Diversion Colitis

Slide 1 · Diversion Colitis

Mechanism · morphology · differential diagnosis · clinical interpretation

A clinicopathologic lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain how fecal diversion creates an excluded colonic segment.

Recognize the characteristic lymphoid follicles and the wider histologic spectrum.

Differentiate diversion-related injury from primary inflammatory bowel disease.

Relate proposed pathogenesis to restoration of bowel continuity.

Slide 3 · Definition: Inflammation of excluded colon

Fecal flow is surgically rerouted away from a distal colonic segment.

Inflammation can develop in the defunctioned mucosa.

An anatomical exposure history is integral to the diagnosis.

Fecal stream

Upstream bowel

Diversion stoma

Functional outflow

Distal colon

Little / no fecal flow

Schematic flow map

The site of diversion is more informative than any isolated microscopic feature.

Slide 4 · The normal fecal stream exposes colonic mucosa

Intraluminal contents

Microbial metabolism

Epithelial substrate supply

Mucosal homeostasis

Bacterial short-chain fatty acids to epithelial signaling and energy provision.

Diversion perturbs the normal luminal environment.

Slide 5 · Diverting ostomy: Anatomy determines exposure

A temporary or permanent ostomy redirects fecal transit.

Upstream bowel remains in the functional fecal stream.

The bypassed distal colon is the site at risk for diversion colitis.

Proximal limb

Feces exit at stoma

Distal limb

Downstream bowel is bypassed

Distal mucosa

Remains in place

Schematic flow map

Do not confuse a functioning proximal segment with the excluded segment.

Slide 6 · End ostomy and blind distal bowel

An ostomy may leave a blind distal colonic segment.

Excluded mucosa remains viable but lacks normal fecal flow.

Biopsy site and the surgical reconstruction must be documented.

Functional bowel

Exits through ostomy

Blind distal stump

Excluded from fecal stream

Mucosal site

Potential inflammation

Schematic flow map

Slide 7 · Clinical situations requiring diversion

Ulcerative colitis

Surgical treatment can create a diverted segment; Baseline IBD complicates interpretation.

Hirschsprung disease

A congenital motility disorder for which staged bowel diversion may be used.

Colon cancer

Selected oncologic operations may include temporary or permanent fecal diversion.

Other disorders

The mechanism depends on diversion itself, not a single underlying diagnosis.

Slide 8 · Anatomical localization is the first clue

Document surgery

Identify bypassed bowel

Identify biopsy location

Interpret inflammation in context

A pathology report without the ostomy history risks overdiagnosis of primary colitis.

Slide 9 · Three clinically distinct bowel environments

Location

Exposure

Interpretive significance

Proximal functioning colon

Continuous fecal flow

Not the typical site of diversion colitis

Diverted distal colon

Excluded from fecal stream

Characteristic anatomical setting

Reconnected segment

Fecal transit restored

Diversion-related inflammation can resolve

Slide 10 · Why diversion causes colitis remains uncertain

Proposed microbiota change

Altered microbial composition or function after fecal-stream interruption.

Proposed nutritional deprivation

Reduced delivery of bacterial metabolites needed by colonocytes.

Observed reversibility

Improvement after flow restoration supports an exposure-dependent process.

Slide 11 · The microbiota hypothesis

Fecal diversion changes the luminal ecosystem.

Changes in microbiota are a proposed contributor to mucosal inflammation.

The exact organism-level and immune pathways are not resolved.

Fecal diversion

Altered luminal milieu

Altered microbial community

Possible mucosal inflammation

Proposed mechanism — not fully established

Microbiota alteration is a hypothesis, not a diagnostic laboratory test.

Slide 12 · Short-chain fatty acids and the epithelium

Luminal bacteria produce short-chain fatty acids.

These metabolites provide an energy source for colonic epithelial cells.

They also participate in epithelial signaling.

Fecal-stream substrate

Bacterial fermentation

Short-chain fatty acids

Colonocyte signaling and energy

Proposed mechanism — not fully established

Slide 13 · Luminal nutrient deprivation: Proposed sequence

Fecal-stream exclusion may limit epithelial access to bacterial metabolites.

This could reduce local metabolic support.

The link to the complete inflammatory phenotype remains incompletely defined.

