Question 16

Write a short note on hypomagnesaemia.

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College Answer

A common electrolyte abnormality in the ICU:
Mg primary intracellular cation and plays a major·role in the transfer, storage and utilization of energy.
Causes: diarrhoea, NG suction, TPN, RTA, alcoholism, malabsorption
Drugs-amphotericin B, Aminoglycosides, Carbenicillins, diuretics.
Pathophysiology: Mg deficiency leads to a drop in ICF potassium and a rise in ICF Na., leading to an elevation in the resting membrane potential. This leads to a rise in the inward Ca current and hence the enhanced neurological and cardiac irritability.
Effects: Confusion, injtability, delirium, tremors, tachyanhytbmias, Torsade,
refractory hypokalemia and hypocalcemia.
Treatment: IV MgS04 in doses of 5-10 mmol/L, given slow IV. Repeated doses may
be required. Rapid administration can lead to hypotension.

Discussion

There are few fellowship questions in this exam which ask the candidate to write a short note about anything. Understandably, somebody who was waiting to critically evaluate something or to discuss your management would have been taken aback by such a question. How does one structure a response?

Using this article, I have attempted a coherent answer.

Causes of hypomagnesaemia

Malnutrition/malabsorption
NG suction
Diarrhoea
Diuretics
Renal tubular acidosis
Aminoglycosides
Hyperparathyroidism
Diabetes
Alcoholism

Consequences of hypomagnesaemia

hypokalemia and hypocalcemia
tetany, muscle cramps
vertigo, nystagmus
delirium
ventricular arrhythmias, particularly Torsades de points
increased sensitivity to digoxin toxicity

Pathophysiology of cardiac consequences

Magnesium is required for the function of Na⁺/K⁺ ATPase.
Na⁺/K⁺ ATPase maintains the Na⁺ and K⁺ concentration gradients
If intracellular K⁺ concentration decreases, the cell membrane potential becomes less negative which increases its vulnerability to cardiac arrhythmias.

Management of hypomagnesaemia

Magnesium replacement
Amelioration of the aetiology of magnesium loss
Careful monitoring of rising levels

In greater detail, from the hypomagnesemia chapter:

Causes of Hypomagnesemia from Oh's Manual
Gastrointestinal disorders Malabsorption syndromes GIT fistulas Short-bowel syndrome Prolonged nasogastric suction Diarrhoea Pancreatitis Parenteral nutrition Endocrine disorders Hyperparathyroidism Hyperthyroidism Conn’s syndrome Diabetes mellitus Hyperaldosteronism	Renal diseases Renal tubular acidosis Diuretic phase of acute tubular necrosis Drugs Alcohol Aminoglycosides Carbenicillin, ticarcillin Amphotericin B Diuretics Cisplatin Cyclosporine

A better way to organise the list of causes would be by pathophysiological disturbance, as below.

Causes of Hypomagnesemia ordered by Pathophysiology
Increased Loss Gastrointestinal loss GIT fistulas Prolonged nasogastric suction Diarrhoea Pancreatitis Renal loss Renal tubular acidosis Diuretic phase of acute tubular necrosis Thiazide diuretics Loop diuretics Alcoholism Hypercalcemia Nephrotoxic drugs, eg. aminoglycosides, amphotericin, cyclosporin etc. Primary renal magnesium wasting (a congenital disorder) Sequestration and chelation Pancreatitis (saponification in fat) Post-operative (chelation with fatty acids) Post cardiopulmonary bypass (adsorption into circuit) Foscarnet (chelation)	Decreased intake Poor intake Fasted patient Improperly prescribed parenteral nutrition Poor absorption Malabsorption syndromes, eg. coeliac disease Short-bowel syndrome Weird congenital disorders of selective magnesium malabsorption Unclear association with low magnesium Hyperthyroidism Conn’s syndrome Diabetes mellitus Hyperaldosteronism

Clinical Features of Hypomagnesemia
Symptoms Physical signs Confusion Delirium Tremors Seizures Tachyarrhythmias (particularly VT and VF) Tetany Chvostek sign Trousseau sign	ECG changes Widening QRS complexes Peaking T-waves (which vanish in very severe hypomagnesemia) Prolonged PR interval Associated biochemical abnormalities Hypokalemia, refractory to replacement Hypocalcemia Low parathyroid hormone levels (in spite of hypocalcemia) Low Vitamin D levels

References

Agus, Zalman S. "Hypomagnesemia." Journal of the American Society of Nephrology 10.7 (1999): 1616-1622.

Kutsal, Ebru, et al. "Severe hypermagnesemia as a result of excessive cathartic ingestion in a child without renal failure." Pediatric emergency care 23.8 (2007): 570-572.

SHILS, MAURICE E. "Experimental human magnesium depletion." Medicine 48.1 (1969): 61.

Grubbs, Robert D., and Michael E. Maguire. "Magnesium as a regulatory cation: criteria and evaluation." Magnesium 6.3 (1986): 113-127.

Martin, Kevin J., Esther A. González, and Eduardo Slatopolsky. "Clinical consequences and management of hypomagnesemia." Journal of the American Society of Nephrology 20.11 (2009): 2291-2295.

Chakraborti, Sajal, et al. "Protective role of magnesium in cardiovascular diseases: a review." Molecular and cellular biochemistry 238.1-2 (2002): 163-179.

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