Congenital Megacolon (Hirschsprung's disease):
A congenital absence of mesenteric ganglion cells in a distal colonic segment results in permanent muscular spasm of the affected area of bowel. This creates a functional obstruction. The colon proximal to the obstruction becomes greatly distended. It is reported in dogs and cats, but is a rare condition, and is not the main cause of megacolon.
Clinical signs can develop at any age, but usually during first few months of life. The severity of signs depends on the degree of obstruction. There is a history of recurrent episodes of constipation for many months. The diagnosis is suspected in young animals with a severe distension of the colon on abdominal palpation and radiographs. The rectum distal to the affected segment usually empty. A barium enema after removal of fecal mass can demonstrate the narrowed segment of colon.
Treatment with laxatives is attempted but is usually unsuccessful. Surgical excision of the narrowed segment of colon with preservation of the anal sphincter is the treatment of choice. A partial colectomy with direct anastomosis is possible if the lesion is not close to the pubis. At the terminal colon a pull-through procedure is necessary.
Obstipation: See notes from Feline Megacolon lecture
Perforation of the Colon
This can be caused by sharp or blunt abdominal trauma such as gunshot wounds, knife wounds, rarely bone fragments, and rarely foreign bodies such as needles. Non-traumatic perforation is reported in dogs after the parental administration of steroids. Seven cases have been reported in dogs given Dexamethasone and all died within 10 days. Perforation leads to rapidly progressive peritonitis and septic shock. A diagnosis of peritonitis can be made by clinical signs, examination of abdominal fluid and radiography, and laparotomy is indicated immediately.
Treatment: The animal is treated for peritonitis and the cause is removed by surgery. The abdomen is irrigated and antibiotics and fluids are given.
Surgical Conditions of the Rectum
Rectal Prolapse:
This occurs in young, unthrifty parasitized animals, in association with tumors, and following perineal hernia repairs. Clinical signs are obvious, however it is necessary to distinguish the prolapse from a prolapse of anus and colon and intussusception of ileum.
The prolapse must be amputated if it is necrotic. In other cases the prolapsed rectum is reduced. The edema is removed by gentle pressure and massage using soft cloth moistened with warm saline. Following reduction a loose purse-string suture is inserted in the anus. This is removed in 3-5 days. A sedative to prevent straining is necessary. No food, then a low residue diet of hamburger and white rice is fed for several days.
Colopexy can be a very useful procedure to prevent recurrence of rectal prolapse. The descending colon is sutured to the abdominal wall by a double row of sutures.
Tumors of the Cecum, Colon and Rectum
Polyps and Carcinomas-in-Situ
Inflammatory polyps are basically composed of branching lamina propria supporting abnormal epithelium which is contiguous with normal rectal mucosa. In carcinoma in situ, malignant cells are present in the mucosa, however, there is no invasion through the basal lamina to either the lamina propria or the submucosa. This is important because both the lamina propria and the submucosa contains lymphatics and blood vessels, which are potential avenues for metastasis. Standard pathological terminology is that malignancy is defined as an invasion of the basement membrane by tumor cells with extension into the lamina propria.
Carcinomatous changes in polyps (ie benign tumors) are rare in animals, but have been reported. Seiler described 17 cases of colorectal polyps in dogs. The histopathologic classifications were hyperplastic polyp, papillary adenomas, tubular adenoma, papillotubular adenoma, and an unclassified type. It was interesting that severe epithelial atypia, likely carcinoma in situ, was apparent in 5 of the papillotubular adenomas, and these were regarded as being more likely to recur or to become malignant.
The question has been raised regarding the relationship between size of polyps and potential for malignancy. In humans, polyps greater than 1 cm in diameter have a higher potential for malignancy. Five dogs in Seiler's series had polyps greater than 1 cm in diameter and multiple polyps or marked epithelial atypia, or both, developed in these dogs.
Polyps in the rectum are associated with clinical signs that are typical but not diagnostic. Most affected dogs have mucus and blood in the feces, and prolonged diarrhea is common. Tenesmus is often present and this can lead to prolapse of the polyp or rectum. Some dogs show few signs and the polyp is not detected until it prolapses during defecation. In my experience, the small pedunculated masses are likely to be polyps and the soft sessile ones are likely to be carcinomas-in-situ.
Most polyps of the rectum and all polyps of the anal canal can be palpated via the anus and rectum. Bleeding and fragmentation of the polyp occur commonly. The pedunculated or sessile mass can be seen early by endoscopy, however, emptying and cleaning of the bowel are usually needed. Colonoscopy or radiography is important in these cases as approximately 30% of cases have multiple masses. A biopsy and histological examination are required for a specific diagnosis. A wedge shaped portion of the polyp can be obtained with the aid of a suitable retractor or endoscope. Small pedunculated polyps can be removed and biopsied after removal.
