Megacolon is a condition in which the colon becomes abnormally dilated and enlarged and loses its ability to contract. It is associated with constipation or obstipation.
There are two forms of megacolon; congenital (present at birth) or acquired. The most common form of acquired megacolon is idiopathic (cause unknown) although it is believed to be improper activation of smooth muscle within the colon and rectum. 
Other causes , dietary (ingesting nondigestible objects which become impacted), injury (ie; pelvic fracture), refusal to defecate because of a dirty litter tray which results in fecal matter building up and distending the colon, anal sac impaction which also leads to a refusal to defecate, tumors and neurological disorders.
Megacolon can occur in cats of any age or breed, however, more cases are seen in middle-aged, male cats.
What are the symptoms of megacolon?
- Crouching and straining for prolonged periods in the litter tray, with either no feces being passed or small, hard, dry stools.
- Defecating outside the litter tray.
- Hunching over, due to discomfort.
- Loss of appetite.
- Weight loss.
- Abdominal pain.
How is megacolon diagnosed?
Your veterinarian will perform a complete physical examination, including a complete neurological exam. Abdominal palpitation will reveal a hard and full colon.
Other tests he may perform include;
- Complete blood count, biochemical profile, and urinalysis to check for metabolic reasons for dehydration such as kidney disease or diabetes mellitus.
- Abdominal/pelvic radiographs - To assess the size of the colon and to evaluate for abnormalities of the lumbar spine and pelvis.
- Rectal examination - This is performed under anesthesia. It helps evaluate for rectal strictures, masses, and perineal hernias.
- Abdominal ultrasound, Contrast studies, and colonoscopy may also be performed to help determine the cause.
- T4 test to check for hypothyroidism.
How is megacolon treated?
Treatment depends on the severity of the condition and includes;
- If possible, find and treat the cause of megacolon.
- Enema and manual removal of the feces: This is performed with the cat under anesthesia.
- Diet: Your veterinarian may choose to put your cat on a high fibre diet which helps create a soft stool.
- Cisapride is an oral medication which stimulates gastrointestinal motility (muscular contractions which push the feces through the intestine).
- Lactulose: This is a synthetic sugar which is used as a stool softener.
- Colectomy: Your veterinarian may recommend a colectomy if medical management treatments have failed. This is an operation to remove part or all of the colon.
 The Cornell Book of Cats - P. 256