Pouchitis: Practical Points for Pathologists

Pouchitis is the most common long-term complication of ileal pouch surgery (affecting up to 60% of patients) and has a significant adverse impact on the patient’s quality of life. Signs and symptoms of pouchitis can include diarrhoea, abdominal pain, joint pain, cramps, fever, an increased number of bowel movements, night-time faecal seepage, faecal incontinence and a strong urge to have a bowel movement.

The aetiology of pouchitis is not entirely understood. Bacterial overgrowth, altered balance of luminal bacteria, mucosal ischaemia, nutritional deficiencies, lack of short chain fatty acids and faecal bile acid toxicity have all been suggested as possible aetiological factors [3].

Diagnosis of pouchitis based on symptoms alone has been shown to be non-specific since symptoms can originate from a myriad of aetiologies, not necessarily inflammatory in nature. As a result, the diagnosis of pouchitis should generally be based on presence of the appropriate constellation of symptoms, combined with endoscopic and histological assessment [4]. There is in fact a good correlation between more severe grades of histological inflammation, frequency of defaecation and endoscopic appearance.

Histopathology of pouchitis

According to ECCO-ESP Histopathology Consensus statement 21: “For a proper histologic evaluation of pouchitis multiple biopsies are recommended. The exact location has not been determined but according to some data it is useful to take biopsies from the anterior and posterior wall avoiding suture lines. Samples from the posterior wall are more likely to show the inflammatory changes” [5].

Biopsies from pouches are best reported descriptively regarding quantity of acute and chronic inflammation, the presence of crypt abscesses, erosions and ulcers, and also loss of villous height, presence of pseudopyloric metaplasia, granulomas and presence of dysplasia (Figure 1). From a histological point of view, pouchitis is a gradable phenomenon, based on extent of active crypt injury.

Various scoring systems have been developed to standardise the diagnosis and assessment of the severity of pouchitis. The Pouchitis Disease Activity Index (PDAI) quantitates clinical symptoms and endoscopic and histological features (acute histological inflammation: crypt abscess and ulceration) on three separate six-point scores, whereby a total score higher than 7 is indicative of pouchitis [6]. The Heidelberg Pouchitis Activity Score (PAS) also includes the histological features of chronic inflammation and distinguishes between three grades of pouch inflammation (mild adaptive inflammation, moderate pouchitis and severe pouchitis). Scores greater than 13 are considered to be pouchitis [7].

Most pouches have some degree of inflammation, which can be patchy or diffuse. However, chronic inflammatory changes should be distinguished from true pouchitis [8]. In fact, mucosal architecture distortion, villous atrophy, crypt hyperplasia and infiltration of the lamina propria by mononuclear cells, eosinophils and histiocytes can be seen in biopsies from “healthy” pouches and are considered adaptive changes. As “colonic metaplasia” occurs more frequently in patients with pouchitis, it has been suggested that it may be a “reparative” rather than an “adaptive” response.

Besides adaptive changes, mild ischaemic changes can be observed in a few patients, while others may show features of mucosal prolapse, such as fibromuscular obliteration of the lamina propria and a disrupted muscularis mucosae. Deep lymphoid follicles can also be seen in excised pouches and should not refute a diagnosis of UC. In fact, diagnosis of CD should only be made when re-examination of the original proctocolectomy specimens shows typical pathological features of CD [9].

Pouchitis should be distinguished from “cuffitis”, i.e. inflammation in the columnar cuff mucosa distal to the pouch, or islands of columnar mucosa that may be left behind. CD10 staining (expressed in ileal samples) or tropomyosin isoform 5 (hTM5) (not expressed in genuine ileal samples) may confirm the biopsy site.

Finally, histological assessment should also help to rule out opportunistic infections (cytomegalovirus) [10] and dysplastic changes.

In conclusion, assessment of mucosa from IPAA for UC can be a valuable tool when diagnosing pouchitis, but pathologists must be familiar with the concept of adaptive changes, be aware that activity of inflammation needs to be categorised and avoid misinterpretation of deep lymphoid follicles as synonymous with CD.

Figure 1:  Mucosal architecture distortion, villous atrophy, cryptitis and crypt abscesses in an ileal pouch biopsy. © Paula Borralho Nunes

References

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