Removal of Polyps in Dogs
Removal via the anus using electrosurgery is recommended for a pedunculated polyp. They must be within the reach of suitable instruments or occur in the terminal rectum or anus and can be prolapsed out the anus. All anal and rectal polyps, and many polyps in the descending colon can be reached via the anus. The polyps are removed by grasping them and dividing the base with an electrode or tonsil snare acting as an electrode. Overzealous coagulation must be avoided. This procedure requires evacuation of feces from the colon and rectum as the only pre-surgical preparation.
Removal of large, sessile polyps by electrocoagulation increases the danger of perforation of the bowel wall, or inadequate removal of the polyp with recurrence. In most cases, resection of the affected segment of colon or rectum is recommended. Antiseptic and antibiotic bowel preparation is needed. The surgical technique is similar to that described for removal of rectal carcinomas.
Prognosis
The prognosis following removal of pedunculated polyps or small sessile ones is excellent.
Probably, the prognosis is excellent if the polyp can be removed completely. This is easy in pedunculated polyps and difficult in large sessile ones unless a resection of bowel is done. For a definite carcinoma-in-situ, prognosis is very poor unless a 360o resection is done.
Carcinomas
Neoplasms of the intestinal tract in animals are uncommon, however, carcinomas appear to be the most common neoplasms of the canine gastrointestinal tract. They compose about 0.5% of all malignant tumors in dogs.
In a series of 14 rectal carcinomas, the average age of the dog was 9 years and the most common type was adenocarcinoma (eight cases). The acinar structures were filled with mucous and the tumors infiltrated the muscularis mucosa. The second most common type (four cases) was the scirrhous carcinoma with diffuse infiltration and marked fibrosis. Metastases were seen in 9 of the 14 cases, by lymphatic or venous channels, and by implantation on serous surfaces (2 cases). Metastases were found in local lymph nodes, lungs, liver and spleen.
Clinical Signs
Signs can resemble those seen with polyps, however, signs are generally more severe, specially pain. There is straining to defecate, passage of blood and mucous with feces, and painful defecation with passage of ribbon like feces. Proctoscopic exam may reveal irregular luminal narrowing and ulcerative foci in a bed of firm grey tissue. Some tumors are annular. The mass can be palpated rectally and seen using contrast radiography.
Treatment
In removal of rectal masses without prior biopsy, a decision may have to be made after local excision to do a more radical excision if the specimen shows malignancy. Excisional biopsy is commonly recommended if a polyp is suspected, however, pre-surgical diagnosis by biopsy is highly recommended if a carcinoma is suspected. The surgeon is able to plan a wider excision and, theoretically, can be more certain of total removal.
Unfortunately, prognosis for long term survival in rectal carcinomas is unfavorable.
Anatomical Aspects of Rectal Removal
Fecal continence depends on the presence of afferent sensory fibers in the rectum, intact nerve tracts to and from the CNS, an intact external anal sphincter and a functional reservoir. As fecal material passes into the rectum, sensory fibers relay the information to the CNS. If the animal cannot defecate, the external sphincter contract (sphincter continence) and the colon relaxes to accommodate the fecal mass (reservoir continence). When considering removal of part of the rectum, possible damage to these vital structures should be considered, and the owner informed of the possibility of incontinence. In dogs 4cm of the rectum can be removed experimentally before fecal incontinence ensues.
A rational plan for management of rectal polyps and carcinomas in the dog must take into account the site and size of the mass and the degree of local invasiveness, as determined crudely by palpation, and perhaps ultrasound of the local lymph nodes. Few objective facts are available in the dog. In humans, lymphatic spread does not generally occur until the muscularis mucosa is penetrated, and the primary route of spread is upward (cranial).
Large tumors with evident local invasion probably cannot be removed successfully by any current technique. After biopsy and confirmation of the diagnosis, euthanasia is indicated.
For malignant tumors in the lower one-third of the rectum that are still mobile and that show no evidence of metastasis, total local resection can be used. These tumors cannot be removed without damage to the anal sphincter, and therefore, the anus and sphincter should be removed. After patient preparation, a circular incision is made around the anus and anal sphincter and the rectum is gradually mobilized and pulled caudally until the rectum can be transected cranial to the tumor. The rectum is sutured to the skin opening by two rows of sutures.
For tumors of the middle third of the rectum which cannot be approached via the abdomen a perineal approach is needed. By means of an inverted U-shaped incision between the anus and the tail, the entire rectum can be mobilized and exteriorized. The affected section is removed and the ends anastomosed. One row of simple interrupted sutures is placed, using 3/0 or 4/0 PDS. The sutures do not penetrate the mucosa. The wound is lavaged thoroughly with saline, a penrose drain is inserted, and the subcutaneous tissues and skin are closed. The drain is placed close to, but not touching, the incision line and is in the shape of an inverted "U". It is removed in three days.
Tumors in the terminal colon, or at the colorectal junction, can usually be resected by an abdominal approach with removal of the pubis, and reattachment of the pubis at the completion of the procedure. A one layer closure for the anastomosis is done, using simple interrupted absorbable sutures of 3/0 or 4/0 PDS in an appositional pattern without inversion or eversion and avoiding the mucosa as much as possible.