Four main mechanisms can be involved:^3,4,5,6

·     - increased capillary hydrostatic pressure due to increased venous pressure or increased plasma volume forcing fluid through the capillary wall into the interstitial space or intracellular space;

·     - decreased oncotic pressure where a reduction in large molecules such as proteins in blood plasma (or excess hydrophilic molecules in the interstitial space) means the osmotic gradient draws fluid out of the capillaries;

·     - endothelial integrity where increased capillary wall permeability lets fluid leak out, often due to vascular damage such as burns, bites or allergy;

·     - obstructed lymphatic drainage.

Ordinarily these four systems are kept in check, such as by increased lymphatic flow or vessel contraction, but disease, injury or medication can alter the balance triggering oedema. Excess sodium (dietary or administered intravenously) can also cause water retention.³

Drainage via the lymph system is the main process for removing fluid and associated large molecules that enter the interstitial space from the capillaries. Lymphatic impairment can also worsen oedema due to other causes.

Symptoms due to hereditary angioedema derive from C1 esterase inhibitor (C1-inh) gene mutation reducing C1-inh protein synthesis causing impaired immune function.¹⁰

Medical literature highlights a wide range of medications associated with a risk of oedema.