Adenomatous polyps and adenomas: Polyps are common in the large bowel (Fig. 27.1). The most significant are adenomas (i.e. benign neoplasms) and all have potential for malignant change. In general, it takes 5–10 years to progress to invasive cancer. Early removal prevents progression from adenoma to adenocarcinoma. The process by which the epithelial cells acquire increasingly severe genetic mutations is termed the adenoma–carcinoma sequence. Thus, if adenomas are found at endoscopy, all of them should be meticulously removed (Fig. 27.1c) and subjected to histological examination.

Most adenomas are typically pedunculated or sessile (stalkless), allowing easy recognition and removal by diathermy snare. The much less common flat adenomas are particularly found in the Far East. These can be small and recognition at colonoscopy requires special dye-spray techniques. Flat adenomas can be removed by injecting saline into the submucosa to ‘lift’ the flat lesion before excising the abnormal mucosa with diathermy, sometimes in several pieces.

Adenomatous lesions examined histologically display a range of epithelial abnormalities from mild to severe dysplasia, to early invasive cancer. In invasive cancer, the cellular abnormality has breached the muscularis mucosae, from where extension progressively occurs into the submucosa. As a rule, the larger the lesion, the more likely it is to be malignant: only 1% of polyps smaller than 1 cm are malignant whereas about half of those larger than 2.5 cm are malignant.

Even in apparently benign lesions, there may be discrete areas of frank malignancy; so thorough histological examination is needed. When pedunculated lesions are removed by colonoscopic snaring, it is crucial to establish whether there is stalk invasion: if it is clear of cancer, further treatment is not usually required.

Classification of colonic adenomas: Three patterns of lesion are recognised histologically: tubular adenomas, villous adenomas and tubulo-villous adenomas.

Tubular adenomas are small pedunculated or sessile lesions in which the adenoma cells retain a tubular form similar to normal colonic mucosa. Tubular adenomas have the least potential for malignant transformation.

Villous adenomas are usually sessile (no stalk) and frond-like (papilliferous) lesions which tend to secrete mucus. The epithelial component of villous adenoma is more dysplastic than tubular adenoma and there is a correspondingly greater potential for malignancy; as with tubulo-villous adenomas, this potential is proportional to size.

Tubulo-villous adenomas are intermediate between tubular and villous adenomas and comprise the majority of colonic polyps (Fig. 27.1d). Most are pedunculated, and the stalk is covered with normal colonic epithelium. The stalk probably develops by peristalsis dragging the tumour mass distally and can range from about 0.5 to 10 cm long.

Distribution of colorectal adenomas: Although adenomatous polyps can occur in any part of the large bowel, three-quarters of them arise in rectum and sigmoid colon. This exactly parallels the distribution of carcinomas and provides verification that most cancers develop from polyps.

Adenomas often arise singly (particularly villous adenoma) but more than 20% of patients with colonic polyps have multiple polyps, most often tubulo-villous. Patients with proven carcinoma often have coexisting benign adenomas (synchronous), likely to become malignant later if not removed (see Fig. 27.2). This explains why the whole colon should ideally be examined before colectomy, preferably by colonoscopy, and why long-term follow-up after treatment of large bowel cancer should include regular colonoscopy.

Symptoms and signs of colorectal polyps: Many polyps cause no symptoms, and are found incidentally on colonoscopy, barium enema or CT colonography. Symptomatic polyps typically present with rectal bleeding and sometimes iron deficiency anaemia from occult blood loss. Mucus production, especially from villous adenomas, may be so copious as to be the main presenting complaint. Very occasionally, symptomatic hypokalaemia may develop because so much potassium-containing mucus is lost. Distal lesions occasionally produce tenesmus (a painful urge to defaecate) or may prolapse through the anus. Rarely, large polyps can cause obstructive symptoms or intussusception.

