Healing of the Large Intestine
The large intestine heals similarly to the small intestine but return of tensile strength is slower than for the small intestine, reaching 75% of normal strength at 4 months postsurgery (Williams 2012). As with any surgical wound, optimal healing depends on gentle tissue handling, accurate mucosal apposition, a tension-free closure and preservation of blood supply. In particular, the blood supply should be maintained at the colorectal junction, where it is important to preserve the cranial rectal artery to avoid ischaemic necrosis.
Surgical closure of the colon requires a thorough understanding of the healing process so that the most appropriate technique is used. Failure to maximize healing potential may lead to catastrophic, life-threatening complications. The colonic lumen contains high numbers of aerobic and anaerobic bacteria (1010 to 1011 bacteria per gram of faeces) and leakage from a colonic incision can result in severe and rapidly fatal peritonitis. The large bacterial population and high intraluminal pressures generated during passage of faecal material mean that the risk of dehiscence is high for the first 4 days after surgery. In humans, colonic anastomoses are more likely to leak than those of the small intestine, with a complication rate of 7 to 40% reported; such a high complication rate has not been reported in dogs and cats (Williams 2012).
The routine use of enemas prior to colotomy or colectomy cannot be recommended. Due to the liquid slurry created, enemas are associated with a greatly increased risk of leakage and gross abdominal contamination.
General Surgical Considerations
Perform a complete abdominal exploration.
Evaluate and perform biopsy on the liver, spleen and lymph nodes in cases of suspected neoplasia.
Isolate the colon from the rest of the abdominal cavity by packing off with laparotomy swabs.
Use a slowly absorbed or permanent suture material for colonic surgery; i.e., polydioxanone, glycomer 631 or polypropylene.
Change gloves and instruments after closure of the colon.
Omentalize the incision site.
Colotomy is performed most commonly to obtain a full-thickness biopsy when other diagnostic procedures (e.g., colonoscopic biopsy) have failed. Rarely, colotomy is performed to remove a foreign body, but foreign bodies that have reached the large intestine are usually expelled with the faeces unless the object has sharp points or the distal colon or rectum is obstructed.
Subtotal colectomy involves removal of 90–95% of the colon. The primary indication is for the treatment of obstipation related to megacolon. Other indications include trauma, perforation, neoplasia or irreducible intussusception.
Preserve the ileocolic valve when possible, since this shortens the postoperative recovery period and decreases the likelihood of intractable diarrhoea from small intestinal bacterial overgrowth. It can, however, be more difficult to achieve a tension-free anastomosis when the valve is preserved, and in cats the ileocolic valve can be resected with very few postoperative complications.
Dogs develop large intestinal tumours more commonly than cats, in which the small intestine is most frequently affected. Adenomas, adenocarcinomas, lymphosarcomas, leiomyomas, leiomyosarcomas, carcinoids and plasmacytomas have all been reported. Adenocarcinoma of the large intestine often occurs in the mid- to distal rectum and in several forms: nodular, pedunculated, or as an annular constriction. Metastasis can occur to the regional lymph nodes, mesentery and liver. Metastasis to the spinal meninges, testes and skin has also been reported (Philips 2003). In cats, the most common tumour types are adenocarcinoma, lymphoma, mast cell tumours and neuroendocrine tumours. Metastatic disease is found in between 75%–80% of cats with colonic neoplasia.
The most common clinical signs in dogs with colonic neoplasia include tenesmus, haematochezia, dyschezia, constipation and rectal prolapse. Diarrhoea, weight loss and vomiting may also be observed. Dogs with mesenchymal caecal tumours can present with fever, collapse and septic peritonitis. In cats, clinical signs associated with colonic neoplasia include weight loss, anorexia, vomiting and diarrhoea. Haematochezia and tenesmus are seen far less commonly than in the dog.
Large intestinal tumours can sometimes be felt on abdominal palpation and distal colonic/rectal tumours may be palpated per rectum. Animals may be cachexic and dehydrated, with or without abdominal pain. A complete blood count can reveal a mild-to-moderate anaemia and leucocytosis. Abdominal radiographs (including contrast radiography), ultrasonography and colonoscopy are all useful to diagnose and stage colonic tumours. Wide en bloc excision with margins of 4–6 cm is recommended for isolated malignant masses, followed by colonic anastomosis. Pelvic osteotomy may be required to achieve adequate margins in some cases.
The mean survival time after surgical resection of colorectal adenocarcinomas in dogs is 6–22 months; whereas in cats, mean survival time following mass resection alone is 68 days vs. 138 days in cats treated with subtotal colectomy. Chemotherapy does significantly increase survival time for cats with adenocarcinomas but does not seem to make a difference to cats with colonic lymphoma (Slawienski 1997).
