Definitions of Histological Abnormalities in Inflammatory Bowel Disease: an ECCO Position Paper

Abstract

Histological assessment of endoscopic biopsies in inflammatory bowel disease [IBD] plays an important role in clinical management, investigative studies, and clinical trials. Scoring schemes consisting of multiple histological items and offering considerable precision are widely available. However, definitions of histological abnormalities are often inconsistent. Furthermore, interobserver variability for their recognition and assessment may be high.

The European Crohn’s and Colitis Organisation [ECCO] formed an expert panel to explore definitions of histological abnormalities in IBD, with the aim of improving the quality of diagnosis and facilitating development of scoring schemes. The process confirmed that the current definitions often have no evidence base and vary between sources. Using available evidence and expert knowledge, the panel produced a series of ECCO consensus position statements on histological features in IBD.

1. Introduction

Microscopic examination and interpretation of ileocolonic biopsies help to optimise the management of inflammatory bowel disease [IBD]. Histology can confirm the diagnosis, aid subclassification, determine severity, estimate the efficacy of drug therapy, and detect complications.^1,2 The pathologist’s focus depends on the setting. For example, basal plasmacytosis and architectural changes support a diagnosis of IBD, and granulomas and spatial distribution help to distinguish between ulcerative colitis [UC] and Crohn’s disease [CD].¹ Clinical trials and observational studies may require assessment of histological activity and other histological changes,^3–5 sometimes relying on precise multi-component scoring systems.^6–11 However, there is often no consensus on basic definitions of histological abnormalities,⁹ and many definitions are inevitably arbitrary. Table 1 outlines broad categories of histological abnormality.

2. Materials and Methods

The project leaders assembled a panel of 15 expert histopathologists and gastroenterologists to develop consensus position statements on histological definitions in IBD. Table 2 summarises the aims. The ECCO Positions appear below with accompanying supporting text. Details of the methodology and additional text are in the Supplementary Materials.

3. ECCO Positions

3.1. Architecture

Normal crypts are parallel to one another and extend from the mucosal surface to the underlying muscularis mucosae [‘test tube’ arrangement; Figure 1].^1,12 Assessment of architecture and of features that rely on spatial distribution requires biopsies of adequate size with optimal orientation that also ideally include muscularis mucosae [Figure 1].

Suggestions for thresholds to define crypt distortion include the following: a minimum of 10% crypts showing any of the features in ECCO Position 1.1. or the presence of more than two branched crypts within a well-oriented biopsy [Figures 1–3].^13–15 However, there is no consensus on these suggestions.

Crypt atrophy encompasses crypt shortening and wider crypt spacing [Figures 2 and 3], but assessment of these features is subjective.^13,16,17 Occasional definitions exist [eg, at least one crypt diameter between two crypts].^13,15–21

Several exceptions are worth noting. Sampling from innominate grooves can demonstrate apparent branching.²² The mucosa up to a distance of three crypts from a lymphoid aggregate is probably unreliable for assessment of architecture because the aggregates can distort adjacent glands. Normal rectal or caecal mucosa may show a minor degree of crypt distortion or shortening [Figure 1].¹

There is no consistent definition of surface irregularity [or the analogous terms villous surface, villiform surface, villous mucosa]. The impression of a villiform surface is partly the result of separation of crypts [Figure 4].^21,23,24

3.2. Epithelium

Definitions of mucin depletion may specify a reduction in the number of goblet cells or a reduction in mucin droplets [Figure 2].^5,13 Definitions describe ‘significant’ or ‘unequivocal’ reduction but do not usually offer quantification. One suggestion is one or less goblet cell in eight to ten enterocytes [anonymous personal communication].

Paneth cells have a distinctive pyramidal shape and supranuclear eosinophilic granular cytoplasm [Figure 5].^25–28 Most sources state that Paneth cells distal to the splenic flexure are abnormal, despite some evidence that they are present throughout the large bowel [with a decrease in number from proximal to distal].^28–30

Pyloric metaplasia [PM; broadly synonymous with pseudopyloric metaplasia / ulcer-associated cell lineage / spasmolytic polypeptide-expressing metaplasia] comprises glands lined by columnar cells that have clear or pale PAS-positive cytoplasm with neutral mucin granules and oval or round basal nuclei, thus resembling normal gastric antral and Brunner glands [Figure 6].^31,32 PM is associated with chronicity, particularly in the small bowel in CD. PM occurs in areas of ulceration and regeneration and may persist long after completion of healing.^33–36

