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What are hemorrhoids?
A precise definition of hemorrhoids does not exist, but they can be described as masses or clumps (”cushions”) of tissue within the anal canal that contain blood vessels and the surrounding, supporting tissue made up of muscle and elastic fibers. The anal canal is the last four centimeters through which stool passes as it goes from the rectum to the outside world. The anus is the opening of the anal canal to the outside world.

Although most people think hemorrhoids are abnormal, they are present in everyone. It is only when the hemorrhoidal cushions enlarge that hemorrhoids can cause problems and be considered abnormal or a disease.

Prevalence of hemorrhoids

Although hemorrhoids occur in everyone, they become large and cause problems in only 4% of the general population. Hemorrhoids that cause problems are found equally in men and women, and their prevalence peaks between 45 and 65 years of age.

Anatomy of hemorrhoids

The arteries supplying blood to the anal canal descend into the canal from the rectum above and form a rich network of arteries that communicate with each other around the anal canal. Because of this rich network of arteries, hemorrhoidal blood vessels have a ready supply of arterial blood. This explains why bleeding from hemorrhoids is bright red (arterial blood) rather than dark red (venous blood), and why bleeding from hemorrhoids occasionally can be severe. The blood vessels that supply the hemorrhoidal vessels pass through the supporting tissue of the hemorrhoidal cushions.

The anal veins drain blood away from the anal canal and the hemorrhoids. These veins drain in two directions. The first direction is upwards into the rectum, and the second is downwards beneath the skin surrounding the anus. The dentate line is a line within the anal canal that denotes the transition from anal skin (anoderm) to the lining of the rectum.

What causes hemorrhoids?

It is not known why hemorrhoids enlarge. There are several theories about the cause, including inadequate intake of fiber, prolonged sitting on the toilet, and chronic straining to have a bowel movement (constipation). None of these theories has strong experimental support. Pregnancy is a clear cause of enlarged hemorrhoids though, again, the reason is not clear. Tumors in the pelvis also cause enlargement of hemorrhoids by pressing on veins draining upwards from the anal canal.

One theory proposes that it is the shearing (pulling) force of stool, particularly hard stool, passing through the anal canal that drags the hemorrhoidal cushions downward. Another theory suggests that with age or an aggravating condition, the supporting tissue that is responsible for anchoring the hemorrhoids to the underlying muscle of the anal canal deteriorates. With time, the hemorrhoidal tissue loses its mooring and slides down into the anal canal.

One physiological fact that is known about enlarged hemorrhoids that may be relevant to understanding why they form is that the pressure is elevated in the anal sphincter, the muscle that surrounds the anal canal and the hemorrhoids. The anal sphincter is the muscle that allows us to control our bowel movements. It is not known, however, if this elevated pressure precedes the development of enlarged hemorrhoids or is the result of the hemorrhoids. Perhaps during bowel movements, increased force is required to force stool through the tighter sphincter. The increased shearing force applied to the hemorrhoids by the passing stool may drag the hemorrhoids downward and enlarge them.

What are the symptoms of hemorrhoids?

There are two types of nerves in the anal canal, visceral nerves (above the dentate line) and somatic nerves (below the dentate line). The somatic (skin) nerves are like the nerves of the skin and are capable of sensing pain. The visceral nerves are like the nerves of the intestines and do not sense pain, only pressure. Therefore, internal hemorrhoids, which are above the dentate line, usually are painless.

As the anal cushion of an internal hemorrhoid continues to enlarge, it bulges into the anal canal. It may even pull down a portion of the lining of the rectum above, lose its normal anchoring, and protrude from the anus. This condition is referred to as a prolapsing internal hemorrhoid. In the anal canal, the hemorrhoid is exposed to the trauma of passing stool, particularly hard stools associated with constipation. The trauma can cause bleeding and sometimes pain when stool passes. The rectal lining that has been pulled down secretes mucus and moistens the anus and the surrounding skin. Stool also can leak onto the anal skin. The presence of stool and constant moisture can lead to anal itchiness (pruritus ani), though itchiness is not a common symptom of hemorrhoids. The prolapsing hemorrhoid usually returns into the anal canal or rectum on its own or can be pushed back inside with a finger, but it prolapses again with the next bowel movement.

Less commonly, the hemorrhoid protrudes from the anus and cannot be pushed back inside, a condition referred to as incarceration of the hemorrhoid. Incarcerated hemorrhoids can have their supply of blood shut off by the squeezing pressure of the anal sphincter, and the blood vessels and cushions can die, a condition referred to as gangrene. Gangrene requires medical treatment.

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For convenience in describing the severity of internal hemorrhoids, many physicians use a grading system:

• First-degree hemorrhoids: Hemorrhoids that bleed but do not prolapse.
• Second-degree hemorrhoids: Hemorrhoids that prolapse and retract on their own (with or without bleeding).
• Third-degree hemorrhoids: Hemorrhoids that prolapse but must be pushed back in by a finger.
• Fourth-degree hemorrhoids: Hemorrhoids that prolapse and cannot be pushed back in.

Fourth-degree hemorrhoids also include hemorrhoids that are thrombosed (containing blood clots) or that pull much of the lining of the rectum through the anus.

In general, the symptoms of external hemorrhoids are different than the symptoms of internal hemorrhoids.

External hemorrhoids can be felt as bulges at the anus, but they usually cause few of the symptoms that are typical of internal hemorrhoids. This is perhaps, because they are low in the anal canal and have little effect on the function of the anus, particularly the anal sphincter. External hemorrhoids can cause problems, however, when blood clots inside them. This is referred to as thrombosis. Thrombosis of an external hemorrhoid causes an anal lump that is very painful (because the area is supplied by somatic nerves) and often requires medical attention. The thrombosed hemorrhoid may heal with scarring and leave a tag of skin protruding from the anus. Occasionally, the tag is large, which can make anal hygiene (cleaning) difficult or irritate the anus.

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