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This sympXXXXX XXXXXkely is caused by venous hypertension or venous insufficiency. This is more common as we age and with obesity, but can also occur due to genetic factors, a sedentary lifestyle, smoking, long periods of standing or sitting at work. The valves in the vein fail to close completely.
In case you are not familiar, veins have valves in them. After the arteries send blood to the periphery, the venous cappilaries pick up the used blood to send it via the larger veins back to the heart and lungs for re-oxygenation. There is no pumping mechinism, except for the movement of the muscles in your legs to get the blood back to the heart. There are one way valves in the veins that prevent gravity from pulling the venous blood back down. If these veins are not closing properly, you can get this type of discoloration from the remaining venous blood in the tissues. Varicose veins and sweeling may also occur. In severe cases ulceration of the skin can develop and there may severe skin breakdown.
In rare cases this can be caused by clots in the veins. If one leg has much of this and the other dosn't or one leg is swollen larger than the others, this would be an indication to get in right away for doppler or other exam to check for clots in the veins.
Complete discussion is available here: http://my.clevelandclinic.org/disorders/venous_insufficiency/hic_venous_insufficiency.aspx
If you do not have any risk factors (obesity, smoking, sitting or standing for long periods) and you have this, I would see the doctor for evaluation in the next several days. Otherwise, it is best to work at the factors that you can control to help prevent further problems.
If you have questions, please reply.