constipation may lead to formation of large hard stools filling up the rectum which cannot be passed and which may cause overflow fecal incontinence and thus may present as diarrhoea
bowel distention from feacal overload may cause ischaemia of the bowel wall and cause stercolic colitis which may require emergent bowel resection
in general, the patient with fecal impaction should not be discharged from the ED without it being resolved - these patients will probably need inpatient care to ensure it is resolved, and this may require manual removal perhaps under GA