Varicose veins are not dangerous but are cosmetically ugly. However their complications cause increased mobility.
Bleeding — this may be profuse from the superficial veins and may happen at random. Should this happen then do not panic. Just put pressure at that spot of bleeding and elevate the leg. Once stopped tie a pressure bandage and consult a vascular surgeon.
Venous ulcers — They will never heal till the veins are sorted out. Once veins are treated surgically the ulcers may heal spontaneously or need skin grafting.
Superficial thrombophlebitis – This is inflammation of the dilated vein and is very painful and tender. It needs treatment which is conservative and once settled would need surgical correction of the veins before recurrence of the phlebitis occurs.
Deep vein Thrombosis. — there is increased incidence of DVT with varicose veins. Once DVT occurs it is mostly a contraindication for varicose veins surgery. So it is advised to not delay varicose veins because they are not giving problems because if DVT happens then may be surgery would not be possible even if you develop ulcers. Now a days endo-venous dilatation and stenting of DVT is possible but the results are sketchy.
Other complications like discolouration, itching pain and swelling are just discomforts.
They are due to weakness in the wall of the vein leading to laxity of the wall. Once the walls are lax they lead to ballooning. This causes the valves to be separated leading to varicose veins. It is purely mechanical. Naturally when we stand due to the pull of gravity more pooling occurs leading to worsening of the situation and making veins more prominent. They collapse on lying because there is no gravity acting.