What is it?
Megacolon is persistent, irreversible colon dilatation and hypomotility. It is preceded by constipation and obstipation, with intermittent diarrhea and/or hematochezia being possible. Dyschezia and inappropriate defecation are also commonly seen
Who is affected?
Cats tend to develop megacolon more commonly than dogs, with Siamese, DSH and DLH over-represented. Males tend to be disproportionately affected (70% of cases)
Causes
Megacolon can have a variety of causes. Idiopathic cases are the most common, representing 2/3 of cases, with pelvic nerve injury and pelvic fracture being the next two commonly seen presentations. Rarer causes include obstructive (ex: neoplasia, stricture) endocrine/metabolic dysfunction (ex: HypoK, HypoT), congenital abnormalities ( ex: Manx cats) and neurological abnormalities (ex: feline dysautonomia).
Diagnosis
Diagnosis is primarily based on a history of chronic constipation/obstipation and physical examination. Common PE findings include a distended and impacted colon on abdominal palpation. Bloods are likely to be normal, excepts in the case. of metabolic or endocrine causes.
Radiology can be helpful in confirming the diagnosis; a ratio of maximum colon diameter to length of L5 vertebrae can be used, with anything >1.48 suggestive of megacolon (Se 77%, Sp 85%)
Treatment/Surgery
Medical treatment can be helpful in the earlier stages, with laxatives, pro-kinetic agents (cisapride), enemas and dietary modification commonly used.
Surgical treatment involves the removal of the colon (colectomy). Subtotal colectomy, with the ileocolic junction being preserved, is typically preferred but is not always possible. Although removal of the junction can result in SIBO (Small Intestinal Bacterial Overgrowth) and subsequently chronic loose stool, many animals adjust to the higher bacterial load with time. Possible complications of colectomy include dehiscence (4-20% depending on study/technique), recurrence of constipation (more common in subtotal colectomy), chronic diarrhea (more common with total colectomy) and post-operative electrolyte disruptions.
