, maximum dosage is 20 g per 48 hours with frequent serum magnesium monitoring. 2
Continuous maternal administration of magnesium sulfate in pregnancy beyond 5-7 days can cause fetal abnormalities and should be avoided. 2