Causes and types of edema

1. Definition

• Edema = abnormal accumulation of fluid in interstitial tissues / body cavities, from a disturbed balance in the microcirculation.
• Determining factors: ↓colloid osmotic (oncotic) pressure, ↑capillary permeability, ↑venous hydrostatic pressure, compromised lymphatics, Na⁺/water retention.
• Cavity terms: hydrothorax (pleural), hydropericardium, ascites (peritoneal), anasarca (subcutaneous, generalized).

2. Types of Edema Fluid

	Transudate	Exudate
Protein	Low	High
Nature	Non-inflammatory	Inflammatory
Specific gravity	< 1.012	> 1.012
Cause	Pressure/volume overload, ↓plasma protein	Inflammation, ↑permeability

3. Pathophysiological Forms

↑ Hydrostatic pressure

• Local: venous obstruction (DVT → leg edema; cirrhosis → ascites), pulmonary edema (acute left HF), SVC syndrome.
• Lymphatic obstruction: irradiation, tumor, surgery, filariasis → lymphedema → elephantiasis.
• Inflammation, allergy, hypoxia (cerebral).
• Cerebral: vasogenic (BBB disruption, perifocal around tumor/abscess) vs cytotoxic (hypoxia/trauma).

Systemic (generalized)

• Hypoalbuminemia / ↓oncotic pressure: nephrotic syndrome (urinary protein loss), cirrhosis, malnutrition → periorbital then generalized.
• Right-sided HF / cor pulmonale: hydropericardium, bilateral hydrothorax, ascites, anasarca.
• Rh incompatibility / infection: fetal hydrops → intrauterine death.

4. Morphology & Clinical

• Subcutaneous (pitting): hypoproteinemic, periorbital, painless depression.
• Dependent: legs (standing) / sacrum (lying) — typical of CHF.
• Pulmonary: lungs 2–3× weight, frothy blood-tinged fluid → impaired gas exchange.
• Cerebral: ↑brain weight, narrowed sulci/distended gyri → foramen magnum herniation → death.

💡 High-yield: 4 mechanisms = ↑hydrostatic, ↓oncotic, ↑permeability, lymphatic obstruction (+ Na⁺ retention). Transudate (low protein, SG <1.012, CHF/nephrosis) vs exudate (high protein, inflammation). Cerebral edema → herniation = life-threatening.