Constipation and Obstipation in Small Animals
Constipation is the infrequent or difficult evacuation of feces, which are typically dry and hard. Constipation is a common clinical problem in small animals. In most instances, the problem is easily rectified; however, in more debilitated animals, accompanying clinical signs can be severe. As feces remain in the colon longer, they become drier, harder, and more difficult to pass. Obstipation is intractable constipation characterized by an inability to evacuate the mass of dry, hard feces; impaction extending from the rectum to the ileocolic valve can result. Megacolon is a pathologic condition of hypomotility and dilation of the large intestine that results in constipation and obstipation.
Peristaltic waves are responsible for the aboral movement of fecal material in the colon. Giant migrating waves that occur intermittently throughout the day move this matter farther and more rapidly. These waves constitute the “gastrocolic reflex” and are common after ingestion of a meal. A reduction or loss of these waves may contribute to constipation. Similarly, an increase in segmentation wave activity may predispose to constipation. However, diet is the most important local factor affecting colonic function.
Chronic constipation may be due to intraluminal, extraluminal, or intrinsic (ie, neuromuscular) factors. Intraluminal obstruction is most common and is due to the inability to pass poorly digested, often firm matter (eg, hair, bones, litter) mixed with fecal material. The lack of water intake or the reluctance to defecate on a regular basis because of environmental (eg, stress) or behavioral (eg, dirty litter box) factors or painful anorectal disease predisposes to formation of hard, dry feces. Intraluminal tumors may also impede the passage of feces. Extraluminal obstruction may be caused by compression of the colon or rectum from a narrowed pelvic inlet after suboptimal healing of pelvic fractures or from enlarged sublumbar lymph nodes or prostate gland. Colonic stricture due to trauma or neoplasia should also be considered. Finally, some animals (usually cats) with chronic constipation or obstipation may have megacolon, likely caused by a lesion of the neuromuscular bed of the colon. The etiology of megacolon often remains undiagnosed. Other diseases that affect neuromuscular control of the colon and rectum include hypothyroidism, dysautonomia, and lesions of the spinal cord (eg, Manx sacral spinal cord deformity) or pelvic nerves. Hypokalemia and hypercalcemia also adversely affect muscular control. Some drugs (eg, opioids, diuretics, antihistamines, anticholinergic agents, sucralfate, aluminum hydroxide, potassium bromide, and calcium channel-blocking agents) promote constipation via differing mechanisms.
The classic clinical signs of constipation are tenesmus and the passage of firm, dry feces. If the passage of feces is hindered by an enlarged prostate or sublumbar lymph nodes, the feces may appear thin or “ribbon-like” in appearance. Abdominal palpation and rectal examination can confirm the presence of large volumes of retained fecal matter. Passed feces are often putrid. Some animals are quite ill and also have lethargy, depression, anorexia, vomiting (especially cats), and abdominal discomfort.
A history of dietary indiscretion and physical evidence of retained feces confirms the diagnosis. Detailed information regarding the duration of constipation and influencing factors may help determine the cause, as will a history of ingestion of indigestible material that may increase fecal bulk or cause pain that can terminate the defecation reflex. Other historical factors that may be relevant include recent surgery, previous pelvic trauma, and possibly radiation therapy. A complete neurologic examination with special emphasis on caudal spinal cord function should be performed to identify neurologic causes of constipation, eg, spinal cord injury, pelvic nerve trauma, and Manx sacral spinal cord deformity.
Abdominal palpation and rectal examination, including evaluation of the prostate and sublumbar lymph nodes, should be performed to determine the presence of perineal hernia, foreign material, pain, or masses. Plain abdominal radiographs may help establish the inciting factor(s) of fecal retention and give some indication of what the feces contain (eg, bones). A barium enema, ultrasonography, or colonoscopy may facilitate demonstration of obstructive lesions or predisposing causes of chronic constipation.
A CBC, biochemical profile including a serum T4 level, urinalysis, and detailed neurologic examination should be completed in cases of chronic or recurring constipation.
Affected animals should be adequately hydrated. Mild constipation can often be treated by dietary adjustment consisting of avoidance of dietary indiscretion, ready access to water and high-fiber diets, and use of suppository laxatives. Continued or longterm use of laxatives should be discouraged unless absolutely necessary to avoid constipation.
A number of pediatric rectal suppositories are available for management of mild constipation. They include dioctyl sodium sulfosuccinate (DSS; emollient laxative), glycerin (lubricant laxative), and bisacodyl (stimulant laxative). The use of suppositories requires a compliant pet and a willing owner. Suppositories can be used alone or in conjunction with oral laxative therapy.
Mild to moderate or recurrent episodes of constipation may require administration of enemas or manual extraction of impacted feces, or both. Types of enemas include warm tap water (5–10 mL/kg), warm isotonic saline (5–10 mL/kg) with or without a mild soap to act as an irritant, DSS (5–10 mL/cat), mineral oil (5–10 mL/cat), or lactulose (5–10 mL/cat). Enema solutions should be administered slowly with a 10–12 French rubber catheter or feeding tube. Phosphate-containing enemas must be avoided in cats.
