Hyponatremia

• Definition: Serum Sodium <135 mEq/L
• Etiology
  • 4 Main groups
    • 1) Pseudohyponatremia (hyperglcemia)
    • 2) Hypervolemic Hyponatremia (Heart Failure)
    • 3) Euvolemic Hyponatremia (SIADH)
    • 4) Hypovolemic Hyponatremia ()
• Symptoms
  • Headache, Delirium, Weakness, Hypoactive DTRs
  • Nausea, vomiting, ileus, watery diarrhea, tremor, hyperreflexia
  • Severe (<115): Confusion, lethargy, psychosis, and seizures, Coma
    • Neurogenic pulmonary edema
    • Hyponatremic Encephalopathy + Non-cardiogenic pulmonary edema Ayus-Arieff Syndrome
• W/U:
  • 1) Confirm Hyponatremia and Correct for Hyperglycemia
  • 2) Calculate Serum Osmolality = 2 x (serum Na+) + (glucose)/18 + (BUN)/2.8
  • 3) Measure Serum Osmolality
    • Get TSH/Cortisol
    • Urine Osm and Urine Lytes
      • Urine Sodium <20: Extra-renal volume loss or true Hypervolemia
      • Urine sodium 20-40: Useless
      • Urine Sodium >40: Euvolemic or Hypovolemic Hyponatremia due to renal salt wasting
  • 4) Check Osmol Gap (Normal <10)
    • (+): Low molecular weight alcohol
    • (-)

• Causes (Measured Osms):

