Small intestine

Small bowel mucosa. H&E stain.

The small intestine, also small bowel, is a relatively well-behaved piece of machinery from the perspective of pathology. It is uncommonly affected by malignancies, relative to its length when compared to the colon and rectum.

Normal small bowel

• AKA normal small intestine.

Anatomy

Consists of three segments:

• Duodenum - can be divided into four parts.
• Jejunum.
• Ileum.

The later two parts keep general surgeons awake at night (quite literally) 'cause they get obstructed and need urgent operations.

Length

• Normal length of small bowel = 3-8 metres.

"Short":

• Less than 200 cm = short bowel syndrome.
  • Less than 100 cm usu. requires parenteral nutrition.

Histology

Main article: Gastrointestinal pathology

The Gastrointestinal pathology article covers basic histology of the GI tract.

Immunohistochemistry

• Normal small intestine is CK20 +ve... while adenocarcinoma of the small bowel may be CK20 -ve.

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Small Bowel, Biopsy:
- Small bowel mucosa within normal limits.

Terminal Ileum, Biopsy:
- Small bowel mucosa within normal limits.

Terminal Ileum, Biopsy:
- Small bowel mucosa with morphologically benign lymphoid aggregates, negative for significant pathology.

Roux-en-Y gastric bypass

Main article: Obese

Main article: Roux-en-Y gastric bypass

Block letters

SMALL BOWEL, BIOPSY:
- SMALL BOWEL MUCOSA WITHIN NORMAL LIMITS.

TERMINAL ILEUM, BIOPSY:
- SMALL BOWEL MUCOSA WITHIN NORMAL LIMITS.

TERMINAL ILEUM, BIOPSY:
- SMALL BOWEL MUCOSA WITH MORPHOLOGICAL BENIGN LYMPHOID AGGREGATES, NO SIGNIFICANT PATHOLOGY.

The segments

Duodenum

Main article: Duodenum

The duodenum is often biopsied by gastroenterologists.

Common reasons for biopsy:

• Suspected giardia.
• Suspected celiac sprue.
• Is this cancer?
• Looks normal... want to dot the i's and cross the t's.

Jejunum

• Uncommonly seen by pathologists.
• May be seen in the context of a resection done for a bowel obstruction.

Ileum

• This is seen occasionally -- often in the context of IBD and more specifically Crohn's disease.
  • Crohn's disease andulcerative colitis are discussed in their respective articles.

Specific diagnoses

Ileitis

Active ileitis and acute ileitis redirect here.

This deals with nonspecific ileitis.

General

• Common.

Microscopic

Features:

• Intraepithelial neutrophils.

DDx:

• Crohn's disease.
• Infectious ileitis.
  • Tuberculosis.
• Benign ileum - may have focal intra-epithelial lymphocytes associated with lamina propria lymphoid nodules.
• NSAID enteropathy.

Images

• 

  Ileitis - very low mag. (WC)

• 

  Ileitis - low mag. (WC)

• 

  Ileitis - intermed. mag. (WC)

• 

  Ileitis - high mag. (WC)

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Terminal Ileum, Biopsy:
- Small bowel with moderate active inflammation, marked villous blunting, basal plasmacytosis 
  and prominent eosinophils, see comment.
- NEGATIVE for granulomas.
- NEGATIVE for dysplasia.

Comment: 
The inflammation is nonspecific; it could be due to infection, inflammatory bowel disease (especially Crohn's disease), ischemia, or therapy/drugs.  Clinical correlation is required.

Small bowel obstruction

• Abbreviated SBO.

Main article: Small bowel obstruction

Small bowel neoplasms

• Adenocarcinoma - like colonic.
• Adenosquamous carcinoma.
• Neuroendocrine tumour.
• GIST.
• Schwannoma.
  • Classically have a peripheral lymphoid cuff.

Mechanical small bowel perforation

See mechanical bowel perforation.

Ileal nodular lymphoid hyperplasia

• AKA nodular lymphoid hyperplasia of the terminal ileum.

General

• An uncommon diagnosis.
• May be associated with hypogammaglobulinemia.

Gross

• Mucosal nodularity.

Microscopic

Features:

• Lymphoid nodules +/- germinal centre formation.

Note:

• Tingible body macrophages suggest an aggregate is benign.

DDx:

• MALT lymphoma.
• Mantle cell lymphoma.
• Other lymphomas.

IHC

• CD20 and CD3 - mixed population of lymphocytes.
• CD23 - follicular dendritic cells.
• Cyclin D1 -ve.

Note:

• IHC should be used if there is:
  • Clinical suspicion.
  • Histologic suspicion - either:
    • Sheets of lymphocytes without apparent germinal centre formation in a larger area (~ 2 mm).
    • Lymphoepithelial lesions.

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TERMINAL ILEUM, BIOPSY:
- SMALL BOWEL MUCOSA WITH LYMPHOID NODULES WITH GERMINAL CENTRE
  FORMATION -- NO SIGNIFICANT PATHOLOGY.
- NEGATIVE FOR ILEITIS.

TERMINAL ILEUM, BIOPSY:
- SMALL BOWEL MUCOSA WITH LYMPHOID HYPERPLASIA -- NO SIGNIFICANT PATHOLOGY.

Micro

The sections show small bowel mucosa with lymphoid hyperplasia.

Immunostains for CD3 and CD20 show a mixed population of lymphocytes. A CD23 immunostain shows follicular dendritic cell networks. A cyclin D1 immunostain is negative.

Alternate

Immunostains for CD3 and CD20 show a mixed population of lymphocytes, with CD20 cells predominating. A cyclin D1 immunostain is negative. A CD23 immunostain shows follicular dendritic cell networks.

Multiple lymphomatous polyposis

• Abbreviated MLP.
• AKA lymphomatous polyposis.

General

• MLP is classically due to mantle cell lymphoma.
  • May be due to other lymphomas, e.g. follicular lymphoma.

Microscopic

Features:

• Lymphoid nodules consisting of (small) atypical lymphocytes with an abnormal architecture.

IHC

See small cell lymphomas.

Small bowel diaphragm disease

• AKA diaphragm disease.

General

• Strictures associated with chronic NSAID use.
• Usu. mid to distal ileum.

Microscopic

Features:

• Focal submucosal fibrosis.
  • +/-Distortion of adjacent muscularis propria.
  • +/-Adjacent mucosal erosions/granulation tissue.

DDx:

• Cryptogenic multifocal ulcerous stenosing enteritis. (???)
• Crohn's disease.

Meckel diverticulum

Main article: Meckel diverticulum

Ischemic enteritis

Main article: Ischemic enteritis

Weird stuff

Autoimmune enteropathy

• Abbreviated as AIE.

Main article: Autoimmune enteropathy

See also

• Stomach.
• Gastrointestinal pathology.
• Colon.