Time to classify: a narrative and scoping review of the old and the new classifications of perianal Crohn’s disease

Perianal Crohn's disease (pCD) is a complex manifestation of Crohn's disease. Classifying this patient cohort for both clinical purposes and for inclusion into research trials is challenging but crucial in order to improve outcomes. This review provides an overview of historical classifications of both fistulising and non-fistulising pCD, including the Park's, Cardiff-Hughes and American Gastroenterological Association (AGA) classifications, as well as recent advances including the Treatment Optimisation and CLASSification of perianal Crohn's disease (TOpClass) classification of fistulising pCD. Secondly, this article provides a scoping review of recent trials in pCD and describes how the cohorts in these trials relate to the TOpClass classification. Of the 19 studies relating to pCD that were identified, four could be confidently classified as class 2a. Seven could be classified as class 2a or 2b, but it was not possible to subdivide further, and seven to class 2a, 2b or 2c, but it was not possible to subdivide further. One study population was classified as class 2a or 2c. In eight studies, it was not specified whether patients with a defunctioning stoma were included or excluded. This review demonstrates the heterogeneous nature of some patient cohorts in previous clinical trials, and how the TOpClass classification may be used to group patients more accurately for clinical use and inclusion in research trials.