Predisposing factors

Congenital:

• absence of valves in the iliac veins
• abnormal vein wall elasticity
• arteriovenous fistulae e.g. Robertson's giant limb

Acquired:

• obesity in women (but not in men)
• prolonged standing
• parity - women are affected six times more often than men, the majority of cases following a second or third pregnancy; important factors may include:
  • impaired venous return, due to pressure on the iliac veins from the pregnant uterus
  • high level of progesterone which alters collagen structure, sometimes irreversibly, and relaxes smooth muscle
• previous deep vein thrombosis - valves damaged when the veins recanalise