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Stepwise Hyponatremia Triage Matrix

Serum Osmolality → Clinical Volume Assessment → Urine Sodium & Osmolality → Targeted Pharmacotherapy.

ODS Safety Gate COMLEX / USMLE Board Grade
Step 1: Tonicity Serum Osm
< 275 mOsm/kg (True Hypotonic)
Rule Outs: • > 295: Hyperglycemia / Mannitol • 275–295: Pseudohyponatremia
Step 2: Volume Bedside Exam
Euvolemia No edema, normal JVP
Volume Spectrum: • Hypovolemic (dry, orthostasis) • Hypervolemic (edema, ascites, JVP)
Critical Board Safety Rule Black Box
Max ≤ 8 mEq/L / 24h
"Low to High, Pons will Die" → Osmotic Demyelination Syndrome (CPM)

Rapid sodium overcorrection dehydrates oligodendrocyte myelin in the basis pontis causing locked-in syndrome, dysarthria, and quadriplegia. High risk (malnutrition, alcoholism, hypokalemia): limit correction to 4–6 mEq/L/24h.

Diagnostic Algorithm Step 3: Urine Studies
Urine Na & Urine Osm
Hypovolemic
Urine Na < 20 mEq/L Extrarenal loss (Vomiting, diarrhea, burns, 3rd-spacing). Kidney holds Na.
Urine Na > 40 mEq/L Renal loss (Diuretics, mineralocorticoid deficiency / Addison, cerebral salt wasting).
Rx: Isotonic Normal Saline (0.9% NaCl) to restore circulating volume.
Euvolemic (SIADH Hub)
Urine Osm < 100 mOsm/kg Dilute urine: Psychogenic polydipsia, beer potomania ("tea & toast").
Urine Osm > 100 + UNa > 40 Concentrated urine: SIADH, secondary adrenal insufficiency, hypothyroidism.
Rx: Fluid restriction (< 1 L/day), oral salt tabs, vaptans (tolvaptan).
Hypervolemic
Urine Na < 20 mEq/L Low effective arterial volume: Congestive Heart Failure, Cirrhosis, Nephrotic syndrome.
Urine Na > 40 mEq/L Renal failure (CKD / AKI) unable to excrete free water or filter sodium.
Rx: Fluid + sodium restriction, loop diuretics (furosemide).
Emergency Protocol Severe Symptoms
Seizures / Stupor / Coma
Hypertonic Saline (3% NaCl)

100 mL IV bolus over 10 min, repeatable up to 3x. Goal: raise serum sodium acutely by 4–6 mEq/L to arrest cerebral herniation and seizures.

Overcorrection Rescue (Desmopressin)

If sodium rises > 8 mEq/L/24h, administer DDAVP (1–2 μg IV) and D5W infusion to re-lower sodium and halt pontine myelinolysis.