Thrombosed hemorrhoids are prolapsed external hemorrhoids containing blood clots, according to WebMD. Thrombosed hemorrhoids can bleed and turn purple or blue, but are rarely serious. Thrombosis of a hemorrhoid can be extremely painful.
External hemorrhoids are swollen rectal blood vessels located within the anus, as stated by WebMD. An external hemorrhoid can prolapse to the outside of the anus, typically during a bowel movement.
Thrombosed hemorrhoids usually heal without treatment in a couple weeks, as reported by WebMD. In cases of unbearable pain, health care providers can remove the thrombosed hemorrhoid. Anal bleeding and pain can indicate the presence of colorectal cancer, but hemorrhoids, which are seldom dangerous, are the leading cause of anal bleeding and pain.