Hyponatraemia Quiz

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Which of the following best describes the underlying fluid balance in euvolaemic hyponatraemia?

A Reduced total body sodium with reduced total body water
B Normal total body sodium with reduced total body water
C Normal total body sodium with increased total body water ✓
D Reduced total body sodium with normal total body water
E Increased total body sodium with markedly increased total body water

The answer is C: Normal total body sodium with increased total body water

Explanation

Why C is correct

See Euvolaemic. Option C is correct because in euvolaemic hyponatraemia the total body sodium is essentially normal, but total body water is increased. The extra water dilutes the plasma sodium concentration, which is why the blood test shows hyponatraemia even though the body is not short of sodium. The patient looks euvolaemic, meaning that their volume status looks normal, with no oedema (swelling from excess fluid) and no signs of dehydration. This fits what the serum sodium actually measures. It is a measure of how much water there is relative to sodium, not of how much sodium the body holds, so a normal amount of sodium dissolved in too much water gives a low concentration. The key mechanism is water retention rather than sodium loss. Common causes are the syndrome of inappropriate antidiuresis (SIADH, in which ADH, the hormone that makes the kidneys retain water, is released inappropriately), hypothyroidism, glucocorticoid deficiency and primary polydipsia (drinking too much water). Treatment follows from the mechanism: restrict fluids and treat the cause, rather than giving salt.

Sodium: total body sodium is essentially normal.
Water: total body water is increased, which dilutes the plasma sodium.
Examination: no oedema and no signs of dehydration.
Causes: SIADH, hypothyroidism, glucocorticoid deficiency and primary polydipsia.

Why the other options are incorrect

A. Reduced total body sodium with reduced total body water: This describes hypovolaemic hyponatraemia, in which sodium loss exceeds water loss, so the patient looks dry. In euvolaemic hyponatraemia the problem is excess water, not sodium loss.
B. Normal total body sodium with reduced total body water: Too little water with normal sodium would concentrate the plasma and raise the serum sodium (hypernatraemia). It is the opposite of hyponatraemia.
D. Reduced total body sodium with normal total body water: This is not one of the three recognised patterns of hyponatraemia. Sodium holds water in the extracellular space, so a patient who has lost sodium has a reduced extracellular volume and would be called hypovolaemic, not euvolaemic.
E. Increased total body sodium with markedly increased total body water: This is the pattern of hypervolaemic hyponatraemia, seen in heart failure, cirrhosis and nephrotic syndrome, where both are raised but water is raised more and the patient is oedematous. In euvolaemic hyponatraemia sodium is not increased and there is no oedema.

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1
In clinical practice, how is hypotonic hyponatraemia usually subdivided?
2
A 32-year-old woman is found to have a serum sodium of 124 mmol/L on routine blood testing. She is asymptomatic. Which investigation is needed first to classify her hyponatraemia?

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