Constipation and Obstipation - The Merck Veterinary Manual

Constipation and Obstipation

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, in which the animal is unable to successfully defecate.

Etiology and Pathophysiology:

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 digestible, 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 due to environmental (stress) or behavioral (dirty litter box) situations or to painful anorectal disease predisposes to the 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 by a narrowed pelvic inlet following inappropriate healing of pelvic fractures or by compression of the colon or rectum by 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 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.

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 this wave activity 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.

Clinical Findings:

The classic clinical signs 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, and abdominal discomfort.

Diagnosis:

A history of dietary indiscretion and physical evidence of retained feces confirms the diagnosis. Abdominal palpation and rectal examination, including evaluation of the prostate and sublumbar lymph nodes, should be performed. 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 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.

Treatment and Control:

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 the use of suppository laxatives. Continued or longterm use of laxatives should be discouraged unless absolutely necessary to deter constipation.

In more severe cases, retained feces must be evacuated using enemas or manual extraction while under general anesthesia. 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. High-fiber bulk-forming laxatives are added to the diet. These products absorb water, soften feces, add bulk, stretch the colonic smooth muscle, and improve contractility. Supplementation of the diet with fiber (eg, 1-6 tsp per feeding of psyllium hydrophilic mucilloid, or 1-4 tbsp of coarse wheat bran) is adequate. For longterm control of constipation, commercial high-fiber diets should be fed. Mineral oil (5-25 mL, PO, bid) and petrolatum products are lubricants and are given to affected animals between meals. Mineral oil should be flavored to avoid accidental inhalation of this otherwise tasteless product. Docusate sodium (cats: 50-mg capsule, sid; 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 emollient laxatives. These mild laxatives soften feces by promoting water absorption. Osmotic laxatives (eg, lactulose, 0.5 mL/kg, PO, bid-tid) osmotically retain water in the bowel to soften fecal material. Lactulose, a nonabsorbable disaccharide, is also useful in management of hepatic encephalopathy because it decreases luminal pH, reduces the bacterial production of ammonia, and favors the 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.

Enema solutions are frequently used to moisten and soften feces making them easier to pass. Warm isotonic saline or tap water (5-10 mL/kg) with or without a mild soap (without hexachlorophene) to act as an irritant is practical and effective. Docusate sodium (cats and small dogs: 5-10 mL; medium-sized dogs: 10-20 mL; large dogs: 20-30 mL) is another option. Sodium phosphate enemas are sometimes used to relieve constipation in dogs but should not be used if dehydration, cardiac disease, nausea, or vomiting are present. They are also contraindicated in small dogs and cats and in animals with renal dysfunction. Clinical signs of toxicity are seen within 1 hr of use and include depression, ataxia, tetany, seizures, vomiting, hemorrhagic diarrhea, tachycardia, pallor, and stupor. Associated biochemical abnormalities may include hyperphosphatemia, hypernatremia, hypocalcemia, hyperglycemia, hyperosmolality, and metabolic acidosis with a high anion gap (increased lactic acid). Death has been reported with the use of these agents in cats. Mineral oil (5-20 mL) can be directly instilled into the rectum to help facilitate passage of hard feces.

To prevent recurrence, animals are encouraged to eat high-fiber diets, ready access to water should be maintained, and frequent opportunities to defecate allowed.

Chronic constipation that has been unresponsive to medical management may respond to subtotal or total colectomy. Cases that present with simple intraluminal obstruction due to dietary indiscretion respond well to bowel evacuation and prevention of this habit in the future. Cats with megacolon that do not respond to medical management alone respond well to subtotal colectomy.