History
Constipation, obstipation, and megacolon may be observed in cats of any age, sex, or breed, however, most cases are observed in middle aged (mean = 5.8 years), male cats (70% male, 30% female) of Domestic Shorthair (46%), Domestic Longhair (15%), or Siamese (12%) breeding. Affected cats are usually presented for reduced, absent, or painful defecation for a period ranging from days to weeks or months. Some cats are observed making multiple, unproductive attempts to defecate in the litter box, while other cats may sit in the litter box for prolonged periods without assuming a defecation posture. Dry, hardened feces are observed inside and outside of the litter box. Occasionally, chronically constipated cats have intermittent episodes of hematochezia or diarrhea due to the mucosal irritant effect of fecal concretions. This may give the pet owner the erroneous impression that diarrhea is the primary problem. Prolonged inability to defecate may result in other systemic signs, including anorexia, lethargy, weight loss, and vomiting.
Physical Examination
Colonic impaction is a consistent physical examination finding in affected cats. Other findings will depend upon the severity and pathogenesis of constipation. Dehydration, weight loss, debilitation, abdominal pain, and mild to moderate mesenteric lymphadenopathy may be observed in cats with severe idiopathic megacolon. Colonic impaction may be so severe in such cases as to render it difficult to differentiate impaction from colonic, mesenteric, or other abdominal neoplasia. Cats with constipation due to dysautonomia may have other signs of autonomic nervous system failure, such as urinary and fecal incontinence, regurgitation due to megaesophagus, mydriasis, decreased lacrimation, prolapse of the nictitating membrane, and bradycardia. Digital rectal examination should be carefully performed with sedation or anesthesia in all cats. Pelvic fracture malunion may be detected on rectal examination in cats with pelvic trauma. Rectal examination might also identify other unusual causes of constipation, such as foreign bodies, rectal diverticula, stricture, inflammation, or neoplasia. Chronic tenesmus may be associated with perineal herniation in some cases. A complete neurologic examination with special emphasis on caudal spinal cord function should be performed to identify neurologic causes of constipation, e.g. spinal cord injury, pelvic nerve trauma, and Manx sacral spinal cord deformity.
Differential Diagnoses
Several authors have emphasized the importance of considering an extensive list of differential diagnoses (e.g., neuromuscular, mechanical, inflammatory, metabolic/endocrine, pharmacologic, environmental, and behavioral causes) for the obstipated cat. A recent review, however, suggests that 96% of cases of obstipation are accounted for by idiopathic megacolon (62%), pelvic canal stenosis (23%), nerve injury (6%), or Manx sacral spinal cord deformity (5%). A smaller number of cases are accounted for by complications of colopexy (1%), and colonic neoplasia (1%); colonic hypo- or aganglionosis was suspected, but not proved, in another 2% of cases. Inflammatory, pharmacologic, and environmental/behavioral causes were not cited as predisposing factors in any of the original case reports. Endocrine factors (obesity, n=5; hypothyroidism, n=1) were cited in several cases, but were not necessarily impugned as part of the pathogenesis of megacolon. Thus, while it is important to consider an extensive list of differential diagnoses in an individual animal, it should be kept in mind that most cases are idiopathic, orthopedic, or neurologic in origin.
Pathogenesis
The pathogenesis of idiopathic megacolon has been variably attributed to a primary neurogenic or degenerative neuromuscular disorder. While it seems clear that a small number of cases (11%) result from neurologic disease, the vast majority (> 60%) of cases have no evidence of neurologic disease. These idiopathic cases may instead involve disturbances of colonic smooth muscle. Recent studies suggest that colonic smooth muscle function is impaired in cats affected with idiopathic megacolon.
Laboratory Data
While most cases of obstipation and megacolon are unlikely to have significant changes in laboratory data (e.g., complete blood count, serum chemistry, urinalysis), these tests should nonetheless be performed in all cats presented for constipation. Metabolic causes of constipation, such as dehydration, hypokalemia, and hypercalcemia may be detected in some cases. Basal serum T4 concentration should also be considered in cats with recurrent constipation and other signs consistent with hypothyroidism.
