Although internal hemorrhoids generally do not cause severe pain, there are some that can. These are known as incarcerated hemorrhoids. If internal hemorrhoids progress, they can cross the dentate line, move outside the anus, and become incarcerated hemorrhoids. These cannot go back inside the anus, form clots, swell to a large size, and cause severe pain.
Incarcerated hemorrhoids do not require immediate surgery, but they do require emergency treatment, and must be treated at a hospital right away.
External thrombosed hemorrhoids
External thrombosed hemorrhoids are sudden thrombi (blood clots) around the anus, and cause severe pain. Although there is no need for immediate surgery, they should be treated at a hospital if the pain is strong or if symptoms continue for a long period of time.