What is melanosis coli?
Melanosis coli is a condition usually associated with chronic laxative use in which dark pigment is deposited in the lamina propria (one of the lining layers) of the large intestine (colon). The pigment deposition results in a characteristic dark brown to black discoloration of the lining of the large intestine. This condition is sometimes called pseudomelanosis coli because the pigment deposits consist of a pigment known as lipofuscin and do not contain melanin as implied by the term "melanosis." Lipofuscin is a cellular pigment that forms when cells are destroyed, often called "wear and tear" pigment that can be found throughout the body.
The dark color of the intestinal lining may be uniform or patterned, and the discoloration may be slight or very pronounced. The intensity and pattern of the discoloration may even vary among different sites in the colon of a single person. The condition may also be reversed upon discontinuation of laxative use. In some cases, the wall of the colon appears normal to the eye, but microscopic evaluation of biopsies by a pathologist reveals areas of pigment in the colon's lining. The pigment in melanosis coli does not accumulate in polyps or tumors of the large intestine.
What causes melanosis coli?
Melanosis coli usually results from chronic use of laxatives of the anthranoid group. Some examples of anthranoid laxatives are senna (sennosides; Senocot, Senokot EXTRA and others) and rhubarb derivatives. Many of these laxatives have been in use for hundreds of years. In 1997, the U.S. Food and Drug Administration (FDA) banned the use of the popular anthranoid laxative phenolphthalein due to fears that it might be carcinogenic (cancer-causing). Animal studies had shown that extremely high doses of phenolphthalein led to tumors in animals, but it has never been shown to cause cancers in humans.
The anthranoid laxatives pass through the gastrointestinal tract unabsorbed until they reach the large intestine, where they are changed into their active forms. The resulting active compounds cause damage to the cells in the lining of the intestine and leads to apoptosis (a form of cell death). The damaged (apoptotic) cells appear as darkly pigmented bodies that may be taken up by scavenger cells known as macrophages. When enough cells have been damaged, the characteristic pigmentation of the bowel wall develops. The condition can develop after just a few months of laxative use.
How do doctors diagnose melanosis coli?
Melanosis coli can be observed during endoscopic procedures that examine the large intestine, such as colonoscopy and sigmoidoscopy. Sometimes the diagnosis is made upon microscopic examination of biopsies taken during endoscopic procedures.
What is the prognosis (outcome) of melanosis coli?
If a person stops using anthranoid laxatives, the changes associated with melanosis coli lessen over time and may disappear.
Early studies suggested that anthranoid laxatives might have carcinogenic or tumor-promoting activities in humans and that the presence of melanosis coli might signal an increased risk for the development of colorectal cancer. However, more recent follow-up studies have failed to show an association between colon cancer and anthranoid laxative use or between colon cancer and the finding of melanosis coli.
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Shazia Ahmed, and Naresh T. Gunaratnam. "Melanosis Coli." N Engl J Med 349:1349 Oct. 2, 2003.