Constipation is defined as difficult, painful or reduced defecation over a period of time ranging from days to weeks or months (Washabau and Day, 2013). Cats should easily pass their faeces daily with stools observed being brown, formed and, if in a litter tray, soft enough so that some litter adheres to them. If the cat is straining in the litter box or if their faeces are dry and hard, this may be an early indication of a problem (Keeler, 2009).
The colon has numerous roles, including the absorption of water and electrolytes and storage for digestive waste, by means of coordinated neuro-hormonal mechanisms including segmental contractions mediated by the sympathetic nervous system and the peristaltic ones generated by the parasympathetic; it also periodically eliminates that waste (Danks, 2015). The fact that the large intestine contains a diverse ecosystem and microbiome of its own can often be overlooked; each gram of faecal matter contains 10^10 microbes (one million times that of the distal small intestine) and it’s these which maintain the delicate environmental balance in short-chain fatty acids, water, hydrogen, methane and carbon dioxide on which colonic function depends (Freiche, 2013). A study conducted across rescue centres in the UK found that 5.6 percent of cats suffer with constipation and that increasing age was a risk factor, as was seasonality, namely winter (German et al., 2015), potentially due to the increased amount of time spent inside and not exercising.
Some cats are observed making multiple, unproductive attempts to defecate in the litter box, while other cats may sit in the litter box for prolonged periods of time without assuming a defecation posture. Dry, hardened faeces are observed inside and outside of the litter box. Occasionally, chronically constipated cats have intermittent episodes of haematochezia or diarrhoea caused by the mucosal irritant effect of the faeces remaining in situ. Owners may observe this as small amounts of loose stools being passed around a hardened impacted stool and misinterpret this as their cat suffering with diarrhoea rather than constipation as the primary issue. Prolonged inability to defecate may result in other systemic signs, including anorexia, lethargy, weight loss and vomiting (Washabau and Day, 2013). If constipation remains untreated for long enough, the condition may progress to obstipation, which is defined as intractable constipation that has become refractory to cure or control (Foley, 2017); this may further develop into acquired secondary megacolon.
One study found that older, overweight cats and cats with chronic kidney disease or previous episodes of constipation were found to be at increased risk of constipation (P < 0.0001, P = 0.0004, P = 0.0046 and P < 0.0001, respectively) (Benjamin and Drobatz, 2019). The same study documented that cats noted to be painful on abdominal palpation were less likely to defecate following an enema and that adjunctive treatments such as fluids and laxatives increased the likelihood of a successful enema. Chronic kidney disease was identified as a prominent risk factor due to the accompanying level of dehydration that will result in decreased fluid in the colon while the body attempts to retain fluids, but inevitably loses them through the diseased kidneys (Cannon, 2016).
A full history should be taken to rule out causes relating to drug therapy, such as use of opioids, anticholinergics and sucralfate, and any behavioural components which may be causing the cat stress or aversion to their litter tray (Little, 2011). Physical examination confirms the presence of large amounts of faeces palpable in the colon, sometimes accompanied by abdominal pain. A careful evaluation (eg musculoskeletal system, caudal spinal cord function and anorectal area) should be made for underlying causes. A rectal exam should be performed, under sedation if necessary, for masses, pelvic fracture malunion and anal gland abnormalities (Washabau and Day, 2013). A minimum database (CBC, serum chemistries/electrolytes, urinalysis) should be assessed, especially to determine hydration and electrolyte status and identify underlying diseases such as chronic renal disease (Cannon, 2016). Survey abdominal radiographs are useful to confirm the diagnosis and assess severity as well as to evaluate for potential underlying causes, such as previous pelvic trauma and arthritis. Studies have suggested that measuring the ratio of radiographic colonic diameter to fifth lumbar vertebral length can be used to differentiate between the colon of a normal or constipated cat (defined as a colon diameter of less than 1.28 times the length of the fifth lumbar vertebra (L5)) and that of a cat suspected of having megacolon (defined as a colon diameter larger than 1.48 times the length of L5), where there is not only generalised distension but a loss of motility as well (Trevail et al., 2011).
The specific therapeutic plan will depend upon the severity of constipation and the underlying cause. Medical therapy may not be necessary with first presentation of constipation as some episodes are often transient and resolve without therapy. Mild to moderate or recurrent episodes of constipation usually require management with dietary modification, water enemas, oral or suppository laxatives and/or colonic prokinetic agents (Washabau, 2001). Follow-up therapy and dietary changes in such cases are directed at correcting predisposing factors and preventing recurrence.
Recurring episodes of constipation may require administration of enemas. Several types of enema solutions may be administered, such as warm tap water (5 to 10 ml/kg), warm isotonic saline (5 to 10 ml/kg), dioctyl sodium sulfosuccinate (5 to 10 ml per cat), mineral oil (5 to 10 ml per cat) or a polysaccharide laxative with poor absorption such as lactulose (5 to 10 ml per cat). Enema solutions should be administered with a well-lubricated 10 to 12 Fr. rubber catheter or feeding tube (Little, 2011).
Cases unresponsive to enemas may require manual extraction of impacted faeces. Cats should be adequately rehydrated and then anaesthetised with an endotracheal tube in place to prevent aspiration should colonic manipulation induce vomiting (Carr and Gaunt, 2010). Water or saline should be infused into the colon while the faecal mass is manually reduced by abdominal palpation. Depending on the severity of the impaction it may be advisable to evacuate the faecal mass over a period of several days to reduce the risks of prolonged anaesthesia and perforation of a devitalised colon (Washabau, 2001). If this approach fails, subtotal colectomy will become necessary in cats suffering from obstipation or idiopathic dilated megacolon which are unresponsive to medical management (White, 2002).
Most of the available bulk-forming laxatives are dietary fibre supplements of poorly digestible polysaccharides and celluloses made primarily from cereal grain, wheat bran and psyllium. Many constipated cats will respond to supplementation of the diet with one of these products (Freiche et al., 2011). Dietary fibre is preferable because it is well tolerated, more effective and more physiologic than other laxatives. Fibre-supplemented diets are available commercially and cats should be well hydrated before commencing fibre supplementation to minimise the impaction of fibre in the constipated colon. Two field trials across 66 cats were conducted to assess the efficacy of a psyllium-enriched diet for management of constipation in cats (Freiche et al., 2011). After investigations and faecal evacuation (by enema if required), all cats were fed on a moderate fibre, psyllium-enriched, dry extruded diet. Additional therapy was either not used (trial one) or initially allowed but was subsequently withdrawn if possible (trial two). The diet was well tolerated, and palatability was excellent. Most cases improved after initial therapy (at two months; trial one: 14/15 [93 percent]; trial two: 42/51 [82 percent]) and faecal consistency improved significantly in both trials (P < 0.001). The diets used in these pilot studies appeared to be beneficial in the management of cats with recurrent constipation (Freiche et al., 2011). Some cats prone to constipation may benefit from a high fibre diet. Other cats perform better on a highly digestible “low residue” formula. The choice between the two is down to individual case presentation and response to treatment and management.