Varicose veins and venous reflux
Varicose veins are twisted, widened leg veins that develop when their one-way valves fail, so blood flows backwards (reflux) and pools in the lower leg.
Remember
When one valve fails, blood leaks backwards and the pressure rises on the valves nearer the foot, so they fail in turn and the problem spreads.
Memory aid
Think of a row of one-way doors: once one gives way, the next one takes more pressure and gives way too.
Use the picture to remember that a failed valve passes extra pressure to the next valve, which then fails as well.
A varicose vein is a vein that is tortuous (twisted) or dilated (widened). The superficial veins of the legs are commonly affected. Older age, pregnancy, obesity and standing for long periods increase the risk.
Normally, blood from the legs flows upwards towards the heart. Valves (small one-way flaps inside the veins) let it move up and stop it falling back down, so flow is one-way.
The wall of a superficial vein weakens and the vein widens. The valve leaflets (the flaps themselves) are pulled apart and can no longer close, so the valve is incompetent, meaning it leaks. Gravity then pulls blood backwards through it. This is called retrograde flow, or reflux. Varicose veins can also develop after a deep vein thrombosis (a clot in a deep vein), but valve failure is the usual mechanism.
Blood that falls back builds up and increases the pressure on the valves nearer the foot. These valves are overwhelmed in turn, which allows still more backward flow. The cycle continues down the superficial veins, and blood pools in the lower leg. The veins become dilated, tortuous and visible under the skin, and the legs often feel heavy, tense and itchy.
Pooling keeps the pressure inside the veins high, a state called venous hypertension (abnormally high pressure inside the veins, not in the arteries). It can eventually lead to oedema and leg ulcers. Because each failed valve overloads the next, varicose veins tend to worsen slowly over the years.
Key facts
- Normal valves
- one-way flaps in the leg veins let blood move up towards the heart and stop it falling back down.
- Mechanism
- a weakened vein wall dilates, the valve leaflets are pulled apart, and blood leaks backwards under gravity (retrograde flow, also called reflux).
- Spread
- blood that falls back puts extra pressure on the valves nearer the foot, so they fail in turn and the cycle extends down the superficial veins.
- Venous hypertension
- pooling of blood keeps the pressure inside the veins high, which can eventually lead to oedema and leg ulcers.
- Risk factors
- older age, pregnancy, obesity and standing for long periods all increase the risk.
- Typical history
- aching or heaviness that is worse after prolonged standing and better with the legs raised.
Exam traps
- High pressure in the arteries does not dilate superficial veins, because it does not reach them; varicose veins come from failed venous valves.
- Inflammation and clotting in a varicose vein are complications of varicose veins, not what starts them, because clots usually form once blood is stagnant in the vein.
- Failure of the calf muscle pump (the calf muscles squeezing blood up the deep veins) raises venous pressure and can make varicose veins worse, but the spreading failure of valves, not pump failure, is the mechanism that makes them progress.
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