Megacolon is a term used to describe a very dilated, flabby, incompetent colon. This usually occurs secondary to chronic constipation and retention of feces, but may be a congenital dysfunction. Megacolon itself is not a specific disease entity, but it will usually result in obstipation (inability to defecate), since feces is retained in the colon in a larger diameter than is able to pass through the pelvis. This feces also becomes very dry and hard, as water is absorbed by the colon. Surgery may be required to treat this condition if medical management has been exhausted.
Megacolon is secondary to colonic inertia (mechanical obstruction) or outlet obstruction (functional obstruction). In each category there are a number of specific causes. The most common cause of colonic inertia is idiopathic (unknown cause) megacolon. Idiopathic megacolon is a disease in cats where the colon loses its normal motility and becomes progressively larger. As the disease progresses, cats lose the ability to defecate. The most common cause reason for outlet obstruction is obstruction due to poorly healed pelvic fractures, which impinge on the outflow tract of the pelvis and prevent the normal passage of feces. Tumors, strictures and hernias of the rectum/anus can also contribute to the development of megacolon.
Symptoms and Clinical Presentation
Cats affected with idiopathic megacolon are usually between 5-9 years old. The condition is usually diagnosed long after constipation has been present, especially in multiple cat household, or when cats are indoor/outdoor. In these cases, owners may not be aware of their cat’s normal bowel habits.
On physical exam, affected cats are often uncomfortable in the abdomen and may have decreased appetite, may be depressed or lethargic. They may also have signs of tenesmus (straining to defecate). The stool present in the colon is large and firm, and is easily palpable (the veterinarian can feel it easily). It is important that the veterinarian perform a rectal exam to check for old collapsed pelvic fractures, obstructive masses or hernias located either inside or outside of the colon/rectum.
Diagnosis of megacolon is based on history and physical exam and is confirmed with radiographs (x-rays) of the abdomen. The diagnostic work-up should also include blood work to rule out any metabolic abnormalities. Radiographs can confirm the presence of a large colon (Figure 1) and can also be used to determine if there are any old pelvic fractures (Figure 2 & 3), masses or spinal deformities. Abdominal ultrasound, contrast (dye or barium) studies of the lower gastrointestinal tract, or colonoscopy may also be needed to determine cause of the condition. The final diagnosis is made by excluding all other causes of constipation and/or obstipation.
Figure 1. Lateral radiograph of abdomen. The arrow points to the extremely enlarged colon filled with feces.
Figure 2. and Figure 3. Lateral radiograph of a patient with pelvic factures. Note the extremely enlarged colon. Ventrodorsal radiograph showing a displaced pelvic fracture (arrow) which has collapsed the pelvic canal resulting in megacolon.
For idiopathic megacolon, initial management is medical. These cats should be appropriately hydrated (IV fluids if dehydrated) and then an enema and deobstipation (manual removal of feces) should be performed. This almost always requires general anesthesia, as it is extremely painful for an awake cat. NEVER attempt to give a cat an enema at home unless instructed by your veterinarian, and NEVER use an over-the-counter Fleet phosphate enema (infant enema), which is toxic to cats. After they have been “cleaned out”, medical management begins. In the past, a high fiber diet and bulking agents such as Metamucil and fiber pills were recommended in an effort to make the cat more “regular.” These are actually contraindicated, and can worsen the signs. Best medical management includes a low-residue diet (your veterinarian can suggest low-residue prescription diets) and prescription medications such as lactulose and Cisapride. Lactulose is a mild cathartic (helps speed defecation) and is a stool softner. Cisapride stimulates colonic motility (propulsion) by increasing the release of a certain neurotransmitter important in nervous supply to the colon. The low residue diet helps to stimulate the colonic cells without increase in bulk. Most cats will respond to this therapy, but some will eventually become refractory to treatment. When medical therapy is no longer effective, surgery to remove the enlarged colon is recommended.
The surgery is referred to as a “subtotal colectomy”(removal of most, but not all, of the colon), although occasionally a total colectomy (removal of the entire colon) is required. Before surgery, cats are started on broad spectrum antibiotics, because the colon, containing feces, is the most bacteria-laden part of the intestinal tract. Antibiotics help to prevent bacterial contamination at surgery.
In subtotal colectomy, the entire affected colon (Figure 4) is resected (cut out) and the two ends are sutured back together (Figure 5). Depending on how dilated the colon is, your cat’s surgeon has the option of performing two slightly different procedures. In one, the ileocecal valve (area which connects the ileum, which is the last part of the small intestine, and cecum to the colon) is left intact. In this case, the short segment of proximal colon is sutured to the distal colo-rectum. In the other, the end of the ileum, cecum and ileocecal valve are removed and the proximal ileum (or distal jejunum – mid-section of the small intestine) is sutured to the distal colo-rectum. Though there is no statistical difference in how the cats do clinically long-term, preservation of the ileo-cecal valve can reduce post-op diarrhea in the short term. Preserving this valve is not optional; if your cats’ surgeon removes it, it is because the area was too diseased to preserve. Although the ileum functions in Vitamin B12 absorption and reabsorption of bile salts, it is rare for cats to have chronic diarrhea or need vitamin replacement treatment even if the ileum has to be removed. Failure to remove an affected portion of bowel can cause formation of a new dilated area of bowel, leading to recurrence of clinical signs of constipation and obstipation.
Sub-total colectomy is a challenging surgical procedure, so your general practice veterinarian may wish to refer you to a surgical specialist for this procedure. Find an ACVS Veterinary Surgeon.
Figure 4. A photograph of the feces filled colon taken at the time of surgery.
Figure 5. This is the patient from Figure 4. The megacolon has been resected and the arrows point to ends that will be sutured together.
Cats with pelvic obstruction secondary to pelvic trauma can be treated by removal of the abnormal pelvic bones (pelvic ostectomy) to allow normal passage of feces again. Unfortunately, if the megacolon has been present for greater than four to six months, dilation and loss of function may be irreversable. The colon is not able to return to normal function after this extended period of time. Therefore most of cats with impinging pelvic fractures are also treated with the same surgery (subtotal colectomy).
Postoperatively, broad-spectrum antibiotics are continued and cats are closely monitored for infection. Soft stools and occasionally diarrhea can occur in the first few months after surgery. Progressively the stool becomes more formed, but rarely ever becomes normal. Cats are not incontinent following subtotal colectomy, but may occasionally drop a small piece of soft stool on the way in or out of the litterbox as they are adjusting to their new life post-constipation. Postoperative constipation has been reported, but is rare and is usually successfully treated medically. Rarely, cats require a second surgery if not enough colon was removed the first time. The vast majority of cats have excellent quality of life and do not need special diets or medication.
Unfortunately there is no way to prevent idiopathic megacolon, nor is there any way to determine which cats will develop it. The best advice is to have cats seen by their veterinarian at least once yearly so that a complete physical exam can be performed. If the cat shows signs of constipation it should be examined and put on the appropriate medical therapies as indicated. Megacolon secondary to pelvic fractures can only be prevented if the fractures are identified soon enough. If a cat has been hit by a car it should be carefully examined by a veterinarian so that pelvic fractures may be identified and treated if necessary.
The prognosis for megacolon is very good with the appropriate therapy.
—Susan Mitchell, DVM
Reviewed and updated 2/19/2009 by Holly S. Mullen, DVM, Diplomate ACVS
Reviewed 10/1/2011 by Mitchell A. Robbins, DVM, Diplomate ACVS