The small and large intestine

Anatomy and function of the small intestine

The small bowel extends from the pylorus to the ileocaecal valve and ranges in length from 3 to 7 metres. The jejunum comprises two-fifths of the small intestine and is of wider calibre than the ileum. The gut diameter narrows progressively from the duodenojejunal flexure to the ileocaecal valve. The small bowel mucosa is supported by a strong submucosa and comprises a single layer of columnar cells in a villiform structure that greatly increases the absorptive surface area. Columnar glandular epithelium is interspersed with mucus-secreting cells, Paneth cells and amine precursor uptake and decarboxylation (APUD) cells derived from the neural crest. Between the inner layer of circular muscle and outer longitudinal layer runs Auerbach myenteric plexus comprising vagal parasympathetic fibres and sympathetic fibres from the lesser and greater splanchnic nerves. This plexus controls orderly propulsive contractions of the muscular layers of the gut wall. The submucosal (Meissner) neuronal plexus of autonomic nerves innervates the glandular cells in the epithelium. The sensation of visceral pain is mediated by the sympathetic nervous system, originating mainly from the thoracolumbar outflow and fed along the arterial supply to the gut.

The arterial supply to the small intestine is via the superior mesenteric artery, which runs in the root of the small bowel mesentery, supplying the bowel with a series of arterial arcades ( Fig. 17.1 ). These midgut vessels communicate with the coeliac axis through the pancreaticoduodenal arcade and with the inferior mesenteric artery by contributing to the colonic marginal artery through the left branch of the middle colic artery, which joins the ascending branch of the left colic artery. Venous blood drains via the superior mesenteric vein to the portal vein. Lymphoid aggregates in the submucosa (Peyer patches) are more numerous in the ileum, and lymph drains to regional nodes in the root of the mesentery before passing to the cisterna chyli.

The main function of the small bowel is nutrient absorption (amino acids, short peptides, sugars, fats, minerals, vitamins and micronutrients). Its secretory and digestive functions supplement those of the upper GI tract. The mucosa is thrown into circular folds (plicae semilunares) and is carpeted by finger-like villi, giving an absorptive area of 200 to 500 m ² . Some 5 to 8 L of fluid enter the jejunum each day, of which only 1 to 2 L normally pass to the colon.

Anatomy and function of the large intestine and appendix

The main function of the large intestine is water absorption and to act as a reservoir until defaecation is appropriate. The large bowel mucosa consists of columnar epithelium interspersed with mucus-secreting goblet cells that secrete lubricating mucus. The villi are shorter than those of the small intestine, and crypts pass down to the muscularis mucosa, which is supported by a strong submucosa. The large bowel extends from the ileocaecal valve to the upper anal canal (approximately 1.6 metres). The caecum is a blind pouch at the most proximal part of the large bowel. The transverse and sigmoid colon are mobile because they have a mesentery, whereas the ascending and descending colon are only partially peritonealised. The true rectum is demarcated by coalescence of the three taeniae coli of the sigmoid colon to form a continuous outer longitudinal muscular tube. The upper third of the rectum has a peritoneal cover anteriorly and on both sides; the middle third is peritonealised only anteriorly; and the lower third is normally wholly extraperitoneal.

The inferior and superior mesenteric arteries supply the colon via a marginal artery ( Fig. 17.2 ) that allows collateral supply in the event of arterial occlusion but is weakest at the splenic flexure. In contrast to the small bowel, each of the terminal arterial branches feeding the large bowel are end arteries that has implications for the risk of ischaemia. The superior rectal artery is the continuation of the inferior mesenteric artery and, with the middle and inferior rectal arteries (branches of the internal iliac arteries), supplies the rectum. The inferior mesenteric vein drains into the splenic vein. Lymph channels run along the course of the arterial supply ( Fig. 17.2 ). Lymph from the rectum, sigmoid and descending colon drains to the superior rectal and inferior mesenteric nodes, whereas anal canal lymph drains to inguinal nodes. Knowledge of the lymphatic drainage has considerable relevance to the management of patients with rectal or anal cancers.

The appendix is lined by colonic epithelium but has no known function in humans. The submucosa contains prominent lymphoid follicles in childhood that regress in adolescence. In older patients, the lumen may be obliterated by fibrosis. The appendix projects from the medial wall of the caecum some 2 cm below the ileocaecal junction as the taeniae coli converge.

