True Hyponatremia Classified by Volume Status

NEPHROLOGY

Info in the following table is from AAFP 2015 (algorithm) and Pocket Medicine

TBW TBS  Causes & Treatment
Hypovolemic hypotonic hyponatremia ↓TBW  ↓↓ TBS Renal losses ( UNa >20 mEq/L, FENa > 1%)
– Diuretics, salt-wasting nephropathy, cerebral salt wasting, mineralocorticoid deficiency.
Extrarenal losses (UNa < 20* mEq/L, FENa < 1%)
GI losses (eg, diarrhea, vomiting), third-spacing (eg, pancreatitis), inadequate intake, insensible losses
Treatment: Slow volume repletion with normal saline (isotonic saline). When the volume is replete, the low blood volume which was the stimulus for ADH production will be removed. The kidneys will again start excreting free water and serum sodium will quickly go back to normal.
-Tx underlying cause. D/c diuretics.
Euvolemic  hypotonic hyponatremia

Also called Isovolemic…

 ↑TBW No change in TBS Here there is an increase in the TBW relative to total body sodium (which doesn’t change).

  • SIADH (Multiple things cause SIADH)
  • Endocrine disorders: increased ADH activity is seen in glucocorticoid deficiency  (co-secretion of ADH & CRH) and severe hypothyroidism (↓CO & ↓GFR)
  • Psychogenic polydipsia (Uosm < 100, ↓ Uric Acid), usually requires intake 12 L/d
  • Low solute:“tea & toast”;“beer potomania”
  • Reset Osmostat: chronic malnutrition (decreased intracellular osmoles) or pregnancy (hormonal effects) lead to ADH physiology reset to regulate a lower [Na] serum

** Here, you have mostly free water going into the intravascular space. Most of it is going to be redistributed into the ECF and ICF by osmosis leaving the patient isovolumic.
Treatment: Free water restriction + treat the underlying cause
Hypervolemic hypotonic hyponatremia ↑↑TBW  ↑TBS Renal Causes (UNa > 20 mEq/L)
–Advanced renal failure (diminished ability to excrete free H2O). Also, Acute Renal Failure can cause this.
Extra-Renal Causes (UNa < 20* mEq/L, FENa <1%)
-CHF
-Cirrhosis
-Nephrotic syndrome
CHF, Cirrhosis, and Nephrotic syndrome each lead to decrease EAV.
Treatment: Free water restriction, sodium restriction, and restrict all fluids.

Primary polydipsia = Psychogenic polydipsia; EAV = Effective arterial volume. TBW = Total Body Water; TBS=Total Body Sodium.
The extra-renal causes can be called pre-renal causes. FENA for pre-renal AKI applies to it.
Note: For the Extra-renal causes, in both hypovolemic and hypervolemic hyponatremia, the Urine sodium is not just <20, you should expect it to be much lower than that and be actually <10 because the kidneys are responding appropriately.

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