Last revised by Henry Knipe on 1 Mar 2022

Edema is the accumulation of fluid in tissues. It is caused by an imbalance between forces holding fluids in the vasculature and tissues 1. Fluids are normally held in a steady state between tissues and vasculature by homeostasis between these forces (Starling equation) 2:

  • capillary hydrostatic pressure: drives fluid out of the vasculature into tissues
  • plasma colloid osmotic pressure: keeps fluid in the vasculature, preventing extravasation into the tissues
  • tissue colloid osmotic pressure: keeps fluid in the tissues
  • increased capillary hydrostatic pressure
  • reduced plasma colloid osmotic pressure
  • increased tissue colloid osmotic pressure
  • increased vascular permeability
    • inflammation, e.g. trauma, hypersensitivity response, infection 4
    • infection
  • reduced lymphatic clearance
  • increased salt retention (causes both increased hydrostatic pressure and reduced plasma colloid osmotic pressure by fluid retention)
    • renal insufficiency

Soft tissue edema presents as non-enhancing, diffuse or focal low T1, high PD/T2 signal reticulated signal in the subcutaneous fat and superficial fascia 4.

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