Reduced luminal input

Less microbial SCFA supply

Possible epithelial metabolic stress

Mucosal vulnerability

Proposed mechanism — not fully established

Describe an explanatory model without converting it into proven causality.

Slide 14 · Butyrate is an illustrative short-chain fatty acid

Colonic fermentation

Short-chain fatty acids

Epithelial uptake / signaling

Mucosal support

Butyrate is one relevant short-chain fatty acid; The broader class.

A single-metabolite model cannot explain every histologic finding.

Slide 15 · Mechanism: Facts versus proposals

Observation

Status

Appropriate interpretation

Inflammation in defunctioned colon

Established

Typical anatomic setting

Prominent lymphoid follicles

Established

Important morphological clue

Altered luminal microbiota

Proposed

Plausible pathogenesis

Loss of SCFA supply

Proposed

Mechanistic support from topical response

Recovery with restored transit

Established

Strong clinical correlation

Slide 16 · A unified pathogenesis model

Surgical diversion changes luminal exposure.

Microbiota and nutrient hypotheses may act together.

Mucosal inflammation can become histologically conspicuous.

Surgical diversion

Altered microbiota and nutrients

Mucosal immune activation

Colitis in excluded bowel

Proposed mechanism — not fully established

Slide 17 · Endoscopy: Mucosal erythema

The diverted mucosa may appear erythematous.

Redness signals mucosal inflammation but is nonspecific.

The anatomical context provides diagnostic specificity.

Endoscopic observations

Erythema

Friability

Nodularity may reflect follicles

Slide 18 · Endoscopy: Mucosal friability

Friability accompanies mucosal inflammation.

A fragile surface is prone to contact-related bleeding.

The finding overlaps with other inflammatory colitides.

Endoscopic observations

Erythema

Friability

Nodularity may reflect follicles

Slide 19 · Endoscopic spectrum in a diverted segment

Robbins core

Erythema and friability are explicitly described.

Reported extensions

Granularity, edema, erosion, ulcers and nodularity occur in published series.

Interpretive rule

Neither endoscopic appearance nor severity alone identifies etiology.

Slide 20 · Histologic hallmark: Lymphoid follicles

Schematic — not a histologic specimen

Expanded lymphoid follicle

Numerous mucosal lymphoid follicles are the most striking Robbins feature.

Enlarged follicular structures create a recognizable low-power pattern.

This clue gains meaning when the biopsy is from diverted colon.

Slide 21 · Lymphoid follicle architecture

Schematic — not a histologic specimen

Expanded lymphoid follicle

A reactive follicle may contain a germinal center.

The lymphoid structure is distinct from epithelial crypts.

Assess follicles together with surface and crypt injury.

Slide 22 · Histology: Lamina propria inflammation

Schematic — not a histologic specimen

Lamina propria inflammatory cells

The lamina propria may contain abundant inflammatory cells.

Lymphocytes, monocytes, macrophages and plasma cells.

The mixture may be conspicuous even without severe crypt injury.

Slide 23 · Cell populations and their location

Lymphocytes

Chronic mucosal immune component; Prominent in follicles and lamina propria.

Plasma cells

May contribute to a dense chronic lamina propria infiltrate.

Monocytes / macrophages

Additional mononuclear inflammatory populations.

Slide 24 · Histology: Active crypt injury

Schematic — not a histologic specimen

Neutrophils within a crypt

Severe disease can include crypt abscesses.

Crypt abscess denotes neutrophils collected inside a crypt lumen.

This pattern is also seen in inflammatory bowel disease.

Slide 25 · Histology: Altered crypt architecture

Schematic — not a histologic specimen

Branching / altered crypt orientation

Architectural distortion may accompany severe diversion colitis.

Abnormal branching and orientation indicate mucosal injury and repair.

Distortion alone must not establish a new diagnosis of chronic IBD.

Slide 26 · Rare finding: Granulomas

Granulomas can occur, but rarely.

Granulomas are not sufficient to identify Crohn disease in a diverted segment.

Their interpretation depends on morphology, background disease and clinical context.

Do not treat rarity as exclusion or proof of a specific mechanism.

