It is generally accepted that a diagnosis of pouchitis should be based, not simply on the presence of symptoms described above, but on the combination of clinical, endoscopic and histological findings.
Two composite scores are used to diagnose pouchitis and to assess disease severity. These are the pouchitis disease activity index (PDAI) and the pouchitis activity score (PAS) (see Table 1 and Table 2). In the PDAI, an overall score is calculated from three separate six-point scales comprising clinical symptoms, endoscopic findings and histological changes. The PDAI incorporates histological features of acute inflammation, and establishes a cut-off of seven for differentiation between 'pouchitis' (≥7 points) and 'no pouchitis' (<7 points). The PAS incorporates elements similar to those of the PDAI but also includes the histological features of chronic inflammation. The resulting total score distinguishes between three grades of pouch inflammation: mild adaptive, moderate pouchitis and severe pouchitis. Only individuals with scores greater than 13 are considered to have pouchitis, as a degree of mild inflammation is an almost universal finding.
Despite the fact that the PDAI and PAS have become commonly used in clinical trials evaluating different therapies of pouchitis, it has been suggested that the clinical components of the scores correlate poorly with endoscopic and histological findings, presumably due to the fact that conditions other than pouchitis can result in pouch dysfunction. Due to the variety of aetiologies that can result in pouch dysfunction, it is important that endoscopy is performed and it is a general consensus among many inflammatory bowel disease centres that pouch endoscopy and histological evaluation should be performed in order to diagnose pouchitis accurately.
Based on aetiology, disease duration and response to treatment, pouchitis may be classified into four categories: (1) idiopathic versus secondary; (2) acute (<4 weeks) versus chronic (≥4 weeks); (3) infrequent episodes (less than two acute episodes) versus relapsing (three or more acute episodes) or continuous; (4) antibiotic responsive versus refractory.
The Diagnostic Work-up
At the initial occurrence of symptoms that might suggest 'pouchitis' infectious aetiologies should be excluded by sending stool for culture and Clostridium difficile toxin assay and, in the appropriate setting, by ruling out cytomegalovirus infection.
Once infectious aetiologies and other possible contributors have been ruled out, pouch endoscopy should be performed if feasible. If not, a trial of antibiotics may be administered as being both therapeutic and diagnostic of 'pouchitis'. However, the use of an empiric course of antibiotics as a means of diagnosis of pouchitis should be practised with caution because, as discussed later, it is our impression and that of others that patients with pouch symptoms may respond quite rapidly to the administration of antibiotics without having evidence of the endoscopic or histological inflammation that is required to make a diagnosis of pouchitis.
On endoscopy, it is important to examine the pouch, the pre-pouch ileum and the rectal cuff. Biopsies from the pouch may be helpful even from normally appearing pouch mucosa, as findings of acute inflammation may still be present. Conversely, the yield of sampling an apparently normal pre-pouch ileum seems to be low.
The endoscopic features of pouchitis may range from anywhere between minimal changes, including erythema, friability and mucus exudate to frank ulcers and bleeding. Histological assessment will focus on acute inflammatory infiltrates and also dysplastic changes to rule out the rare case of progression to malignant transformation.
More complicated cases, presenting with systemic symptoms and signs of inflammation, fistulas, leaks or abscesses, may warrant additional modalities, such as abdominal and pelvic CT scan, pelvic MRI, perineal or transanal ultrasound, pouchography or examination under anaesthesia.