A 76-year-old female presents with seizures. She takes no regular medications. On examination she weighs 60 kg, has no evidence of cardiac failure or liver disease, and appears euvolaemic. Her results in the Emergency Department reveal the following:

Parameter

Patient Value

Adult Normal Range

Blood Results:

Na+

110 mmol/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 mmol/L*

50 – 90

Osmolality

237 mmol/kg*

274 – 289

Urine Results:

Na+

23 mmol/L*

10 – 20

Osmolality

488 mmol/kg

40 – 1200

  1. What is the likely cause of the hyponatraemia?                                                
    (10% marks)
  1. Approximately how many mmol of NaCl would need to be given to raise her serum sodium to 120 mmol/L? Show your calculations.                                              
    (20% marks)

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College answer

a)    SIADH

b)    (An answer between 300 - 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 concentrated urine and a high urine sodium. The college also told us the patient was euvolaemic. There are several possibilities which do not fit the scenario:

  • Diuretic therapy (thiazides) - but she's not on any regular meds
  • Polyuric phase of ATN - but her creatinine is normal
  • Chronic renal failure - but her creatinine is normal
  • Hypoadrenalism - but the potassium is not raised and there is no acidosis

It could still be

  • Hypothyroidism - 
  • Pan-hypopituitarism

These are less likely from the history; which is to say, if the college had wanted you to go down that road, they'd have given you red flags for myxoedema like hypothermia and bradycardia. SIADH is chosen by the examiners probably because cerebral salt wasting (the other possible cause of this electrolyte pattern) is less likely in somebody who has not had a severe head injury or intracranial haemorrhage. 

For a diagnosis of SIADH, one needs to have:

  • Hypoosmolar hyponatremia
  • Urine osmolality greater than plasma osmolality
  • Urine sodium excretion greater than 20mmol/L
  • Normal renal, hepatic, cardiac, pituitary, adrenal and thyroid function
  • Absence of hypotension, hypovolemia, oedema and ADH-influencing drugs
  • Hyponatremia corrected with water restriction

So, most of these are covered in the provided material. 

As for the calculation of the sodium deficit:

Sodium deficit = 0.6 ×body weight × (desired concentration - current concentration) 

The multiplier of body weight is 0.6 for men and 0.5 for women (whose fraction of body water is smaller). For this elderly 60kg woman, assuming you want to get her back to a sodium level of 135mmol/L the equation calls for 30L  × 10mmol = 300 mmol of sodium. 

References

References

Spasovski, Goce, et al. "Clinical practice guideline on diagnosis and treatment of hyponatraemia." European Journal of Endocrinology 170.3 (2014): G1-G47.

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.