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.
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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 in 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, 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 the 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 the
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.
If
enemas are unsuccessful, manual extrac-tion 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, 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 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 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.
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) is effective in stimulating 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 on
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, 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 (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.
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