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:

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