"Melanosis" in the small and large intestine

Abstract

Deposition of pigment in the intestinal mucosa is commonly observed by the endoscopist, especially within the colon, and particularly during investigations for constipation. Pigment may also be detected in the small intestine. Although labeled as melanosis, electron microscopy and X-ray analytical methods have provided evidence that this pigment is not melanin at all, but lipofuscin. Often, herbal remedies or anthracene containing laxatives are often historically implicated, and experimental studies in both humans and animal models have also confirmed the intimate relationship with these pharmacological or pseudo-pharmacological remedies. The appearance of melanosis coli during colonoscopy is largely due to pigment granule deposition in macrophages located in the colonic mucosa. The pigment intensity is not uniform, being more intense in the cecum and proximal colon compared to the distal colon. Possibly, this reflects higher luminal concentrations of an offending agent in the proximal compared to distal colon, differential absorption along the length of the colon, or finally, differences in macrophage distribution within the colon. Mucosal lymphoid aggregates normally display a distinct absence of pigment producing a "starry sky" appearance, especially in the rectosigmoid region. Interestingly, some focal, usually sessile, colonic mucosal neoplastic lesions, rather than submucosal lesions, may be better appreciated as pigment deposition may be absent or limited. If detected, removal and further histopathologic analysis of the polyp may be facilitated.

Figure 1

Typical alligator or snake-skin appearance of melanosis coli. Despite routine colon preparation, residual fecal debris is common, likely reflecting reduced colonic propulsive activity.

Figure 2

Rectosigmoid mucosa illustrating focal areas of patchy intense pigmentation as well as focal areas of absent pigment, the latter reflecting the presence of normal mucosal aggregates of lymphoid cells (so-called “starry sky” appearance of melanosis coli).

Figure 3

Ileal and cecal mucosa in melanosis coli. Melanosis is generally confined to the cecal mucosa although there is a very limited area of transition into ileal mucosa that is not pigmented.

Figure 4

Photomicrograph showing typical pigment granule laden macrophages in the lamina propria.

Figure 5

Easily visualized small sessile polypoid lesion in ascending colon with adjacent background pigmented mucosa typical of melanosis coli. Resected specimen confirmed absence of pigmented macrophages in the body of the resected adenoma.

Figure 6

Small pigmented polypoid lesion similar to background pigmented colonic mucosa. Resected specimen revealed a submucosal leiomyoma along with pigmented macrophages in the overlying colonic mucosa due to melanosis coli.