Microsclerotherapy is a technique of injecting thread or spider veins with a special substance known as a "sclerosant", usually a solution called sodium tetradecyl sulphate that causes swelling in the veins wall.
If the vein is small enough, the swelling will destroy the vein over several weeks, making it much less noticeable or even invisible.
Microsclerotherapy is normally used for the treatment of leg thread veins and is rarely used for the treatment of facial veins, where laser or IPL treatments are often favoured, owing to the small possibility of scarring that can occur.