Sometimes, as a result of heavy lifting, coughing, or straining for any reason, one of the small blood vessels adjacent to the edge of the anus ruptures - blood escapes and collects under the surface of the skin. Why this should happen in this area so often is not clear, but it is fairly common. The blood clots into a lump or hematoma, usually the size of a blueberry, and much the same color. These clots are called perianal hematomas (or sometimes thrombosed external hemorrhoids) and can cause severe pain. The pain is constant, and can last for up to 4 days, after which it gradually subsides. The pattern of pain (constant and lasting 4 days) is very much like the pattern of pain of a thrombosed hemorrhoid inside the anus, leading to confusion between the 2 quite different problems.
Perianal hematomas do not cause short jabs of pain or any sharp pain. Sharp pain is more likely to be an anal fissure.
The blood clot under the skin is usually easy to feel. It will often grow in size over a week and can eventually erode through the skin. If it does erode through the skin, dark blue blood will be noticed on the underwear or on the toilet tissue. The blood is not actually coming from inside the anus, but from the skin overlying the clot next to the entrance to the anus.
Treatment of a perianal hemotoma is to leave it alone or surgically remove it
A perianal hematoma can be left alone to heal, if it is already subsiding, or it can be surgically removed if it is still painful or if it is eroding through the skin. A small surgical procedure can be done, usually in the office or emergency department of a hospital under local anesthetic. The surgeon freezes the area and then simply cuts off the hematoma with a pair of scissors. The entire lump including the overlying skin should be removed rather than just opened up as it often comes back otherwise.
The only way to prevent perianal hematomas is to avoid straining or lifting. However, many perianal hematomas happen in young people who like to lift weights at the gym or whose job requires them to lift (movers, etc.). For such individuals, avoiding lifting is not practical - but sometimes an avoidance of "straining" while lifting can lessen the chances of a perianal hematoma.