A 55 yo male with a history of significant alcohol intake presents with a 2-week history of lethargy. He takes no regular medications and has no other medical disorders. Clinically, he appears malnourished and euvolaemic. Investigations  reveal:

Plasma  
           Normal Range
Na+            115 mmol/L 134-143
K+ 3.7 mmol/L 3.5-5.0
Cl- 80 mmol/L   97-107
HCO3-   22 mmol/L 24-34
Urea 3.0 mmol/L   3.1-8.1
Creatinine 46 micromol/L 50-90
Glucose 4.1 mmol/L 4.4-6.8
Osmolality 241 mmol/Kg 274-289

Urine

Na+ <5mmol/L  
Osmolality 53 mmol/Kg  

a)  What is the most likely cause of the hyponatraemia?

[Click here to toggle visibility of the answers]

College Answer

Water intoxication.

Discussion

This is a hypoosmolar hyponatremia with low urine osmolality and low urine sodium.

I.e. the kidneys are making an excellent effort to excrete copious amounts of water while preserving sodium, which means that not only is the renal response appropriate, but that the whole body systems are actively trying to defend tonicity. This is the sort of response one would expect from a sudden massive excess of water.

So. how did this happen?

Well: the differentials in this scenario would include

  • Psychogenic polydipsia
  • Beer potomania
  • Dietary solute deficiency ("crash diet potomania")

References

References

Hariprasad MK, Eisinger RP, Nadler IM, Padmanabhan CS, Nidus BD. Hyponatremia in psychogenic polydipsia. Arch Intern Med. 1980 Dec;140(12):1639-42.

Hilden T, Svendsen TL. Electrolyte disturbances in beer drinkers. A specific "hypo-osmolality syndrome". Lancet. 1975 Aug 9;2(7928):245-6.

Thaler SM, Teitelbaum I, Berl T. "Beer potomania" in non-beer drinkers: effect of low dietary solute intake. Am J Kidney Dis. 1998 Jun;31(6):1028-31.

Fox BD.Crash diet potomania. Lancet. 2002 Mar 16;359(9310):942.

Lipschutz JH, Arieff AI. Reset osmostat in a healthy patient. Ann Intern Med. 1994 Apr 1;120(7):574-6.