Definition

Hyponatremia is defined as a serum sodium concentration <135 mmol/L. It reflects an imbalance between water and sodium homeostasis and can be acute or chronic, with varying severity and clinical consequences.

Epidemiology

• Most common electrolyte disorder in hospitalized patients (15–30%)
• Higher prevalence in elderly and patients with comorbidities such as heart failure, liver disease, or renal failure
• Can occur in outpatient settings, especially in patients on diuretics or with SIADH

Etiology

• Hypovolemic hyponatremia: GI losses (vomiting, diarrhea), renal losses (diuretics, adrenal insufficiency), skin losses (sweating, burns)
• Euvolemic hyponatremia: SIADH, hypothyroidism, glucocorticoid deficiency, drugs (SSRIs, thiazides)
• Hypervolemic hyponatremia: heart failure, liver cirrhosis, nephrotic syndrome, chronic kidney disease
• Other: psychogenic polydipsia, post-operative hyponatremia

Pathophysiology

• Hyponatremia results from excess water relative to sodium or loss of sodium relative to water
• Hypovolemic: sodium loss exceeds water loss → volume depletion stimulates ADH
• Euvolemic: inappropriate ADH secretion → water retention without edema
• Hypervolemic: fluid retention with dilutional hyponatremia
• Chronic hyponatremia allows cerebral adaptation; acute hyponatremia can cause cerebral edema and neurological symptoms