While muscular arteries carry your blood away from your heart, it is the thin walled veins that bring the blood back to the lungs and heart. Because there is no muscle in veins, the muscles of your legs and feet are what squeeze the blood back up to your heart. One way valves in the veins help keep the blood moving up by preventing it from flowing backwards, down into your legs.
When the veins are damaged, as can happen from excess pooling of blood, from diabetes, and from genetic factors, then the valves fail and the blood begins to pool in the vessels. The result is dilated baggy veins that pouch out from the skin. Over time smaller veins grow to try to redirect the blood flow. These tiny vessels are the spider veins that develop.
Sclerotherapy works by causing the treated veins to scar down, sealing them tight and preventing the blood from filling them again. This is done by inserting a tiny needle into the vein and injecting a substance that causes damage to the blood vessel walls. The two most commonly used substances are hypertonic saline and detergents.
Hypertonic saline is super salty water. The high salt damages the red blood cells in the veins causing a clot to form. Once a clot fills the whole vein, the blood cannot flow back into it. The vein eventually atrophies and disappears. The advantage of hypertonic saline is that it is readily available and often works. The disadvantage is that it can be painful, and if the saline is injected into the tissue outside of the vein, then tissue damage can occur, leading to an ulcer.
Detergents are, just as it sounds, soaps. Injecting detergents into the veins also causes a clot to form and damages the vessel walls. Again, this leads to a sealing of the vessel and eventually the vein disappears. There are several detergents available for physicians to use including sotradecol, which is FDA approved, and polidocanol, which is not FDA approved at this time, but is often preferred because it is effective and less painful than other detergents. These detergents also have a low likelihood of causing a serious allergic reaction which can occur when these substances are injected right into your bloodstream.
Ironically, the larger blue veins (3 mm or less in diameter) are often easier to treat than the smaller ones. Sclerotherapy of larger blue veins usually leads to good results in just a few days. Larger veins can, however, lead to brown pigmentation of the skin. This pigment is from the iron in the red blood cells that were trapped in the vein. The brown color can take 6 months to 2 years to fade. The smaller, red spider veins are less likely to cause this brown discoloration when treated. However, they can be more difficult to treat. It often takes several treatments and can take months to see improvements. They can also regrow more easily. The more times these small red veins have been treated, the less likely that they will go away completely with therapy.
Veins higher on the leg and thigh can also be easier to treat than those lower on the leg or foot. Varicose veins on the inside ankle are notriously difficult to treat and are the most likely to lead to an ulcer of the skin after treatment with sclerotherapy.