The dentate line divides two sets of nerves in the rectum canal; visceral nerves (above the dentate line), and somatic nerves (below the dentate line). Somatic nerves are skin nerves, meaning they are capable of sensing pain. The visceral nerves are like intestinal nerves, they sense pressure instead of pain. Internal hemorrhoids (are above the dentate line) usually are painless because of the visceral nerves.
An internal hemorrhoid is caused by the hemorrhoidal cushion enlarging, and bulging into the rectum canal. In some cases it even pulls down a portion of the rectum above, loses its normal anchoring, and can protrude from the anus. This is referred to as a prolapsing internal hemorrhoid. Internal hemorrhoids are exposed to passing stools in the rectum canal. The trauma of passing a stool, particularly a hard stool, can cause bleeding and pain. The rectal lining that has been pulled down secretes a mucus, and moistens the anus and surrounding areas. Sometimes stool can also leak out onto the anus skin. Constant moisture and the presence of stool can cause rectum itchiness. However, rectum itchiness is not a common symptom of hemorrhoids. After a bowel movement, a prolapsing hemorrhoid usually returns back into the rectum canal, or can be pushed back in with a finger. It will normally prolapse again on the nextbowel movement.
Sometimes the hemorrhoid will protrude from the rectum canal and cannot be pushed back inside. This condition is known as an incarceration of the hemorrhoid. Incarcerated hemorrhoids can lose their blood supply from the squeezing of the anus sphincter. This can cause the blood vessels and cushions to die, a conditon referred to as gangrene. Gangrene requires immediate medical treatment.
Physicians have come up with a grading system for describing the severity of internal hemorrhoids.
First Degree Hemorrhoids-Hemorrhoids that bleed without prolapsing.
Second Degree Hemorrhoids-Hemorrhoids that prolapse and retract by themselves (bleeding may or may not be present).
Third Degree Hemorrhoids-Prolapsed hemorrhoids that must be pushed back in by a finger.
Fourth Degree Hemorrhoids-Prolapsed hemorrhoids that cannot be pushed back in. (Fourth Degree Hemorrhoids also include a Thrombosed Hemorrhoid (containing blood clots) or hemorrhoids that pull much of the rectum lining through the anus)
Generally, the symptoms of External Hemorrhoids differ from Internal Hemorrhoids. External hemorrhoids can generally be felt as bulges at the anus. They usually do not cause many of the symptoms that are typical of internal hemorrhoids. This is because they are low in the rectum canal and do not really have an effect on the function of the anus or sphincter. Many external hemorrhoids cause problems when clots form inside of them. This is called thrombosis. Thrombosis of an external hemorrhoid can be very painful and often require medical attention. External hemorrhoids are supplied by somatic nerves, which makes them painful, as opposed to internal hemorrhoids. Thrombosed hemorrhoids may heal with scarring and leave a tag of skin protruding from the anus. When the tag is large, it can make hygiene difficult or irritate the anus.