Diagnosis and management of colorectal polyps: For symptomatic patients, visualisation of the colon is needed. Most colorectal investigation is somewhat invasive and uncomfortable and the benefits have to be explained to patients, whilst their dignity and privacy is respected as far as possible. In the outpatient clinic, rigid sigmoidoscopy (which actually visualises the rectum) is often performed initially, as nearly half of all polyps lie within reach of the 25 cm instrument. Flexible sigmoidoscopy, usually performed without bowel preparation or after a phosphate enema, reaches past the sigmoid and descending colon to the splenic flexure, covering 75% of the area at risk, but to view the remainder of the bowel requires colonoscopy. Well performed colonoscopy is the ‘gold standard’ investigation: it allows visualisation of the entire large bowel mucosa, and polyps may be removed at the same time. For this reason, it is the first-line investigation in many centres. However, there are disadvantages: it requires a full day’s bowel preparation and the procedure often requires sedation because of the discomfort. Furthermore, colonoscopy carries a 1 : 1000 risk of major haemorrhage or perforation. Sessile, small or flat adenomas in any location can be difficult to recognise even at colonoscopy and these potentially malignant lesions can be missed, especially if bowel preparation has been poor.

An alternative investigation growing in popularity and accuracy is CT colonography. Some studies have shown it to be virtually as sensitive for detecting polyps. It is less invasive and does not require sedation but bowel preparation is needed. Other alternative investigations include double contrast barium enema or unprepared CT scan.

Once an adenomatous polyp is found or a colorectal cancer has been treated, that patient is at risk of forming further polyps elsewhere in the large bowel and needs follow-up colonoscopies. Intervals between colonoscopies are set according to guidelines determined by the number, size and pathology of polyps at each investigation and vary between 1 and 5 years.

Ideally, people at risk would undergo colonoscopic surveillance to detect asymptomatic polyps (as well as invasive cancers) so they can be removed before undergoing malignant change. The UK NHS national bowel cancer screening programme employs faecal occult blood testing every 2 years for people between 60 and 75 years of age. This test has been shown to aid detection of about 50% of asymptomatic cancers and is predicted to reduce mortality from colorectal cancer by at least 15%. Greater reductions could undoubtedly be achieved if patient compliance could be improved, and plans are under way to supplement this programme with a single flexible sigmoidoscopy offered to people aged 55–60.

Polyposis syndromes: The most important is familial adenomatous polyposis (FAP), because it is reasonably common (1 : 30 000 people) and because large bowel cancer is inevitable if untreated. An autosomal dominant defect in the APC gene causes 100 or more adenomatous polyps to develop in the large bowel by the mid teens. Affected patients usually have one parent with the condition. Each affected individual is certain to develop colorectal cancer, at an average age of 40, unless preventative measures are taken. Ideally, prophylactic surgery to remove the area at risk should be performed in early adulthood. One option is subtotal colectomy and ileorectal anastomosis which removes nearly all the large bowel but has the retained rectum requires careful very long-term surveillance for malignancy. The alternative is to remove the rectum as well (panproctocolectomy) and perform an ileostomy or an ileal pouch restorative procedure.

Another polyposis syndrome is Peutz–Jeghers syndrome, which causes hamartomatous polyps throughout the gastrointestinal tract. Patients often have freckles around the mouth and on hands, feet and genitalia. Half of these patients are likely to die by the age of 50 because of polyp-related emergencies such as bowel intussusception or cancer. Patients are prone to develop cancers of small and large bowel, stomach, pancreas, testis and breast.

Blood loss and anaemia: Carcinomas of caecum and ascending colon rarely obstruct unless the ileocaecal valve is involved. This is because the right colon has a larger diameter than the left and the faecal stream is more fluid. However, occult bleeding from the tumour surface commonly causes iron deficiency anaemia, and these patients typically present with anaemia and a palpable mass in the right iliac fossa.

Change of bowel habit and large bowel obstruction: Colorectal cancers usually secrete mucus and bleed into the lumen, which tends to change the bowel habit towards a looser stool. Thus a recent history of loose stool is more likely to predict cancer than increasing constipation, especially since constipation is so common in the elderly population. Faeces in the left colon are more solid and the intraluminal pressure is higher, thus distal cancers here are more likely to obstruct. Colonic cancers tend to progressively encircle the bowel wall, encroaching on the lumen and producing an annular stenosis, taking perhaps a year to involve each quarter of the circumference.

