Decision support only — not a substitute for the European hyponatraemia guideline, eTG, or your renal/endocrine team. Two opposing dangers: acute hyponatraemia threatens cerebral oedema (treat promptly), while over-rapid correction of chronic hyponatraemia causes osmotic demyelination (correct slowly). Severe symptoms override the cause — give 3% saline. Verify all doses.
1 Diagnostic algorithm
Hypotonic hyponatraemia — serum osmolality <275 mOsm/kg
Exclude first: ↑glucose / mannitol = hypertonic (correct Na for glucose) · ↑lipids / protein = pseudohyponatraemia. Confirm with a measured serum osmolality.
Urine osmolality?
>100 mOsm/kg · ADH active
Volume status?
+ spot urine sodium
≤100 mOsm/kg · ADH appropriately off
Dilute urine — water intake exceeds excretion: primary polydipsia, low solute (beer potomania, "tea & toast"), reset osmostat.
HYPOVOLAEMIC
UNa <30 Extra-renal loss — GI losses, skin, third-spacing, prior vomiting.
UNa ≥30 Renal loss — diuretics (esp. thiazides), salt-wasting nephropathy, adrenal insufficiency, cerebral salt wasting.
EUVOLAEMIC
UNa ≥30 SIADH — diagnosis of exclusion (urine osm >100, euvolaemic). First exclude hypothyroidism, adrenal insufficiency, diuretics, drugs.
Causes: malignancy (SCLC), pulmonary, CNS, drugs (SSRI, carbamazepine), pain / nausea / post-op.
HYPERVOLAEMIC
UNa <30 Low effective volume — heart failure, cirrhosis, nephrotic syndrome.
UNa ≥30 Renal failure — AKI / CKD (water excretion impaired, not ADH-mediated).
2 Management — severity decides first, cause decides next
Severe symptomsany acuity — emergency
Seizures, coma, ↓GCS, vomiting, cardiorespiratory distress.
3% saline 150 mL IV over 10–20 min, repeat ×up to 3 until Na ↑~5 mmol/L or symptoms resolve. ICU/close monitoring. Don't wait for the cause.
Moderate symptomsact, then investigate
Nausea, confusion, headache.
A single 3% saline infusion + prompt diagnostic workup + cause-specific treatment.
Mild / asymptomaticcause-directed
Chronic, minimal symptoms — manage by the diagnostic bucket.
Hypovolaemic → isotonic saline. SIADH → fluid restriction first-line (± salt/loop, urea, vaptan). Hypervolaemic → fluid + salt restriction, treat HF/cirrhosis.
Management is severity-first: severe symptoms get 3% saline immediately, whatever the cause; the flowchart then directs the non-emergency arm. Intermittent 3% saline boluses match a continuous infusion for efficacy with less overcorrection SALSA.
3 Rate of correction & osmotic demyelination
The limits (chronic / unknown duration)
- TargetAim ~5 mmol/L per 24h. Limit ≤10 mmol/L in the first 24h, ≤8 mmol/L per 24h thereafter (<18 over 48h).
- High ODS riskNa <105, hypokalaemia, alcohol use, malnutrition, liver disease → be more conservative (≤6–8/24h); consider a proactive DDAVP clamp.
- MonitorRecheck Na every 2–4h during active correction.
Overcorrection & rescue
- AnticipateWhen the cause resolves (volume repleted, cortisol replaced, DDAVP stopped, K⁺ corrected), ADH switches off → brisk aquaresis → overshoot.
- Re-lowerOvercorrecting → DDAVP 2–4 mcg ± 5% dextrose (D5W 3 mL/kg/h drops Na ~1 mmol/L/h).
- ODSDelayed (days) and devastating (dysarthria, quadriparesis, locked-in). Prevention is the rate limit — there's no treatment.
4 SIADH & the traps
SIADH essentials
- CriteriaEuvolaemic, hypotonic, urine osm >100, UNa ≥30, normal thyroid/adrenal/renal, no diuretics.
- First-lineFluid restriction. Add salt + loop, urea, or a vaptan if inadequate.
- VaptansOvercorrection, cost, tolvaptan hepatotoxicity — not first-line.
Classic traps
- Saline in SIADHNormal saline can worsen SIADH — if urine osm exceeds the infusate, you retain water and excrete the salt. Restrict fluid instead.
- GlucoseCorrect Na for hyperglycaemia (translocational) before calling it true hyponatraemia.
- PseudoAn osmolar gap → pseudohyponatraemia (lipids/protein) — a lab artefact, no treatment.
- Don't restrictThe hypovolaemic patient needs volume, not fluid restriction.
Sources.
Spasovski et al. "Clinical practice guideline on diagnosis and treatment of hyponatraemia," European Society of Endocrinology / ERA-EDTA / ESICM, Eur J Endocrinol / Intensive Care Med 2014 (osmolality → urine osmolality → volume status + urine sodium algorithm; 3% saline for severe symptoms; correction limits). US expert panel recommendations (Verbalis 2013). eTG.
Key evidence: SALSA (rapid intermittent vs slow continuous 3% saline bolus — comparable efficacy, less overcorrection); DDAVP-based re-lowering / clamp strategies for overcorrection.
Caveats: the acute-vs-chronic distinction is often unclear in the ED — when in doubt, treat as chronic (respect the rate limit) unless symptoms are severe. Urine sodium cut-offs (≤30 / ≥30) are guides, blurred by diuretics and renal impairment. Recent literature debates how strict the correction limits must be, but the conservative limits remain standard. Verify all doses.