Hypomagnesemia

Hypomagnesemia can lead to hypocalcemia as it interferes with the action of parathormone. 

 

More than 60% of the body’s  magnesium stores reside in bone, and nearly 70% of serum magnesium circulates freely, not bound to plasma proteins.

 

Can be responsible for neurological, respiratory and cardiovascular symptoms.

2% of the general population, 10-20% in hospitalized patients, 50-60% in intensive care unit patients, 60-80% of alcoholics, and 25% of diabetics.

Associated with impaired intake in alcoholics, undernourished, with intestinal malabsorption syndrome, prolonged vomiting and with the use of nasogastric tubes and in ICU setting.

Excessive renal losses seen with loop diuretics, thiazide and saline infusions, and interstitial nephritis.

In patients with heart failure, the critically ill, and patients recently undergoing dialysis is associated with the high mortality rate.

For every 0.49 mg of deciliter serum reduction there is a 30% increase in cardiovascular disease.

Seen with hyperaldosterone states such as primary hyperaldosteronism, cirrhosis, congestive heart failure and fluid depletion.

Can be seen with osomotic diuresis associated with diabetes mellitus.

Seen with nephrotoxic drugs cisplatin, amphotericin, aminoglycoside antibiotics.

Associated with hypokalemia, hypercalcemia and hypercalciuria.

Associated with lesions of the proximal tubules, such as Fanconi’s disease and interstitial nephritis.

Magnesium absorbed in the small intestine, mainly in the ileum, via active and passive transport mechanisms.

Magnesium excreted in the stool and urine.

Regulation of serum levels of magnesium under renal control.

Most renal absorption of magnesium occurs in the proximal tubule and the ascending limb of the loop of Henle.

Magnesium in the presence of hypomagnesememia the kidney may excrete as little as 1 mEq/L.

Magnesium in times of deficiency may be removed from bone.

Low blood magnesium makes spontaneous cardiac muscle depolarization more likely.

Hypomagnesemia is sometimes caused by the proton-pump inhibitor (PPI) inhibitors.

Treatment depends on the degree of deficiency and the clinical status.

Non-Renal hypomagnesaemia:

Redistributive such as refeeding or insulin therapy

Reduced magnesium intake

Reduced gastrointestinal absorption as with proton pump inhibitors

Alcohol use

Diabetes

 Renal wasting hypomagnesaemia:

Diuretics

Antimicrobials as with amphotericin B, aminoglycosides

Calcineurin Inhibitors as with cyclosporine, tacrolimus

Epidermal growth factor receptor inhibitors cetuximab

 Cytotoxic drugs as with cis-platinum, carboplatinum

Hypercalcemia

Hyperthyroidism

Hyperparathyroidism

Alcohol use

Diabetes

In evaluating hypomagnesaemia, the initial step is to differentiate renal versus non-renal losses using urine magnesium testing.

24 hour urine magnesium levels are recommended as spot urines for magnesium very with diet, diurnal circadian rhythm, and because the 24 hour measure allows an assessment of daily urine volume.

Renal magnesium wasting is defined as a fraction excretion of magnesium greater than 4%, or a daily urinary magnesium excretion of more than 1 mmol per day.

24 hour urine creatinine is estimated to be 15 mg per kilogram per deciliter in females and 20 mg per kilogram per day in males.

Polyuria is a common cause of renal magnesium wasting with increased urinary transit time, reducing the contact time of electrolytes and tubular epithelial cells, attenuating the ability to establish a transepithelial voltage gradient and reducing the tubular reabsorption of magnesium.

Oral replacement is appropriate for a mild process, while IV replacement is indicated for severe clinical disease.

Impaired renal function may increase toxicity of magnesium administration.

Diarrhea is most common adverse effect of oral magnesium.

Intravenous therapy is with 2-4 g of 50% magnesium sulfate diluted in saline or dextrose IV over 30-60 minutes.

In cases of life-threatening arrhythmias intravenous push of the magnesium sulfate is appropriate.

Intravenously administered magnesium may alter cardiac conduction, leading to heart block in digitalized patients decrease respiratory rate, impair deep tendon reflexes.

Renal function should be monitored when magnesium is administered parenterally.

Administering magnesium intravenously may produce significant hypertension or asystole.

If an overdose occurs, calcium gluconate, 10-20 mL of 10% solution IV, can be given as antidote for hypermagnesemia.