Question 14.4

A 76 yo female presents with seizures. She takes no regular medications. On examination she weighs 60kg, has no evidence of cardiac failure or liver disease, and appears euvolaemic. Her blood results in the emergency department reveal:

Plasma  
           Normal Range
Na+            110mmol/L 134-143
K+ 3.8 mmol/L 3.5-5.0
Cl- 81 mmol/L   97-107
HCO3-   24 mmol/L 24-34
Urea 5.7 mmol/L   3.1-8.1
Creatinine 36 micromol/L 50-90
Glucose 4.1 mmol/L 4.4-6.8
Osmolality 237 mmol/Kg 274-289

Urine

Na+ 23 mmol/L  
Osmolality 488 mmol/Kg  

a) What is the likely cause of the hyponatraemia?

b) Approximately how many mmol of NaCl would need to be given to raise her serum sodium to 120mmol/L? . Show your calculations.

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College Answer

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

b) Approximately how many mmol of NaCl would need to be given to raise her serum sodium to 120mmol/L? . Show your calculations.
An answer between300 – 360 mmol was acceptable) (Sodium deficit = TBW x (desired Na – Actual Na)
= 0.5/0.6 x 60 x (120-110) 
= 30/36 x 10
= 300/360)

Discussion

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

The urine osmolality suggests that the kidneys are retaining water in spite of decreased body tonicity, and the inappropriately high urine sodium (>20mmol/L) suggests that are negligently wasting sodium. Of course, nobody is wasting anything - they are merely excreting a normal daily load (150-250mmol in the Western world), keeping up with intake.

This smells like SIADH.

Alternative differentials might include

  • Corticosteroid deficiency
  • Cerebral salt wasting
  • Thiazide diuretics
  • Hypothyroidism
  • Oliguric chronic renal failure

But wait. Is this really SIADH, with that urinary sodium? One needs to point out that the 20mmol/L cutoff is actually from the old Bartter & Schwartz criteria (dating back to 1967), that call for:

  • Decreased plasma osmolality (<275 mosm/kg)
  • Inappropriately concentrated urine (>100 mosm/kg)
  • Euvolemia
  • Urinary sodium >20 mEq/L
  • A patient who is euthyroid, eucortisolemic and not on diuretics 

This is supported by the modern RCPA cutoffs and the 2015 European guidelines.  Other authors  give a urinary sodium cutoff of 30 mmol/L.  Modern international sources with  paywall-level authority aim as high as 40 mmol/L, and even though they give a 2001 textbook chapter as a supporting reference, that value is supported by the most recent entry at the time of writing, a 2018 European consensus statement from enough acronyms to sound very authoritative and official (SIE, SIN, AIOM).

The question about sodium replacement relies on the candidate's ability to recall the formula for total body sodium deficit.

The formula is as follows:

NaDeficit = Total Body Water × Weight in kg × (desired Na+ - measured Na+)

( where TBW = 0.6 if male and 0.5 if female)

Thus, for this 60kg lady, the calculation would be:

(60) × (0.5) × (140-110) = 900mmol ... to bring the sodium back to a normal range

But, of course, a more sensible target would be a sodium level where seizures are no longer a problem. 120mmol/L would suffice. Thereafter, one can rely on water restriction to maintain the steady rise of body tonicity. In this case, the lady only needs 300mmol of sodium, which is two bags of isotonic saline.

References

Palmer, Biff F. "Hyponatremia in patients with central nervous system disease: SIADH versus CSW." Trends in Endocrinology & Metabolism 14.4 (2003): 182-187.

Milionis, Haralampos J., George L. Liamis, and Moses S. Elisaf. "The hyponatremic patient: a systematic approach to laboratory diagnosis."Canadian Medical Association Journal 166.8 (2002): 1056-1062.

Bartter, Frederic C., and William B. Schwartz. "The syndrome of inappropriate secretion of antidiuretic hormone." The American journal of medicine 42.5 (1967): 790-806.

Pliquett, Rainer U., and Nicholas Obermüller. "Endocrine testing for the syndrome of inappropriate antidiuretic hormone secretion (SIADH).Endotext [Internet] (2022).