Background

Cats present with obstructive conditions of the large intestines relatively frequently. Unfortunately, accurate diagnosis can be difficult, and successful management of the subsequent constipation/obstipation can be challenging and ongoing.

Faeces will usually transit through the normal colon in 12–24 hours. Some variability in transit time is permitted, due to the distensible nature of the colon. Prolonged faecal retention in the colon promotes continued water absorption and faecal desiccation/concretion. This can precipitate a vicious chain of events, with further slowing or cessation of faecal transit, pathologic colonic distention and altered smooth muscle tone, ultimately prohibiting normal defecation. This stage of colonic disease and dysfunction is termed "megacolon."

Megacolon occurs almost exclusively in cats, and can be caused by congenital, neurologic, mechanical obstruction and idiopathic aetiologies. Feline idiopathic megacolon is a disorder of generalized colonic smooth muscle dysfunction, resulting in severe colonic distention and subsequent faecal impaction. Secondary megacolon is the result of any lesion that affects defecation over a prolonged period of time; for example, pelvic fracture malunion, space-occupying lesions or Manx deformity of the sacral spinal column. Initially, secondary megacolon develops as an obstructive hypertrophic megacolon. With early aggressive treatment, the condition may be reversible at this stage. If left untreated, however, animals' progress to irreversible dilated megacolon.

Anatomy

The colon consists of ascending, transverse and descending parts. The colorectal junction is anatomically indistinct, but is loosely delineated by the entrance to the pelvic canal, the seventh lumbar vertebra and cranial (origin) extent of the cranial rectal artery. Similar to the intra-abdominal intestinal tract, the colon wall is composed of four layers: mucosa, submucosa, muscularis and serosa.

The colon receives blood supply from the ileocolic and caudal mesenteric arteries and veins. These vessels course parallel to the colon, liberating short vasa recta, which penetrate the intestinal wall. Parasympathetic innervation to the colon is effected via the vagus and pelvic nerves, while sympathetic innervation is via the paravertebral sympathetic trunk and the sympathetic ganglia.

Diagnosis

Cats with megacolon typically present with a history of chronic constipation, tenesmus, anorexia, vomiting and weight loss. Any animal that presents with megacolon should be thoroughly evaluated to rule out concurrent diseases such as pelvic fracture malunions, colorectal neoplasia or congenital/acquired stricture. A thorough neurological examination and routine laboratory evaluation (haematology, biochemistry, and urinalysis) should be performed. Thyroid function should be checked in younger cats, as obstipation can be seen with hypothyroidism. Plain radiographs should be taken of the abdomen and pelvis, with subsequent contrast studies and ultrasound examination if required. Most affected cats have megacolon that is determined to be either idiopathic (~ 60 per cent), or caused by pelvic canal stenosis (~ 20 per cent), nerve injury (~ 5 per cent) or Manx deformity (~ 5 per cent).

Recently, lateral abdominal radiographs have been evaluated to differentiate between normal, constipated, and cats with megacolon. The ratio of maximal colonic diameter to the length of the fifth lumbar vertebra was both reliable and accurate. It was found that a ratio greater than 1.5 (maximal colonic length 1.5 times the length of L5) was predictive of megacolon.

Abdominal ultrasound is useful at screening for concurrent intra-abdominal pathology, and should be part of every workup. Colonoscopy, on the other hand, has little utility due to the difficulty in appropriately preparing the constipated and friable colon before this procedure. Inadequate colonic preparation therefore makes visualization during colonoscopy almost impossible.

Medical Treatment

Initial medical treatment of megacolon is directed at medical stabilization (fluid resuscitation, analgesia, etc.) and relieving the colonic impaction with enemas. Depending on the degree of colonic distention, the diseased and distended colon may be more friable than normal. An overzealous enema (fluid volume, manipulation) can cause vomiting or colonic perforation. Once the faecal material has been softened it can be gently manipulated towards the rectum. Severely affected animals may require multiple enemas. Clean tap water, or water mixed 50/50 with sterile bacteriostatic lubricant is most commonly used as an enema solution. It should be ensured that soap is not added, as this can induce colitis. Hypertonic sodium phosphate enemas should also be avoided due to their potential to induce electrolyte disturbances.

Ongoing prevention of recurrent constipation can be attempted using a combination of low residue, highly digestible diets; hyperosmotic laxatives (lactulose, polyethylene glycol 3350) and/or prokinetic agents effective on colonic musculature (cisapride). Cisapride has been withdrawn from use in man because of potentially fatal cardiac arrhythmias, so supplies may now be difficult to obtain. We are still able to obtain cisapride from a compounding pharmacy (Optimus) in New Zealand. Cisapride has been replaced in human medicine by prucalopride and tegaserod; however, these medications have not been studied in depth in veterinary medicine.

Surgical Treatment

Cats with chronic constipation and colonic impaction refractory to medical management require surgical intervention. Many surgical techniques have been described over the years, with subtotal colectomy being considered the current standard of care. Because the entire colon of affected cats is histologically (and [dys]functionally) similar, removing only the grossly distended portion of colon results in recurrence of colonic dilation and impaction of the remaining colon.

