Edema
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Edema
Definition
Edema is the accumulation of excess interstitial fluid within tissues. When extravascular fluid collects in body cavities it is called an effusion - specifically:
- Hydrothorax - pleural cavity
- Hydropericardium - pericardial cavity
- Hydroperitoneum (Ascites) - peritoneal cavity
- Anasarca - severe, generalized edema with profound subcutaneous swelling and body cavity fluid accumulation
Pathophysiology
Approximately 60% of lean body weight is water; two-thirds is intracellular, and most of the rest is interstitial fluid. Only ~5% of body water is in plasma.
Fluid movement between the vasculature and interstitium is governed by two opposing forces:
- Vascular hydrostatic pressure - pushes fluid out
- Plasma colloid osmotic pressure - draws fluid back in
Normally, small net fluid outflow into the interstitium is drained by lymphatics. When either hydrostatic pressure increases or colloid osmotic pressure falls beyond lymphatic drainage capacity, edema results.
Transudate vs. Exudate:
- High hydrostatic pressure or low colloid pressure → protein-poor transudate
- Increased vascular permeability (inflammation) → protein-rich exudate
Causes
| Mechanism | Examples |
|---|---|
| Increased Hydrostatic Pressure | Congestive heart failure, constrictive pericarditis, cirrhosis, DVT, venous compression, prolonged limb dependency, heat, arteriolar dilation |
| Reduced Plasma Osmotic Pressure | Nephrotic syndrome (protein loss), liver cirrhosis (reduced synthesis), malnutrition, protein-losing enteropathy |
| Lymphatic Obstruction | Filariasis (elephantiasis), breast cancer (peau d'orange), post-axillary dissection/irradiation |
| Sodium and Water Retention | Acute renal failure, poststreptococcal glomerulonephritis - expands intravascular volume and dilutes plasma proteins |
| Increased Vascular Permeability | Inflammation, acute lung injury, anaphylaxis |
The Vicious Cycle in Heart Failure
In congestive heart failure, multiple factors amplify edema:
- Reduced cardiac output → blood pools in venous circulation → increased capillary hydrostatic pressure
- Reduced cardiac output → renal hypoperfusion → activation of the renin-angiotensin-aldosterone axis → secondary hyperaldosteronism → Na+ and water retention
- The failing heart cannot increase output in response to increased filling, so retained fluid worsens venous pressure further
- This creates a downward spiral unless treated with salt restriction, diuretics, or aldosterone antagonists
The same secondary hyperaldosteronism also occurs in nephrotic syndrome and cirrhosis, making increased salt retention actually worsen edema in those settings (because the primary defect - low serum protein - persists).
Morphology and Sites
Subcutaneous edema: Preferentially occurs in dependent areas (legs with standing, sacrum with recumbency) - called dependent edema. Finger pressure displaces fluid, leaving a depression = pitting edema. In nephrotic syndrome/renal failure, edema may first appear in loose connective tissues (e.g., periorbital - eyelids).
Pulmonary edema: Lungs become 2-3x normal weight. On sectioning, frothy blood-tinged fluid exudes. Most common in left ventricular failure, also in renal failure, acute lung injury, and lung infections.
Brain edema: Localized (abscess, tumor) or generalized. Sulci narrow, gyri swell against the skull. Can be rapidly fatal due to herniation through the foramen magnum, compressing the medullary respiratory centers.
Clinical Features
| Site | Consequences |
|---|---|
| Subcutaneous | Signals cardiac or renal disease; impairs wound healing and skin infection clearance |
| Pulmonary | Interferes with ventilation, can be fatal; edema fluid promotes secondary infection |
| Brain | Life-threatening; herniation → brainstem compression → death from respiratory failure |
Key Histological Finding
Microscopic examination shows clearing and separation of the extracellular matrix (ECM) elements in affected tissues. This is subtler than the gross appearance but confirms the diagnosis histologically.
Treatment Principles
- Salt restriction
- Diuretics (reduce intravascular volume)
- Aldosterone antagonists (spironolactone) - especially useful when secondary hyperaldosteronism is contributing
- Treating the underlying cause (e.g., improving cardiac function, correcting hypoalbuminemia)
Sources: Robbins & Kumar Basic Pathology, 11th ed.; Robbins, Cotran & Kumar Pathologic Basis of Disease, 10th ed.
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