Imaging Studies
Abdominal radiography should be performed in all constipated cats to characterize the severity of colonic impaction, and to identify predisposing factors such as intraluminal radio-opaque foreign material (e.g., bone chips), intraluminal or extraluminal mass lesions, pelvic fractures, and spinal cord abnormalities. The radiographic findings of colonic impaction cannot be used to distinguish between constipation, obstipation and megacolon in idiopathic cases. First or second episodes of constipation in some cats may be severe and generalized, but may still resolve with appropriate treatment.
Ancillary Studies
Ancillary studies may be indicated in some cases. Extraluminal mass lesions may be further evaluated by abdominal ultrasonography and guided biopsy, whereas intraluminal mass lesions are best evaluated by endoscopy. Colonoscopy may also be used to evaluate the colon and anorectum for suspected inflammatory lesions, strictures, sacculations, and diverticula.
Therapeutic Plan
The specific therapeutic plan will depend upon the severity of constipation and the underlying cause. Medical therapy may not be necessary with first episodes of constipation. First episodes are often transient and resolve without therapy. Mild to moderate or recurrent episodes of constipation, on the other hand, usually require some medical intervention. These cases may be managed, often on an outpatient basis, with dietary modification, water enemas, oral or suppository laxatives, and/or colonic prokinetic agents. Severe cases of constipation usually require brief periods of hospitalization to correct metabolic abnormalities and to evacuate impacted feces using water enemas, manual extraction of retained feces, or both. Follow-up therapy in such cases is directed at correcting predisposing factors and preventing recurrence. Subtotal colectomy will become necessary in cats suffering from obstipation or idiopathic dilated megacolon. These cats, by definition, are unresponsive to medical therapy. Pelvic osteotomy without colectomy may be sufficient for some cats suffering from pelvic canal stenosis and hypertrophic megacolon.
A number of pediatric rectal suppositories are available for the management of mild constipation. These include dioctyl sodium sulfosuccinate (emollient laxative), glycerin (lubricant laxative), and bisacodyl (stimulant laxative). The use of rectal suppositories requires a compliant pet and pet owner. Suppositories can be used alone or in conjunction with oral laxative therapy.
Recurring episodes of constipation may require administration of enemas. Several types of enema solutions may be administered, such as warm tap water (5–10 mL/kg), warm isotonic saline (5–10 mL/kg), dioctyl sodium sulfosuccinate (5–10 mL/cat), mineral oil (5–10 mL/cat), or lactulose (5–10 mL/cat). Enema solutions should be administered with a well-lubricated 10–12 Fr. rubber catheter or feeding tube.
Cases unresponsive to enemas may require manual extraction of impacted feces. Cats should be adequately rehydrated and then anesthetized with an endotracheal tube in place to prevent aspiration should colonic manipulation induce vomiting. Water or saline is infused into the colon while the fecal mass is manually reduced by abdominal palpation. Sponge forceps may also be introduced rectally (with caution) to break down the fecal mass. It may be advisable to evacuate the fecal mass over a period of several days to reduce the risks of prolonged anesthesia and perforation of a devitalized colon. If this approach fails, colotomy may be necessary to remove the fecal mass. Laxative and/or prokinetic therapy may then be instituted thereafter.
Most of the available bulk-forming laxatives are dietary fiber supplements of poorly digestible polysaccharides and celluloses derived principally from cereal grains, wheat bran, and psyllium. Many constipated cats will respond to supplementation of the diet with one of these products. Dietary fiber is preferable because it is well tolerated, more effective, and more physiologic than other laxatives. Fiber supplemented diets are available commercially, or the pet owner may wish to add psyllium (1–4 tsp per meal), wheat bran (1–2 tblsp per meal), or pumpkin (1–4 tblsp per meal) to canned cat food. Cats should be well hydrated before commencing fiber supplementation to minimize the impaction of fiber in the constipated colon.
