What Is Melanosis Coli?

Pigmentation of the Colon and Rectum

Doctor and patient looking at colonoscopy results
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Melanosis coli is a condition in which there is a discoloration of the membranes lining your colon (large intestine) and rectum resulting from the deposit of pigment in the lamina propria, a layer of the intestinal lining. Because it has no symptoms, this discoloration is usually identified during a colonoscopy. Also called pseudomelanosis coli, melanosis coli got its name because it was originally believed that the color change was the result of the pigment melanin. However, research now shows that lipofuscin, a pigment associated with aging, is the pigment responsible for the dark color.

Typically, melanosis coli is a dark brown or black color that can vary in darkness and intensity. The color change tends to be more evident in the beginning and middle parts of the colon rather than spread uniformly throughout, but this can be different for every individual with the condition. On rare occasions, pigment changes may also be seen in the small intestine, a condition known as melanosis ilei.

Causes

Doctors most often see melanosis coli in patients who experience chronic constipation and frequently use laxatives. This has led to confirmation of a solid link between the use of herbal laxatives that contain organic compounds found in some plants called anthraquinones and the appearance of melanosis coli. These laxatives are also commonly called anthranoids.

The intestinal discoloration may be seen as early as four months after you start regularly using herbal laxatives. Melanosis coli may be seen in patients who do not have constipation or use the anthraquinone-containing laxatives as well, though this is rare.

Anthraquinone-containing laxatives include:

Scientists don't know exactly why these particular laxatives cause the pigment changes of melanosis coli, but the best theory is that the purging effects of the laxatives damage the epithelial cells on the lining of the colon, creating pigmentation. As the damaged cells accumulate, the pigmentation also accumulates, and melanosis coli occurs.

Diagnosis

Since it has no symptoms, melanosis coli is usually found during an endoscopic procedure such as a colonoscopy or a sigmoidoscopy, a procedure that looks at just your sigmoid (lower) colon rather than the entire colon as in a colonoscopy. A doctor may see melanosis coli himself or it may be found during the examination of a tissue biopsy taken during one of these procedures.

Women are more likely to be diagnosed with melanosis coli than men, probably because constipation is three times higher in women, which may result in more laxative use. Constipation is more frequent in women most likely because stool tends to move more slowly through the female digestive tract and because some women seem to experience constipation during their monthly periods.

Cancer Risk

Most gastroenterologists consider melanosis coli to be a harmless condition. Studies have not found an association between melanosis coli and an increased risk of colorectal cancer in humans. This doesn't mean that there isn't one, but that a potential association is difficult to find and hasn't been proven yet.

In 1997, the U.S. Food and Drug Administration (FDA) banned the use of the anthraquinone phenolphthalein in over-the-counter laxatives because of concerns that prolonged or excessive use could cause cancer. The reason for the ban was that there were animal studies that showed that using large amounts of phenolphthalein caused tumors. However, phenolphthalein hasn't been shown to cause tumors in humans.

One study of melanosis coli in patients with colon cancer found that the presence of melanosis coli increases with age, similar to colon cancer itself. This makes sense considering that the pigment responsible for melanosis coli, lipofuscin, is a product of aging (cell death). The study also found that there was no melanosis inside the tumors studied.

The reason for the lack of clarity on the subject has to do with the unclear association of constipation and cancer risk. Studies have shown an association between constipation and colon cancer, but it isn't known why. Some scientists think it's due to the slow motility, or movement, of bowel movements during constipation, which could result in carcinogens having prolonged contact with intestinal cells. Or it may be that the factors that contributed to constipation itself, such as a diet that's too low in fiber, are the reason why this possible higher risk of cancer exists.

A Word From Verywell

There is no definitive protocol for eliminating melanosis coli, but your doctor will likely recommend that you take steps to prevent constipation, including staying hydrated and eating a high-fiber diet, and that you stop using laxatives, especially those that contain anthraquinone. Once these laxatives are discontinued, it's possible that the condition will disappear within six to 12 months.

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