Melanosis coli and colorectal cancer: Coincidence or correlated?

Melanosis coli (MC) is the benign hyperpigmentation of the colorectal mucosa.1 Known risk factors for MC include the use of anthraquinone laxatives, and chronic constipation.1 Prior studies have suggested a link between MC and colorectal cancer (CRC), though this remains unresolved.2 Here, we describe a case of MC coinciding with CRC in a patient lacking obvious risk factors for MC. A 71-year-old gentleman with underlying chronic lymphoid leukaemia presents with intermittent generalised abdominal pain, bloating, vomiting, tenesmus and altered bowel habit. He denies previous or current use of anthraquinone laxatives and traditional medicines. His abdominal and rectal examination were insignificant. His colonoscopy revealed extensive MC along nearly the full colonic length, in addition to a circumferential tumour at the hepatic flexure, alongside multiple small polyps. Histopathological investigations identified the tumour as an adenocarcinoma. He subsequently underwent a right hemicolectomy. A study proved that that proteins identified in CRC are similarly up-regulated in MC, suggesting a contributory role.2 It has also been postulated that the aberrant Hedgehog signalling pathway activation seen in CRC is also detected in MC, albeit at a different cellular level, possibly suggestive of a link between the pathophysiology of both conditions.3 MC is associated with higher detection rates of colonic polyps during colonoscopy, which are the precursor lesions for CRC.1 This is attributable to the 'enhance effect', where the visibility of non-pigmented polyps is increased against the pigmented colonic mucosa in MC.2 Our case report demonstrates that MC may arise in the absence of known risk factors such as the use of anthraquinone laxatives or traditional medicines. While some studies have demonstrated a possible link between MC and colorectal malignancy, a true pathophysiological link has yet to be established. More studies are warranted to demonstrate a possible pathophysiological relationship between MC and CRC. Irrespective of causality, the enhanced detection of colonic polyps in coincidental MC may have implications for enabling earlier interventions in colonic malignancies, and its presence on colonoscopy should prompt increased vigilance by clinicians.