Hyponatremia

Definition:

  • < 135 mmol\L
  • Low serum osmolality

 

Causes:

1. Hypotonic hyponatremia: “true hyponatremia”, serum osmol <280 mOsm\kg

  • Hypervolemic: intravascular volume depletion -> ↑ ADH
    • CHF
    • Nephrotic
    • CirrhosisHyponatremia 1
  • Euvolemic:
    • SIADH
    • Psychogenic polydipsia (15-20 L\d): *no nocturia
    • Hypothyroidism
    • Post-op
  • Hypovolemic
    • Low urine Na <10 mEq\L: diarrhea, vomiting, burns, sweating -> kidneys retain Na to compensate
    • High urine Na >20 mEq\L: renal loss: diuretics, ACE, Addison’s (↓ aldosterone), cerebral sodium wasting

2. Isotonic hyponatremia: “pseudohyponatremia”

  • Lab artifact from high proteins or lipids

3. Hypertonic hyponatremia:

  • High glucose, mannitol, sorbitol. Radiocontrast agents -> increases serum osmolality -> water shifts in ECF -> dilutional hyponatremia

Hyponatremia 2

Hyponatremia 3

 

Presentation:

  • [CNS]: headache, irritability, confusion, high ICP, seizures, coma
  • The severity of hyponatremia is defined according to the severity of symptoms, not to sodium levels

 

Treatment:

  • Asymptomatic\mild -> fluid restriction
  • Symptomatic -> NS + furosemide
  • Refractory -> 3% hypertonic saline + V2 receptor antagonist (tolvaptan)

Hyponatremia 4

  • Rate of correction:
    • Asymptomatic\mild: < 0.5 mEq\L\h
    • Symptomatic: initial rapid correction 2 mEq\L\h
    • Rate of Na rise shouldn’t exceed 0.5 mEq\L\h or 12 mEq\L\d
  • Cerebral salt-wasting disease -> give fludrocortisone
  • SIADH -> give demeclocycline or lithium

 

Complication of treatment:

  • If sodium levels are brought up too rapidly -> central pontine myelinolysis

Hyponatremia 5

 


Download the PDF version: here


 

References:

  • Step up to medicine
  • Toronto notes
  • Master the boards

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