Slide 27 · Spectrum of mucosal damage

Schematic — not a histologic specimen

Expanded lymphoid follicle

Follicular hyperplasia may dominate the biopsy.

Inflammatory infiltrates and crypt activity can coexist.

Severe examples may resemble primary IBD.

Slide 28 · Hallmark versus nonspecific findings

Finding

Role in diversion colitis

Important caution

Prominent follicles

Characteristic / high-yield

Not absolutely specific

Lamina propria infiltrate

Common accompanying feature

Many causes possible

Crypt abscesses

Severe activity pattern

IBD and infections may show this

Crypt distortion

Severe / chronic injury

Cannot establish IBD alone

Granulomas

Rare possible feature

Do not automatically diagnose Crohn disease

Slide 29 · Why histology alone may mislead

Crypt abscess or distortion

IBD-like biopsy appearance

Surgical diversion not disclosed

Incorrect etiologic label

Histologic resemblance does not establish the same disease mechanism.

The surgical history must be reconciled with the tissue findings.

Slide 30 · No single slide feature is diagnostic

Morphology

Follicles and inflammation strongly suggest a possibility.

Anatomy

A bypassed distal colonic segment makes the diagnosis plausible.

Clinical setting

Pre-existing disease and timing alter the differential.

Slide 31 · Diagnostic triangulation

Surgical history establishes a diverted segment.

Endoscopy documents mucosal inflammation.

Histology identifies the pattern and its severity.

A combined assessment supports diversion colitis.

Surgical history

Diverted colonic segment

Endoscopic appearance

Erythema / friability

Biopsy

Lymphoid follicles ± active injury

Integrated diagnosis

Diversion colitis favored

Slide 32 · What the requisition should communicate

Type and approximate date of ostomy.

Site of each biopsy relative to the fecal stream.

Underlying disease: Ulcerative colitis, cancer, motility disorder, etc.

Current symptoms and endoscopic findings, when known.

Clinical procedure

Was fecal diversion performed?

Specimen provenance

Is the biopsy from bypassed colon?

Morphology

Follicles with variable active/chronic injury

Interpretation

Correlate before labeling IBD

Slide 33 · Interpretation: Pre-existing IBD matters

No antecedent IBD

Inflammation confined to diverted mucosa strongly supports diversion-associated injury.

Known ulcerative colitis

Residual UC and superimposed diversion changes may overlap.

Known Crohn disease

Patchy active injury or granulomas require careful correlation; Avoid single-feature inference.

Slide 34 · Diversion colitis versus ulcerative colitis

Feature

Diversion colitis

Ulcerative colitis

Defining setting

Diverted bowel segment

Primary chronic inflammatory disease

Anatomical distribution

Excluded segment

Continuous colonic / rectal disease

Lymphoid follicles

Often striking

May occur but not defining

Crypt abscesses

Possible if severe

Common active-disease feature

Interpretive pivot

Fecal-stream status

Overall chronic clinical course

Slide 35 · Diversion colitis versus Crohn disease

Feature

Diversion colitis

Crohn disease

Precipitating setting

Surgical diversion

Primary immune-mediated bowel disease

Granulomas

Rarely possible

Can occur as hallmark of Crohn

Crypt changes

Possible with severe injury

Related to repeated damage / repair

Critical discriminator

Affected diverted segment

Distribution and transmural / extraintestinal features

Slide 36 · IBD-like morphology does not equal IBD

Severe diversion injury

Crypt abscess ± distortion

Resemblance to IBD

Etiology determined by context

Robbins explicitly cautions that severe lesions can look like IBD.

Restoration of fecal flow produces a different causal test from immunologic labeling.

Slide 37 · Microscopic colitis: A distinct pattern

Feature

Diversion colitis

Microscopic colitis

Required context

Fecal diversion

No diversion requirement

Endoscopy

Often erythema / friability

Typically normal

Typical biopsy clue

Prominent follicles

Thick collagen band or excess intraepithelial lymphocytes

Clinical emphasis

Excluded segment

Chronic watery nonbloody diarrhea

Slide 38 · Graft-versus-host disease: Different lesion

Feature

Diversion colitis

GI graft-versus-host disease

Setting

Excluded colon

Allogeneic stem-cell transplant

Dominant histology

Follicles + variable inflammation

Epithelial / crypt apoptosis

Lamina propria

Often cellular infiltrate

May be relatively sparse

Diagnostic key

Fecal diversion

Transplant history

Slide 39 · Drug-related apoptotic enterocolitis

Checkpoint inhibitors

Can induce apoptotic enterocolitis in treated oncology patients.

