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Megacolon  

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by Arnold Plotnick MS, DVM, ACVIM, ABVP

Megacolon is a condition of extreme and irreversible dilation and poor motility of the colon, usually combined with accumulation of fecal material and the inability to evacuate it. The majority of cases (62 percent) have no known cause, however, disorders that lead to recurrent episodes of constipation can also result in megacolon.  For years, there was debate as to whether the problem was neurological vs. muscular, however, recent studies have now shown that cats with megacolon have an impairment of the colonic smooth muscle.

Constipation is a clinical sign characterized by absent, infrequent or difficult defecation associated with retention of feces within the colon and rectum. There are many potential causes of constipation, for example: obstruction of the colon due to a foreign object, tumor or stricture, bone impaction, neurologic disease, or pelvic canal narrowing due to pelvic fractures. Most cats respond with medical therapy but some have recurrent episodes, leading to obstipation. Obstipation occurs when the colon becomes so severely impacted with a large amount of hard feces that it is completely impossible to defecate.  The longer the feces remain in the colon, the more fluid is extracted by the colonic lining, resulting in a progressively drier and firmer fecal mass. Repeated episodes of constipation or prolonged obstipation can lead to megacolon.

Diagnosis of Megacolon

Causes and Risk Factors

Causes
Most cases of megacolon (62 percent) are idiopathic (they occur for no clearly defined reason). Other causes include pelvic canal narrowing (23 percent), neurologic injury (6 percent), and as a condition in Manx cats born with sacral spinal cord deformity (5 percent). Other potential causes include colon cancer or complications associated with previous colon surgery. 

Risk factors
Age – Megacolon is most often seen in middle-aged cats with an average age of 6 years.

Breed/genetics – Domestic shorthaired cats are most often affected (46 percent). It has been found that 12 percent of cases are Siamese. Megacolon can also occur in Manx cats with sacral spinal cord deformity.

Sex – Megacolon is more common in males (70 percent) than females.

Other medical disorders – Cats with recurrent episodes of constipation or disorders that lead to constipation are more at risk of developing megacolon.

Prevention – Prevention of megacolon can be difficult. The underlying cause must be eliminated, if possible. Cats prone to constipation may benefit from a fiber-supplemented diet, laxatives and/or enemas. To add fiber to the diet, you can administer psyllium (Metamucil powder) at a dosage of 1 to 2 teaspoons per meal, wheat bran (1 to 3 teaspoons per meal) or canned pumpkin (1 to 3 teaspoons per meal) added to the food once a day. (Many cats actually like the taste of canned pumpkin).  Manhattan Cat Specialists has a veterinary fiber formulation called Vetasyl that works well in constipated cats.  In addition to adding fiber, laxatives may be necessary. Commonly used laxatives include white petrolatum (Laxatone) or lactulose. Manhattan Cat Specialists keeps a supply of these medications available at all times. Clean fresh water should be available at all times.

History and Clinical Signs

Presenting signs and historical problems – Straining to defecate, painful defecation, blood in the stool, passage of small amounts of dry, hard feces or absence of feces are common signs of megacolon. Most often, cats have a history of repeated episodes of constipation. Other systemic signs of illness may be present as a result of prolonged inability to defecate, such as anorexia, lethargy, weight loss, abdominal pain and vomiting.

Physical Examination Findings

General
Attitude – Most affected cats have a normal attitude but those with chronic untreated megacolon may be depressed due to metabolic changes.

Body condition – Some cats may be obese since obesity is a risk factor.

Hydration status – Cats with chronic megacolon may be dehydrated.

Abdominal examination will reveal a large colon full of hard feces. It may be difficult to differentiate a megacolon from an abdominal mass without x-rays. Affected cats may appear unkempt. Digital rectal exam may reveal a narrowed pelvic canal, old pelvic fractures, a colonic or rectal tumor, stricture and/or dry, hard feces.

The remainder of the physical examination is often unremarkable.

Diagnostic Studies
Special examination techniques - Colonoscopy allows evaluation of the colon non-invasively using an endoscope, although this is usually not necessary. The lining of the colon can be examined for inflammation, dilations, irregularities, and strictures. If indicated, biopsies can be obtained.

Clinical laboratory tests – cats that may need to be anesthetized so that the feces can be extracted manually may need blood tests performed as a pre-anesthetic screen.

Complete Blood Count – Usually unremarkable, although a high white blood cell count may indicate inflammation or infection.

