The large intestine starts at the caecum, on the posterior medial wall of which is the appendix. The colon is made up of ascending, transverse, descending and sigmoid parts, which join the rectum at the rectosigmoid junction. The muscle wall consists of an inner circular layer and an outer longitudinal layer. The outer layer is incomplete, coming together to form the taenia coli, which produce the haustral pattern seen in the normal colon. The mucosa of the colon is lined with epithelial cells with crypts but no villi, so that the surface is flat. The mucosa is full of goblet cells. A variety of cells, mainly lymphocytes and macrophages, are found in the lamina propria. The blood supply to the colon is from the superior and inferior mesenteric vessels. Generally there are good anastomotic channels, but the caecum and splenic flexure are areas where ischaemia can occur.
The rectum is about 12 cm long. Its interior is divided by three crescentic circular muscles producing shelf-like folds. These are the rectal valves and can be seen at sigmoidoscopy. The anal canal has an internal and an external sphincter.
The main role of the colon is the absorption of water and electrolytes (Table 4.11). Approximately 2 litres of fluid passes the ileocaecal valve each day. The absorption of fluid and electrolytes takes place mainly in the right side of the colon, and only about 150 rnl is passed in the faeces. The role of the rectum and anus in defecation is complex. The rectum is usually empty and collapsed; the entry of faeces from the colon produces relaxation of the internal sphincter and the puborectalis muscle. This decreases the acute angle between the rectum and the anal canal. When the rectum contains approximately 100 ml of faeces the urge to defecate is experienced. The rectum is emptied by relaxation of the external anal sphincter (under voluntary control) and an increase in intraabdominal pressure.
Diverticula are frequently found in the colon and occur in 50% of patients over the age of 50 years. They are most frequent in the sigmoid, but can be present over the whole colon.
The term diverticulosis’ indicates the presence of diverticula; diverticulitis implies that these diverticula are inflamed. It is perhaps better to use the more general term diverticular disease, as it is often difficult to be sure whether the diverticula are inflamed. The precise mechanism of diverticula formation is not known. There is thickening of the muscle layer and, because of high intraluminal pressures, pouches of mucosa extrude through the muscular wall through weakened areas near blood vessels to form diverticula. Diverticular disease seems to be related to the low-fibre diet eaten in the western hemisphere.
Diverticulitis occurs when faeces obstruct the neck of the diverticulum causing stagnation and allowing bacteriato multiply and produce inflammation. This can then lead to bowel perforation (peridiverticulitis), abscess formation, fistulas into adjacent organs, or even generalized peritonitis.
Diverticular disease is asymptomatic in 90% and is usually discovered incidentally on a barium enema examination; no treatment is required. Left iliac fossa pain, constipation and diarrhoea are often attributed to diverticular disease, but as these symptoms are very similar to those seen in the irritable bowel syndrome, it is debatable whether they are due to diverticular disease. In practice, both conditions are treated symptomatically with a high-fibre diet, antispasmodic drugs (e.g. mebeverine 135 mg three times daily) and agents to regulate the bowel. A barium enema is often performed to exclude colonic carcinoma. Diverticular disease can produce rectal bleeding, which is sometimes massive, particularly from right-sided diverticula. In most cases the bleeding stops and the cause of the bleeding can be established by X-ray, colonoscopy and sometimes angiography. In rare cases emergency colectomy is necessary. It is unwise to ascribe an iron deficiency anaemia to a bleeding diverticulum unless all other causes, e.g. piles or carcinoma, have been excluded.
ACUTE DIVERTICULITIS almost always affects diverticula in the sigmoid colon. It presents with severe pain in the left iliac fossa, often accompanied by fever and constipation. These symptoms and signs are similar to appendicitis but on the left side. On examination there is tenderness, guarding and rigidity on the left side of the abdomen. Tachycardia and pyrexia are present and the white cell count shows a leucocytosis. Chest and abdominal X-rays are necessary to exclude free air under the diaphragm and an ultrasound is performed to detect an abscess following localized perforation.
Complications of acute diverticulitis
ABSCESS FORMATION, causing pain, pyrexia and a palpable tender mass in the left iliac fossa. Ultrasound or CT scanning can show the mass. Surgical drainage with or without a defunctioning colostomy may be required. Antibiotics are always given.
PERFORATION, leading to generalized peritonitis.
FISTULA FORMATION into the bladder, causing dysuria or pneumaturia, or into the vagina, causing discharge; the diverticular disease is often chronic, without evidence of acute inflammation. Surgery is usually required.