Nephrology Clinical guideline

Hyponatremia in Children - Complete Reference

Definitions & Classifications

  • Defined as serum sodium < 135 mEq/L (mmol/L). Most common electrolyte disorder in children < 4 years old.
  • Mild: 130-134. Moderate: 125-129. Severe: < 125 mEq/L.
  • Acute: < 48 hours. Chronic: > 48 hours.
  • Hypotonic (True): Plasma Osm < 270-280 mOsm/kg H2O.
  • Isotonic (Pseudo): 280-295 mOsm/kg. Check for hyperproteinemia, hypertriglyceridemia. Use direct ion-selective electrodes.
  • Hypertonic: > 295 mOsm/kg. Check for hyperglycemia, mannitol.

Clinical Presentation

  • Children are at significantly higher risk for symptomatic hyponatremia due to a higher brain-to-intracranial volume ratio.
  • Signs: Irritability, agitation, weakness, nausea/vomiting.
  • Severe (<125) Manifestations: Cerebral edema, hyponatremic encephalopathy, herniation, seizures, coma, respiratory arrest, Cushing triad (bradycardia, irregular breathing, hypertension).
  • Neurogenic pulmonary edema may occur due to sympathetic discharge and pulmonary vasoconstriction.

Diagnostic Approach & Formulas

  • 1. Measure Plasma Osmolality (mOsm/kg) = 2 × Na + (Glucose/18) + (BUN/2.8).
  • 2. Measure FENa = (U_Na × P_Cr) / (U_Cr × P_Na) × 100.
  • Hypovolemic + FENa < 0.5%: Extrarenal loss (GI, burns, sweating).
  • Hypovolemic + FENa > 0.5%: Renal loss (Diuretics, mineralocorticoid deficiency, salt-wasting syndrome).
  • Euvolemic + FENa > 0.5%: Check Urine Osmolality. If > 200 = SIAD/Thiazides. If < 100 = Primary polydipsia / Reset osmostat.
  • Hypervolemic + FENa < 0.5%: Nephrotic syndrome, liver/heart failure.
  • Hypervolemic + FENa > 0.5%: Kidney dysfunction (AKI/CKD).
  • Urine-to-Plasma Tonicity = (U_Na + U_K) / P_Na. If < 1, indicates free water deficit requiring compensation.

Management & Correction Targets

  • Severe Symptomatic (Seizures/Encephalopathy): 3% NaCl 2-5 mL/kg IV bolus over 10-15 mins (up to max 100mL). Goal is to stop neurologic symptoms; may repeat bolus x2.
  • Rate Limit: Increase Na by strictly ≤ 6-10 mEq/L in first 24 hours, and ≤ 20 mEq/L in 48 hours to absolutely prevent central pontine myelinolysis.
  • Calculation: Free water deficit (L) = Total body water % * Lean weight (kg) * ((Serum Na / 140) - 1).
  • TBW %: 90% extreme preterm, 80% moderate preterm, 70% term infant, 60% child ≥ 1 year.
  • Infusate calculation: Change in Na = (Infusion Na concentration - Serum Na) / (TBW + 1 L).
  • Content of Infusates: 5% NaCl=856, 3% NaCl=513, 0.9% NaCl=154, LR=130, 0.45% NaCl=77 (mEq/L).

Diagnostic & Management Algorithm

  • Hyponatremia Symptomatic Resuscitation: [START] Na < 120 OR Seizures from hyponatremia -> [ACTION] 3% Hypertonic Saline 3-5 mL/kg IV over 20-30 mins -> [MONITOR] Stop seizures -> [GOAL] Increase Na by 2-4 mEq/L acutely

References for this guideline are being added; see the methodology for our source hierarchy.

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