Q1 LONG ANSWER 10M ★ High-yield

Define oedema. Describe its pathogenesis with the mechanisms involved. Differentiate transudate from exudate.

Pathology · 2nd Professional — Para-Clinical

Definition

Oedema is the abnormal accumulation of fluid in the interstitial spaces or body cavities.

Term	Meaning
Anasarca	Severe, generalised oedema with widespread subcutaneous swelling
Ascites	Fluid in the peritoneal cavity
Hydrothorax	Fluid in the pleural cavity
Hydropericardium	Fluid in the pericardial sac
Lymphoedema	Oedema from lymphatic obstruction

Normal Fluid Exchange — the Starling Forces

At the arteriolar end, hydrostatic pressure exceeds oncotic pressure → Fluid moves out into the interstitium → At the venous end, oncotic pressure exceeds hydrostatic pressure → Most fluid is reabsorbed → The small residue is returned by the lymphatics → Oedema results when this balance is disturbed at any of these points

Pathogenesis — the Five Mechanisms

Mechanism	Explanation	Causes
1. Increased hydrostatic pressure	Raised venous pressure opposes reabsorption	Congestive cardiac failure (generalised); deep vein thrombosis, varicose veins, cirrhosis with portal hypertension (localised)
2. Reduced plasma oncotic pressure	Hypoalbuminaemia — albumin below about 2.5 g/dL	Nephrotic syndrome (loss), cirrhosis (reduced synthesis), protein malnutrition — kwashiorkor, protein-losing enteropathy
3. Lymphatic obstruction	Interstitial fluid and protein cannot be returned	Filariasis (elephantiasis), malignant infiltration, surgical removal of nodes, radiotherapy, post-inflammatory scarring
4. Sodium and water retention	Expands the plasma volume and dilutes the plasma protein	Acute glomerulonephritis, renal failure, excessive salt intake, activation of the renin–angiotensin–aldosterone system
5. Increased capillary permeability	Protein-rich fluid escapes — an exudate	Inflammation, burns, anaphylaxis, sepsis

CLINICAL PEARL

Cardiac failure produces oedema by three of these mechanisms at once. Raised venous pressure is the obvious one, but reduced renal perfusion also activates the renin–angiotensin–aldosterone system, retaining sodium and water, and hepatic congestion eventually impairs albumin synthesis. Each worsens the others — which is why diuretics alone are seldom enough.

Transudate and Exudate

Feature	Transudate	Exudate
Mechanism	Altered Starling forces; permeability normal	Increased permeability — inflammation
Protein	Low, under 3 g/dL	High, over 3 g/dL
Specific gravity	Under 1.012	Over 1.020
Cells	Few	Many, including neutrophils
Clots on standing	No	Yes — contains fibrinogen
Appearance	Clear, straw-coloured	Turbid, may be purulent or blood-stained
Examples	Cardiac failure, nephrotic syndrome, cirrhosis	Empyema, tuberculous effusion, peritonitis

Morphology and Clinical Types

Type	Features
Subcutaneous oedema	Pitting on pressure; dependent — ankles when upright, sacrum when supine. Indicates cardiac or renal disease
Periorbital oedema	Characteristic of renal disease, especially in children, because the tissue is lax and there is no dependent effect at night
Pulmonary oedema	Heavy, wet lungs; frothy, blood-tinged fluid; "heart failure cells" (haemosiderin-laden macrophages). Life-threatening
Cerebral oedema	Brain swollen with narrowed sulci and flattened gyri; risk of herniation
Lymphoedema	Non-pitting and firm, because the fluid is protein-rich and fibrosis follows; skin thickened and warty — elephantiasis

Pitting versus non-pitting is the single most useful bedside distinction — low protein content pits, high protein content does not

Oedema in Specific Diseases

Disease	Chief mechanism	Distribution
Congestive cardiac failure	Raised hydrostatic pressure + sodium retention	Dependent and pitting; ankles, sacrum; with raised JVP
Nephrotic syndrome	Hypoalbuminaemia from urinary protein loss	Periorbital first, then generalised (anasarca)
Acute glomerulonephritis	Salt and water retention	Periorbital, with hypertension and haematuria
Cirrhosis	Portal hypertension + hypoalbuminaemia + hyperaldosteronism	Ascites predominates, with leg oedema
Kwashiorkor	Severe hypoalbuminaemia	Generalised; the child appears deceptively well-nourished
Filariasis	Lymphatic obstruction	Non-pitting; leg and scrotum; elephantiasis
Hypothyroidism	Deposition of glycosaminoglycans	Non-pitting myxoedema
Angio-oedema	Increased permeability (bradykinin or histamine)	Face, lips, larynx; sudden and dangerous

The two non-pitting oedemas are lymphoedema and myxoedema — both because the interstitium contains protein or mucopolysaccharide rather than free water

Applied Aspects

Pulmonary oedema is a medical emergency — the patient is breathless, orthopnoeic, with crepitations and pink frothy sputum; treated by sitting up, oxygen, diuretics and nitrates
Filariasis remains an important cause of lymphoedema in India; Wuchereria bancrofti obstructs the lymphatics, giving elephantiasis of the leg and scrotum
Nephrotic syndrome — proteinuria above 3.5 g a day, hypoalbuminaemia, oedema and hyperlipidaemia; the oedema is typically periorbital first
Ascites in cirrhosis arises from portal hypertension, hypoalbuminaemia and secondary hyperaldosteronism together; the serum-ascites albumin gradient distinguishes portal from other causes
Cerebral oedema kills by herniation, not by the swelling itself; the rigid skull permits no expansion
Oedema is a sign, not a diagnosis — its distribution points to the cause: generalised suggests cardiac, renal or hepatic disease, localised suggests venous or lymphatic obstruction
Diuretics remove fluid but do not correct the cause, and over-vigorous diuresis causes hypovolaemia, hypokalaemia and renal impairment
Daily weight is the most reliable measure of fluid balance, more so than intake and output charts, and a kilogram corresponds to about a litre
Pleural effusion is detected clinically only above about 500 mL, and on a chest radiograph above about 200 mL, so a small effusion is easily missed
Albumin infusion is rarely justified for hypoalbuminaemic oedema, since it leaks into the interstitium and the effect is transient

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