Mycophenolate

Another association with gastrointestinal epithelial apoptosis.

Diversion distinction

Fecal-stream exclusion and follicles support a different clinicopathologic process.

Slide 40 · Practical comparative differential

Clinical clue

Expected emphasis

Most useful discriminator

Diversion surgery

Follicular hyperplasia ± colitis

Excluded bowel anatomy

Long-standing continuous colitis

Crypt injury and chronicity

UC distribution and history

Patchy transmural phenotype

Granulomas / Crohn features

Full Crohn phenotype

Watery diarrhea; Normal scope

Collagen / IEL change

Microscopic colitis biopsy

HSCT or relevant drug

Crypt epithelial apoptosis

Exposure history

Slide 41 · Symptoms can be absent

Endoscopic or histologic diversion colitis may be clinically silent.

Visible inflammation does not imply disabling symptoms.

Clinical decisions should consider burden and indication for the ostomy.

Endoscopic observations

Erythema

Friability

Nodularity may reflect follicles

Slide 42 · Symptoms when diversion colitis is symptomatic

Rectal discharge

Mucus and sometimes blood may pass from the distal stump.

Anorectal symptoms

Tenesmus or discomfort may occur.

Abdominal / rectal pain

Symptom severity varies across patients.

Slide 43 · Differentiate symptoms from biopsy activity

Domain

What is observed

What cannot be inferred

Clinical

Pain, discharge, bleeding or none

Microscopic severity from symptoms alone

Endoscopic

Erythema, friability, other changes

Exact histology without biopsy

Microscopic

Follicles, infiltrate, active injury

Patient burden without clinical history

Slide 44 · Report findings before causal attribution

Description

Prominent lymphoid follicles; Variable chronic / active mucosal inflammation.

Context

Biopsy is from a diverted distal colonic segment.

Interpretation

Findings compatible with diversion colitis, if clinical and endoscopic context supports it.

Slide 45 · A safe reporting formulation

“Colonic mucosa with prominent lymphoid follicular hyperplasia.”

“Associated chronic inflammation; Active crypt injury if present.”

“In a diverted segment, these changes are compatible with diversion colitis.”

“Correlate with prior IBD and current anatomic distribution.”

Clinical procedure

Was fecal diversion performed?

Specimen provenance

Is the biopsy from bypassed colon?

Morphology

Follicles with variable active/chronic injury

Interpretation

Correlate before labeling IBD

Slide 46 · Therapeutic target: Restore the fecal stream

Re-anastomosis restores bowel continuity.

Diversion colitis resolves after restoration of flow.

The response supports the importance of luminal exposure.

Diverted segment

No fecal transit

Re-anastomosis

Continuity restored

Mucosal recovery

Inflammation resolves

Schematic flow map

Restoration of continuity is the defining mechanistic intervention.

Slide 47 · Short-chain fatty-acid enemas

Deliver microbial metabolites directly to excluded mucosa.

That these enemas can promote mucosal recovery.

Their use is infrequent because of unpleasant odor.

Fecal-stream substrate

Bacterial fermentation

Short-chain fatty acids

Colonocyte signaling and energy

Proposed mechanism — not fully established

Slide 48 · How much does topical response prove?

Supported observation

Topical short-chain fatty acids can promote recovery.

Biological inference

Colonocyte substrate supply may contribute to pathogenesis.

Boundary

Response does not establish one exclusive causal pathway or universal efficacy.

Slide 49 · Additional treatments: Evidence is limited

Published series describe topical mesalamine or corticosteroids in selected patients.

There is no universally established severity scale or single standardized medical regimen.

The surgical plan and feasibility of restoring continuity remain central.

Avoid extrapolating efficacy from case reports into treatment guarantees.

Slide 50 · Therapeutic decision: Anatomy and symptoms

Confirm diverted segment

Assess symptoms / complications

Review surgical feasibility

Tailor management to patient

Anatomical restoration is etiologically directed when feasible.

