Constipation is defined as the infrequent or difficult evacuation of stool. It is a common problem in cats and may be acute or chronic but does not inherently imply a loss of colonic function. Often the underlying cause is dehydration and is easily managed by supportive hydration, via oral, nutritional or parenteral means. Obstipation refers to intractable constipation that is unresponsive to therapy or cure and implies permanent loss of function. When obstipation results in dilatation of the colon or hypertrophy of the colon, then the condition is described as megacolon.

Dilated megacolon is the end-stage condition of idiopathic colonic dysfunction. The resulting disease has diffuse colonic dilatation and hypomotility. Hypertrophic megacolon is a result of pelvic fracture malunion and stenosis of the pelvic canal or another obstructive mechanism including neoplasm, polyp, or foreign body. Colonic impaction is the accumulation of hardened feces in the pelvic colon and is the consequence of constipation, obstipation or megacolon. It does not, in itself, imply loss of function or reversibility of the problem. This distinction is critical in considering treatment plans as well as prognosis.

Etiology

Unlike humans with megacolon, in cats there is no evidence to show any deficiency in enteric neuronal network. Histologic studies fail to show significant abnormalities in density or morphology of ganglia. Studies by Washabau, et al. have shown that in naturally occurring cases of feline megacolon, sections of longitudinal and circular smooth muscle of all parts of the colon show abnormalities of function when submitted, in vitro, to isometric stress measurements. These tests further showed that the smooth muscle was less responsive to neurotransmitters (acetylcholine, substance P and cholecystokinin), membrane depolarization using potassium chloride and electrical field stimulation when compared to colonic segments from healthy controls. The conclusion drawn by these workers was that the disorder involves abnormal intracellular activation of smooth muscle myofilaments.

Signalment

Constipation, obstipation and megacolon may be seen in cats of any age, breed and gender, however middle aged (mean 5.8 years), male (70%) domestic short-haired (46%) cats appear to be more at risk.

History

Cats are presented because of a client's concern about reduced, absent or painful, elimination of hard stool in or outside of the litterbox. There may be mucous or blood passed associated with irritative effects of impacted stool, and even, intermittently, diarrhea. Vomition is frequently associated with straining. Inappetance, weight loss, lethargy and dehydration become features of this condition if unresolved. Dilated megacolon is preceded by repeated episodes of recurrent constipation and obstipation. In the cat with hypertrophic megacolon, there may be a known history of trauma resulting in pelvic fracture.

Physical Examination

Impaction and enlargement of the colon is the underlying finding in all cases of megacolon. Cats with dysautonomia will have signs referable to other autonomic defects, such as urinary incontinence, regurgitation, mydriasis, prolapse of the nictating membrane and bradycardia. Digital rectal examination under sedation or anaesthesia should be performed in all cats to rule-out pelvic fracture, malunion, rectal diverticulum, perineal hernia, anorectal stricture, foreign body, neoplasia or polyps. A neurological examination should be performed to detect any neurological causes of constipation, including pelvic nerve trauma, spinal cord injury, or sacral spinal cord deformities of Manx cats.

Diagnostics

Serum biochemistries and a complete blood count characteristically are normal, however, these should be performed in order to detect those cats with electrolyte abnormalities (hypokalemia, hypercalcemia, dehydration). A baseline serum T4 should be checked in obstipated kittens suspected of being hypothyroid.

Abdominal radiography should be performed to characterize the mass and verify that it is, indeed, colonic impaction. Radiographs will also help to identify predisposing factors such as pelvic fracture, extra-luminal mass, foreign body, and spinal cord abnormalities.

Therapeutics

There are five components to medically managing the megacolon patient.

1.  Achieve and maintain optimal hydration

2.  Remove impacted feces

3.  Dietary fiber

4.  Laxative therapy

5.  Colonic prokinetic agents

1) As long as cellular dehydration is present, the need will exist to resorb water from renal and gastrointestinal systems. Systemic rehydration must be addressed and may be achieved through parenteral fluid therapy, including regular subcutaneous fluids in the home, feeding canned foods, adding water or broth to the food, feeding meat broths, or the use of running water fountains in the home.

2) Removal of impacted feces is required to reduce the toxic and inflammatory stress on the bowel wall. Pediatric rectal suppositories may be used to help with mild constipation. They include dioctyl sodium sulfosuccinate (DSS, Colace^TM), glycerin or bisacodyl (Dulcolax^TM).

Enemas are another way to soften hardened stool. Solutions that may be used include warm tap water, DSS (5-10 ml/cat), mineral oil (5-10 ml/cat) or lactulose (510 ml/cat). Enemas should be administered slowly through a well- lubricated 10-12 French rubber catheter. Mineral oil and DSS should not be given together as the DSS promotes mucosal absorption of the mineral oil. Sodium phosphate containing enemas (e.g., Fleet^TM) are contraindicated because they predispose to lifethreatening electrolyte imbalances (hypernatremia, hyperphosphatemia and hypocalcemia) in cats. Hexachlorophene containing soaps should be avoided in enemas because of potential neurotoxicity. Enemas given too rapidly may cause vomiting, pose a risk for colonic perforation and may be passed too rapidly for the fecal mass to be softened by them.

