Approach to Hyponatremia

Pathogenesis of hyponatremia

Approach to Hyponatremia

Type

Etiologies

Normal Sodium Regulation

Osmoreceptors in hypothalamus sense serum sodium
Elevated serum sodium stimulates ADH (vasopressin) release from pituitary

ADH

- Stimulates water reabsorption in kidney to lower serum sodium concentration, which turns off osmoreceptors in brain

Thirst reflex stimulated to cause water intake to lower serum sodium to normal

Possible mechanisms of hyponatremia

Increased fluid intake overwhelms hypothalamus-kidney feedback loop

- Primary polydipsia

- Beer potomania

Malfunction in loop turns on continuous ADH secretion

- SIADH

Receptors in kidney increase water uptake either due to ADH or independent of ADH (Angiotensin II)

Angiotensin II

- Stimulates pituitary to release ADH, which causes water retention in kidney

- Constricts blood vessels to increase blood pressure

- Stimulates adrenal gland to release aldosterone, which leads to

- - Sodium retention

- - Potassium wasting (hypokalemia)

- - Hydrogen loss (metabolic alkalosis)

- - Further water retention leading to hyponatremia

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