Hypomagnesemia & Hypermagnesemia

Magnesium is the fourth most abundant cation in the body and the second most abundant intracellular cation. It is a cofactor for over 300 enzymatic reactions, including ATP synthesis, DNA replication, and protein synthesis. It is also essential for the function of the Na-K-ATPase pump — which means that magnesium deficiency directly impairs the ability to maintain normal potassium and calcium levels.

Despite its importance, magnesium is frequently not included in routine electrolyte panels and is often the last electrolyte checked when a patient has refractory hypokalemia or hypocalcemia. This is a clinical error. Hypomagnesemia should be on the differential for any patient with unexplained or refractory hypokalemia, hypocalcemia, or cardiac arrhythmia.

The serum magnesium is a poor reflection of total body stores — only about 1% of total body magnesium is in the extracellular space. A normal serum magnesium does not exclude intracellular magnesium depletion. Patients with chronic alcoholism, malnutrition, or prolonged diuretic use may have significant total body depletion with a serum level that appears normal or only mildly low.

Hypomagnesemia — a serum magnesium below 1.7 mg/dL — results from decreased intake, increased GI losses, increased renal losses, or redistribution.

Decreased intake is common in patients with poor nutrition, chronic alcoholism, and prolonged parenteral nutrition without adequate magnesium supplementation. Alcoholism causes hypomagnesemia through multiple mechanisms: poor intake, increased renal wasting (alcohol directly impairs tubular magnesium reabsorption), and GI losses from vomiting and diarrhea.

Increased GI losses occur with prolonged diarrhea, malabsorption syndromes, and short bowel syndrome. Magnesium is absorbed primarily in the small intestine, so conditions that impair small intestinal absorption (Crohn's disease, celiac disease, surgical resection) cause magnesium deficiency.

Increased renal losses are caused by loop and thiazide diuretics (the most common cause in hospitalized patients), osmotic diuresis, hypercalcemia (which competes with magnesium for tubular reabsorption), and medications including cisplatin, amphotericin B, aminoglycosides, and — importantly — proton pump inhibitors. Long-term PPI use impairs intestinal magnesium absorption through a mechanism that is not fully understood but is a class effect.

The clinical consequences of hypomagnesemia include neuromuscular excitability (tremor, muscle cramps, tetany, seizures), cardiac arrhythmias (particularly torsades de pointes and other ventricular arrhythmias), and the downstream effects of refractory hypokalemia and hypocalcemia.

Mild asymptomatic hypomagnesemia (1.2–1.7 mg/dL) can be treated with oral magnesium supplementation. Magnesium oxide is the most commonly used oral form, but it has poor bioavailability and causes diarrhea at higher doses. Magnesium glycinate and magnesium citrate have better GI tolerability. Oral repletion is appropriate for outpatients and hemodynamically stable inpatients without active arrhythmia.

Moderate to severe hypomagnesemia (below 1.2 mg/dL), symptomatic hypomagnesemia, or hypomagnesemia with active cardiac arrhythmia requires IV magnesium sulfate. A typical dose is 2–4 g IV over 2–4 hours for moderate deficiency, with additional doses as needed based on repeat levels. For torsades de pointes, 2 g IV over 1–2 minutes is the immediate treatment.

Renal magnesium wasting limits the efficiency of IV repletion — a significant fraction of IV magnesium is excreted in the urine before it can be taken up by cells. Slow infusion rates improve cellular uptake. Repeat magnesium levels 4–6 hours after IV repletion to assess response, and plan for multiple rounds of repletion in patients with ongoing losses.

Address the underlying cause: stop or reduce the offending medication if possible, treat the GI condition, provide adequate nutritional support. In patients on chronic PPIs who develop recurrent hypomagnesemia, consider switching to an H2 blocker if clinically appropriate.

Hypermagnesemia — a serum magnesium above 2.2 mg/dL — is almost exclusively iatrogenic or due to renal failure. The kidney is highly efficient at excreting magnesium; hypermagnesemia in patients with normal renal function requires a substantial exogenous magnesium load.

The most common causes are: IV magnesium for eclampsia or preterm labor (therapeutic hypermagnesemia is intentional, but toxicity can occur); magnesium-containing antacids and laxatives (particularly in patients with renal impairment who cannot excrete the load); magnesium-containing enemas (Fleet enemas contain magnesium); and renal failure with normal or increased magnesium intake.

The clinical consequences of hypermagnesemia follow a predictable progression with rising magnesium levels. Mild hypermagnesemia (2.5–4 mg/dL) causes nausea, flushing, and headache. Moderate hypermagnesemia (4–7 mg/dL) causes hyporeflexia, somnolence, and hypotension. Severe hypermagnesemia (above 7 mg/dL) causes respiratory depression, cardiac conduction abnormalities (bradycardia, heart block), and cardiac arrest. Loss of deep tendon reflexes is an early clinical sign of toxicity and is used to monitor patients receiving therapeutic magnesium infusions.

The first step is to stop all magnesium-containing medications and supplements. In patients with normal renal function, the kidney will excrete the excess magnesium and levels will normalize over hours.

For symptomatic hypermagnesemia — respiratory depression, cardiac conduction abnormalities, hemodynamic instability — IV calcium gluconate is the immediate intervention. Calcium antagonizes the neuromuscular and cardiac effects of magnesium by competing for the same calcium channels. The typical dose is 1–2 g IV over 5–10 minutes, repeated as needed. This is a temporizing measure — it does not remove magnesium from the body.

Definitive treatment requires magnesium elimination. In patients with adequate renal function, IV fluids and loop diuretics can enhance renal magnesium excretion. In patients with renal failure or severe toxicity, hemodialysis is the most effective method of magnesium removal.

In patients receiving therapeutic magnesium infusions (for eclampsia or preterm labor), monitor deep tendon reflexes, respiratory rate, and urine output continuously. Loss of patellar reflex is the first sign of toxicity and should prompt dose reduction or discontinuation. Respiratory depression requires immediate calcium gluconate and supportive care.