Clinical history taking

Painful contraction of the midgut secondary to obstruction or inflammation results in periumbilical colic due to referred pain related to its the embryologic origin. Nausea, vomiting and pain are early and predominant features of many small bowel disorders, particularly obstruction. Disorders affecting the hindgut frequently present with poorly defined features including abdominal distension, colicky lower abdominal pain and altered bowel habit. Vomiting is a late feature of large bowel obstruction. Normal stool frequency is variable and ranges from one motion in 3 days to 3 motions/day. Hence, it is important to establish if there has been any change in frequency from the patient’s normal habit. Passage of blood or mucus by rectum is a common feature of large bowel disease. It is important to differentiate ‘outlet-type’ bleeding (from the anal canal) from sinister blood loss when the blood is mixed with stool; there may also be associated altered bowel habit or tenesmus (a painfully urgent but ineffectual attempt to pass stool). Outlet bleeding is typically bright red and may be present only on toilet paper or spattered in the pan, separate from the stool. There may be associated perianal pain due to fissure or prolapsed piles. Blood originating from the distal bowel is usually bright red, whereas blood coming from the upper GI tract is usually altered by gut bacteria and digestive enzymes, becoming black or tarry (melaena). Weight loss, malaise and anaemia are common nonspecific features of intestinal disease. A careful drug history is essential as many medicines can cause GI upset, both directly and indirectly.

Examination

The hands, fingernails, eyes, conjunctivae and oral mucous membrane should be inspected. Abdominal examination may reveal distension, a mass or visible peristalsis. In thin subjects, the caecum is often palpable, and the descending and sigmoid colon may be palpable when loaded with faeces. Hepatomegaly due to metastatic disease should be excluded. Abdominal auscultation is rarely of any value and has been shown to be nondiscriminatory. Digital rectal examination may detect a rectal tumour and reveal blood or mucus. In patients with lower GI symptoms, there is no rationale for checking the faeces for occult blood, as the sensitivity of the faecal occult blood (FOB) test is low. The more specific and sensitive faecal immunologic testing (FIT) may have a place in stratifying symptomatic patients for investigation.

Investigation of the luminal gastrointestinal tract

Investigation of persisting diarrhoeal illnesses should include stool samples for faecal calprotectin (nonspecific test of intestinal inflammation), culture and testing for Clostridium difficile toxin. For recent foreign travel, hot stool should be assessed for cysts, ova and parasites. Imaging modalities for small bowel comprise plain radiography, magnetic resonance imaging (MRI) enteroclysis, computed tomography (CT), capsule video endoscopy, labelled white cell radionuclide scanning and labelled red cell radionuclide scanning. Barium follow-through and contrast small bowel enema have largely been superseded by MRI. Fibreoptic small bowel enteroscopy allows direct inspection of the proximal jejunum, whereas the terminal ileum can be inspected and biopsied at colonoscopy. Coeliac disease may be diagnosed by serum ELISA assays for autoantibodies, including antiendomysial IgA antigliadin or tissue transglutaminase (tTG) antibodies. Duodenal biopsy taken at endoscopy is the gold standard investigation and would reveal the characteristic subtotal villous atrophy of coeliac disease. Tests of absorptive capacity are rarely performed. Bacterial overgrowth can be assessed using the glucose, ¹⁴ C-xylose and ¹⁴ C-glycocholate breath tests. Small bowel aspiration can be carried out by nasojejunal tube or at enteroscopy for bacterial culture.

Following digital rectal examination, direct inspection includes proctoscopy, rigid sigmoidoscopy, flexible sigmoidoscopy and colonoscopy. These techniques allow biopsy and snare removal of colorectal polyps using cauterising diathermy. Plain radiography is used extensively in an emergency. CT double-contrast colonography has superseded double-contrast barium enema. CT also has considerable utility in the assessment of the acute abdomen. CT of the chest, abdomen and pelvis is routinely used in staging of colon and rectal cancer, along with MRI for rectal cancer. Positron emission tomography (PET) after administering a ¹⁸ F-labelled tracer (fluorodeoxyglucose) that is metabolised by tumours is reserved for staging of colorectal cancer when multiorgan resection is being considered. Colonic transit can be assessed in cases of suspected megacolon or slow-transit constipation by administering radioopaque markers or ingestion of radionucleotide ( ¹¹¹ In or ⁹⁹ Tc)-labelled feed to assess large bowel transit time.

Appendicitis

Acute appendicitis remains the most common acute abdominal emergency in childhood, adolescence and early adult life ( Chapter 13 ).

