What is Hyponatremia?

Hyponatremia is defined as serum sodium <135 mEq/L. It is the most common electrolyte disorder and can range from mild/asymptomatic to severe/life-threatening.

Symptoms (Depend on Severity & Acuity):

• Mild (130-134 mEq/L): Often asymptomatic or subtle (fatigue, nausea)
• Moderate (120-129 mEq/L): Headache, confusion, weakness
• Severe (<120 mEq/L): Seizures, coma, respiratory arrest (cerebral edema)
• Acute Onset (<48 hours): More symptomatic
• Chronic: Often well-tolerated

Classification by Volume Status:

1. Hypovolemic Hyponatremia (Low Total Body Sodium & Water; Sodium Loss > Water Loss):

• Renal Losses: Diuretics, salt-wasting nephropathy, adrenal insufficiency, cerebral salt wasting
• GI Losses: Vomiting, diarrhea
• Clues: Dry mucous membranes, low BP, tachycardia, low urine sodium (<20 mEq/L if extrarenal losses)

2. Euvolemic Hyponatremia (Normal Total Body Sodium; Excess Water):

• SIADH (Syndrome of Inappropriate ADH): Most common cause; cancer, lung disease, CNS disorders, medications
• Hypothyroidism (Severe)
• Glucocorticoid Deficiency (Adrenal Insufficiency)
• Psychogenic Polydipsia: Excessive water intake
• Clues: Normal volume status, urine osmolality >100 mOsm/kg (SIADH), urine sodium >40 mEq/L

3. Hypervolemic Hyponatremia (Excess Total Body Sodium & Water; Water Excess > Sodium Excess):

• Heart Failure, Cirrhosis, Nephrotic Syndrome, Advanced CKD
• Clues: Edema, ascites, low urine sodium (<20 mEq/L)

Diagnosis Steps:

1. Confirm True Hyponatremia: Rule out pseudohyponatremia (hyperglycemia, hyperlipidemia)
2. Assess Volume Status: Physical exam (hypovolemic, euvolemic, hypervolemic)
3. Urine Studies:
   • Urine osmolality
   • Urine sodium
4. Check Thyroid (TSH), Cortisol (AM Cortisol or Cosyntropin Stim)

Treatment Principles:

• Treat Underlying Cause
• Correction Rate:
  • Acute (<48h), Symptomatic: 1-2 mEq/L/hour initially (until symptoms improve), then slower
  • Chronic (>48h) or Asymptomatic: Slow correction (6-8 mEq/L in 24 hours; MAX 10-12 mEq/L in 24h)
  • Risk of Overcorrection: Osmotic demyelination syndrome (ODS) → irreversible brain damage

Specific Treatments:

• Hypovolemic: IV normal saline (0.9% NaCl)
• SIADH: Fluid restriction (500-1000 mL/day), salt tablets, demeclocycline, vaptans (tolvaptan)
• Hypothyroidism: Levothyroxine
• Adrenal Insufficiency: Hydrocortisone
• Hypervolemic: Fluid/salt restriction, diuretics, treat underlying condition
• Severe/Symptomatic: Hypertonic saline (3% NaCl) with close monitoring