  • Usually due to increased circulating AVP and/or increased renal sensitivity to AVP with intake of free water
    • Iatrogenic
    • PostOp: Pre-menopausal women
    • Colonoscopy Prep
  • MCC Thiazides (30% Risk), SSRIs, Carbamazepine
    • Polydipsia, MDMA, Exercise
  • Hypertonic: High Serum Osmolarity (>295)
    • Osmotically active solutes are present
      • Marked Hyperglycemia
      • Mannitol Infusion
      • Contrast Agents (High urine specific gravity)
      • Post-TURP (Glycine irrigation: TURP/uterine surgery)
    • Advanced Renal Failure
  • Isotonic: Normal Serum Osmolarity (285-295)
    • Isotonic Hyponatremia (Pseudohyponatremia)
      • Hyperlipidemia
      • Hypertriglyceridemia (>1500)
      • Hyperproteinemia (MM, IVIG)
    • Exogenous Osmols
      • Contrast dye, Mannitol, IVIG, Sorbitol/glycine for surgery
  • Hypotonic: Low Serum Osmolarity (<285)
  • True hyponatremia
    • Assess ECV (Hypovolemic, Euvolemic, Hypervolemic) - A) Hypovolemic (dry mucous membranes) - Increased circulating AVP - Total body water decreased < sodium
      • Typically Urine sodium <50, give isotonic saline
    • UNa > 20-40: (Renal Losses: Loss of NaCl in the urine)
      • Diuretic Excess (thiazides)
      • Post-Obstructive Diuresis
        • ATN
      • Osmotic Diuresis (ACEI)
      • Mineralocorticoid Insufficiency
        • Cerebral Salt Wasting
    • UNa < 10-20: (Extrarenal Loss)
      • Vomiting
      • Diarrhea
      • Hemorrhage
      • Dehydration/Sweating
      • Third Spacing of fluids, Burns, Pancreatitis, Trauma
        • B) Euvolemic (No Edema)
        • Total body water increased
        • Total body sodium normal
      • Measure Urine Osms
    • UNa > 20, Urine Osmolality < 300, usually <100
      • Primary Polydipsia
        • Defect in thirst regulation
      • Malnutrition (Beer Drinker’s Potomania)
      • Low Solute diet
    • UNa ≥ 20-25, Urine Osmolality >100, usually >300
      • SIADH (MC)
        • Meds: Carbamazepine, cyclophosphamide, SSRIs
        • NSAIDs, Chemo (cyclophosphamide, vincristine)
        • Oxcarbazepine, valproate
        • Oxytocin, bromocriptine
        • Amiodarone, Opioids
        • Ecstasy
        • Serum urate <4.0 usually
        • Typically UNa > 50 (82%)
      • Stress
      • Lung cancer (Especially Small Cell Lung Carcinoma)
      • Respiratory failure
      • Pain or nausea
      • Early Adrenal Insufficiency
        • Hypovolemia, increased ADH and hyponatremia
      • Hypothyroidism
        • C) Hypervolemic (edematous, JVD, rales, S3)
        • Total body water increasedx2
        • Total body sodium increased
    • Variable
      • Acute or Chronic Renal Failure
        • Kidney unable to absorb sodium
    • UNa < 20 (Impaired Kidney Sensing)
      • Nephrotic Syndrome
      • Cirrhosis
      • Cardiac Failure (CHF): Increase ACEI
• Complications
  • Overcorrection
    • Osmotic Demyelination Syndrome
      • RF: Hypokalemia, Cirrhosis, Alcoholism, Malnutrition, severe and asymptomatic, chronic
• Treatment
  • Avoid KCL
    • 50 mEq of oral KCL will have about the same effect as 100mL of 3% NaCl
    • Estimated sodium increase = mEq of oral K+ / (.55x(weight in kg))
  • Acute Hyponatremia (<48h): High risk of Brain Herniation
    • Na+ <130 and any symptoms of elevated ICP
    • Low risk of osmotic demyelination syndrome due to lack of neural adaptation activation
    • A) Hypertonic (3%) saline (513mM) boluses + O2 + IV Loops
      • 3x50ml boluses or 2ml/kg, may give via peripheral line
      • Goal is 4-6mEq/L rise in sodium over a period of hours
      • 8mEq/L max over 24 hours
    • B) 2 amps of Hypertonic bicarbonate 50ml
      • Same tonicity as 6% NaCl = 200mL of 3% saline = 3mM rise
      • CI: Metabolic alkalosis
        • DDVAP Clamp
        • 2mcg IV q8hr
        • Restrict free water <1L
        • Trajectory 6mEq/L rise per day
          • 1) Infuse 3% NaCl to raise Na
          • 2) Infuse D5W to lower Na
          • 3) Hold everything to keep equal
  • Chronic Hyponatremia (>/48h): Better Tolerated
    • Na + <120, severe symptoms, concurrent intracranial pathology of any kind: 3% hypertonic saline boluses 100mL
      • <1L/24hrs of fluid
  • Don’t correct faster than 12-24 mEq/day
    • SIADH: Tolvaptan (Vasopressin Receptor antagonist)
    • Otherwise: DDAVP + Hypertonic Saline + O2
  • Iatrogenic Hyponatremia
    • RF: Hypotonic fluids, hypoxia, CNS disorders
    • Headache, Nausea, vomiting, Encephalopathy
    • Treatment: Hypertonic (3%) saline, serial electrolytes, increase serum sodium 6-8 in first 24hrs
  • Hypovolemic Hyponatremic (mild): Normal saline
  • Hypovolemic Hyponatremic (severe): Hypertonic saline
  • Euvolemic Hyponatremic (mild): Fluid Restriction
  • Euvolemic Hyponatremic (Acute): Hypertonic saline
    • 100ml bolus of 3% NS to increase Na by 2-3
    • May repeat once or twice at 10minute intervals
  • Euvolemic Hyponatremic (Chronic): Hypertonic saline
    • 30ml/hr infusion of 3% NS +/- simultaneous desmopressin
  • Hypervolemic Hyponatremic (mild): Fluid restriction +/- loop diuretic
  • Hypervolemic Hyponatremic (severe): Normal saline
  • Urea
    • Euvolemic or hypervolemic
    • Abscense of severe renal failure or hepatic encephalopathy
    • No reversable cause
    • Not emergent
    • Urgent
    • Urea 30 grams PO w/fluid restriction 1.0-1.5L daily - Not Urgernt
    • Urea 15g PO x1 daily
• References
  • https://emcrit.org/ibcc/hyponatremia/#hypokalemic_hyponatremia