Benign leiomyoma and adenomatous polyps are commonly seen in dogs, with adenomatous polyps being the most frequently reported colorectal tumour. There is evidence that, as with familial polyposis in humans, malignant transformation can occur in approximately 25% of cases. In the dog, malignant tumours include adenocarcinoma, leiomyosarcoma and lymphosarcoma. The ratio of benign-to-malignant tumours is approximately 40:60. In the cat, adenocarcinoma is the most commonly reported tumour, followed by lymphoma and mast cell tumours. The median age for colorectal tumours is 6–9 years in dogs and 12 years for cats.
Patients with large intestinal and rectal tumours consistently present with dyschezia and/or haematochezia, though signs such as anal discharge, frank haemorrhage, increased faecal frequency or constipation are also seen. Occasionally, owners will notice a temporary prolapse of a mass after the patient has defecated.
Lower colorectal lesions are readily detected by digital rectal examination, which allows the site and extent of the lesion to be identified. Open illuminated proctoscopes are invaluable in the investigation of all colorectal lesions, and are preferred to fibreoptic endoscopes for lower colonic examination. Proctoscopes allow a direct view of the lesion so that deep representative biopsy samples can be obtained. Radiography (including survey films and barium enemas) often adds little information to that gained by rectal and proctoscopic examination. Survey films for evidence of metastatic spread to the sublumbar (iliac) lymph nodes and thorax should be carried out routinely. Positive contrast radiographs (barium enema) may reveal mural filling defects and classically an 'apple core' appearance is seen.
Due to their potential for malignant transformation, all colorectal polyps, as well as malignant lesions, should be excised as early as possible and with a wide surgical margin of 1–2 cm. Access to a significant number of colorectal masses is often limited as they are intrapelvic, making it difficult to achieve adequate surgical margins. A number of techniques have been described for management of intrapelvic colorectal tumours. The majority of these techniques involve an anal or perineal approach, which requires the patient to be placed in sternal recumbency with the tail tied upward and forward, with the hind limbs hanging over the end of a table (perineal stand). To facilitate surgery, the table can be tipped at an angle, taking care not to exceed 25–30 degrees. A tilt greater than this may compromise respiration, due to cranial shifting of the abdominal organs.
Local Excision and the Anal 'Pull-Out' Procedure
Eversion of small nonmalignant rectal masses adjacent to the anus is a simple and practical procedure. Following eversion, stay sutures can be placed in the rectal mucosa around the lesion. The lesion is excised along with the mucosa. A full-thickness excision is usually not required. The mucosal defect is closed with simple appositional absorbable sutures.
It is also often possible to evert larger distal colorectal lesions by traction and then carry out a full-thickness resection, with adequate margins. The resulting defect is closed with simple interrupted sutures, whilst for distal annular lesions, a standard end-to-end anastomosis is performed.
Rectal 'Pull-Through' Technique
The rectal 'pull-through' technique is preferable for large or annular lesions that are located in the caudal-to-mid part of the pelvic canal. It involves incising the rectum circumferentially, immediately cranial to the anus. The author finds this technique preferable to anal pull-out for large lesions, as the incidence of stricture formation or dehiscence is less.
An inverted horseshoe-shaped incision between the anus and the base of the tail allows good access to the rectum. The paired rectococcygeus muscles must be transected to allow separation of the rectum from its pelvic attachments so that it may be exteriorized. In most cases, a standard resection and end-to-end anastomosis is performed. Care must be taken not to resect more than 6 cm of rectum, as the risk of faecal incontinence becomes much greater.
In this modification of the dorsal approach, a unilateral incision is made from the base of the tail to the ischial tuberosity (see below). This gives limited access to the lateral aspect of the pelvic canal; separation of levator ani and coccygeus muscles may be required for more cranial lesions. The technique is useful only for excision of small serosal lesions of the rectum and well-encapsulated benign lesions (e.g., fibroleiomyoma, lipoma).
In some cases, colorectal tumours may be exposed via a posterior midline laparotomy, which may be combined with a pubic symphysiotomy or osteotomy. This gives limited access and requires considerable postoperative analgesia. It is generally only considered when other techniques are not feasible.
Complications and Prognosis
Complications that may be encountered following colorectal surgery include stenosis, persistent tenesmus, dehiscence, incontinence and pararectal abscessation. Stenosis at the surgical site is most likely when more than half the circumference of the rectum is excised.
Rectal polyps have a very favourable prognosis, but with malignant lesions local recurrence and regional metastasis are common after surgery. Mean postsurgical survival time for dogs with rectal adenocarcinoma is reported as 6–9 months.
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3. Williams JM. Colon. In: Tobias, Johnston, eds. Veterinary Surgery: Small Animal. St Louis, MO: Elsevier; 2012:1542–1563.