Serum biochemical tests – Usually unremarkable, although some cats may have an elevated total protein due to dehydration, or excessively low potassium or excessively high calcium levels. Although uncommon, serum thyroxine levels should be determined to rule-out hypothyroidism as an underlying cause.

Urinalysis – Usually normal

Diagnostic Imaging
Radiographs (abdominal) – Abdominal x-rays are recommended to determine the severity of the colonic impaction and to determine the presence of any predisposing conditions, such as foreign bodies, pelvis fractures or a mass.

Ultrasound (abdominal) – Abdominal ultrasound is not commonly performed but may be indicated if cancer is suspected.

Contrast radiographs – A barium enema may be necessary if colonoscopy is not possible. This procedure is rarely necessary.

Myelography – If a neurologic disorder is suspected to be the underlying cause of the cat’s condition, particularly a spinal disorder, myelography (evaluation of the spinal cord by injecting a dye into the spinal column) may be indicated. 

Pathology
Biopsy – Ultrasound guided biopsy may be indicated if a colonic tumor or cancer is suspected.

Treatment of Megacolon

Treatment Principles
Megacolon is a dysfunction of the smooth muscle of the colon. The goal of treatment is to maintain a soft stool and to improve colonic motility by administering medications that stimulate contraction of the colonic smooth muscle.

Initial/Hospital Therapy
Symptomatic therapy - The therapy for megacolon depends on several factors including the severity of the constipation and fecal impaction and the underlying cause. Initial episodes of constipation, if mild, may not need any therapy at all. Mild or moderate episodes that recur usually require some kind of treatment. There are many treatment options. The most successful therapy usually involves a combination of treatment interventions. Cats that are dehydrated or have electrolyte abnormalities should be placed on intravenous fluids to correct metabolic disorders. Once hydrated, enemas and possible manual removal of feces can be attempted.

Enemas - Mild or moderate episodes of constipation that recur will sometimes require the administration of enemas. Although many cat owners are often willing and able to administer a packaged enema to their cat at home, enemas are best performed in a veterinary setting. Manhattan Cat Specialists uses safe, pre-packaged enemas that work very well in cats.

If enemas are not sufficient to remove the impacted feces, manual extraction can be attempted.  Some cats will tolerate this procedure under heavy sedation, however, the majority will need to be completely anesthetized for the procedure to be performed properly. It sometimes requires repeated enemas and manual extraction to remove the majority of the feces.

Following evacuation of the feces, laxatives and medications that increase the motility of the colon should be prescribed.

Surgical therapy – Some cases of megacolon respond poorly to medical therapy. In these patients, a surgical procedure to remove most of the defective colon may be beneficial.

Long-term/Home Therapy

Diet is an important part of therapy.  There are two schools of thought when it comes to dietary therapy.  Traditionally, cats with megacolon have been fed diet high in fiber to bulk up the feces and help attract water to the stool, improving its consistency.  This can be accomplished in several ways. Fiber can be added to the cat’s regular diet, using products such as Metamucil (psyllium, 1 to 2 teaspoons per meal), or a high fiber prescription diet or over the counter diet can be fed. Wheat bran (1 to 3 teaspoons per meal) and canned pumpkin (1 to 3 teaspoons per meal) are other natural sources of fiber. The other school of thought is that a colon that is having trouble propelling the stool would benefit from a diet that doesn’t result in the production of much stool, i.e. a highly digestible diet.  The Iams Company makes such a diet, called “Low Residue” formula, and we’ve had fairly good success with this diet.  There’s no way to predict whether a low residue or a high fiber diet is best for any particular cat.  It must be determined through trial and error.

The occasional use of laxatives when excessively firm stool is noticed in the litter box may be helpful. There are a variety of laxatives that can be used.

Drugs that help the colon contract better are often prescribed for cats with megacolon.  Cisapride is a motility drug that was recently withdrawn from the human market, but can still be obtained for veterinary use.  Many cats benefit from this medication.

Follow-up Care

Cats with megacolon will initially require frequent rechecks to monitor response to therapy. Owners will need to carefully monitor their cats at home for straining, painful defecation, or blood in the stool. Timely treatment for future constipation episodes can help increase the quality of the cat’s life. 

Prognosis

The prognosis is guarded to fair. Most cats will respond to therapy.  A few will continue to suffer from occasional episodes of constipation that may or may not require brief hospitalization and manual removal of feces.  Some are refractory to treatment, unfortunately, and may require surgery. However, the success rate for surgery is high and the vast majority of cats that require surgery do well afterward.         

Updated 2/9/06