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Megacolon in Dogs

The colon is the section of the large intestine that begins at the cecum, the sac that joins the colon to the end of the small intestine (ileum), and ends where the rectum begins. All are part of the larger digestive tract. The main purpose of the colon is to serve as a temporary storage conduit while it performs the function of extracting useful water and salt from the waste. Megacolon is a condition in which waste is remaining in the colon, causing the diameter of the colon to become abnormally enlarged. It is typically associated with chronic constipation, or obstipation – severe, obstinate constipation that blocks the passage of gas as well as feces. Medically, the colon is described as exhibiting minimal colonic activity – that is, the colon is not releasing its contents.

Megacolon can be a congenital or acquired condition. Dogs with congenital megacolon are born with an abnormal lack of normal smooth muscle function of the colon. Megacolon can also be acquired, such as when feces is chronically retained, fecal water is completely absorbed, and the water and matter bond, causing the feces to become solidified within the colon. If concreted feces remains lodged in the colon for a prolonged period of time, distention of the colon will occur, resulting in irreversible colonic inertia (inactivity). Colonic inertia is characterized by the smooth muscle of the colon no longer contracting or expanding to void feces.

Symptoms and Types

• Constipation: feces trapped in the colon
• Obstipation: severe blockage that impedes both feces and gas, keeping them trapped in the colon
• Infrequent defecation
• Straining to defecate with small or no fecal volume
• Small amount of diarrhea may occur after prolonged straining
• Hard, dry feces
• Hard colon felt with abdominal examination (palpation)
• Fecal impaction can be felt when a gloved finger is inserted into the rectum
• Occasional vomiting, anorexia and/or depression
• Weight loss
• Dehydration
• Scruffy, unkempt hair coat

Causes

• Idiopathic (unknown)
• Congenital (present at birth)
• Mechanical obstruction of feces
• Trauma to the body
  • Limb and/or pelvic fractures
• Metabolic disorders
  • Low serum potassium
  • Severe dehydration
• Drugs
  • Vincristine: used for lymphoma and leukemia
  • Barium: used for enhancing x-ray images
  • Sucralfate: used for treating ulcers
  • Antacids
• Neurologic/neuromuscular disease
  • Spinal cord disease
  • Intervertebral disk disease
  • Disease of the anus and/or rectum

Diagnosis

Your veterinarian will perform a thorough physical exam on your dog. A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, an electrolyte panel, and a urinalysis. Your doctor will also perform an abdominal palpation (examination by touch) of the colon, and a manual examination of the rectum, by digital rectal penetration (by finger). You will need to give a thorough history of your dog's health, including a background history, onset of symptoms, and possible incidents that might have precipitated this condition.

Abdominal radiograph imaging is useful for a visual examination of the colon. Recorded images will show whether the colon is full of feces, if there is a mass blockage in the colon, or if there are any other underlying causes for the megacolon. An internal examination of the colon using a lighted tubular instrument (colonoscope) may be performed if obstructive lesions inside the colon, or in the wall of the colon, cannot be ruled out.