Hyponatremia: Volume Status, Safe Correction & DDAVP Clamp

Hyponatremia management starts with classification and ends with safe correction. Identify tonicity, assess volume, use urine studies to pinpoint mechanism, treat the symptoms first, and cap the rise in Na⁺ to avoid osmotic demyelination.

Step 1 — Confirm Hypotonic Hyponatremia

• Serum osmolality:
  • Isotonic (pseudohyponatremia: severe hyperlipidemia/proteins) — no true hypotonicity.
  • Hypertonic (e.g., hyperglycemia, mannitol) — water shifts; treat the driver.
  • Hypotonic (<275 mOsm/kg) — proceed below.
• Check glucose; correct Na⁺ when markedly hyperglycemic.

Step 2 — Volume Status & Urine Clues

• Urine osmolality (Uosm):
  • <100 mOsm/kg → excess water intake/low solute (primary polydipsia, beer potomania, tea & toast).
  • ≥100 mOsm/kg → ADH effect present (SIAD, hypovolemia, hypervolemia).
• Urine Na⁺ (spot):
  • <30 mmol/L → extrarenal Na⁺ loss/low effective arterial volume (vomiting/diarrhea, HF, cirrhosis).
  • ≥30 mmol/L → renal salt loss (diuretics, adrenal insufficiency, salt-wasting) or SIAD.
• Clinical volume:
  • Hypovolemic: orthostasis, dry mucosa, ↑BUN/Cr; often Uosm ≥100, Urine Na⁺ varies.
  • Euvolemic: SIAD, hypothyroidism, adrenal insufficiency, primary polydipsia.
  • Hypervolemic: HF, cirrhosis, nephrosis (edema/ascites).

Step 3 — Treat the Patient in Front of You

• Severe symptoms (seizure, coma, herniation risk) or Na⁺ <120 with neuro signs: give hypertonic saline 3% NaCl as 100 mL IV bolus, repeat up to 2 more times at 10-minute intervals if symptoms persist; target initial rise of ~4–6 mEq/L.
• Moderate symptoms (vomiting, confusion, marked headache): similar 3% strategy with close monitoring.
• Chronic, mild/asymptomatic: correct slowly with cause-directed therapy (see below).

Step 4 — Safe Correction Limits

• Do not exceed ~8–10 mEq/L in 24 h and ~18 mEq/L in 48 h.
• High-risk ODS (alcoholism, malnutrition, liver disease, hypokalemia, very low starting Na⁺): aim for ≤6 mEq/L in 24 h.
• Check Na⁺ every 2–4 h initially; hourly in actively treated severe cases.

Step 5 — Stop Overcorrection: The DDAVP Clamp

When water diuresis begins, Na⁺ can shoot up. Prevent or arrest overcorrection by “clamping” free water excretion.

• Proactive clamp (high-risk or unpredictable cases): start DDAVP 1–2 µg IV/SC q6–8 h once hypertonic therapy begins; pair with calculated hypertonic saline to raise Na⁺ at a controlled rate.
• Rescue clamp (Na⁺ rising too fast): give DDAVP 2 µg IV/SC and infuse D5W to re-lower to target trajectory.
• Reassess Na⁺ frequently and adjust 3%/D5W to stay within goals.

Cause-Directed Therapy

• Hypovolemic hypotonic hyponatremia: isotonic saline (0.9%) to restore volume → ADH falls → water diuresis (watch for overcorrection; consider clamp).
• SIAD (euvolemic): fluid restriction (e.g., 800–1200 mL/d), ↑solute intake (salt + protein; oral urea where available), consider loop diuretic + salt, vasopressin antagonists in select inpatients with monitoring.
• Hypervolemic (HF/cirrhosis/nephrosis): Na⁺/fluid restriction, loop diuretics, treat underlying hemodynamics; specialist input for advanced cirrhosis/HF.
• Endocrine: check TSH and morning cortisol; replace if hypothyroid/adrenal insufficient.
• Low solute intake (beer potomania/tea-toast): carefully add solute (protein/salt); use clamp to avoid rapid free-water excretion.

Useful Calculations (guide—not a substitute for labs)

• Total body water (TBW) ≈ 0.5 × weight (kg) in women; 0.6 × weight in men (adjust with age/obesity).
• Sodium deficit ≈ TBW × (target Na⁺ − current Na⁺).
• Expected ΔNa⁺ (Adrogué–Madias) with 1 L infusate ≈ (Na⁺infusate − Na⁺serum) / (TBW + 1). For 3% NaCl, Na⁺infusate ≈ 513 mEq/L.

When to Escalate/ICU

• Severe symptoms, need for 3% NaCl infusion, rapidly changing Na⁺, high ODS risk, or unclear diagnosis with instability.

Common Pitfalls

• Missing hyperglycemia/tonicity correction; treating pseudohyponatremia.
• Giving large volumes of isotonic saline in SIAD (can worsen Na⁺).
• Failure to anticipate water diuresis → overcorrection; not using DDAVP clamp when indicated.
• Ignoring potassium: K⁺ repletion raises Na⁺ (moves water intracellularly)—account for it in your targets.

Patient FAQs

“Why can’t we fix my sodium fast?” Rapid correction risks permanent nerve damage (osmotic demyelination). Slow and steady is safer.

“Why am I not allowed to drink water?” In SIAD and some other causes, extra water makes sodium fall; we limit fluids until the underlying problem is controlled.

References & Notes

Practical approach: confirm hypotonicity, interpret urine osm/Na⁺, treat symptoms with targeted hypertonic saline, and cap the daily correction using a DDAVP clamp when needed. Local protocols vary—follow institutional guidance. Educational only.