Recognize
- Almost always = free water loss + impaired access to water
- Symptoms: AMS, lethargy, seizures, intracranial hemorrhage (esp. peds)
- Causes: dehydration, DI (central / nephrogenic), osmotic diuresis (DKA/HHS), GI losses, hypertonic feeds
Resuscitate first
- If hemodynamically unstable: NS 20 mL/kg bolus until perfusion restored
- Then switch to hypotonic fluid for free-water deficit
Free water deficit
- FWD (L) = TBW × ([Na]/140 − 1) · TBW = 0.6 × kg (♂), 0.5 × kg (♀)
- Replace over 48–72 h · cap Na ↓ to ≤ 10 mEq/L per 24 h (≤ 0.5 mEq/h)
- Add ongoing losses + insensible (~30–40 mL/kg/d)
- Fluid: D5W (no Na access), ½ NS, or PO water
Diabetes insipidus
- Central DI: DDAVP 1–2 mcg IV/SC or 10–40 mcg intranasal
- Nephrogenic DI: stop offending drug (lithium), thiazide, low-Na diet
- Confirm with paired serum/urine osm, water deprivation if stable
Disposition
- Admit most — frequent Na monitoring (q2–4h initially)
- ICU for AMS, seizure, severe (Na > 160)