Constipation is common in small animals, particularly in cats. It is characterized by painful or absent defecation and an impacted colon on physical examination and radiographs. Adding to or changing the diet is usually successful. However, in more severe cases, such as obstipated patients, enemas are necessary. When the colon is irreversibly distended and hypomotile (megacolon), surgery may be required.
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 pathological condition of hypomotility and dilation of the large intestine that results in constipation and obstipation.
Etiology and Pathophysiology of Constipation, Obstipation, and Megacolon in Small Animals
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 animals to constipation. However, diet is the most important local factor affecting colonic function.
Older, overweight cats (with poor colonic muscle contraction), cats with chronic kidney disease (that are chronically dehydrated), and those with previous episodes of constipation are at increased risk of constipation.
Chronic constipation may be due to intraluminal, extraluminal, or intrinsic (ie, neuromuscular) factors.
Intraluminal obstruction is most common. Intraluminal obstruction develops due to the inability to pass poorly digested, often firm matter (eg, hair, bones, litter) mixed with fecal material and is secondary to a lack of sufficient water intake (such as in cats with chronic kidney disease) or reluctance to defecate (due to stress, a dirty litter box, pain, or a tumor). A colonic stricture could also result in intraluminal obstruction.
Extraluminal obstruction may be caused by compression of the colon or rectum from a narrowed pelvic inlet (caused by poorly healed pelvic fractures, enlarged sublumbar lymph nodes or prostate gland, or colonic stricture).
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, lesions of the spinal cord (eg, Manx sacral spinal cord deformity) or pelvic nerves, hypokalemia, and hypercalcemia. Some drugs (eg, opioids, diuretics, antihistamines, anticholinergic agents, sucralfate, aluminum hydroxide, potassium bromide, and calcium channel blocking agents) promote constipation via differing mechanisms.
Clinical Findings of Constipation, Obstipation, and Megacolon in Small Animals
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, mass, 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.
Diagnosis of Constipation, Obstipation, and Megacolon in Small Animals
History
Physical examination
Rectal examination
Diagnostic imaging
Constipation is diagnosed based on a history of inability to defecate and the presence of retained feces in the colon palpable on physical examination or visible on radiographs (especially in obese animals).
A thorough history is essential to rule out iatrogenic causes (eg, medication, radiation) and previous trauma (eg, pelvic fracture).
Rectal examination, neurological examination, blood work, and diagnostic imaging (ie, radiography, ultrasonography, colonoscopy) help rule out specific causes of constipation. Some diseases, such as megacolon in cats, are idiopathic.
A history of dietary indiscretion and physical evidence of retained feces confirms the diagnosis.
Detailed information regarding the duration of constipation and influencing factors can 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 neurological examination, with special emphasis on caudal spinal cord function, should be performed to identify neurological 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 factors of fecal retention, assess the severity of constipation, 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 (thyroxine) level, urinalysis, and detailed neurological examination should be completed in cases of chronic or recurring constipation.
Treatment and Control of Constipation, Obstipation, and Megacolon in Small Animals
Dietary changes
Laxatives
Enemas
Fecal removal under anesthesia
Depending on the suspected cause of constipation, diet changes, laxatives, and a prokinetic agent may be recommended. More severe or recurrent cases of constipation or cases of obstipation may additionally require enemas or mechanical removal of feces under general anesthesia.
Mild constipation can often be treated by avoiding dietary indiscretion, ensuring access to water, and providing high-fiber diets. Suppository laxatives also may be helpful. Continued or long-term use of laxatives should be discouraged unless necessary to avoid recurrent constipation. Affected animals should be adequately hydrated.
A number of pediatric rectal suppositories are available for management of mild constipation. They include dioctyl sodium sulfosuccinate (DSS; emollient laxative), sodium citrate/sodium lauryl sulfoacetate/glycerol (osmotic 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.
Moderate to severe or recurrent episodes of constipation, or cases of obstipation, may require enema administration, manual extraction of impacted feces, or both. Types of enemas include warm tap water (5–10 mL/kg), warm saline (0.9% NaCl) solution (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 through a 10–12 French rubber catheter or feeding tube passed per rectum under sedation. Phosphate-containing enemas must be avoided in cats due to the potential for developing severe electrolyte imbalances and potentially life-threatening hyperphosphatemia and subsequently, hypocalcemia, hypernatremia, and hyperosmolality.
Pearls & Pitfalls
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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. Laxatives should be avoided in dehydrated animals because they can make dehydration worse.
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 physiological than other laxatives. Commercial fiber-supplemented diets are available, or the pet owner may add psyllium (1–4 teaspoon/meal, depending on the animal's size—eg, cats and small dogs need lower dosages), wheat bran (1–2 tablespoon/meal), or pumpkin (1–4 tablespoon/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 dioctyl calcium sulfosuccinate are emollient laxatives available in oral and enema form. Docusate sodium (cats: 50-mg capsule every 24 hours; dogs: 50-mg capsule, 1–4 every 24 hours) and docusate calcium (cats: 50-mg capsule, 1–2 every 24 hours; dogs: 50-mg capsule, 2–3 every 24 hours) are other examples of emollient laxatives.
Mineral oil (rectally) 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 liquid 0.5 mL/kg, or crystals ¼–½ teaspoonful, PO, or in food, every 8–12 hours), 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, decreases bacterial production of ammonia, and favors formation of ammonium ions that are poorly absorbed.
Polyethylene glycol 3350 (1/8–1/4 teaspoonful, PO or in food, every 12 hours, adjusted to achieve soft stool consistency) can also be used.
Stimulant laxative products (eg, bisacodyl [cats and small dogs: 5 mg/animal; medium-sized dogs: 10 mg/dog; large dogs: 15–20 mg/dog]) 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 suggests that cisapride (0.1–0.5 mg/kg, PO, every 8–12 hours) 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 major adverse effects have been reported in cats treated with cisapride at dosages of 0.1–1 mg/kg, PO, every 8–12 hours. Cisapride is not conventionally manufactured and must be compounded in the US. Cats with long-standing obstipation and megacolon are not likely to improve with cisapride therapy.
Ranitidine (3.5 mg/kg, PO, every 12 hours) and nizatidine (2.5–5 mg/kg, PO, every 24 hours), 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. One uncontrolled study demonstrated that psyllium-enriched extruded dry diets were efficient in the management of chronic colitis in cats (1). Randomized controlled trials are necessary before recommending this type of diet routinely.
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 disease. Mild to moderate diarrhea can 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 months duration. In such cases, pathological 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 months. In these cases, hypertrophy is followed by muscular degeneration and pathological dilatation, and pelvic osteotomy alone will not provide relief from obstipation.
Key Points
Constipation is frequent in small animals, particularly cats.
Diagnosis is based on history and physical examination. A thorough history is needed to rule out iatrogenic causes.
Constipation is treated by dietary change, along with enemas or mechanical removal under anesthesia if needed.
For More Information
Washabau RJ, Holt D. Pathogenesis, diagnosis, and therapy of feline idiopathic megacolon. Vet Clin North Am Small Anim Pract. 1999;29(2):589-603.
Benjamin SE, Drobatz KJ. Retrospective evaluation of risk factors and treatment outcome predictors in cats presenting to the emergency room for constipation. J Feline Med Surg. 2020;22(2):153-160.
Also see pet owner content regarding constipation in dogs and cats.
References
Freiche V, Houston D, Weese H, et al. Uncontrolled study assessing the impact of a psyllium-enriched extruded dry diet on faecal consistency in cats with constipation. J Feline Med Surg. 2011;13(12):903-911. doi:10.1016/j.jfms.2011.07.008
