ANSWER 79
This woman’s confusion is due to hyponatraemia. There are many causes of confusion in
the elderly but the very low sodium level of 113 mmol/L in this case is an adequate
explanation. Her serum is profoundly hypo-osmolar. In her case, osmolality can be calcu-
lated from the following equation:
2 %([Na']'[K'])'[urea]'[glucose])241 mosmol/kg (normal range:
278–305 mosmol/kg).
Hyponatraemia may be asymptomatic, but when it falls rapidly or reaches very low levels
(below 120 mosmol/kg) it can cause confusion, anorexia, cramps, fits and coma. Hypo-
natraemia is associated with hypovolaemia when there is excess loss of fluid and sodium
(sweating, burns, diarrhoea and vomiting), or when there is renal loss of sodium and water
(diuretic use, Addison’s disease).
Hyponatraemia with hypervolaemia occurs when there is excess retention of water.
Normally, the dilutional fall in plasma osmolality suppresses arginine vasopressin (AVP,
antidiuretic hormone) secretion which allows excretion of excess water. In congestive car-
diac failure and cirrhosis with ascites, baroreceptors register reduced perfusion causing AVP
secretion, but in most other cases of hyponatraemia there is an inability to suppress AVP
secretion normally. In rare cases of primary polydipsia, the huge water intake may over-
whelm this mechanism, and in severe renal failure the kidneys cannot excrete a water load.
The syndrome of inappropriate anti-diuretic hormone secretion (SIADH) occurs in relation
to malignancy, neurological disorders or pneumonia. Circulating volume is usually normal.
Normovolaemia with hyponatraemia also occurs after administration of too much intra-
venous hypotonic fluid and in hypothyroidism.
The low plasma sodium, potassium and urea in this patient are consistent with water excess.
Measurement of urinary sodium and osmolality is useful. In primary polydipsia the urine
can be maximally diluted to!100 mosmol/kg, whereas in states with excess AVP the urine
osmolality is usually#320 mosmol/kg while plasma osmolality is low. Urinary sodium is
usually!25 mmol/L in hypovolaemic states, but#40 mmol/L in SIADH where patients are
normovolaemic and the rate of sodium excretion depends on dietary intake and taking of
diuretics. Diuretic-induced hyponatraemia tends to occur within a few weeks of starting
treatment, and occurs mainly in elderly women often concurrently on non-steroidal anti-
inflammatory drugs (NSAIDs) which inhibit water excretion. The clinical and biochemical
picture in this woman is consistent with diuretic-induced hyponatraemia.
- Low plasma osmolality with high urinary osmolality suggests excess ADH production.
- Volume depletion with urinary sodium#20 mmol/L suggests water and sodium loss
through the kidneys (renal failure, diuretic use, Addison’s disease). - Volume depletion and low urinary sodium (!20 mmol/L) suggests volume and sodium
loss extrarenally, e.g. vomiting, diarrhoea, sweating.