Pearl of the Day: Hypomagnesemia

Hypomagnesemia Pathophysiology - second most abundant intracellular cation - acid-base imbalance affects levels of ionized magnesium - may coexist with hypokalemia - essential to many enzymes, including membrane-bound ATPase; metabolism; regulation of PTH secretion - magnesium blocks release of acetylcholine, interferes with release of catecholamines from adrenal medulla - magnesium is absorbed principally in small intestine - associated with hypokalemia due to similar underlying etiologies

Causes - redistribution: IV glucose, IV hyperalimentation, refeeding syndrome, acute pancreatitis, hypoalbuminemia, postparathyroidectomy, osteoblastic metasis - extrarenal losses: lactation, profuse sweating/burns, sepsis, intestinal or biliary fistula, diarrhea - decreased intake: alcoholism, malnutrition, small bowel resection, malabsorption - renal loss: saline or osmotic diuresis, potassium depletion, phosphorus depletion, familial hypophosphatemia, tubulointerstitial renal disease - drugs: loop diuretics, aminoglycosides, amphotericin B, vitamin D intoxication, alcohol, cisplatin, theophylline, PPIs, calcineurin inhibitors - endocrine disorders: SIADH, hyperthyroidism, hyperparathyroidism, hypercalcemic states, hyperaldosteronism

Signs/Symptoms - neuromsucular: tetany, muscle weakness, cerebellar (ataxia, nystagmus, vertigo), confusion, coma, seizures, depression, paresthesias - GI: dysphagia, anorexia - CV: heart failure, dysrhythmias, hypotension, coronary artery vasospasm - hypokalemia, hypocalcemia, anemia - Chvostek and Trousseau signs (traditionally associated with hypocalcemia)

Diagnosis - likely underdiagnosed as levels are rarely drawn - BMP, LFTs, phosphorus, calcium, magnesium, EKG - EKG changes similar to hypokalemia (tachyarrhythmias, afib, torsades de pointes, ventricular tachycardia, ventricular fibrillation) and hypocalcemia  (prolonged QT interval, T wave inversions) due to alteration of intracellular potassium content - enhances digitalis toxicity -> may also contribute to EKG changes - correction for hypoalbuminemia corrected Mg (mmol/L) = measured total Mg + [0.005 x (40 - serum albumin in g/L)] corrected Mg (mEq/L) = measured total Mg x 0.42 + 0.05 x (4 - serum albumin in g/dL)

Treatment - monitor for hypokalemia, hypocalcemia, hypophosphatemia - if asymptomatic, oral magnesium in multiple low doses - for severe and symptomatic, urgent MgSO4 IV replacement - torsades de pointes or eclampsia -> 1 - 4 g or diluted in 100 mL D5 or NS over 10 - 60 min under continuous cardiac monitoring - chronic deficiency -> 6 g MgSO4 per day - chronic alcoholics with delirium tremens -> 8 - 12 g MgSO4 IV on first day (1.5 - 2 g Iv MgSO4 over 1 to 2 hours) - spironolactone maintains magnesium homeostasis and reduces arrhythmias in CHF patients

Resources Tintinalli's Emergency Medicine, 8th Edition