Megacolon

Megacolon describes a colon that is markedly enlarged, flaccid, and functionally ineffective. It most often develops secondary to long-standing constipation and prolonged retention of feces, although in some cases it may be congenital. Megacolon is not a single, specific disease; however, it commonly leads to obstipation (inability to pass stool) because feces are retained within a colon that has expanded to a diameter that cannot pass effectively through the pelvis. As water is absorbed by the colon, the retained stool becomes increasingly dry and hard. When medical management is no longer successful, surgery may be necessary.

Megacolon results from either colonic inertia (a functional obstruction) or an outlet obstruction (a mechanical obstruction). Each category has multiple potential causes. The most frequent cause of colonic inertia is idiopathic megacolon (meaning the cause is unknown). Idiopathic megacolon is a feline disease in which the colon progressively loses normal motility and becomes increasingly dilated; as it advances, cats progressively lose the ability to defecate. The most common outlet obstruction cause is narrowing due to poorly healed pelvic fractures, which collapse or encroach on the pelvic outflow and prevent normal stool passage. Tumors, strictures, and rectal/anal hernias can also contribute to constipation or the development of megacolon.

Cats with idiopathic megacolon are most commonly 5–9 years of age. Diagnosis is often delayed and made well after constipation has been present, particularly in multi-cat households or in cats that go both indoors and outdoors, where owners may not recognize typical bowel patterns.

Affected cats may show:

• abdominal discomfort
• decreased appetite
• lethargy
• tenesmus (straining to defecate)

The stool retained in the colon is typically large and firm and can be readily felt on palpation (a veterinarian can easily feel it). A rectal examination is important to assess for old collapsed pelvic fractures, obstructive masses, or hernias located within or outside the colon or rectum.

Megacolon is suspected from history and physical examination and is confirmed with abdominal radiographs (x-rays). The diagnostic evaluation should also include blood testing to rule out metabolic abnormalities. Radiographs can verify an enlarged colon (Figure 1) and can help identify old pelvic fractures (Figures 2 and 3), masses, or spinal deformities. Abdominal ultrasound, contrast studies of the lower gastrointestinal tract, or colonoscopy may also be required to determine the underlying cause. A final diagnosis of idiopathic megacolon is made only after all other constipation causes have been excluded.

Figure 1. Lateral abdominal radiograph: arrow indicates a markedly enlarged colon packed with feces.
Figure 2. Lateral radiograph of a patient with pelvic fractures; note the markedly enlarged colon.
Figure 3. Ventrodorsal radiograph showing a displaced pelvic fracture (arrow) collapsing the pelvic canal, resulting in megacolon.

Medical Treatment

For idiopathic megacolon, initial therapy is medical. Cats should be adequately hydrated (including IV fluids if dehydrated). An enema and deobstipation (manual removal of stool) are then performed. This nearly always requires general anesthesia because the procedure is extremely painful in an awake cat. Owners should never administer an enema at home unless specifically instructed by a veterinarian, and over-the-counter Fleet phosphate enemas (infant enemas) should never be used because they are toxic to cats.

Once feces have been removed, ongoing medical management is started. Historically, high-fiber diets and bulking agents such as Metamucil and fiber pills were recommended to improve “regularity,” but these are actually contraindicated and may worsen signs. Optimal medical management includes a low-residue diet (a veterinarian can recommend prescription low-residue diets) along with prescription medications such as lactulose and cisapride. Lactulose is a mild cathartic that also softens stool. Cisapride enhances colonic motility (propulsion). A low-residue diet supports colonic stimulation without increasing stool bulk. Many cats respond initially, but some eventually stop responding. If medical therapy fails, surgical removal of the enlarged colon is recommended.

Surgical Treatment

The operation is most commonly a subtotal colectomy (removal of most, but not all, of the colon), although a total colectomy (removal of the entire colon) is sometimes necessary. Preoperatively, antibiotics may be started because the colon—filled with feces—contains the highest bacterial load in the intestinal tract, and antibiotics help reduce surgical infection risk.

During subtotal colectomy, the diseased colon is resected and the remaining ends are sutured together. If any affected segment is left behind, a new dilated portion can develop, leading to recurrence of constipation and obstipation. Subtotal colectomy is considered a demanding surgical procedure.

Cats with pelvic outlet obstruction from prior pelvic trauma may be treated by removing abnormal pelvic bone (pelvic ostectomy) to restore stool passage. However, if megacolon has been present longer than four to six months, the dilation and loss of function may be irreversible and the colon may not recover normal function after that duration. For this reason, most cats with pelvic fractures that impinge on the outflow tract are also treated with subtotal colectomy.

After surgery, antibiotics are often continued and cats are closely monitored for infection. Soft stools and occasional diarrhea are common during the first few months. Over time, stools usually become more formed, but they rarely return to a completely normal character. Cats are not typically incontinent after subtotal colectomy, although during adjustment they may occasionally drop a small piece of soft stool while entering or leaving the litter box. Postoperative constipation has been reported but is uncommon and is usually managed successfully with medical therapy. Rarely, a second surgery is needed if insufficient colon was removed initially. Overall, the vast majority of cats achieve an excellent quality of life and usually do not require special diets or ongoing medication.