Manual extraction may be required in recalcitrant cases. Caution must be used to reduce the risk of perforation. Anytime a cat is anaesthetized for manipulations of the colon, an endotracheal tube should be in place, in case the cat vomits.

3) Dietary fiber acts as a bulk-forming laxative. The benefits of insoluble (poorly fermentable) fiber, such as from wheat bran, cereal grains and psyllium, are to improve or normalize colonic motility by distending the colonic lumen, they increase colonic water content, they dilute luminal toxins (such as bile acids, ammonia and ingested toxins) and they increase the rate of passage of ingested materials thereby reducing the exposure of the colonocyte to toxins, while increasing the frequency of defecation. Suggested doses are: psyllium (Metamucil^TM, 1-4 tsp mixed with food q12-24h), canned pumpkin (1-4 tbsp PO q24h), coarse wheat bran (1-2 tbsp with food PO q24h).

Soluble (highly fermentable) fibers (oat bran, pectin, beet pulp, vegetable gums) are readily digested by bacteria and provide large quantities of short chain fatty acids, which are beneficial for colonic health, but they are not suitable as laxatives.

4) Besides bulk forming, laxatives may be categorized as emollient, lubricant, hyperosmotic and stimulant, based on their method of action. Emollient laxatives are anionic detergents that increase the miscibility of water and lipid in ingesta, enhancing lipid absorption and impairing water absorption. DSS (Colace^TM, 50 mg PO q24h) and dioctyl calcium sulfosuccinate (Surfax^TM, 50 mg PO q12-24h) are examples of emollient laxatives that have been used in cats.

Lubricant laxatives impede water absorption as well as enabling easier passage of stool. Mineral oil (10-25 ml PO q24h) or petrolatum (hairball remedies, 1-5 ml PO q24h) are best suited to mild cases of constipation.

Hyperosmotic laxatives stimulate colonic fluid secretion and propulsive motility. While there are three types (poorly absorbed polysaccharides [lactulose, lactose], magnesium salts [magnesium citrate, magnesium sulfate, magnesium hydroxide] and polyethylene glycols [GoLYTELY^TM, Colyte^TM]), lactulose (0.5 mg/kg PO q8-12h, prn) is the safest and most consistently effective agent in this group.

The stimulant laxatives enhance propulsive motility by a variety of actions. One example, which has been used in cats, is bisacodyl (Dulcolax^TM, 5 mg PO q24h), which acts by stimulating nitric oxide-mediated epithelial call secretion and myenteric neuronal depolarization. Long-term use may result in myenteric neuron damage.

5) Colonic prokinetic agents are a relatively new class of drug, which have the ability to stimulate motility from the esophagus aborally. Cisapride (Propulsid^TM, Prepulsid^TM) is one of the new group of benzamide prokinetic drugs and has been shown, anecdotally, to be beneficial in cases of mild to moderate constipation. Cats with longstanding obstipation or megacolon are not likely to be helped much by cisapride. Published dose recommendations are 2.5 mg PO q8-12h; anecdotally, doses of 5-7.5 mg/cat PO q12h are used without noted side effects.

Cisapride has been withdrawn from the pharmaceutics market because of cardiac toxicity in a small, select group of human patients. Veterinarians may request cisapride from compounding pharmacists. Washabau et al, have shown that nizatidine and ranitidine, stimulate colonic smooth muscle, in vitro. They appear to work by inhibition of acetylcholinesterase. Suggested doses to be given in conjunction with cisapride are ranitidine (12 mg/kg PO q12h) or nizatidine (2.5-5.0 mg/kg PO q12h). Other H2 receptor antagonists, cimetidine and famotidine are not effective. Two new drugs, prucalopride and tegaserod are new prokinetic agents that have been shown to be effective in feline colonic motility. Their release into the market is imminent.

Surgery

Cats with chronic obstipation or megacolon should be considered as candidates for colectomy. Chronic fecal impaction results in mucosal ulceration and inflammation and risk of perforation. Surgery should be done before bowel wall and patient health are compromised and debilitated. At the time of resection, small intestinal biopsies are advised, as concurrent, underlying disease (e.g., lymphosarcoma, feline infectious peritonitis) may be identified. Post-operatively, diarrhea will be present for 4-6 weeks. As anal tone isn't compromised, this does not result in house soiling. The prognosis is very good for recovery.

If the megacolon is of the hypertrophic type, then pelvic osteotomy is required in addition to the colectomy. Should the megacolon be less than 6 months duration subsequent to the pelvic fracture, then the pelvic osteotomy may be all that is required.