The term subtotal colectomy describes the removal of the majority of the colon. There are two variations of the technique depending on whether the ileocaecocolic valve is preserved or removed. The ileocaecocolic valve normally prevents reflux of colonic contents into the small intestine; and removing the valve may predispose to bacterial overgrowth in the small intestine, steatorrhoea, deconjugation of bile salts and diarrhoea. Many surgeons advocate excision of the valve, as this minimizes the recurrence of segmental megacolon. Removing the majority of the colon will undoubtedly have certain metabolic effects on the animal, however, the significance of these findings is unknown. Further, enteric function is ultimately unchanged with subtotal colectomy compared to normal cats. I, personally, evaluate each patient individually at the time of surgery. If I can perform a tension-free colocolectomy (preserving the ileocaecocolic junction) then that is my preference. If the ileocaecocolic junction simply will not mobilize to the level of the pelvic inlet, then I will resect the ileocaecocolic junction and perform an enterocolostomy. I routinely perform a single layer of full-thickness appositional sutured anastomosis using polydioxanone in a modified Gambee pattern.

Many different suture patterns have been described for colonic surgery. Simple interrupted suture patterns are generally preferred as they result in less interference with blood flow and thus improved tissue oxygenation. Many experienced surgeons, however, prefer simple continuous. Two-layer closure is no longer recommended as it substantially decreases luminal diameter and provides no advantage over single-layer appositional repair. Crushing appositional patterns are certainly discouraged, due to the disruption to tissue oxygenation, and the profound inflammatory response that follows. An absorbable synthetic monofilament suture material should be used (polydioxanone or similar). While braided suture materials (polygalactin 910) are popular in human gastrointestinal surgery, their use is discouraged in veterinary medicine due to the theoretical risk of harbouring bacteria in the interstices of the braided suture. Chromic gut should never be used as its rate of absorption is related to enzymatic degradation (collagenase) and is unpredictable at best.

Over the past 10 years, numerous investigators have studied the safety and efficacy of stapling devices for colonic anastomosis. Stapled anastomoses typically exhibit less inflammatory reaction than sutured anastomoses, although the technique may be more cumbersome and the equipment is expensive. Subtotal colocolostomy and enterocolostomy can be performed using circular end-to-end anastomosis (CEEA) stapling instruments. Depending on the instrument used, and the surgical technique, the instrument is either inserted per rectum (trans-anal) or via a transcaecal approach. Most surgical staplers utilize titanium staples, although biofragmentable anastomotic rings are available. Regardless of the technology used, short- and long-term complication rates are similar to sutured colonic anastomoses.

Operative complications are uncommon. Where reported, complications are generally attributable to surgical technique. That is, technical errors leading to dehiscence, or localized peritonitis resulting from intra-operative contamination. The most commonly reported complication in the long term is recurrence of constipation. The majority of these individuals can be managed medically, but some cases will require revision surgery.

Intra-operative contamination can be minimized by appropriate occlusion of the colon during surgery, and avoidance of pre-operative enemas. Pre-operative enemas will convert whatever faecal material is present in the colon to a semi-liquid, that is much more difficult to control in surgery than inspissated faeces. Therefore, pre-operative enemas should not be used.

Peri-operative antibiotics with activity against anaerobes (especially gram-negative organisms) as well as aerobes must be used. Suitable antibiotic choices include: a first generation cephalosporin, such as cefazolin, combined with metronidazole; a second or third generation cephalosporin, such as cefuroxime or cefotaxime; or amoxicillin/clavulanate combined with metronidazole. Antibiotics should be administered intravenously within 30 minutes before the first skin incision, and repeated every 90 minutes for the duration of surgery.

The colon has an enormous load of potentially pathogenic organisms, and therefore the consequences of colonic leakage or dehiscence are potentially. Further, the colon is uniquely more susceptible to iatrogenic injury and dehiscence than other areas of the gastrointestinal tract. In the first day or two after surgery, collagenase is produced at the site, causing a decrease in both breaking and bursting strength. Both then start to increase two days after surgery. Excessive suture tension will also impair tissue oxygenation, and prolong the lag-phase of anastomotic healing. Therefore, choosing appropriate suture material and paying close attention to atraumatic and watertight surgical technique can avoid most complications in the immediate post-operative period.

Tenesmus and loose faeces are often seen immediately after surgery. Before closing the abdomen after subtotal colectomy, I always ensure the intra-pelvic rectum is not impacted with faeces. Intra-operative digital manipulation can usually evacuate the rectum. If this is not performed, all cats with have persistent tenesmus post-operatively, and place undue strain on their anastomosis. Most cats will begin to develop soft but formed faeces by about three months. If the ileocaecocolic junction is resected, cats may have loose stools or diarrhoea indefinitely.

Acquired Megacolon

For cats in which a definitive cause for megacolon can be found, this should be treated primarily. If the megacolon is still hypertrophic (rather than dilated), then colonic function may return after the obstructive condition is relieved. Treatment of the primary condition most commonly involves removal of colorectal masses or strictures, or widening of the pelvic canal in cases with pelvic fracture malunion. Bilateral pubic and ischial osteotomies, and pubic symphyseal spacing devices have both been recently described to facilitate exposure and widening of the pelvic canal, respectively.

Prognosis

Currently, the surgical management of megacolon consists of subtotal colectomy with the recommendation that the ileocolic junction be preserved. The procedure, in general, is associated with few life-threatening complications although the majority of individuals will experience a transient period of loose stool formation in the immediate post-operative period. In the majority of cases, the long-term outcome following subtotal colectomy is considered excellent.