HYPONATRAEMIA - a step-wise approach to clinical cases
concepts:
1. calculated serum osmolality= 2Na + BUN/3 + Glu/20
2. measured serum osmolality= measured in a lab, includes ALL osmoles
* a value >10 indicates that there are other osmoles not included in the formula.
* normal serum osmolality [275-295]
A patient w/ hyponatraemia, what do we do?
1. calculate the serum osmolality, then measured osmolality:
a. hyperosmolar [ddx: DKA -> true dilutional hyponatraemia, Glycine -> used in uro & gyne surgeries, Mannitol -> used in neurosurgery]
* Glycine & Mannitol are NOT detected by the serum osmolality
formula, that's why we order Measured osmolality.
* Hyperglycaemia decreases serum Na: for each 100 glucose above the normal value (100), Na decreases 2.
b. iso-osmolar - true pseudo-hyponatraemia [ddx: multiple myeloma, hyperlipidaemia]
* if the calculated & measured serum osmolalities are within the normal range -> order lipid panel and MM work-up.
c. hypo-osmolar -> assess the volume status
a. hypervolemic: CHF, renal failure, cirrhosis or malabsorption [hypoproteinaemia] -> activates the RAAS & ADH. Look for oedema, ascites, and increased JVP. Treat the underlying cause.
b.hypovolemic: vomiting, diarrhoea .. etc. Manage w/ fluids.
c.isovolumic -> check the urine osmolality [normal 50-1200 mosm] & urine Na+.
1. water intoxication [psychogenic polydipsia, beer potomania, tea-toast syndrome .. etc]: low serum AND urine osmolalities [kidneys are dumping water to compensate], urine Na+ <10.
2. SIADH: low serum osmolality, high urine osmolality, urine Na [20-40]. We have to r/o hypothyroidism & adrenal insufficiency.