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 the colon (large intestine) and the rectum. The condition is usually identified during a colonoscopy.

Typically, melanosis coli presents itself with the color of dark brown or black. The color change is not seen uniformly throughout the colon, but rather tends to be more evident in the beginning and middle parts of the colon.

On rarer occasions, pigment changes may also be seen in the small intestine, a condition known as melanosis ilei.

Melanosis coli got its name because it was thought that the color change was the result of the pigment melanin. Microscopic research now points to the lipofuscin as the pigment-producing the dark color.

Causes

Doctors most typically see melanosis coli in patients who are experiencing chronic constipation. This has led to confirmation of a solid link between the use herbal laxatives that contain anthraquinones and the appearance of melanosis coli. Melanosis coli may be seen as early as four months after regular use of herbal laxatives, but will disappear within six to 12 months after laxative use has stopped.

Anthraquinone-containing laxatives include:

It is just not yet well known as to why these laxatives cause the pigment changes of melanosis coli.

Some theorists propose that the purging effects of the laxatives cause a dying-off of the epithelial cells on the lining of the colon. The remnants of these cells are thus theorized to be acted upon by other cells, with the result of the formation of a substance with a dark granular appearance that now covers the intestinal lining.

Interestingly, in rare cases, melanosis coli may be seen in patients who do not have constipation nor use the above laxatives.

Melanosis Coli and 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. This doesn't mean that there isn't one, just that it is hard to detect such an association.

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 think it is due to the slow motility of constipation resulting in carcinogens having prolonged contact with intestinal cells. Or, it may be that the factors that contributed to constipation itself, such as a diet low in fiber, are the reason why this risk exists.

Treatment

There is no definitive protocol for eliminating melanosis coli. Typically, the recommendation is for improved management of constipation itself, along with a recommendation to stop using anthraquinone-containing laxatives. Once these laxatives are discontinued, it is possible that the condition will be either completely or partially reduced.

Sources:

Biernacka-Wawrzonek D, Stępka M, Tomaszewska A, et al. Melanosis coli in patients with colon cancer. Gastroenterology Review. 2017;1:22-27. doi:10.5114/pg.2016.64844.

Freeman, H. “Melanosis” in the small and large intestine World Journal of Gastroenterology 2008 14:4296–4299.

Li X, et. al. Histopathology of melanosis coli and determination of its associated genes by comparative analysis of expression microarrays Molecular Medicine Reports 2015 12:5807–5815.

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