Megacolon is a term used to describe a very dilated, flaccid, 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. The feces also become very dry and hard, as the colon absorbs water. Surgery may be required to treat this condition once medical management has been exhausted.
Megacolon is secondary to colonic inertia (functional obstruction) or outlet obstruction (mechanical obstruction). In each category, there are a number of specific causes. The most common cause of colonic inertia is idiopathic (meaning 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 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 or constipation.
Cats affected with idiopathic megacolon are usually between 5–9 years old. Diagnosing the condition usually happens long after constipation has been present, especially in a multiple cat household, or when cats are indoor/outdoor. In these cases, owners may not be aware of their cat’s normal bowel habits.
Cats affected by megacolon may show the following signs:
- abdominal discomfort
- decreased appetite
- tenesmus (straining to defecate)
The stool present in the colon is large and firm, and is easily palpable (a veterinarian can feel it easily). It is important that your veterinarian perform a rectal exam to check for old collapsed pelvic fractures, obstructive masses, or hernias located either inside or outside of the colon or 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 be used to determine if there are any 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 needed to determine cause of the condition. The final diagnosis of idiopathic megacolon is made by excluding all other causes of constipation.
Postoperatively, antibiotics are often continued and cats are closely monitored for infection. Soft stools and occasionally diarrhea can occur for the first few months after surgery. Progressively, the stools become more formed, but rarely ever become 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 litter box as they are adjusting to their new life post-constipation. Postoperative constipation has been reported, but is rare and is usually treated successfully with medical management Rarely, cats require a second surgery if not enough colon was removed the first time. The vast majorities of cats have excellent quality of life and do not need special diets or medication.