Hypomagnesemia

Background

Magnesium, the fourth most common cation in the body, has been the recent focus of much clinical and scholarly interest. Previously underappreciated, this ion is now established as a central electrolyte in a large number of cellular metabolic reactions, including DNA and protein synthesis, neurotransmission, and hormone-receptor binding. It is a component of GTPase and a cofactor for Na⁺/K⁺ –ATPase, adenylate cyclase, and phosphofructokinase. Magnesium also is necessary for the production of parathyroid hormone. Accordingly, magnesium deficiency has an effect on multiple body functions.

Magnesium is present in greatest concentration within the cell and is the second most abundant intracellular cation after potassium. The total body content of magnesium is 2000 mEq. The intracellular concentration of magnesium is 40 mEq/L, while the serum concentration is 1.5-2 mEq/L. Most of the body's magnesium is found in bone. Only 1% of the total body magnesium is extracellular. Of this amount, one half is ionized, and 25-30% is protein bound.

Magnesium, a component of chlorophyll, is absorbed in the small bowel by active and passive transport mechanisms. Absorption of dietary magnesium takes place mainly in the ileum. It is excreted in stool and urine, but regulation of serum magnesium is under renal control. Most renal reabsorption of magnesium occurs in the proximal tubule and the thick ascending limb of the loop of Henle. In hypomagnesemic patients, the kidney may excrete as little as 1 mEq/L of magnesium. Additionally, magnesium may be removed from bone stores in times of deficiency.

View Image

A: Magnesium reabsorption in the thick ascending limb of the loop of Henle. The driving force for the reabsorption against a concentration gradient is....

• References

Pathophysiology

Clinical effects of hypomagnesemia are greatest in the CNS, neuromuscular, GI, and cardiac systems.

• References

Epidemiology

Frequency

United States

Risk of incidence is as follows:

• 2% in the general population
• 10-20% in hospitalized patients
• 50-60% in ICU patients
• 30-80% in alcoholics
• 25% in diabetic outpatients

Sex

Incidence is equal in males and females.

Age

Magnesium deficiency may contribute to many age-related diseases.^[1]

• References

History

• Clues to the presence of hypomagnesemia can be found by obtaining a history of potential causes (see Causes).

  • Historical complaints related to hypomagnesemia are nonspecific.
  • Patients may report weakness, muscle cramping, or rapid heartbeats.
  • Altered mental status may be present in severe cases. Less severe cases may result in vertigo, ataxia, depression, and seizure activity.

   

  

   

• References

Physical

The primary clinical findings are neuromuscular irritability, CNS hyperexcitability, and cardiac arrhythmias. The severity of symptoms is not related directly to the magnesium level. The reference range for serum magnesium level is 1.8-3 mEq/L. Usually, patients become symptomatic at 1.8 mEq/L. However, the physical findings may not be present in all cases. In one study of patients who were severely depleted of magnesium, abnormal physical findings were present in only 2 of 21 patients.

• Neuromuscular irritability

  • Hyperactive deep tendon reflexes
  • Muscle cramps
  • Muscle fibrillation
  • Trousseau and Chvostek signs
  • Dysarthria and dysphagia from esophageal dysmotility

   

  

   
• CNS hyperexcitability

  • Irritability and combativeness
  • Disorientation
  • Psychosis
  • Ataxia, vertigo, nystagmus, and seizures (at levels < 1 mEq/L)

   

  

   
• Cardiac arrhythmias that may be caused by hypomagnesemia alone or concomitant hypokalemia result from decreased activity of ATPase.

  • Paroxysmal atrial and ventricular dysrhythmias
  • Repolarization alternans

   

  

   
• Neonates

  • Apnea
  • Weakness
  • Seizures
  • Jitteriness

   

  

   

• References

Causes

The causes of hypomagnesemia are numerous. Most causes are related to renal and GI losses.

• GI losses

  • Malabsorption of magnesium in the ileum results in hypomagnesemia. Situations of decreased absorption include malabsorption syndromes (eg, celiac sprue), radiation injury to the bowel, bowel resection, or small bowel bypass.
  • GI secretions in large amounts may cause hypomagnesemia. Upper GI secretions contain 1 mEq/L of magnesium, while lower GI secretions contain 15 mEq/L of magnesium. Significant losses of magnesium that result in hypomagnesemia may result from chronic diarrhea, laxative abuse, inflammatory bowel disease, or neoplasm.

   

  

   
• Malnutrition leads to hypomagnesemia when dietary intake of magnesium is low. Dietary sources include green vegetables, fruits, fish, fresh meat, and cereal. Alcoholics are classically hypomagnesemic in part due to poor nutrition.^[2] Diabetic patients who are not receiving magnesium supplements may have dietary deficiencies in magnesium.
• Renal losses from primary renal disorders or secondary causes (eg, drugs, hormones, osmotic load) may result in magnesium wasting and subsequent hypomagnesemia.

