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 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.
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.
Megacolon is most often seen in middle-aged cats with an average age of 6 years. It is more common in males (70%) than females. 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. Cats with recurrent episodes of constipation or disorders that lead to constipation are more at risk of developing megacolon.
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. Abdominal examination will reveal a large colon full of hard feces. 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. Abdominal x-rays are helpful 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.
Therapy for megacolon depends on several factors including the severity of the constipation and fecal impaction and the underlying cause. 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 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. 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.
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. Cisapride is a motility drug that was withdrawn from the human market, but can still be obtained for veterinary use. Many cats benefit from this medication.
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.
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.
Prevention of megacolon can be difficult. The underlying cause must be eliminated, if possible. Some cats prone to constipation may benefit from high fiber diet. Other cats perform better on a highly digestible “low residue” formula. Laxatives may be necessary. Commonly used laxatives include white petrolatum (Laxatone) or lactulose.
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.