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