Hyponatremia: Pathophysiology, Hypervolemic, and Hypovolemic Forms

Definition

General Pathophysiology

The Core Concept: Water Excess Relative to Sodium

1. Arginine Vasopressin (AVP) - the primary antidiuretic hormone (also called ADH)
2. Thirst - which drives water intake

The Role of AVP (ADH) - Central Mechanism

• Increased circulating AVP and/or
• Increased renal sensitivity to AVP
• ...combined with free water intake

Two Mechanisms Causing Impaired Water Excretion

1. Low distal delivery of filtrate to the diluting segment of the nephron
   • Occurs when GFR is reduced or fractional NaCl reabsorption in the proximal tubule (PT) is increased
   • Increased PT reabsorption is driven by a decreased Effective Arterial Blood Volume (EABV)
2. Actions of vasopressin on the inner medullary collecting duct (via renal water pores, RWP)
   • AVP drives water reabsorption along the osmotic gradient created by the hypertonic medullary interstitium

Key teaching point: Decreased EABV can result from an actual total-body NaCl deficit (true hypovolemia) OR from a low cardiac output / splanchnic vasodilation state (as in heart failure or cirrhosis) - in both cases, the kidney "perceives" underfilling and responds accordingly. - Brenner & Rector's The Kidney

Brain Adaptation and Risk of Osmotic Demyelination

• Acute hyponatremia (<48 hours): Brain cells swell due to osmotic water influx - causing cerebral edema, headache, seizures, coma, and potentially death
• Chronic hyponatremia (>48 hours): Brain cells adapt by exporting effective osmoles - mainly K+ (and accompanying anions) and organic solutes (taurine, myoinositol). This returns brain cell volume toward normal
• Risk of rapid correction: If Na+ is corrected too rapidly in chronic hyponatremia, cerebral vascular endothelial cells shrink, the blood-brain barrier opens, lymphocytes/complement/cytokines enter the brain, oligodendrocytes are damaged, and osmotic demyelination syndrome (ODS) results - Brenner & Rector's The Kidney

Classification by Volume Status

Hypovolemic Hyponatremia

Pathophysiological Mechanism

1. Volume depletion triggers baroreceptor activation
2. Non-osmotic (volume-mediated) AVP release from the posterior pituitary - this is a "non-osmotic" AVP stimulus, where the body sacrifices osmolality to preserve volume
3. Thirst activation drives the patient to drink water (often hypotonic)
4. Renal water retention via V2 receptor activation
5. Net result: water is retained but sodium is not replaced at the same rate - Na+ concentration falls

"Hypovolemia causes a marked neurohumoral activation, increasing circulating levels of AVP. The increase in circulating AVP helps preserve blood pressure via vascular and baroreceptor V1A receptors and increases water reabsorption via renal V2 receptors; activation of V2 receptors can lead to hyponatremia in the setting of increased free water intake." - Harrison's 22E

Causes

• Gastrointestinal losses: vomiting, diarrhea, tube drainage (secretory diarrhea is Na+-rich)
• Insensible losses: sweating (especially when replaced with plain water), burns
• Third spacing: ascites, pancreatitis, trauma, ileus

• Thiazide diuretics - block NaCl cotransporter in cortical diluting segment; impair urinary dilution; lead to polydipsia; this is the most common pharmacological cause
• Mineralocorticoid deficiency (Addison's disease / primary adrenal insufficiency) - lack of aldosterone causes inappropriate urinary Na+ wasting; suspect when hyperkalemia + hyponatremia + hypotension coexist with high urine Na+
• Salt-losing nephropathies - reflux nephropathy, interstitial nephritis, post-obstructive uropathy, medullary cystic disease, ATN recovery phase
• Cerebral salt wasting (CSW) - rare, associated with subarachnoid hemorrhage, TBI, craniotomy, encephalitis; characterized by inappropriate renal Na+ wasting + clinical hypovolemia (distinguish from SIADH, which is euvolemic)

Key Laboratory Findings

A urine Na+ <20 mM (in the absence of hypervolemic causes) predicts a rapid rise in plasma Na+ in response to IV normal saline, as saline infusion suppresses AVP, inducing a water diuresis. - Harrison's 22E

Special note on Thiazides

1. Block NaCl reabsorption in the cortical diluting segment (not the medullary concentrating mechanism)
2. Therefore, the kidney loses its ability to dilute urine but retains its ability to concentrate urine
3. Any hypotonic fluid ingested is retained while salt is lost - leading to hyponatremia
4. Can mimic SIADH clinically; presentations can be indistinguishable without careful history - Goldman-Cecil Medicine

Hypervolemic Hyponatremia

Pathophysiological Mechanism

"The pathophysiology of hyponatremia in the sodium-avid edematous disorders (congestive heart failure [CHF], cirrhosis, and nephrotic syndrome) is similar to that in hypovolemic hyponatremia, except that arterial filling and circulatory integrity is decreased due to the specific etiologic factors (e.g., cardiac dysfunction in CHF, peripheral vasodilation in cirrhosis)." - Harrison's 22E

The Common Neurohumoral Cascade

• Baroreceptor activation (carotid sinus, aortic arch, renal afferent arterioles)
• ↑ Renin-Angiotensin-Aldosterone System (RAAS) → Na+ retention → edema worsens
• ↑ Sympathetic nervous system activation → renal vasoconstriction, ↓ GFR
• ↑ Non-osmotic AVP release → V2 receptor activation → water retention > Na+ retention
• ↑ Thirst → increased water intake
• Net: Na+ is retained but water is retained even more → plasma Na+ falls → hyponatremia

Causes and Specific Mechanisms

• Reduced cardiac output → reduced stroke volume → arterial underfilling
• Despite high total body Na+ and water (edema, pleural effusions), arterial baroreceptors "sense" inadequate filling
• The degree of hyponatremia in CHF reflects the degree of neurohumoral activation and is an independent prognostic indicator for mortality

• Splanchnic vasodilation (due to nitric oxide and other vasodilators released by the diseased liver) causes arterial underfilling
• Portal hypertension promotes fluid shift into peritoneal space (ascites)
• Both mechanisms reduce EABV → same neurohumoral cascade as in CHF
• Hyponatremia in cirrhosis is a sign of advanced disease and carries poor prognosis

• Massive urinary protein loss → hypoalbuminemia → reduced plasma oncotic pressure → fluid shifts from intravascular to interstitial space
• Intravascular volume depletion → decreased EABV → AVP release, RAAS activation
• "Underfill theory" of nephrotic edema

• Unique among hypervolemic causes: urine Na+ is >20 mM (kidney cannot conserve Na+ due to tubular damage)
• Hyponatremia results from the kidney's inability to excrete the water load (loss of diluting ability)
• Total solute excretion is impaired

Key Laboratory Findings

"Urine Na+ concentration is typically very low, i.e., <10 mM, even after hydration with normal saline; this Na+-avid state may be obscured by diuretic therapy. The degree of hyponatremia provides an indirect index of the associated neurohumoral activation and is an important prognostic indicator in hypervolemic hyponatremia." - Harrison's 22E

Summary Comparison Table

Sources

• Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 56 - Fluid and Electrolyte Disorders
• Goldman-Cecil Medicine, Chapter 102 - Disorders of Sodium and Water Homeostasis
• Brenner and Rector's The Kidney, 2-Volume Set, Concepts 9 and 10 - Hyponatremia