Introduction:

• Sodium concentration is a water balance issue in the extracellular fluid (ECF):

• Water excess leads to hyponatremia.
• Water deficit leads to hypernatremia.

• Sodium concentration doesn’t necessarily reflect a deficit in total body sodium.
• Total body sodium is almost equivalent to the sodium in the ECF ( a very small amount of sodium in the intracellular fluid).
• Total body sodium (TBS) corresponds with the ECF size:

• Increased TBS→ Larger ECF size→ Volume overload (That’s why we restrict sodium intake in these patients).
• Decreased TBS→ Smaller ECF size→ Volume depletion.

• Water in the ECF is regulated by ADH and the thirst mechanism:

• Water deficit activates the thirst mechanism and ADH release.
• Water excess deactivates the thirst mechanism and ADH release.

ADH (Antidiuretic hormone)

• ADH release leads to renal water retention.
• Excess ADH leads to excess water in the ECF.
• ADH is appropriately stimulated by intra-vascular depletion (Volume overload and volume depletion).
• ADH release is inappropriately released in SIADH.
• Water excess in the ECF may, also, result from a water osmotic shift from intra-cellular to extra-cellular space. This requires an increase in the osmotic pressure in the ECF (Hypertonic ECF) from an accumulation of an osmotically active substance in the ECF (Glucose, hypertonic mannitol).
• For a clinically significant hyponatremia to develop, two elements are required:
  • Continued water intake.
  • Excess ADH release.

Diagnosis:

• History and physical exam is key in hyponatremia diagnosis.
• Hyponatremia detected on blood work→check the osmolality.
  • High osmolality (>290 mosm/kg) → Hypertonic hyponatremia
  • Normal osmolality (270-290 mosm/kg) → isotonic hyponatremia (Pseudohyponatremia), rare, old lab techniques.
  • Low osmolality→ Hypotonic hyponatremia (The most common in clinical practice).

Hypertonic hyponatremia:

• Accumulation of actively osmotic substance in the ECF → increase osmotic pressure in ECF → water shift from the ICF to the ECF → Excess water in the ECF → Hyponatremia.
• Actively osmotic substances: Glucose, Mannitol, Ethanol, ethylene glycol.
• Substances other than glucose will create an osmolar gap.
• Osmolar gap = Measured osmolality – calculated osmolality.

Isotonic hyponatremia:

• Due to old lab techniques that read low sodium levels in the presence of extremely high triglycerides and protein levels.

Hypotonic hyponatremia (Continued water intake + impaired renal water excretion)

• Thiazides and severe renal failure (GFR is severely reduced) are major causes of hypotonic hyponatremia.
• Check volume body status.
• Signs of volume overload→ Hypervolemic hypotonic hyponatremia.
• Signs and symptoms of volume depletion→ Hypovolemic hypotonic hyponatremia.
• The distinction between mild volume depletion and euvolemia can be difficult and only possible after a challenge of IVF.
• Euvolemic hypotonic hyponatremia:
  • SIADH.
  • Severe hypothyroidism
  • Adrenal insufficiency
  • Psychogenic polydipsia, beer potomania, excessive water drinking in marathon runners.
  • Decreases sodium and solute load.

Urine Na, CL, and osmolality:

• Urine Na & Cl and osmolality can be altered by IVF resuscitation and recent diuretic use.
• Intravascular volume depletion (Volume overload & volume depletion):
  • Urine Na < 25 meq/l except in *vomiting (> 40) and recent diuretic use.
  • Urine Cl < 25 meq/L except in recent diuretic use.
  • Urine osmolality is elevated.

     

• Excess water intake:
  • Urine Na < 25 meq/L except.
  • Urine Cl < 25 meq/L.
  • Urine osmolality < 100 Moso/kg.

   

• SIADH:
  • Urine Na> 40 meq/L except.
  • Urine Cl >40 meq/L.
  • Urine osmolality > 300 Moso/kg.

     

• TSH and cortisol levels if hypothyroidism or adrenal insufficiency are suspected.