Large bowel obstruction may be partial or complete. Partial obstruction may present as a change in bowel habit, often noticed as constipation with intermittent ‘overflow’ diarrhoea. Complete obstruction precipitates emergency hospital admission (see Ch. 19).

Rectal bleeding: Carcinomas distal to the splenic flexure often cause visible blood to be passed per rectum. The character of the blood and the nature of its mixing with stool depend on how far proximally the lesion is from the anus.

Tenesmus: Carcinomas or polyps in the lower two-thirds of the rectum may be perceived as masses of faeces. This stimulates a persistent defaecation response, causing an unpleasant sensation of incomplete evacuation known as tenesmus.

Perforation: A cancer penetrating the bowel wall may stimulate a vigorous local inflammatory process resulting in a pericolic abscess which contains the perforation, at least for a while. This most often occurs in the rectosigmoid area and usually presents with left iliac fossa pain and tenderness and a swinging fever. The differential diagnosis is acute diverticulitis or a diverticular abscess.

A carcinoma anywhere in the colon (but rarely in the rectum) may perforate and present as an acute abdomen with peritonitis. Occasionally a carcinoma may erode into a nearby organ creating a malignant fistula. Fistulation can occur into stomach, bladder, uterus or vagina, or direct to the skin.

Blood tests: Bowel neoplasms often cause anaemia but liver function tests remain normal until almost all parenchyma is replaced by tumour. Hypokalaemia rarely results from lesions producing excess mucin. Tumour markers are neither sensitive nor specific for a primary diagnosis of colorectal cancer but carcino-embryonic antigen (CEA) is used to monitor cancer recurrence.

Imaging for staging: When colorectal malignancy is diagnosed, surgery is likely to be necessary to relieve symptoms irrespective of distant spread. Staging is performed to guide oncological planning and counselling. Liver and lung metastases are sought, along with other evidence of spread within the abdomen or to bone. CT scanning is the most useful investigation for these but liver ultrasound scanning may be more sensitive for detecting small liver metastases. MRI scanning can add important information about the precise extent of local spread of rectal cancer.

In a patient presenting as an emergency with complete large bowel obstruction, plain abdominal X-rays often show large bowel dilated by gas down to the level of obstruction and empty of gas beyond it. The level is often at the sigmoid colon or rectosigmoid junction. CT scan or sigmoidoscopy may confirm the likely diagnosis of carcinoma. Similarly, an ‘instant’ Gastrografin enema (i.e. without bowel preparation) can confirm the diagnosis and at the same time exclude pseudo-obstruction.

• Before elective operations, the bowel may be prepared by giving a low residue diet and enemas. Oral purgatives are no longer given because of the potential dehydration

• Perioperative prophylactic antibiotics (e.g. gentamicin and metronidazole) are given

• Operative access is achieved laparoscopically, or by laparotomy, usually via a midline incision

• The affected segment of bowel is removed with a margin of normal bowel, usually 5 cm clear each side of the tumour. There must be a good blood supply to the cut ends of bowel to ensure healing so, in practice, lines of resection are determined by the distribution of mesenteric blood vessels (see Fig. 27.7). For example, ascending colon lesions are treated by removing the whole right colon (right hemicolectomy), as the right colic artery must be ligated in order to remove a section of the right colon

• A wedge-shaped section of colonic mesentery is removed with the bowel. This contains the primary field of lymph node drainage. If there are other obvious lymph node metastases, these are usually included in the resection specimen

• Rectal cancers are a special case and an outline of standard operations is given in Figure 27.8. The preferred operation is a sphincter-saving anterior resection of rectum; provided the lower edge of the tumour is 1–2 cm above the anal sphincters, the sphincter can usually be preserved. This operation involves excising the tumour with an appropriate length of bowel plus an intact envelope of fat around it (the mesorectum containing local lymph nodes). The proximal end of bowel is then anastomosed to the distal stump. Alternatively, a pelvic reservoir is created using a J-pouch technique (Fig. 27.8). This has been shown to reduce the frequency and urgency of defaecation without increasing surgical complications. A temporary defunctioning ileostomy or colostomy is sometimes used to aid healing of a low anastomosis. If the sphincter is involved, the entire rectum and anus has to be removed via an abdomino-perineal excision (APE), with the proximal end of bowel brought out as a colostomy

