Summary

Compilation review comparing the 2013 US expert panel and 2014 European hyponatremia guidelines on diagnosis and treatment of hyponatremia (SNa <136 mmol/L). Both guidelines converge on bolus hypertonic saline for acute/symptomatic hyponatremia and fluid restriction as first-line for chronic hyponatremia, but diverge on diagnostic algorithm (European prioritizes urine osmolality/sodium over volume status), pharmacologic second-line therapy for SIAD, correction-rate limits, and use of vaptans (US permits; European recommends against in profound hyponatremia). Emerging diagnostic markers include fractional uric acid excretion and plasma copeptin.

Key Recommendations

Exclude nonhypotonic hyponatremia (hyperglycemia, mannitol, contrast, pseudohyponatremia from hyperlipidemia/hyperproteinemia) before proceeding with the hypotonic workup.
Use the European algorithm prioritizing urine osmolality and urine sodium over clinical volume assessment, which has poor sensitivity (50–80%) and specificity (30–50%) for volume status.
Urine osmolality <100 mOsm/kg indicates primary polydipsia, low solute intake, or beer potomania; ≥100 mOsm/kg points to vasopressin-driven causes.
Urine sodium <30 mmol/L suggests low effective arterial blood volume (hypovolemia or hypervolemic states); ≥30 mmol/L suggests SIAD, diuretic use, or adrenal insufficiency.
For acute or severely symptomatic hyponatremia, give a bolus of 3% NaCl (US: 100 mL over 10 min ×3 PRN; European: 150 mL over 20 min ×2–3 PRN) regardless of duration.
First-line treatment for chronic SIAD is fluid restriction (<1 L/d, often <500 mL/d if urine-to-serum electrolyte ratio <1); predictors of failure are UNa ≥130 mmol/L and UOsm ≥500 mOsm/kg.
Second-line SIAD therapy differs: US permits vaptans, demeclocycline, or urea; European recommends urea or loop diuretics plus oral NaCl and recommends against vaptans and demeclocycline.
Treat hypovolemic hyponatremia with isotonic saline (or balanced crystalloid); treat hypervolemic hyponatremia with fluid restriction (US allows vaptans in heart failure; European recommends against).
Do not exceed correction of 10 mmol/L in 24 h (European) or 10–12 mmol/L/d, 8 mmol/L/d if high ODS risk (US); US suggests a minimum of 4–8 mmol/L/d.
If overcorrection occurs (especially baseline SNa <120 mmol/L), relower SNa with electrolyte-free water (e.g., D5W) and/or desmopressin.
Screen euvolemic hyponatremia for secondary/primary adrenal insufficiency, which can mimic SIAD and be missed without dynamic testing.
Vaptans are not indicated for acute or severely symptomatic hyponatremia—hypertonic saline is the treatment of choice.

Thresholds & Doses

Hyponatremia definition: SNa <136 mmol/L.
Moderate hyponatremia: 125–129 mmol/L; severe (US)/profound (European): <125 mmol/L.
Acute vs chronic cutoff: 48 hours.
Urine osmolality cutoff for diagnostic branching: 100 mOsm/kg.
Urine sodium cutoff: 30 mmol/L (low EABV vs SIAD/diuretic).
Hypertonic saline (US severe symptoms): 3% NaCl 100 mL over 10 min, repeat up to 3× as needed.
Hypertonic saline (US moderate symptoms): 3% NaCl continuous infusion 0.5–2 mL/kg per h.
Hypertonic saline (European): 3% NaCl 150 mL over 20 min, 2–3 times as needed (severe) or once (moderate).
Alternative protocol (Ayus): 500 mL 3% NaCl over 6 hours for hyponatremic encephalopathy.
Correction limit (US): 10–12 mmol/L per day; 8 mmol/L per day if high ODS risk.
Minimum correction (US): 4–8 mmol/L per day; 4–6 mmol/L per day if high ODS risk.
Correction limit (European): 10 mmol/L per day; no minimum specified.
Trigger to relower SNa (US): baseline SNa <120 mmol/L with correction exceeding 6–8 mmol/L per day.
Fluid restriction target: <1 L/day; <500 mL/day if urine-to-serum electrolyte ratio <1.
Predictors of fluid-restriction failure in SIAD: UNa ≥130 mmol/L or UOsm ≥500 mOsm/kg.
SNa reference change value: only changes ≥4 mmol/L are reliably real.
European vaptan boundary: ‘do not recommend’ when SNa <130 mmol/L; ‘recommend against’ when SNa <125 mmol/L.

Citations

Table 1 — Classification schemes for hyponatremia (severity, acuity, symptoms, tonicity, volume status).
Figure 1 (Diagnostic algorithm) — European algorithm prioritizing UOsm then UNa over volume status.
Table 2 — Side-by-side comparison of US (2013) vs European (2014) recommendations for acute/chronic SIAD/hypervolemic/hypovolemic hyponatremia and correction rates.
Treatment of Acute Hyponatremia section — bolus 3% NaCl dosing and indications.
Treatment of Chronic Hyponatremia section — fluid restriction thresholds and Winzeler predictors of nonresponse.
Vaptans section — divergent US vs European recommendations and overcorrection/hepatotoxicity concerns.
Copeptin section and Figure 2 — copeptin-based SIAD subtypes and differentiation from hypovolemic hyponatremia.
Verbalis JG et al., Am J Med 126[Suppl 1]: S1–S42, 2013 (US guideline) and Spasovski G et al., Eur J Endocrinol 170: G1–G47, 2014 (European guideline).