  • Primary renal disorders cause hypomagnesemia by decreased tubular reabsorption of magnesium by the damaged kidneys. This condition occurs in the diuretic phase of acute tubular necrosis, postobstructive diuresis, and renal tubular acidosis.
  • Drugs may cause magnesium wasting.
    • Diuretics (eg, thiazide, loop diuretics) decrease the renal threshold for magnesium reabsorption in addition to wasting of potassium and calcium.
    • Cisplatin causes dose-dependent kidney damage in 100% of patients receiving this drug.
    • Pentamidine and some antibiotics also cause renal magnesium wasting.
    • Fluoride poisoning similarly causes hypomagnesemia.
  • Endocrine disorders may cause hypomagnesemia.
    • Primary aldosteronism decreases magnesium levels by increasing renal flow.
    • Hypoparathyroidism and hyperthyroidism may cause renal wasting.
  • Osmotic diuresis results in magnesium loss in the kidney.
    • Diabetic patients, especially those with poor glucose control, develop hypomagnesemia from a glucose-induced osmotic diuresis.
    • Alcoholics become hypomagnesemic partially by an osmotic diuresis from alcohol. Urinary losses have been reported to be 2-3 times control values.

   

  

   
• Miscellaneous

  • Extracellular volume expansion, as in cirrhosis or intravenous (IV) fluid administration, may decrease magnesium levels.
  • Redistribution of magnesium into cells may cause lower magnesium levels. Insulin causes this effect.
  • Excessive lactation may create a significant amount of magnesium loss.
  • Hungry bone syndrome may lead to lower serum magnesium concentrations.
  • Pregnant women have been found to be magnesium depleted, especially those women who experience preterm labor.

   

  

   

• References

Laboratory Studies

• Serum magnesium, calcium, potassium, and phosphorus levels

  • The serum magnesium level is not a reliable way to determine total body magnesium depletion because only a small fraction of magnesium in the body is extracellular. Consider obtaining an ionized magnesium level.
  • Body stores of magnesium may be depleted markedly before the serum level drops.
  • Nevertheless, a deficiency of magnesium is clearly present if the serum level is low.
  • In one study, alcoholics were found to have total body magnesium depletion (via measurement of their 24-hour urinary magnesium excretion) despite having normal serum levels.
  • Because extracellular magnesium is protein bound, the patient's protein status is an important consideration in interpreting magnesium levels.
  • Hypomagnesemia contributes to hypokalemia. This condition may be due to defective membrane ATPase or urinary losses of potassium.
  • Hypocalcemia is caused by magnesium depletion, but the reason is not known.
    • Some studies link hypomagnesemia to decreased parathyroid hormone levels and end-organ resistance to parathyroid hormone.
    • Alterations in vitamin D metabolism contribute to hypocalcemia.
  • Hypophosphatemia has been found in patients with hypomagnesemia.

   

  

   
• BUN and creatinine levels
• Blood glucose level

• References

Other Tests

• ECG and cardiac monitor

  • Findings in hypomagnesemia are nonspecific.
  • Findings include ST segment depression; tall, peaked T waves; flat T waves or depression in the precordium; U waves; loss of voltage; PR prolongation; and widened QRS.

   

  

   

• References

Prehospital Care

Be attentive to the ABCs. At this point, the diagnosis usually is not known; therefore, advanced cardiac life support (ACLS) protocol should be followed. Seizures should be treated with benzodiazepines.

• References

Emergency Department Care

• Treat life-threatening dysrhythmias according to ACLS protocol or as outlined below if hypomagnesemia is known.
• Treat seizures with benzodiazepines.
• Perform history, physical examination, and appropriate laboratory tests.
• Treat hypomagnesemia appropriately.

• References

Medication Summary

Treatment of hypomagnesemia depends on the degree of deficiency and the clinical effects. Oral replacement is appropriate for mild symptoms, while IV replacement is indicated for severe clinical effects.

• References

Class Summary

These agents are used to replace an existing magnesium deficit.

• References

Magnesium gluconate (Almora)

Clinical Context:  Oral supplementation should be given when patient is mildly depleted of magnesium (ie, magnesium level >1 mEq/L and patient is asymptomatic).

Other oral supplements (eg, magnesium oxide, magnesium hydroxide) may be used.

• References

Magnesium sulfate

• Dosing, Interactions, etc.

Clinical Context:  Supplementation via IV infusion should be given to patients with moderately severe to severe depletion.

• References

Further Inpatient Care

• Urine should be collected and sent for measurement of 24-hour excretion of magnesium. In a hypomagnesemic patient with healthy kidneys, the excretion of magnesium should be less than 1 mEq/L.
• Continue IV therapy.

• References

Inpatient & Outpatient Medications

• Oral supplementation should be considered in patients who do not have a correctable cause for their hypomagnesemia.

• References

Deterrence/Prevention

• Education of high-risk populations may decrease recurrence.

• References

Prognosis

• Patients have an excellent prognosis once the deficiency is corrected. For the most part, the symptoms are reversible with treatment.

• References