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.