Histological features and inflammatory cytokine profiles in anal fistula: A prospective cohort study

Traditionally, anorectal abscesses and anal fistulas have been considered to originate from a common site within the fistula tract (the intersphincteric space), according to the cryptoglandular hypothesis. Understanding the distribution of cells and inflammatory mediators in different parts of the anal fistula tract is of significant interest. This study aimed to analyze the inflammatory characteristics of anal fistula tract specimens.

An observational study was conducted on specimens retrieved from consecutive cases. We analyzed samples of chronic anorectal fistulas, which were topographically divided into three parts: superficial part (including the external fistula opening, close to the perianal skin), middle (including the intersphincteric space), and deep part (close to the anal canal, including the internal fistula opening). These samples were reviewed under microscopy to describe cell types using hematoxylin-eosin staining and the presence of pro-inflammatory cytokines TNF-alpha and IL-36 using immunohistochemical staining.

Fistula tract samples were obtained surgically from 15 patients. No epithelial lining was observed in any part of the samples. We found a predominance of granulation tissue with chronic inflammatory components and no signs of acute inflammation, with no differences among the different parts of each sample studied. Immunohistochemistry revealed the presence of TNF-alpha throughout the fistulous tract, with an increasing trend towards the deep third (internal opening), although this was not statistically significant. In contrast, IL-36 was uniformly distributed across all three parts of the fistula tract.

There is no specific site within the anal fistula tract that shows distinct findings related to the inflammatory process.