What is megacolon?
Megacolon is a condition in which the colon becomes abnormally dilated and enlarged and loses its ability to contract. Constipation and obstipation are both associated with megacolon.
There are two forms of megacolon, congenital (present at birth) or acquired. The most common form of acquired megacolon is idiopathic, which means the cause is unknown. However, improper activation of smooth muscle within the colon and rectum has been suggested as a possible cause. 
Other causes include the following:
- Dietary, such as ingesting nondigestible objects which become impacted
- Injury, from a pelvic fracture which causes a narrowing of the pelvis
- Refusal to defecate because of a dirty litter tray resulting in fecal matter building up and distending the colon
- Anal sac impaction which causes painful defecation
- Tumours of the colon or anus which can lead to a blockage or painful defecation
- Neurological disorders such as Manx syndrome
Megacolon can occur in cats of any age or breed, however, more cases are seen in middle-aged, male cats.
- 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.
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 evaluate the overall health of your cat and 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 – To look for rectal strictures, masses, and perineal hernias.
- Abdominal ultrasound, contrast studies, and colonoscopy – These procedures can help your veterinarian evaluate the colon and internal organs for abnormalities.
- T4 test to check for hypothyroidism.
Treatment depends on the severity, finding and treating the cause of megacolon are important as well as treating the condition itself. This may include:
- Enema and manual removal of the feces.
- Diet: Your veterinarian may choose to put your cat on a high fibre diet which helps create a soft stool.
- Medication: Cisapride or Zantac are medications which help to stimulate gastrointestinal motility, muscular contractions which push the feces through the intestine.
- Stool softeners: Lactulose to soften the stools, making them easier to pass.
- 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