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 |
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
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:
It could still be
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:
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.
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.