Paper I
Question
Define edema and describe the various pathogenetic mechanisms involved in different clinical situations associated with edema.
Answer
Definition
- Oedema is the accumulation of excess fluid in the interstitial tissue spaces or body cavities, resulting from an imbalance in the Starling forces governing fluid movement across capillary walls
Pathogenetic mechanisms
1. Increased hydrostatic pressure
- Impaired venous return: congestive cardiac failure, constrictive pericarditis, venous obstruction/thrombosis
- Arteriolar dilatation: heat, neurohumoral dysregulation
- Example: leg oedema in congestive heart failure
2. Reduced plasma oncotic pressure (hypoproteinaemia)
- Nephrotic syndrome (protein loss in urine)
- Liver cirrhosis (reduced albumin synthesis)
- Protein-losing enteropathy, malnutrition
- Example: generalized (anasarca) oedema in nephrotic syndrome
3. Lymphatic obstruction
- Inflammatory or neoplastic obstruction of lymphatics
- Post-surgical (e.g., axillary lymph node dissection causing arm lymphoedema)
- Filariasis (elephantiasis)
4. Sodium and water retention
- Excessive salt intake with renal insufficiency
- Increased renin-angiotensin-aldosterone activation (reduced renal perfusion in heart failure)
- Example: contributes to generalized oedema in congestive heart failure and renal disease
5. Increased vascular permeability
- Inflammation (histamine, other mediators increase capillary permeability)
- Example: localized inflammatory oedema
Clinical classification
- Localized oedema: venous/lymphatic obstruction, inflammation
- Generalized oedema (anasarca): cardiac, renal, hepatic, or nutritional causes