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A 60-year-old woman with a family history of colon cancer and a history of adenomatous colorectal polyps underwent surveillance colonoscopy. Her last endoscopy, three years previously, had revealed two polyps and normal colonic mucosa (Panel A). Since then, increasing constipation had developed, for which the patient took a daily dose of senna, a laxative that contains anthraquinone. Endoscopy revealed diffuse dark brown and black pigmentation throughout her colon, which is consistent with the presence of melanosis coli (Panel B). Biopsies of the affected areas showed pigment in macrophages of the lamina propria, without any evidence of inflammation or cancer (Panel C).
Melanosis coli is dark brown pigmentation of the colon that occurs with the use of laxatives containing anthraquinone, such as senna. It can develop within a few months of use, and it can disappear in a few months if the use of the laxative is discontinued. Anthraquinones have a direct toxic effect on the epithelial cells of the colon that results in the production of lipofuscin, the dark pigment seen in macrophages in melanosis coli (Panel C, arrow). The cecum and rectum are the most common sites of melanosis coli, but it can affect the entire large bowel. Melanosis coli is considered a benign lesion because no association with colorectal cancer has been shown.
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