Emollient laxatives are anionic detergents that increase the miscibility of water and lipid in digesta, thereby enhancing lipid absorption and impairing water absorption. Dioctyl sodium sulfosuccinate and dioctyl calcium sulfosuccinate are examples of emollient laxatives available in oral and enema form. As with bulk-forming laxatives, animals should be well hydrated before emollient laxatives are administered. It should be noted that clinical efficacy has not been definitively established for the emollient laxatives. Dioctyl sodium sulfosuccinate, for example, inhibits water absorption in isolated colonic segments in vitro, but it may be impossible to achieve tissue concentrations great enough to inhibit colonic water absorption in vivo.
Mineral oil and white petrolatum are the two major lubricant laxatives available for the treatment of constipation. The lubricating properties of these agents impede colonic water absorption, as well as permit greater ease of fecal passage. These effects are usually moderate, however, and, in general, lubricants are beneficial only in mild cases of constipation. Mineral oil usage should probably be limited to rectal administration because of the risk of aspiration pneumonia in depressed or debilitated cats.
The next group of laxatives consists of the poorly absorbed polysaccharides (e.g., lactose, lactulose), the magnesium salts (e.g., magnesium citrate, magnesium hydroxide, magnesium sulfate), and the polyethylene glycols. Lactulose is the most effective agent in this group. The organic acids produced from lactulose fermentation stimulate colonic fluid secretion and propulsive motility. Lactulose, administered at a dosage of 0.5 mL/kg body weight q8–12h, fairly consistently produces soft feces in the cat. Many cats with recurrent or chronic constipation have been well managed with this regimen of lactulose. The dosage may have to be tapered in individual cases if flatulence and diarrhea become excessive. Magnesium salts and polyethylene glycols are not currently recommended in the treatment of feline constipation and idiopathic megacolon.
The stimulant laxatives are a diverse group of agents that have been classified according to their ability to stimulate propulsive motility. Bisacodyl, at a dosage of 5 mg q24h PO, is the most effective stimulant laxative in the cat. It may be given individually or in combination with fiber supplementation for long-term management of constipation. Daily administration of bisacodyl should probably be avoided, however, because of injury to myenteric neurons with chronic usage.
Cisapride enhances colonic propulsive motility through activation of colonic smooth muscle 5-HT2a receptors in a number of animal species. In vitro studies have shown that cisapride stimulates feline colonic smooth muscle contraction. Unfortunately, Janssen Pharmaceutica has withdrawn cisapride from the American and European markets. Our laboratory has shown that ranitidine and nizatidine, classic histamine H-2 receptor antagonists, stimulate feline colonic smooth muscle contraction in vitro. These drugs mediate contraction apparently through inhibition of tissue acetylcholinesterase. It’s not yet clear how effective these drugs are in vivo. Two new drugs, e.g., prucalopride and tegaserod, may be available by December 2001. Both of these drugs have effects on feline colonic motility, and may be useful in the management of feline constipation.
Colectomy should be considered in cats that are refractory to medical therapy. Following midline abdominal incision, the colon is exteriorized, fecal contents are moved from the operative site by gentle digital pressure, and the colonic lumen is occluded with a non-crushing clamp. Colectomy with colocolonic, ileocolonic, or jejunocolonic anastomosis may be performed depending upon the extent of disease. With routine colectomy, the ascending colon is transected 2–4 cm distal to the cecum, and the descending colon is transected 2–4 cm proximal to the pubis. The caudal mesenteric artery and vein should be left intact to maximize blood supply to the colon distal to the anastomosis. The colonic segments are anastomosed using a single layer of simple interrupted, appositional 4-0 polydioxone sutures. The ileocolic junction should be preserved wherever possible. Cats have a generally favorable prognosis for recovery following colectomy, although mild to moderate diarrhea may persist for weeks to months postoperatively in some cases. Pelvic osteotomy without colectomy has been recommended for cats with pelvic fracture malunion and hypertrophic megacolon of less than six months duration. Pathologic hypertrophy may be reversible with early pelvic osteotomy in such cases. Some surgeons still prefer colectomy in this instance because of the technical difficulty of some pelvic osteotomies.
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