Introduction

Brief overview of pouchitis

Pouchitis is the inflammation of the ileal pouch, an internal reservoir created from the small intestine after removal of the large intestine. Ileal pouch-anal anastomosis (IPAA) is recommended for patients with severe, treatment-resistant inflammatory bowel disease (IBD), including ulcerative colitis (UC) or sometimes Crohn’s colitis, where Crohn’s disease only affects the large intestine. An ileal pouch is created by connecting the end of the small intestine (ileum) to the anus when the large intestine has been removed, with a J-shaped pouch being the most common type.¹ Chronic pouchitis affects 13% to 17% of patients with ileal pouch–anal anastomosis and ulcerative colitis, and 20% with a history of acute pouchitis.²

Patients with an ileal pouch, particularly those who develop pouchitis, may experience a wide range of gastrointestinal symptoms. These commonly include increased bowel movement frequency, urgency, abdominal cramping, bloating, pelvic pressure (a feeling of heaviness or fullness in the pelvic area), and nocturnal faecal incontinence (the involuntary passing of stool).³

Importance of histological (microscopic) examination in diagnosis

The Pouchitis Disease Activity Index (PDAI) is the most commonly used tool to assess inflammation in the ileal pouch. It considers symptom severity, endoscopic findings, and histological changes. Histological studies include the microscopic observation of tissues from the biopsy of the pouch (taking out a sample of the pouch lining). It helps in identifying different characteristics of inflammation, such as neutrophilic infiltration (neutrophils from blood entering the tissues), crypt abscesses (the accumulation of inflammatory cells within the pockets of the gastrointestinal tract), erosion and ulceration (presence of ulcer), in addition to a chronic inflammatory infiltrate (immune cells). ⁴

Goals of evaluating inflammation under the microscope

The histologic scoring is based on the degree of neutrophilic inflammation and the extent of ulceration observed. Neutrophilic infiltration is graded on a scale of 1 to 3, with a score of 1 indicating mild infiltration, 2 indicating moderate infiltration accompanied by crypt abscesses, and 3 indicating severe infiltration with crypt abscesses.

Ulceration is similarly scored based on the average area affected per low-power field of microscope: a score of 1 corresponds to less than 25% ulceration, 2 to 25–50% involvement, and 3 to greater than 50% involvement.⁴

Normal histology of the ileal pouch

Description of normal pouch mucosa post-surgery

Histologic evaluation distinguishes normal pouch mucosa from pouchitis by assessing structural and inflammatory changes. In a normal pouch biopsy, the small bowel mucosa has preserved, slender villous projections into the lumen with minimal mononuclear inflammatory cells in the lamina propria (a thin layer of connective tissue lining the ileal pouch), which is not expanded. There is no evidence of basal lymphoplasmacytosis (abnormal increase in the number of lymphocytes and plasma cells), neutrophilic infiltration, epithelial injury, erosion, or ulceration. In contrast, specimens from patients with pouchitis show characteristic features of chronic inflammation, including villous blunting, erosion, and both chronic and active inflammatory infiltrates. These are evident at higher magnifications and are consistent with chronic pouchitis.

Differences from the native small intestine (colonic metaplasia development)

The pouch is exposed to higher bacterial concentrations due to faecal stasis (stool buildup) during the first year after ileostomy closure. This results in gradual adaptive changes occurring in the pouch microbiota and mucosa. The pouch microbiota and mucosa shift to a colon-like composition (colonic metaplasia), with goblet cells and columnar epithelial cells acquiring colonic morphological and functional characteristics.⁵ Flattening of intestinal villi (villous atrophy) is a common feature when the ileal pouch adapts to new conditions.

The presence of granulomas in pouch biopsies is typically indicative of Crohn’s disease (CD), helping to distinguish it from other forms of pouch inflammation. On the other hand, viral inclusion bodies suggest cytomegalovirus (CMV) infection. The diagnosis of CMV pouchitis can be confirmed through immunostaining for CMV antigens or by using a tissue PCR (polymerase chain reaction) test to detect CMV DNA, providing definitive evidence of viral involvement.⁵

Histological changes in pouchitis

Pouchitis can be categorised as acute, relapsing or chronic. The histological features associated with them are shown below:

Table 1: Difference between acute and chronic pouchitis

Severity grading

While symptoms and endoscopic findings are key indicators of pouchitis, histologic evaluations provide a detailed picture of the inflammatory changes within the pouch. The severity of pouchitis is divided into the following classes based on cellular infiltrate and tissue damage.⁶

• Class 1:
  
  • Normal histology or only minor changes
  • Slightly increased mononuclear cells in the lamina propria
  • Rare neutrophil in surface epithelium or stroma
    
• Class 2:
  
  • Focal or diffuse increase in neutrophils
  • Neutrophils are confined to the lamina propria
    
• Class 3:
  
  • Neutrophilic infiltration in the lamina propria
  • Evidence of cryptitis and/or crypt abscesses
    
• Class 4:
  
  • Features of Class 1–3
  • Presence of superficial erosions or frank ulcerations

Specimens classified as Class 3 or 4 were considered diagnostic for pouchitis.