If enemas are unsuccessful, manual extraction of impacted feces may be needed. After adequate rehydration, the animal should be anesthetized with an endotracheal tube in place to prevent aspiration in case the colonic manipulation induces vomiting. Complete removal of all feces may require 2–3 attempts over as many days. Concurrent fluid and electrolyte abnormalities should also be corrected.
Laxatives are classified as bulk-forming, lubricant, emollient, osmotic, or stimulant types. Most act on fluid transport mechanisms and colonic motor stimulation. They should be avoided in the presence of dehydration. Bulk-forming laxatives are added to the diet. These products are dietary fiber supplements of poorly digestible polysaccharides and celluloses derived principally from cereal grains, wheat bran, and psyllium. They absorb water, soften feces, add bulk, stretch the colonic smooth muscle, and improve contractility. Many constipated cats respond to dietary supplementation with one of these products. Dietary fiber is preferable because it is well tolerated, more effective, and more physiologic than other laxatives. Commercial fiber-supplemented diets are available, or the pet owner may add psyllium (1–4 tsp/meal), wheat bran (1–2 tbsp/meal), or pumpkin (1–4 tbsp/meal) to canned food. Animals should be well hydrated before starting fiber supplementation to minimize the potential for impaction of fiber in the constipated colon.
Emollient laxatives are anionic detergents that increase the miscibility of water and lipids in digesta, thereby enhancing lipid absorption and impairing water absorption. DSS and disoctyl calcium sulfosuccinate are emollient laxatives available in oral and enema form. Docusate sodium (cats: 50-mg capsule/day; dogs: 50-mg capsule, 1–4/day) and docusate calcium (cats: 50-mg capsule, 1–2/day; dogs: 50-mg capsule, 2–3/day) are other examples of emollient laxatives.
Mineral oil and white petroleum are lubricant laxatives that impede colonic water absorption and permit greater ease of fecal passage. These effects are moderate, and lubricant laxatives are beneficial only in mild cases of constipation. Mineral oil use should be limited to rectal administration because of the risk of aspiration pneumonia with oral administration.
Hyperosmotic laxatives consist of poorly absorbed polysaccharides (eg, lactulose, 0.5 mL/kg, PO, bid-tid), magnesium salts (eg, magnesium citrate, magnesium hydroxide, magnesium sulfate), and the polyethylene glycols. Lactulose is the most effective agent of this group. The organic acids produced from lactulose fermentation stimulate colonic fluid secretion and propulsive motility. Lactulose osmotically retains water in the bowel to soften fecal material. It is also useful in management of hepatic encephalopathy because it decreases luminal pH, reduces bacterial production of ammonia, and favors formation of ammonium ions that are poorly absorbed. Stimulant laxative products (eg, bisacodyl [cats and small dogs: 5 mg; medium-sized dogs: 10 mg; large dogs: 15–20 mg]) increase the propulsive activity of the bowel. They are contraindicated in the presence of bowel obstruction.
Colonic prokinetic agents (eg, cisapride) enhance colonic propulsive motility by activating colonic smooth muscle 5-hydroxytryptamine-2A receptors in a number of species. Anecdotal experience suggest that cisapride (0.1–0.5 mg/kg, PO, bid-tid) effectively stimulates colonic propulsive motility in cats with mild to moderate idiopathic constipation. Higher dosages (up to 1 mg/kg) may be necessary in cats with moderate to severe constipation. No significant adverse effects have been reported in cats treated with cisapride at dosages of 0.1–1 mg/kg, PO, bid-tid). Cats with longstanding obstipation and megacolon are not likely to improve with cisapride therapy.
Ranitidine and nizatidine, H2-receptor antagonists, are reported to stimulate colonic motility by inhibiting acetylcholinesterase. They stimulate motility by increasing the amount of acetylcholine available to bind smooth muscle muscarinic cholinergic receptors.
To prevent recurrence, high-fiber diets are recommended, ready access to water should be maintained, and frequent opportunities to defecate allowed.
Cases of simple intraluminal obstruction due to dietary indiscretion respond well to bowel evacuation and prevention of this habit in the future. Chronic constipation unresponsive to medical management (eg, some cats with megacolon) may respond to subtotal or total colectomy. Colectomy with colocolonic, ileocolonic, or jejunocolonic anastomosis may be performed depending on the extent of the disease. Mild to moderate diarrhea may occasionally persist for weeks to months after surgery, and some cats may have recurrent constipation. Pelvic osteotomy without colectomy has been recommended for cats with pelvic fracture malunion and hypertrophic megacolon of <6 mo duration. In such cases, pathologic hypertrophy may be reversible with early pelvic osteotomy. Subtotal colectomy is recommended in cats with pelvic fractures if hypertrophy and clinical signs have persisted for >6 mo. In these cases, hypertrophy is followed by muscular degeneration and pathologic dilatation, and pelvic osteotomy alone will not provide relief from obstipation.