Oedema is an accumulation of interstitial fluid. The volume of fluid in the interstitial space is normally kept constant at around 20% of body weight. Normally leakage from capillaries and lymphatic drainage keep this in balance. However, a number of different pathological processes can disturb the balance, causing an excess of fluid to collect.

Oedema is referred to as “pitting” when after pressing on the affected skin, an indentation remains after the source of pressure has been removed. This is the classic type of oedema caused by fluid accumulation. With non-pitting oedema the pressure on the affected area does not persist. This type of oedema is more complex than pitting oedema, as the process involves more than fluids simply filling up interstitial spaces.

Pitting-dependent oedema

  • Immobility: Increased fluid pressure from venous stasis.

  • Varicose veins

  • Obesity: Increased fluid pressure from sodium and water retention; should not to be confused with non-pitting lymphoedema.

  • Cardiac: Increased fluid pressure: right heart failure, constrictive pericarditis.

  • Drugs: Increased fluid pressure from sodium and water retention: calcium antagonists, non-steroidal anti-inflammatory drugs (NSAIDs), prolonged steroid therapy, insulin.

  • Hepatic (liver): Decreased oncotic pressure due to hypoalbuminaemia. Also increased capillary permeability from systemic venous hypertension.

  • Renal (kidney): Decreased oncotic pressure from protein loss, and increased fluid pressure from sodium and water retention: acute nephritic syndrome, nephrotic syndrome.

  • Gastrointestinal: Decreased oncotic pressure: starvation, malnutrition, malabsorption, protein-losing enteropathy (eg, Crohn’s disease, ulcerative colitis, tumours of stomach and colon, coeliac disease and other intestinal allergies).

  • Obstructive sleep apnoea: Increased capillary hydrostatic pressure due to pulmonary hypertension.

  • Pregnancy: Increased fluid pressure both from sodium and water retention and venous stasis from pelvic obstruction.

  • High-altitude illness

  • Idiopathic oedema: unknown reasons.

  • Post-thrombotic syndrome: Late complication of deep vein thrombosis (DVT) which occurs in up to two thirds of patients.

Pitting Localised Oedema

  • DVT.

  • Compression of large veins by tumour or lymph nodes.

  • Following hip replacement or knee replacement.

  • Chronic venous insufficiency. May be unilateral or bilateral. Usually unilateral predominance.

  • Local infection, trauma (including burns, which may also cause generalised oedema because of protein loss), animal bites or stings.

Non-pitting lower limb oedema

  • Hypothyroidism (mucopolysaccharide deposition).

  • Lymphoedema: Blocked lymph channels: surgical damage, radiation, malignant infiltration, infectious (eg, filariasis), congenital (eg, Milroy’s disease).

  • Lipoedema.

  • Allergy: Increased capillary permeability: angio-oedema.