Appendiceal tumours

Crohn disease

Although originally described as affecting the terminal ileum, any part of the GI tract can be involved, from mouth to anus. In 50% of cases, both small and large bowel are involved, whereas in 25% of cases, large bowel alone is affected. The incidence is increasing in developed countries, and the annual incidence rate is 5 to 7 cases per 100,000 in the UK. Due to the lifelong nature of the condition, the estimated UK prevalence is around 620,000 people. At the time of initial presentation, the clinical and histologic features may be indistinguishable from those of ulcerative colitis, leading to the diagnostic category of inflammatory bowel disease unclassified (IBDU). The outcome from IBDU is worse and may follow a course that eventually leads to overt evidence of Crohn disease.

Cigarette smoking is the single most important risk factor, being associated with increased disease severity, likelihood of relapse and further need for surgical interventions. There is also evidence for the involvement of immunologic factors and the gut bacterial flora in Crohn disease pathogenesis. There is a substantial heritable contribution to disease aetiology, variously estimated at 20–50%.

Pathology

Macroscopically, Crohn disease produces a cobblestone appearance, in which oedematous islands of mucosa are separated by fissures that can extend through all coats of the bowel wall. Circumferential ulceration and associated fibrosis can result in multiple strictures of varying length. Multiple areas of inflammation are common with intervening normal bowel (skip lesions, Fig. 17.3 ). Full thickness involvement of the bowel wall leads to serosal inflammation, adhesion to neighbouring structures and sinus or fistula formation. Microscopically, there are deep fissuring ulcers, oedema and inflammatory cell infiltrates, with foci of lymphocytes and noncaseating granulomas in 50% of cases.

Clinical features

Crohn disease is a chronic disorder with exacerbations, remissions and a varied clinical presentation. Continuous or episodic diarrhoea is associated with recurring abdominal pain and tenderness, lassitude and fever. There may be declining general health, malabsorption, weight loss and metabolic bone disease (osteoporosis or osteomalacia). Failure to thrive and reach developmental milestones are common in affected children.

Examination may reveal malnutrition, and there may be a palpable abdominal mass. Intestinal obstruction may be due to gross thickening of active disease occluding the lumen, stricturing of ‘burnt out’ disease or adhesions from previous surgical intervention. Fistula formation occurs in 20% of patients with small and large bowel disease but less in those with disease restricted to the large bowel. Fistulae may communicate with adjacent loops of bowel, other viscera (e.g., bladder, vagina) or the skin. Enterocutaneous fistulae may result from surgical intervention and commonly involve the anterior abdominal wall or perineum ( Fig. 17.4 ). Abscesses can result from chronic bowel perforation, but free perforation is relatively uncommon because the inflamed segment usually adheres to surrounding structures. Although less common than in ulcerative colitis, toxic dilatation can complicate colonic disease. Fulminant Crohn colitis with severe inflammation, mucosal oedema, deep linear ulcers and fibrosis is shown in Fig. 17.5 .

There is an elevated risk of colorectal adenocarcinoma with long-standing Crohn disease: 2.5-fold overall and 4.5-fold for colonic disease, with a 10-year cumulative risk following diagnosis of 2.9%. Lymphoma risk may also elevated, likely due to immunomodulatory therapy (e.g., thiopurines) and biological therapy (anti-TNF and antiintegrin). Evidence is limited that surveillance provides protection, and many patients with long-standing colonic Crohn eventually come to colectomy. Crohn disease is also associated with ∼30-fold excess risk of small bowel adenocarcinoma, but because that cancer is rare; the absolute risk only amounts to 0.2% at 10 years and 2.2% at 25 years after diagnosis.

Anorectal involvement is common (25% in small bowel Crohn disease; 75% in large bowel disease), including abscess, fistula, fissures, ulceration, oedematous skin tags and anorectal stricturing. Anal fissures are often multiple and indolent, and extend to involve any part of the perineum, including the vagina or scrotum. Systemic manifestations include anterior uveitis, iritis, polyarthropathy, ankylosing spondylitis, liver disease (e.g., sclerosing cholangitis) and erythema nodosum. Terminal ileal involvement or ileocaecal resection may result in gallstone formation owing to poor absorption of bile salts.

