BackgroundChronic intestinal pseudo-obstruction refers to neuromuscular disorders causing ineffective motility without blockage. One rare cause is segmental additional muscle coat, a sharply defined smooth muscle band between the muscularis mucosae and muscularis propria. Segmental additional muscle coat is seldom reported and has been linked to developmental disorders like Hirschsprung's disease. We report a rare specimen of colonic segmental additional muscle coat with emphasis on quantitative assessment.SpecimenIn our specimen, the patient had lifelong severe constipation and multiple surgeries, including subtotal colectomy due to imminent perforation. Macroscopically, the colon showed marked dilation with mucosal thinning and a distal stenotic segment with wall thickening.MethodsWe performed histological, immunohistochemical, and novel quantitative analysis to characterize the additional muscle layer's structure and cellular composition-an approach not previously described.ResultsHistology revealed a sharply defined smooth muscle layer within the submucosa, distinct from the muscularis propria, and containing fibrosis. The additional layer was identified exclusively in the distal colon, corresponding to the region of pseudo-obstruction. Quantitative desmin immunostaining showed significantly lower smooth muscle content in the segmental additional muscle coat (43.3%) compared to the muscularis propria (95.5%), with the remainder being fibrous tissue. Calretinin highlighted nerve elements, while DOG1 demonstrated interstitial cells of Cajal in the muscularis propria (2.99%) but only minimally in the segmental additional muscle coat (0.75%).ConclusionThis specimen highlights segmental additional muscle coat as a rare structural abnormality causing colonic pseudo-obstruction. The novel use of quantitative immunohistochemistry provides new insight into its cellular and stromal composition, supporting a developmental origin and expanding the pathological understanding of chronic intestinal pseudo-obstruction.