Hyponatremia

Hypotonic Hyponatremia (Serum Osmolality <275 mOsm/kg)

Hypovolemic (Decreased ECV):

• Urine Sodium (UNa) < 40 mEq/L:
  • Causes: Nonrenal salt loss, e.g., vomiting, diarrhea, third spacing (e.g., burns, pancreatitis).
• UNa > 40 mEq/L:
  • Causes: Renal salt loss, e.g., diuretic use, mineralocorticoid deficiency (primary adrenal insufficiency, Addison’s disease), cerebral salt wasting, salt-wasting nephropathy.

Euvolemic (Normal ECV):

• Urine Osmolality (UOsm) < 100 mOsm/kg:
  • Causes: Primary polydipsia (excessive water intake), malnutrition (e.g., beer potomania where low solute intake leads to diluted urine).
• UOsm > 100 mOsm/kg & UNa > 40 mEq/L:
  • Causes: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), hypothyroidism, certain medications (e.g., SSRIs, carbamazepine), ectopic ADH production (e.g., small cell lung cancer).

Hypervolemic (Increased ECV):

• Urine Findings Variable:
  • Causes: Heart failure, cirrhosis, nephrotic syndrome, renal failure. These conditions lead to volume overload with dilutional hyponatremia due to retention of both salt and water, but water retention predominates.

Isotonic Hyponatremia (Serum Osmolality 275-295 mOsm/kg)

• ECV Variable:
  • Causes: Pseudohyponatremia, where there’s an increase in plasma proteins or lipids (paraproteinemia, hyperlipidemia) which do not contribute to osmolality but displace water, making serum sodium appear lower than it is when measured.

Hypertonic Hyponatremia (Serum Osmolality >295 mOsm/kg)

• ECV Variable:
  • Causes: Translocational hyponatremia due to an increase in effective osmolality from:
    • Hyperglycemia (glucose draws water from cells into the extracellular space, diluting serum sodium).
    • Exogenous solutes like mannitol or glycine (used in irrigation solutions during surgery) which similarly shift water out of cells.

Interpretation and Management:

• Serum Osmolality helps classify the type of hyponatremia, guiding towards the appropriate diagnostic workup and management.
• ECV Assessment (via clinical examination, jugular venous pressure, orthostatic vital signs) directs towards the likely cause and whether the patient needs fluid resuscitation or restriction.
• Urine Studies (sodium and osmolality) further pinpoint the underlying cause by differentiating between renal and non-renal losses or inappropriate ADH secretion.

Management strategies will vary but might include:

• Fluid restriction or administration based on volume status.
• Correction of underlying causes (e.g., treating heart failure, stopping medications causing SIADH).
• Careful correction of sodium levels to avoid osmotic demyelination syndrome, especially in chronic hyponatremia.

This classification aids in a systematic approach to diagnosing and managing patients with hyponatremia.