Topical approaches may be considered when restoration is not practical; Evidence remains variable.

Slide 51 · Vignette 1: Excluded rectum

A patient has a prior colostomy for colon cancer. Endoscopy of the defunctioned rectal stump shows erythema and friability. Biopsy shows numerous lymphoid follicles with germinal centers.

What diagnosis is most consistent with these combined observations?

Interpret location, clinical history and morphology together.

Slide 52 · Vignette 1: Clinicopathologic reasoning

Surgical diversion + inflamed distal stump + follicular mucosal hyperplasia

1. Diversion colitis is favored.

2. The diagnostic anchor is the defunctioned segment, not a unique histologic cell.

3. Consider alternative or superimposed disease if the history indicates it.

Slide 53 · Vignette 2: The Crohn disease trap

In a diverted segment, biopsy shows crypt abscesses, marked crypt distortion and a rare granuloma. The referral says only “rule out Crohn disease” and omits the ostomy history.

What information is needed before assigning a new Crohn diagnosis?

Slide 54 · Vignette 2: Avoid a false disease label

Severe diversion colitis may resemble inflammatory bowel disease histologically.

1. Obtain surgical and pre-diversion diagnostic history.

2. Map specimens to the diverted and functioning bowel.

3. Reassess morphology in the complete distributional and clinical context.

Slide 55 · Pitfalls that change patient care

Unreported diversion

Biopsy interpreted as de novo IBD despite excluded-bowel anatomy.

Overweighting granulomas

Rare granuloma mistaken for sufficient proof of Crohn disease.

Overweighting crypt abscesses

Active injury treated as etiologically specific.

Assuming all symptoms reflect inflammation

Clinical and microscopic severity can diverge.

Slide 56 · A practical diagnostic checklist

Confirm the ostomy configuration and which segment was biopsied.

Describe follicular hyperplasia and other mucosal changes precisely.

Identify overlapping patterns of active or chronic injury.

Compare with prior histology and established IBD, when relevant.

Give a conditional integrated diagnosis rather than a stand-alone morphologic label.

Slide 57 · Integrated model: Lesion to intervention

Diverted distal colon

Altered luminal exposure

Follicles + variable colitis

Recovery with restored flow

The definitive teaching relationship is anatomical exposure → reversible mucosal injury.

Intermediate microbial and nutrient mechanisms remain incompletely established.

Slide 58 · Five points to retain

Setting

Fecal stream surgically diverted from distal colon.

Endoscopy

Erythematous, friable mucosa.

Histology

Prominent mucosal lymphoid follicles.

Severity

Can include crypt abscesses, distortion and rare granulomas.

Resolution

Restoration of fecal flow reverses diversion colitis.

Slide 59 · Knowledge check: Morphology

A biopsy from a defunctioned colonic segment shows reactive lymphoid follicles, chronic inflammation, and scattered crypt abscesses.

Which finding is the distinctive clue, and what prevents overdiagnosing IBD?

Slide 60 · Knowledge check: Interpretation

Characteristic clue: Prominent mucosal lymphoid follicles.

1. Crypt abscesses are not etiology-specific.

2. Diverted anatomy strongly supports diversion-associated inflammation.

3. Prior IBD and disease distribution must still be evaluated.

Slide 61 · Knowledge check: Pathogenesis

Short-chain fatty-acid enemas improve mucosal inflammation in some diverted colonic segments.

What mechanistic hypothesis does this support, and what is still uncertain?

Slide 62 · Knowledge check: Evidence boundary

The finding supports a role for microbiota-derived epithelial nutrients.

1. Short-chain fatty acids are microbial metabolites and epithelial energy sources.

2. Restoration of normal flow is associated with disease resolution.

3. The complete pathogenic pathway is not definitively established.

Slide 63 · Selected references

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. 2026. Chapter 17, p. 733.

Tominaga K, et al. Diversion colitis and pouchitis: A mini-review. World J Gastroenterol. 2018;24:1734–1747.

Geraghty JM, Talbot IC. Diversion colitis: histological features after defunctioning colostomy. Gut. 1991;32:1020–1023.

Villanacci V, et al. Histopathology of Non-IBD Colitis: GIPAD practical approach. Pathologica. 2021.