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MEGACOLON IN CATS
Overview
Megacolon is a term used to describe a very large colon that has increased in size secondary to chronic constipation. Megacolon itself is not a specific disease entity but can result in obstipation (inability to defecate). 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 pelvic trauma (pelvic fractures that when healed obstruct the rectum). Cats usually sustain these fractures from being hit by a car.
Symptoms and Clinical Presentation
The most common age of cats affected with idiopathic megacolon is six years old. Because most owners are not aware of their cat's bowel habits, the disease is usually diagnosed long after constipation has been present.
On physical exam these cats can be uncomfortable (they have been chronically constipated) 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 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 or masses located either inside or outside of the colon/rectum.
Diagnosis
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 bloodwork 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 studies of the lower gastrointestinal tract (dye in the GI tract), or colonoscopy may also be necessary to determine cause of disease. 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 enlarged colon filled with feces.
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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.
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Treatment Options
Medical Treatment
For idiopathic megacolon, initial management is medical. These cats should be appropriately hydrated and then an enema and deobstipation should be performed (manual removal of feces). After they have been “cleaned out”, medical management with lactulose, Cisapride and low residue diet may be initiated. Cisapride stimulates colonic motility by increasing the release of a certain neurotransmitter important in nervous supply to the colon. Lactulose is a cathartic so helps to propel stool out of the colon and also acts as a stool softener. The low residue diet helps to stimulate the colonic cells without increase in bulk. Bulking agents are not a worthwhile treatment for cats with an already oversized colon. Most cats will respond to this therapy but will eventually become refractory to this treatment. When medical therapy is no longer effective surgery to remove the affected segment is recommended.
Surgical Treatment
The surgery is referred to as a subtotal colectomy. Before surgery, cats are started on broad spectrum antibiotics (cephazolin and metronidazole) because the colon is the dirtiest part of the intestinal tract. Peri-operative antibiotics help to prevent bacterial contamination at surgery. The entire colon (Figure 4) except for a small segment at the beginning and end of the colon is resected (cut out) and the two ends are sutured back together (Figure 5). Your surgeon has the option of performing two slightly different procedures. In one, the ileocecal valve (valve which connects the ileum, cecum to the colon) is left intact and in the other, it is removed (the valve separates the small and large intestine). Though there is no statistical difference in how the cats do clinically (i.e., recurrence of constipation) salvation of the valve prevents bacterial overgrowth in the small intestine and therefore prevents diarrhea. The ileum is also important in Vitamin B12 absorption and bile salts reabsorption. Bile salts are important for fat metabolism. Therefore, keeping the valve is preferable. Your general practice veterinarian may wish to refer you to a surgical specialist for this procedure. To find an ACVS certified surgeon in your area, click here: Find a Surgeon.
Figure 4. A photograph of the feces filled colon taken at the time of surgery.
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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.
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Cats with pelvic obstruction secondary to pelvic trauma can be treated by removal of the pelvic bones (pelvic osteotomy) but this treatment is not effective if the megacolon has been present for greater than six months. The colon is not able to return to normal function after this extended period of time. Therefore most of these cats are also treated with the same surgery, subtotal colectomy.
Postoperative Care
Postoperatively broad-spectrum antibiotics are continued and cats are closely monitored for infection. Diarrhea and soft stool is expected for up to three months after surgery. Progressively the stool becomes more formed. Biopsied tissue shows no abnormality. Postoperative constipation has been reported and is usually successfully treated medically. Rarely, cats require a second surgery.
Prevention
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 seen and put on the appropriate medical therapies. 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.
Prognosis
The prognosis for both types of megacolon is good with the appropriate therapy.
—Susan Mitchell, DVM
Diplomate ACVS
Posted 8/13/2004
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