Investigations

Assessment of nutritional status, including serial weight measurement, is essential. Anaemia may be due to iron deficiency from chronic blood loss and (rarely nowadays) malabsorption from short gut syndrome, a normocytic anaemia of chronic disease, macrocytic anaemia from vitamin B ₁₂ or folate malabsorption. Assays for circulating acute-phase proteins (C-reactive protein) is not a diagnostic test but is useful for monitoring disease activity. Until recently, the diagnosis was most frequently made on barium follow-through ( Fig. 17.6 ), revealing rose-thorn ulcers or long irregular terminal ileal stricture at the site of previous ileocaecal resection. Active disease produces radiologic evidence of thickening, luminal narrowing and separation of bowel loops, and is often associated with mucosal ulceration, deep fissuring ulcers and cobblestone appearance. Skip lesions and fistula formation may be apparent. MRI enteroclysis (image enhanced by administering oral osmotically active agent, e.g., polyethylene glycol) is now the investigation of choice ( Fig. 17.7 ) and has the advantage of limiting radiation exposure. Rectal examination, proctoscopy, sigmoidoscopy and colonoscopy determine disease extent, and biopsy of inflamed bowel is mandatory. Newer investigative techniques include video capsule endoscopy ( Fig. 17.8 ), enteroscopy and CT colonography. Double-contrast radiography still has a place for delineation of fistulous tracts.

Ulcerative colitis

The annual incidence of ulcerative colitis is ∼10/100,000 population in Westernised countries but rare in developing countries. The aetiology is incompletely understood, but genetic, immunologic and dietary factors all play a part. Any age group may be affected, but peak incidence is in early adulthood. The disease may be contiguous but almost universally affects the rectum and extends proximally ( Table 17.1 ). In a minority, the rectum may be spared, and in untreated patients, this should raise suspicion about underlying Crohn disease. There is an elevated risk of adenocarcinoma in ulcerative pancolitis. Although ulcerative colitis is primarily a disease of the large bowel, systemic manifestations (iritis, polyarthritis, sacroiliitis, hepatitis, erythema nodosum, pyoderma gangrenosum) can occur. Sclerosing cholangitis affects 2–5% of patients with ulcerative colitis and may necessitate liver transplantation.

Pathology

The characteristic feature is inflammation restricted to the mucosa and submucosa of the large bowel. In severe episodes, there may be full-thickness involvement with inflammatory infiltrate. Abscesses develop at the base of the colonic crypts, which burst and unite to form crypt abscesses. These undermine the mucosa, resulting in ulceration

Lamb CA et al. Gut. 2019;68:s1–s106. https://doi.org/10.1136/gutjnl-2019-318484 .

EBM 17.1

Crohn disease and ulcerative colitis

‘IBD patients with colonic disease should be offered ileocolonoscopy 8 years after symptom onset to screen for neoplasia, to determine disease extent and decide on the frequency of ongoing surveillance.’

‘Surgery in acute severe ulcerative colitis is indicated when the disease is medically resistant, there are intolerable side effects of medication, or when there is life-threatening haemorrhage, toxic megacolon or perforation.’

‘Surgical resection of the colon and rectum in ulcerative colitis should be offered to patients who have chronic active symptoms despite optimal medical therapy. Ileoanal pouch reconstruction or end ileostomy provide equivalently good quality of life, and are a matter of patient choice.’

( Fig. 17.9 ) and oedema of the intervening mucosa, with formation of inflammatory pseudopolyps. Histologically, there is also chronic inflammatory cell infiltrate, crypt architectural distortion and goblet cell depletion. Granulomas are absent but are occasionally present in severe cases, causing diagnostic difficulties. The thickened colon loses its haustrations and becomes rigid. Stricturing is rare, so its presence should raise the possibility of cancer or Crohn disease.

Investigations

In the acute phase, stool cultures are essential to exclude supervening bacterial infection (especially C. difficile ). Colonoscopy is the mainstay of diagnosis and assessment of disease extent/severity ( Fig. 17.10 ). Abdominal CT should be reserved for suspected perforation to minimise radiation exposure. Typical changes include loss of haustrations, fluffy granularity of the mucosa and pseudopolyps. Undermining ulcers may create a double contour to the edge of the colon. Widening of the retrorectal space due to perirectal inflammation and reduced distensibility of the rectum is common. In long-standing colitis, the bowel may become short and featureless, resembling a smooth tube (‘lead-pipe’ colon). In an acute attack, plain films of the abdomen may reveal a dilated gas-filled colon in which pseudopolyps are evident (‘thumb printing’). When toxic dilatation is suspected, daily plain x-rays are mandated ( Fig. 17.11 ). ‘Backwash ileitis’ may produce a dilated and featureless terminal ileum in which the mucosa appears granular and can lead to diagnostic difficulties with Crohn disease.

Small bowel neoplasms

Small bowel tumours account for less than 5% of all GI neoplasms.