Step 1: Make sure its really hypotonic hyponatremia
●Pseudohyponatremia (lab error)=HLD, multiple myeloma
●Hypertonic hyponatremia=hyperglycemia, mannitol, IVIG etc.
Confirm with serum osmolarity if not sure (normal is 280-300)

 
Step 2: What is the volume status? Use the H&P!

 
●History: CHF, nephrotic syndrome, cirrhosis, ask about poor PO intake, vomiting, diarrhea, excessive water intake, and diuretic use
●Physical exam: Look at JVP, skin turgor, orthostasis, vital signs

Step 3: If euvolemic, next step is checking urine osm

If LOW <100, and low urine sodium, think of two things

●Primary polydipsia (eg: psychiatric patient, marathon runner, or MDMA use)
●Low solute diet (eg: tea & toast diet, beer potamania)

If HIGH, >100, and urine sodium >20

●SIADH
●Hypothyroidism
●Adrenal insufficiency
●Nausea/pain/stress

Treatment
●Water restriction + close monitoring of urine output, urine osm, and serum sodium if primary polydipsia
●If from low solute diet, replace solute in diet (eg: NS) but watch for rapid correction
●Goal is no more than 6-8 meQ/24 hour period. Highest risk of osmotic demyelination syndrome if liver disease, malnutrition, hypokalemia, or alcohol use.

What if you over-correct?
●Use D5W to slow down rate of increase by matching or higher rate than urine output
●If unable to keep up with urine ouput, hit the brakes by giving Desmopressin but make sure patient is fluid restricted to avoid further hyponatremia

See article below on osmotic demyelination syndrome!
Osmotic Demyelination