Abnormal rectum morphology Megarectum 2016-08-26T11:18:38Z Patients with idiopathic megarectum have a dilated rectum but the proximal colon is usually of normal diameter. The condition usually starts in childhood or adolescence, and fecal impaction is common. By contrast, patients with idiopathic megacolon usually do not experience impaction, and the symptoms often begin in adult life [PMID:9301507]. An abnormal dilation of the rectum. There is a large filled rectum as a result of underlying innervation or muscular abnormalities, which remains after disimpaction of the rectum.