Microscopic colitis: what's in a name? An overview from clinical and histological perspectives

Microscopic colitis presents as chronic watery diarrhoea and leads to a significant disease burden. Specific histological characteristics are key to diagnosing microscopic colitis and to specify its subtypes collagenous colitis or lymphocytic colitis. Histological hallmarks of microscopic colitis are an increased inflammatory infiltrate in the lamina propria without significant crypt architectural distortion and intraepithelial lymphocytosis, which is generally more prominent in lymphocytic colitis. Next to these similarities, collagenous colitis is defined by a thickened collagen band underneath the surface epithelium, which is absent or only slightly present in lymphocytic colitis. Besides these two well defined subtypes of microscopic colitis, incomplete forms of microscopic colitis are acknowledged which give the same symptom burden and should be recognized and treated accordingly. Both clinical and histopathological research in the field of microscopic colitis is ongoing. Recent studies have focussed on where to take biopsies to best establish the diagnosis, the implications of supplementary stain usage and interobserver agreement. Furthermore, insight into the potential underlying pathophysiological mechanisms increases. This review aims to provide an overview of the condition of microscopic colitis, with a focus on diagnosis. In addition, it aims to highlight that the increasing incidence and associated significant disease burden imply that this condition is not so ‘microscopic’ at all.