3.3. Chronic Inflammation

Histological manifestations of mucosal chronic inflammation include basal plasmacytosis, lymphoid aggregates, and a diffuse or variable increase in lamina propria chronic inflammatory cells. Basal plasmacytosis is the most objective of these, is the earliest sign of de novo IBD, is the strongest predictor of an IBD diagnosis, and can resolve fully or persist.^37–40 When basal plasmacytosis is present, the density of plasma cells at the base of the mucosa is high and may be similar to or higher than the density of plasma cells in the upper mucosa [Figures 2, 7, and 9].^1,13,20 Basal plasmacytosis can be diffuse and band-like, focal, or patchy.^1,41,42Basal plasmacytosis is more difficult to recognise in the small bowel mucosa than in the colorectal mucosa because there is no plasma cell gradient in normal small bowel mucosa.^43–45 There is no agreement on how to measure the number of mucosal chronic inflammatory cells or on grading of basal plasmacytosis.^40,43,46 Although IBD is a chronic inflammatory process,^47,48 there is less emphasis in clinical practice and in scoring systems on chronic inflammation than on acute or active inflammation.⁶

Some authors refer to basal lymphoplasmacytosis, for which definitions are even less precise than for basal plasmacytosis. Determining colorectal lamina propria lymphocyte numbers or density or defining an increase in lymphocytes is more difficult and subjective than assessing plasma cells.

According to some sources, up to two basal lymphoid aggregates may be present in a normal colorectal mucosal biopsy [Figure 8]. However, distinction between normal and abnormal numbers is difficult. Scoring schemes that include lymphoid aggregates do not quantify these aggregates.^49,50

Architectural distortion and epithelial metaplasia are the result of crypt destruction and injury associated with chronic and active inflammation, but they do not define ‘chronic inflammation’ in IBD.⁵¹

3.4. Eosinophils

There is limited information about the normal number of intestinal mucosal eosinophils, the definition of an eosinophil crypt abscess,^50,49 and the criteria for a mild, moderate, or severe increase in intestinal mucosal eosinophils [Figures 7, 9]. The significance of focal eosinophil cryptitis in the absence of other changes is also uncertain. The panel agreed that an eosinophilic crypt abscess comprises at least two eosinophils. If neutrophils are also present, the term for the lesion is a crypt abscess [ie, neutrophil crypt abscess; Figure 10].

3.5. Granulomas

The number of histiocytes necessary to define a granuloma in IBD is at least five.^13,44,52 The threshold may vary with the setting.¹⁵ Histiocyte counts may depend partly on section thickness, number of serial sections examined, and the individual observer. In CD, a granuloma is almost always non-necrotising, never has caseous necrosis, and typically shows no confluence with other granulomas [Figure 11].^{44,45,51,53,54} Giant cells are acceptable. Conventional practice classifies granulomas as either present or absent, with no quantification.

A cryptolytic granuloma characteristically lies adjacent to a crypt that exhibits rupture [Figure 12].^55,56 In addition to mucin, neutrophils, and histiocytes, the lesion may contain lymphocytes, multinucleate foreign body-type giant cells, and parts of epithelial cells.^44,57 It may consist largely of histiocytes. Close approximation of a granuloma to a crypt suggests that it is cryptolytic, but identification of crypt injury may require serial tissue sections and is sometimes not possible.⁵¹

Although granulomas help distinguish CD from UC, most biopsy samples from CD do not contain granulomas.⁵⁸ Absence of a demonstrable association with crypt injury or with ulceration strengthens the discriminatory value of a granuloma towards CD. Cryptolytic granulomas can occur in both CD and UC,¹ but appear to be more common in CD.

The definition of microgranuloma varies in the literature and there is no widely accepted definition.

3.6. Acute inflammation

Most definitions state that the lamina propria and epithelium of normal colorectal mucosa do not contain neutrophils. However, some authors assert that one, two, or even three neutrophils are acceptable, particularly in the lamina propria. There is lack of agreement.^22,24 Bowel preparation probably accounts for some instances of small numbers of neutrophils. Neutrophils in the capillary lumen in isolation do not indicate acute inflammation.

The panel agreed that one neutrophil is sufficient to define cryptitis, although some publications specify more than one. At least one neutrophil must be demonstrably intraepithelial [Figure 7]. Similarly, there is no universal agreement on the minimum number of neutrophils in a crypt abscess.¹⁵ Definitions of a crypt abscess may refer to a cluster or chain of neutrophils. General definitions of ‘abscess’ usually require more than one neutrophil [Figures 10, 12]. The panel agreed that the presence of at least two neutrophils is necessary. By convention, the terms ‘cryptitis’ and ‘crypt abscess’ without further qualification refer to neutrophil cryptitis and neutrophil crypt abscesses, respectively. Cryptitis or a crypt abscess can include other types of inflammatory cells in addition to the neutrophil[s] [Figures 10, 12].