• In most cases, the two cut ends of bowel can be anastomosed without the need for a temporary or permanent colostomy (the indications for stomas and their types and management are described on p. 361 onwards). The method used to rejoin the bowel depends on the site of the anastomosis, the preference of the surgeon and whether there is much disparity in diameter between the ends to be joined. Methods of large bowel anastomosis are shown in Figures 27.9 and 27.10.

• Postoperatively, an ‘enhanced recovery’ program may be employed to encourage early eating and mobilisation (see Ch. 2, p. 26). These have been shown to lower complication rates and shorten hospital stays.

Permanent stomas: These are necessary when there is no distal bowel segment remaining after resection or when for some reason the bowel is not to be rejoined. A colostomy is required after abdomino-perineal excision of a low rectal or anal canal tumour. An ileostomy (see Fig. 27.12) is employed after excision of the whole colon and rectum (pan-proctocolectomy) unless a pouch reconstruction is performed. Sometimes in patients with severe and permanent incontinence, a colostomy may make a better life possible.

Temporary stomas:

Loop stoma: This type of stoma is designed so that both proximal and distal segments of bowel drain onto the skin surface (see Fig. 27.11a). This deflects proximal effluent to the skin surface and provides a ‘blow-off’ valve for the distal loop. Loop stomas are used mainly for temporary defunctioning to protect a distal anastomosis. It is straightforward to reanastomose the ends at reversal; the loop is then dropped back into the abdomen. The most common form of loop stoma is the loop ileostomy; occasionally a loop transverse colostomy is used.

Split or ‘spectacle’ stoma: This is the ultimate form of defunctioning stoma but has been largely superseded by the loop stoma. After resection, both proximal and distal bowel ends are brought separately to the skin surface. The proximal end stoma passes stool into a stoma appliance; the distal stoma (or mucous fistula) defunctions the bowel beyond it and produces just a little mucus.

End stoma: This type of stoma is usually permanent. An end colostomy is most commonly used to ‘resite the anus’ on to the abdominal wall after removal of the rectum and anal sphincter (i.e. abdomino-perineal excision—Fig. 27.11b). An ileostomy may be employed after subtotal or pan-proctocolectomy, particularly in fulminant colitis (Fig. 27.11c). Later, some form of reconstruction may be considered, involving an ileal pouch or reservoir in the pelvis composed of loops of ileum sutured side-to-side connected to the anus. The anal sphincter mechanism is preserved so that the patient is usually continent and can control evacuation.

Hartmann’s procedure: end colostomy and rectal stump: Hartmann’s procedure is a relatively safe technique, particularly for less experienced surgeons, and carries less overall risk than primary anastomosis. It is employed after emergency resection of rectosigmoid lesions where primary anastomosis is inadvisable because of obstruction, inflammation or faecal contamination or surgical inexperience. It may be the choice of treatment for frail or debilitated patients. At Hartmann’s operation, the lesion is resected, the proximal bowel is made into an end colostomy (the same as that employed after an abdomino-perineal excision) and the cut end of the distal remnant is closed with sutures or staples (see Fig. 27.11d). Secretions from the residual rectum still pass through the anus. Several months later when local inflammation has resolved, a decision may be made to reconnect the bowel, depending on fitness and the preference of the patient. However, the colostomy is so well tolerated that some patients prefer to keep it permanently rather than undergo another major operation.

Irrigation technique for managing a colostomy: An ileostomy tends to work continuously during the day whereas a colostomy is intermittent. Some patients with a colostomy prefer to dispense with a stoma bag by using a technique of colonic irrigation. Once every few days the patient passes a litre or more of water into the colostomy via a special spout, then the water is allowed to drain out, with the aim of emptying the entire colon. After this, the stoma is covered with a dry dressing as a stoma bag is not needed until the next irrigation.