Standardized recording of parameters related to the natural history of inflammatory bowel disease: from Montreal to Paris.

Similar to adults, there is heterogeneous phenotypic expression of inflammatory bowel disease (IBD) in children. Thus, a classification system for disease characteristics is obligatory if one seeks to understand and eventually change the natural history of IBD. Extrapolation of adult clinical trial results to children also depends upon comparable classifications of disease. Features that can differentiate IBD in children from adults include more extensive and severe disease at presentation, frequent corticosteroid dependency, change in location and behavior over time, and the implications of disease for growth and sexual maturation. In contrast to the Montreal classification where all patients <17 years were grouped together, the Paris classification recognizes the different expression of pediatric IBD between those patients aged <10 years and those 10-17 years of age. The recent identification of monogenic disorders in very young children (<2 years) with severe IBD-like disease has further clouded the issue of where appropriate pediatric age guidelines should be drawn, though it is clear these infantile-onset cases should not be grouped with older children. The Paris classification recognizes the importance of upper tract disease on natural history by dividing it into L4a and L4b (proximal and distal to the ligament of Treitz, respectively), while the Montreal system groups all upper-tract patients together. Complicated disease behavior in the Montreal system mandated a single category preventing the concomitant designation as stricturing and penetrating, whereas the Paris classification recognizes that both stricturing and penetrating behavior may occur at the same or different times. Growth delay is recognized only in the Paris classification as a serious manifestation of IBD in children affecting therapeutic decisions. As our understanding of the basic molecular mechanisms of disease pathogenesis in IBD changes over time, it is likely that the IBD classification will change as well. A single classification system that reflects both pediatric and adult disease is needed.