Grading of acute inflammation may refer to none, mild, moderate, and severe,⁵⁹ although this is often subjective. Many histology scoring schemes include grading of acute inflammation despite the lack of concordance on methodology. Examples of mild acute inflammation in scoring schemes include the following: few/rare neutrophils in the lamina propria and/or epithelium [Nancy histological index]; lamina propria neutrophils with no cryptitis or crypt abscesses [modified Riley score]; or <5% crypts and/or surface epithelium infiltrated by neutrophils [Geboes score and Robarts Histopathology Index].^8–11,60 Furthermore, there is no consensus about whether to score all biopsies, one biopsy per anatomical site, an average, or the biopsy with the most severe changes.^60–62

Distinction between an erosion and an ulcer depends on the depth of penetration into the wall.⁶³ Erosions and ulcers may be associated with granulation tissue formation, fibrin, a surface exudate that includes neutrophils, vascular proliferation in the lamina propria, and fibroblast proliferation in the submucosa.⁶³ Ulcers and erosions may show evidence of re-epithelialisation of the adjacent mucosa. Distinction between erosions and ulcers may be difficult or impossible in practice, especially if biopsies are superficial and lack muscularis mucosae and submucosa [Figures 4, 13].

The normal distal ileal mucosa contains confluent dense lymphoid tissue, lymphoid aggregates, and lymphoid follicles. Accordingly, an increase in chronic inflammatory cells alone is usually insufficient to confirm chronic ileitis.⁶⁴ Other features, such as villous distortion, villous atrophy and, in particular, pyloric metaplasia support chronicity in this location.

Clinicians may equate ‘chronic colitis’ with ‘ulcerative colitis’. This term requires qualification, particularly in the conclusion of a pathology report.

Biopsies that show chronicity are categorised as inactive chronic or active chronic, the latter requiring at least one feature of activity. The term ‘activity’ has different meanings depending on the setting [clinical, endoscopy, histology].

Focal crypt injury by neutrophils and focal active colitis [FAC] are common in endoscopic colorectal biopsies. FAC is associated with epithelial injury. If there is an increase in mononuclear inflammatory cells, the term focal active chronic colitis may be applicable.^65–69

3.8. Distribution of abnormalities

Assessment of disease distribution macroscopically and microscopically can assist with the distinction between UC and CD.^1,21

3.9. Dysplasia

Assessment of dysplasia in IBD relies on both cytological and architectural features in haematoxylin and eosin [H&E]-stained sections and adheres to the criteria of Riddell et al. [Figures 14, 15].^70,71 The Vienna system and WHO classification propose replacement of the term dysplasia with ‘intraepithelial neoplasia’.⁷² The features of conventional IBD dysplasia are analogous to those characterising gastrointestinal [GI] neoplasia in general [eg, in colorectal adenomas].^14,70,73 However, the criteria for recognising and grading newer ‘non-conventional’ forms of dysplasia may be different.^74–76

The category ‘indefinite for dysplasia’ includes lesions that are ‘probably negative’ or ‘probably positive’ for dysplasia and is particularly useful when there is significant acute inflammation or ulceration or erosion with epithelial changes that are difficult to classify [Figure 16]. ‘Indefinite for dysplasia’ is not an intermediate category between negative for dysplasia and low-grade dysplasia.

4. Discussion

The ECCO Positions in this article represent expert consensus with the support of evidence where available. Table 3 summarises additional considerations. Further development of this process will include attempts to grade histological changes, aiming to improve the consistency of pathological diagnosis and to facilitate development of new scoring schemes for IBD.

Conflict of Interest

ECCO has diligently maintained a disclosure policy of potential conflicts of interests. The conflict of interest declaration is based on a form used by the International Committee of Medical Journal Editors. The conflict of interest statement is not only stored at the ECCO Office and the editorial office of Journal of Crohn’s and Colitis, but also open to public scrutiny on the ECCO website [https://www.ecco-ibd.eu/about-ecco/ecco-disclosures.html], providing a comprehensive overview of potential conflicts of interest of authors.

Disclaimer

ECCO Position Statement projects are based on an international consensus process. Any treatment decisions are a matter for the individual clinician and should not be based exclusively on the content of the ECCO Position Statement. The ECCO and/or any of its staff members and/or any consensus contributor may not be held liable for any information published in good faith in an ECCO Position Statement.

Author Contributions

This article is a joint expert consensus activity. Hence, all authors participated intellectually and practically in this work and take public responsibility for the content of the article, including concept, design, data interpretation, and writing. All authors and the ECCO Governing Board approved the final version for submission.

Data Availability

No new data were generated in support of this research.

References