Differential diagnosis

Crohn’s disease of the pouch

A few key features can distinguish Crohn’s disease (CD) of the pouch. One hallmark finding is the presence of non-caseating granulomas (localised inflammatory clusters of immune cells) on histologic examination, located in the lamina propria, submucosa, or lymphoid aggregates (clusters of cells from the lymph nodes), and not associated with crypt rupture or foreign body reactions. 

Another distinguishing feature is the development of fistulas or strictures in the pouch, afferent (incoming) limb, or perianal (surrounding the anus) area, particularly when these occur more than three months after ileostomy closure and in the absence of NSAID use. A third important feature is persistent inflammation or ulceration involving the proximal small bowel, including the afferent limb, that does not resolve after at least four weeks of antibiotic therapy and is not consistent with diffuse pouchitis. These features help differentiate CD of the pouch from other forms of pouch inflammation or complications.⁴

Cuffitis (inflammation of the retained rectal cuff)

In cuffitis, inflammation occurs in the residual rectal mucosa (the "cuff") left behind after an ileal pouch-anal anastomosis. The cuff is constructed using a stapled technique without mucosectomy (removal of a portion of mucosa). This method preserves a 1–2 cm segment of rectal columnar epithelium (the cells lining the rectum), which increases the risk of cuffitis and necessitates surveillance for dysplasia. Clinically, cuffitis presents with symptoms similar to pouchitis, including urgency and frequency, but is often distinguished by the presence of blood in the bowel movements.^5,7

Infectious pouchitis

Histologic examination of mucosal biopsies plays an important role in the diagnosis of certain types of infectious pouchitis.

1. C. difficile pouchitis is associated with prior antibiotic use, hospital stays, and low immunoglobulin levels; diagnosis involves stool toxin/PCR testing and shows non-specific histologic inflammation and villous blunting
2. Fungal and rare infections like Candida (surface colonisation without tissue invasion) and Histoplasma capsulatum (chronic inflammation, granulomas, intracellular yeast) can occur in stubborn cases, diagnosed via special stains and antigen tests
3. CMV (Cytomegalovirus) pouchitis mimics idiopathic pouchitis but often includes fever; diagnosis requires histology (owl’s eye inclusions), CMV immunostaining, or PCR from tissue or blood

Thus, it is important to correlate clinical symptoms with histological findings to differentiate between different types of pouchitis.

Histological scoring systems

Various scoring systems have been developed to standardise the diagnosis and assessment of pouchitis severity. The Pouchitis Disease Activity Index (PDAI) is one of them, proposed in 1994.⁴ Several other scoring systems have been developed to assess pouchitis severity. The Monash Pouchitis Score⁸ combines both clinical and histologic components. In contrast, the Japanese Diagnostic Criteria for Pouchitis⁹ incorporate clinical symptoms, such as increased stool frequency, urgency, cramps, fever, and bleeding, alongside endoscopic features.

Table 2: The Pouchitis Disease Activity Index (PDAI)¹⁰

The Pouchitis Disease Activity Index (PDAI) evaluates disease severity across three domains: clinical symptoms (stool frequency, bleeding, cramps, fever), endoscopic findings (oedema, friability, ulceration, etc.), and histologic evidence of acute inflammation (neutrophilic infiltration and ulceration). Each component is scored individually, with higher scores indicating more severe pouch inflammation, which aids in diagnosis, monitoring, and therapeutic decision-making.

Summary

Histological examination plays a unique role in the diagnosis, classification, and management of pouchitis, providing crucial insights beyond clinical symptoms and endoscopic findings. Microscopic evaluation helps differentiate acute, chronic, and relapsing forms of pouchitis, while also distinguishing it from similar presentations such as Crohn’s disease, cuffitis, or infections like CMV or C. difficile. Incorporating histological scoring systems, such as the PDAI, ensures a standardised assessment of disease severity and helps personalise therapeutic strategies and improve patient outcomes in those with ileal pouch-anal anastomosis.

FAQs

What is the role of histology in diagnosing pouchitis?

Histological analysis of biopsy samples from the ileal pouch helps confirm pouchitis by identifying key microscopic features such as neutrophilic infiltration, crypt abscesses, and mucosal ulceration. It also aids in distinguishing pouchitis from other conditions, such as Crohn’s disease, cuffitis, or infectious causes.

How does the pouchitis disease activity index (PDAI) help in assessing pouchitis?

The PDAI is a scoring tool that evaluates pouchitis severity across three domains: clinical symptoms, endoscopic appearance, and histologic findings. It provides a standardised method to diagnose, grade inflammation, and monitor response to treatment.

How can histology help differentiate pouchitis from crohn’s disease of the pouch?

Histology in Crohn’s disease often reveals non-caseating granulomas, fistulae, or strictures, especially in the afferent limb, which are typically absent in pouchitis. Persistent inflammation that does not respond to antibiotics, along with the specific location of the lesions, further supports a diagnosis of Crohn’s disease.