Background

  • Normal range = 1.5 to 3.0 mEq/L
    • NB: This is a poor measure of total body Mg2+ as only a small fraction is within serum
  • 2nd most abundant cation in the body
  • Involved in >300 enzymatic reactions
    • Na+,K+-ATPase pump
  • Distribution:
    • 53% Bone
    • 27% Muscle
    • 19% Soft Tissues
    • 3% Serum

Epidemiology

  • Relatively common, especially in the critically ill
    • Reported incidence as high as 65%
    • Doubling of mortality rates (Tong 2015, PMID: 15665255)
  • High prevalence in:
    • Alcoholic population 60-85% (Tso 1992, PMID: 1491157)
    • Patients with CHF due to loop diuretics
    • Patients on thiazides
    • DKA
      • Increased osmotic diuresis
      • Acidosis increases renal excretion of magnesium
      • Insulin shifts Mg2+ intracellularly

Causes

Medications Gastrointestinal Renal Endocrine Inflammatory Hepatic
Aminoglycosides Diarrhea Diuretic phase of ATN Diabetes Infection/sepsis Alcoholism
Amphotericin B Gastric suctioning Postobstructive diuresis Hyperparathyroidism Postoperative period Cirrhosis
Cisplatin Intestinal fistula Hyperthyroidism Trauma
Cyclosporin Malnutrition Hyperaldosteronism
Digoxin Pancreatitis
Diuretics
Insulin
Tacrolimus
Proton pump inhibitors
(Table adapted from Chang 2014)

Clinical Manifestations

Biochemical

  • Hypokalemia
    • Renal K+ wasting
    • Decreased intracellular K+
  • Hypocalcemia
    • Impaired parathyroid hormone secretion
    • Renal and skeletal resistance to parathyroid hormone
    • Resistance to vitamin D

Neuromuscular

  • Tetany
  • Spontaneous carpal-pedal spasm
  • Seizures
  • Vertigo, ataxia, nystagmus, athetoid, and choreiform movements
  • Muscular weakness, tremor, fasciculation, and wasting
  • Psychiatric: depression, psychosis

Cardiovascular

  • Dysrhythmias
    • Ventricular tachycardia (torsade de pointes)
    • Atrial fibrillation
    • Supraventricular tachycardia
  • Hypertension
  • Vasospasm
  • Electrocardiographic changes
    • Prolonged QT interval
    • Prolonged PR interval
    • Wide QRS
    • Peaked T waves
    • ST depression

(Table adapted fromTong 2015)

Diagnosis

  • Difficult to accurately determine total body Mg2+amount
    • Largely distributed in bone and intracellularly while only extracellular serum levels measured
    • Significant portion of extracellular Mg2+ is protein bound
      • Ionized Mg2+ biologically active
    • Patient’s may be intracellularly depleted with normal extracellular levels (Tso 1992, PMID: 1491157)

Management

  • Serum magnesium level alone should not dictate therapy
  • Stop medication if iatrogenic
  • Dose reduction recommended if patient with renal failure
  • If concomitant hypokalemia is present, attempt to replete Mg2+ concurrently or before repleting K+
    • Hypomagnesemia is associated with enhanced renal K+ excretion and may result in refractory hypokalemia
  • For severe symptoms + hemodynamically unstable
    • Magnesium Sulfate 1 to 2 grams bolus
  • For severe symptoms + hemodynamically stable
    • Magnesium Sulfate 1 to 2 grams over 10 to 60 minutes
  • For concurrent repletion with potassium
    • 5 g/hr infusion (Rosen 2014)
  • NB: Increases in serum Mg2+ leads transient increase in renal excretion of magnesium
    • ~50% of IV Mg2+ given will be excreted

Take Home Points

  • <1% of total Mg2+ is extracellular, making accurate measurement difficult
  • Hypomagnesemia leads to increased K+ renal wasting and concurrent repletion of these electrolytes should be considered, especially if hypokalemia is refractory
  • The more you replete, the more you excrete (renally)

References

Chang WT, Radin B, McCurdy MT. Calcium, magnesium, and phosphate abnormalities in the emergency department. Emerg Med Clin North Am. 2014;32(2):349-66. PMID: 24766937

Marx JA, Rosen P. Rosen’s emergency medicine : concepts and clinical practice. 8th ed. Philadelphia, PA: Elsevier/Saunders; 2014.

Nelson L, Goldfrank LR. Goldfrank’s toxicologic emergencies. 9th ed. New York: McGraw-Hill Medical; 2011. xxviii, 1940 p.

Rubeiz GJ, Thill-Baharozian M, Hardie D, Carlson RW. Association of hypomagnesemia and mortality in acutely ill medical patients. Crit Care Med. 1993;21(2):203-9. PMID: 8428470

Tintinalli JE, Stapczynski JS, Ma OJ, Cline D, Meckler GD, Yealy DM. Tintinalli’s emergency medicine : a comprehensive study guide. Eight edition. ed. New York: McGraw-Hill Education; 2016. xliii, 2128 pages p.

Tong GM, Rude RK. Magnesium deficiency in critical illness. J Intensive Care Med. 2005;20(1):3-17. PMID: 15665255

Tso EL, Barish RA. Magnesium: clinical considerations. J Emerg Med. 1992;10(6):735-45. PMID: 1491157