Oedema

Introduction

Oedema is NOT a disease; it is the sign of a disease state.

• Oedema is defined as :"The swelling of tissues resulting from accumulation of excess fluid in the intercellular tissue spaces and serous cavities."

• Small amounts of fluid are normally present to lubricate cavities and viscera - this is not oedema.
• Excess fluid may accumulate in:
  • Subcutaneous tissue and between muscle.
  • Serous cavities.
    • Depending on the cavity, this has different terms.
      • Thorax - hydrothorax.
      • Pericardium - hydropericardium
      • Abdomen - hydroperitoneum (ascites).
    • Anasarca is when there is generalised body cavity accumulation plus subcutaneous involvement.
  • Lungs.

Local oedema

• Local oedema is the local accumulation of excess interstitial fluid.
• Caused by disturbance of the balance betwen fluid extravasation and resorption at the level of the capillaries.
  • Outwards Forces - arteriolar
    • Vasuclar hydrostatic pressure - 35 mmHg
    • Interstitial osmotic pressure - 3 mmHg
  • Inwards forces - venular
    • Plasma protein osmotic pressure - 25 mmHg
    • Interstitial hydrostatic pressure - 4 mmHg
• May be of inflammatory or non-inflammatory origin.

Types of Local Oedema

Inflammatory oedema

• Generated by one or more of the following:
  1. Increased vascular permeability
  2. Increased arteriolar blood pressure
  3. Breakdown of tissue protein or transfer of plasma proteins into ECF.
     • Results in raised osmotic pressure of tissue fluid.
  4. Obstruction to lymphatic drainage.
     • Usually by fibrin.

Lymphatic oedema

• Results in accumulation of high protein fluid.
• May provoke a granulation or fibrous tissue response.
• Due to:
  1. Lymphangitis/ lymphadenitis
     • Acute inflammation of lymphatics/ lymph nodes. R
     • Caused by stasis in lymphatics and/or bacterial infection.
     • E.g. “Monday Morning leg” in horses.
  2. Chronic inflammation caused by persistent or granuloma-producing bacterial infection.
     • E.g. Johne's disease, actinobacillosis.
  3. Tumour spread.
     • Metastasis of tumour cell plugs lymphatics and nodes
     • e.g. mammary carcinoma.
  4. Parasitic migration
     • Larvae may be following their normal pathway (e.g. Schistosomiasis), or may be aberrant.

Local venous obstruction

• Obstruction to venous drainage may be mechanical or inflammatory-mediated.
  • Causes raised hydrostatic pressure.
  • Endothelial permeability increases due to hypoxia.
  • There may be inflammatory damage.
• Mechanical obstruction, e.g.
  • Torsions of bowel
  • Misplaced organs.
  • Pressure from outside vein from adjacent structures.
    • Tumours.
• Venous inflammation (phlebitis)
  • May be associated with thrombosis (thrombophlebitis).

"Allergic" oedema

• Results from immediate (Type I ) or delayed (Type IV) hypersensitivity.
• Vasular permeability is increased due to release of histamine and vaso-dilating substances.
• E.g.
  • Insect stings (immmediate).
  • Vaccination (delayed).
  • Food reaction (delayed).

Composition of oedema fluid

• Inflammatory oedema which produces an exudate.
  • This is a protein rich fluid containing many inflammatory cells.
• Non inflammatory oedema which produces a transudate.
  • This fluid is low in protein and cells.
• Transudates and exudates are distinguished by the following criteria:

Criteria	Transudate	Exudate
Appearance	Clear/ pale yellow ("straw coloured")	Dark yellow, red or brown. Often cloudy or opaque.
Consistency	Thin, serous	Viscous
Protein content	0.05 - 0.5%, mainly albumin	Usually 2 - 4%
Coagulability	No fibrinogen, no coagulation	Contains fibrinogen, coagulates
Specific gravity	Low (< 1.012)	High (1.012 - 1.020) +
Cell content	Very low. mesothelial cells, some macropahges and lymphocytes/Monocytes.	High. Often macrophages, neutrophils, lymphocyes etc. Depends on cause and chronicity

• Examples of transudates:
  • Ascites
    • Excessive fluid in abdominal cavity.
  • Hydrothorax
    • Excessive fluid in the thorax.
  • Hydropericardium
    • Excessive fluid in the pericardium.
  • Anasarca
    • Generalised tissue oedema most noticeable in subcutaneous tissues.
  • Ventral subcutaneous oedema
• Seen in heart failure in horses and cattle.