Feline megacolon

What it is

Megacolon is the end stage of chronic faecal retention: the colon becomes permanently dilated, full of dry, hard faeces, and can no longer propel them. Two mechanisms are recognised:

• Dilated megacolon: the more common. In the idiopathic form, which makes up most cases, there is generalised dysfunction of colonic smooth muscle, which contracts weakly to normal stimuli; content is retained, the colon absorbs more and more water from the faeces, and they harden and accumulate.
• Hypertrophic megacolon: secondary to an obstruction to the passage of faeces, mainly narrowing of the pelvic canal from malunited pelvic fractures; the colon hypertrophies trying to overcome the obstacle and eventually dilates. If the obstruction is corrected early, it may be reversible.

Other causes are neurological lesions affecting the pelvic nerves and sacral plexus (sacrocaudal luxation, sacral malformations of the Manx cat, spinal cord trauma), dysautonomia, masses compressing the colon and chronic dehydration, for example from kidney disease.

Who it affects

Middle-aged and older cats, more often males and short-haired cats, although any cat can be affected. Secondary megacolon occurs in cats with old pelvic fractures or injuries of the sacrocaudal region.

Anatomy involved

• Colon: markedly dilated, with a thin wall and hard faeces; the descending colon is the most distended part.
• Pelvic canal: bounded by the sacrum dorsally, the ilia laterally and the pubis and ischium ventrally; a reduction in its diameter prevents faeces from passing.
• Sacrum and pelvic nerves: the pelvic nerves (parasympathetic, from the sacral spinal cord segments) control motility of the descending colon and rectum and defecation; their injury causes faecal retention.

Clinical signs

Prolonged, unproductive straining to defecate (tenesmus), small, hard, dry stools, or no defecation for days. Sometimes a little fluid or mucus passes around the impacted faeces and can be mistaken for diarrhoea. In prolonged cases: anorexia, vomiting, dehydration, weight loss, lethargy and an unkempt coat. Abdominal palpation reveals a colon full of hard faeces.

Diagnosis

• Abdominal palpation and rectal examination: colon distended by firm faeces; masses or a narrowed pelvic canal.
• Abdominal radiographs: dilated colon full of faeces; they show old pelvic fractures, the pelvic canal and the lumbosacral and caudal spine. Comparing colonic diameter with the length of the fifth lumbar vertebral body has been proposed as an indicator of megacolon.
• Neurological examination of the tail, perineum and bladder.
• Blood work: dehydration, electrolyte disorders (hypokalaemia reduces motility), kidney disease, hypercalcaemia, hypothyroidism.

Treatment

• Obstipation: rehydration, warm water or lactulose enemas and often manual extraction of faeces under anaesthesia. Sodium phosphate enemas must not be used in cats, because they cause severe electrolyte disorders.
• Maintenance medical treatment: osmotic laxatives such as lactulose or polyethylene glycol 3350, prokinetics (cisapride), an appropriate diet (fibre-supplemented or highly digestible, depending on the case), plenty of water and correction of hypokalaemia.
• Refractory idiopathic megacolon: subtotal colectomy, which gives good long-term results in most cats; faeces stay softer for weeks or permanently.
• Pelvic stenosis: pelvic osteotomy to widen the canal if the colon has not yet lost function; if it has, colectomy.

Key studies

• Washabau and Stalis (1996), in vitro comparison of colonic smooth muscle from cats with idiopathic megacolon and from healthy cats. Contraction in response to acetylcholine, substance P, cholecystokinin, potassium chloride and electrical stimulation was reduced in longitudinal and circular muscle of both the proximal and distal colon, with few histological changes in the muscle cells or enteric neurons. This supports a generalised dysfunction of colonic smooth muscle.
• Trevail et al. (2011), radiographs of 50 normal cats, 13 constipated cats and 26 cats with megacolon. The ratio of maximal colonic diameter to the length of L5 was the most repeatable and accurate measurement: below 1.28 it strongly indicates a normal colon (sensitivity 96%, specificity 87%), and above 1.48 it is a good indicator of megacolon (sensitivity 77%, specificity 85%).
• Benjamin and Drobatz (2020), case-control study of 189 cats treated in an emergency service for constipation and 99 controls. Older age, overweight, chronic kidney disease and previous episodes of constipation were associated with constipation, and cats that were painful on abdominal palpation were less likely to defecate after an enema.
• Rochon et al. (2026), randomised, blinded 6-month trial in 49 cats with chronic constipation, comparing a gastrointestinal diet with 6% psyllium against a similar diet with 0.5%. Diet failure was 27% with the psyllium diet and 73% with the control diet, and stool consistency improved more with psyllium. Cats could switch diets during the study, and it was carried out by the diet's manufacturer.
• Grossman et al. (2021), multicentre retrospective cohort of 166 cats treated by subtotal colectomy for idiopathic megacolon. Major perioperative complications occurred in 9.9% and median survival was not reached. Constipation recurred in 32% (24 of 74) at a median of 344 days. Removing the ileocaecocolic junction did not change recurrence but was associated with long-term liquid faeces and a worse outcome as judged by owners.