It may be poor venous reflux in the lower limbs, such as unilateral venous valve dysfunction and deep vein thrombosis.
In addition, blockage of lymphatic vessels in the lower limbs can cause poor lymphatic return and can also lead to lower limb oedema. For example, filariasis, a parasitic disease, can lead to unilateral lower limb lymphatic vessel obstruction and poor lymphatic return resulting in unilateral lower limb swelling and the characteristic clinical manifestation of elephantiasis.