Megacolon is a condition of cats characterized by persistent colonic dilatation and clinical signs of chronic constipation. The majority of cases are idiopathic. However mechanical obstructions (Pelvic fractures, neoplasia, stricture and foreign bodies) and obvious functional abnormalities such as spinal cord lesions should be ruled out before the diagnosis of "idiopathic" megacolon is made.
Cats present with a history of constipation and tenesmus. Other associated complaints include anorexia, vomiting and weight loss. The colon is palpably distended with feces on physical examination. A digital rectal examination is a vital part of the examination, and will detect strictures, fractures, masses and concurrent perineal hernia. The thyroid glands should be carefully palpated in cats with weight loss. A complete blood count, chemistry screen, T4 and abdominal radiographs comprise an initial workup. Contrast radiographs or an abdominal ultrasound are indicated if an abdominal mass is suspected.
Surgery is indicated in chronic cases of constipation refractory to medical therapy. Preoperative enemas and oral antibiotic therapy to prepare the large bowel for surgery are of little value in most cases. Bacteriocidal parenteral antibiotics with anaerobic and gram negative aerobic activity should be administered perioperatively to give adequate serum levels during surgery A midline incision is made from cranial to the umbilicus to the pubis. A thorough examination of the abdominal contents is performed and the colon is exteriorized and packed off with moistened sponges.
Colectomy with colocolonic, ileocolonic or jejunocolonic anastomosis has been described for megacolon in the cat. Feces distending the colon are moved from the operative site by gentle digital pressure and the colonic lumen is occluded with a non-crushing clamps. The author prefers to ligate the vasa recti supplying the bowel rather than ligating the caudal mesenteric, cranial rectal, left, middle, and right colic vessels. The colon is transected 2-4 cm cranial to the pubis. If the ileocolic junction is preserved, the ascending colon is transected approximately 3 cm from the cecum to ensure a tension free anastomosis. The proximal end is anastomosed to the remaining descending colon. If the ileum is partially or completely removed, the ileum or jejunum may be spatulated by incising the antimesenteric boarder, or the antimesenteric portion of the colon may be oversewn to approximate the lumen sizes for an end to end anastomosis. The author prefers an end to end colocolostomy using a single layer of simple interrupted, appositional 4-0 polydioxone sutures. An end to end stapling instrument can also be used to anastomose the colon, either per rectum or inserted through the cecum.
Surgery is an effective treatment for the majority of cats with either idiopathic megacolon, or megacolon secondary healed pelvic fractures. Most cats remain on intravenous fluids for one to three days after surgery. Fluid therapy is discontinued one the cat begins eating and drinking. Loose to watery stool should be anticipated for weeks to months after surgery.
Discussion
Etiology
In megacolon, the normal colonic motility patterns are disrupted. Speculation on the cause of "idiopathic" megacolon has focused on possible neural defects in the colon. However histologically, megacolon specimens generally have normal neuronal numbers and morphology. Complete functional studies of the colonic neurones in affected cats are currently lacking. Recent studies on both longitudinal and circular smooth muscle from affected cats suggest an intrinsic smooth muscle defect. Muscle strips showed decreased contractility in response to various neuropeptides, and potassium, which depolarizes the muscle membranes independently of any cellular receptors. These decreased responses were associated with decreased myosin light chain phosphorylation, which suggests a disturbance of intracellular calcium mobilization (Washabau, 1991).
Medical therapy
Cisapride, a benzamide prokinetic agent, is now widely recommended as part of the medical management of megacolon in cats. Until recently, it was believed to act by enhancing cholinergic neurotransmission through effects on the myenteric plexus neurones. Recent experimental work (Washabau, 1994) using feline colonic smooth muscle has demonstrated that:
i. Cisapride stimulates contraction of both longitudinal and circular smooth muscle from both the proximal and distal colon;
ii. Cisapride's actions are only partially dependent on enteric cholinergic neurones;
iii. Cisapride can activate a direct smooth muscle response;
iv. Cisapride's actions on colonic smooth muscle are dependent on extracellular calcium; and
v. Other "motility enhancing" drugs such as domperidone, motolin, erythromycin, and metoclopramide have no effect on feline colonic smooth muscle in vitro.
From clinical experience it seems that some cats with chronic constipation are refractory to all medical management, including Cisapride.
Surgery: What level of resection and how?
The cause of megacolon is unknown in many cats. Colectomy is used as an empirical treatment in cases which no longer respond to medical management. In essence, the surgeon removes the majority of the colonic "reservoir", minimizing colonic water reabsorption and creating a soft stool. Resection of the colon has been performed at different levels, and various suture patterns and stapling techniques have been used to anastomose the bowel ends. In a long term retrospective study (Sweet, 1994) comparing preservation and excision of the ileocolic junction, no significant difference in the incidence of recurrent constipation was found between the two groups. Cats with excision of the ileocolic junction had significantly looser stool. Preservation of the ileocolic junction was recommended by the authors. Retaining the ileum and the ileocolic junction has significant theoretical physiologic advantages, including water and vitamin B12 absorption, and bile salt resorption for the enterohepatic biliary cycle and minimizing access of colonic bacteria to the small intestine.
Surgery: Should the Cranial Rectal artery be preserved?
In dogs, experimental ligation of the cranial rectal artery resulted in significant compromise of concurrent distal colonic anastomoses. In cats, many surgeons routinely ligate the cranial rectal artery without any adverse effects on colonic anastomosis healing. However, there is a population of cats where the cranial rectal artery terminates in the colon just cranial to the pubis, and there is no apparent communication with the middle rectal vessels. In these cats, I prefer to preserve the cranial rectal artery.
Surgery: What else should we be doing?
Recently we have seen two cats with megacolon and intermittent vomiting. No gross abnormalities of the small intestine were seen at laparotomy. Small intestinal biopsy of the first cat revealed lymphosarcoma; the second cat had granulomatous small intestinal disease which was positive for FIP on immunohistochemistry. It is not at all clear that the infiltrative bowel disease was the cause of the megacolon in either of these cases. However, many cats with megacolon have vomiting as a presenting complaint. In light of these two cases, small intestinal biopsy is probably indicated at the same time as colectomy in cats with a history of vomiting.
References
1. Washabau RJ. Feline colonic motility. Proc Am Coll Vet Int Med 1991: 637-639.
2. Washabau RJ, Zhukovskaya N. Effect of smooth muscle pro-kinetic agents on feline colonic smooth muscle function. Proc Am Coll Vet Intern Med 199; 8:149.
3. Sweet DC, Hardie EM, Stone EA. Preservation versus excision of the ileocolic junction during colectomy for megacolon: A retrospective study of 22 cats. J Small Anim Pract 1994; 35:358-363.