Usually red or blue in colour, they are often hereditary in nature, and are more likely to occur during hormonal surges, for example during pregnancy. Thread veins most often appear on the thighs, although they can appear anywhere on the body.
Microsclerotherapy treatment video
You can see a short video of Dr Mayoni administering microsclerotherapy below. As you can see, the thread vein literally disappears as the injection is given:
(please note that this video contains footage of a needle)