Feline megacolon

The term megacolon is used to describe a pathologic and persistent dilation of the colon followed by a progressive deterioration in bowel evacuation function. The incidence rate for this condition is far higher in cats than in dogs.

AETIOLOGY

The aetiopathogenesis of feline idiopathic megacolon is not yet totally understood. All possible differential diagnoses must be considered (of inflammatory, endocrine, metabolic, pharmacologic, environmental or behavioural origin) (Table 1), however in most retrospective studies the origin of feline megacolon was of idiopathic (62%), orthopaedic (23%) or neurological (6%) nature._¹

Orthopaedic conditions responsible for megacolon usually include previous pelvic bone fractures in which the healing resulted in axial deviations, causing a narrowing of the pelvic canal. Traumatic lesions of the spinal cord or of pelvic nerves, neoplasms, spinal disc hernias and cauda equina syndrome are among the most frequent acquired neurological causes of megacolon. Apart from megacolon, other neurological symptoms may be present in these neurological patients, such as reduced tail and anal tone sensitivity, bladder atony and paraparesis.

A less common cause of acquired neurogenic megacolon is feline dysautonomia, or Key-Gaskell syndrome, a progressive polyneuropathy of the autonomous nervous system, which prevalently affects young cats; in the initial stages of the disease constipation is often the main symptom.

In Manx breed cats, congenital deformities of sacral and caudal vertebrae as well as sacral spinal cord malformations have been reported; such conditions may be the cause of paraparesis, paraplegia, bladder atony, urinary and fecal incontinence, anal areflexia and perineal insensitivity, all conditions which may be associated with megacolon;1 in these subjects the presence of megacolon may be considered of congenital nature.

Another rare cause of congenital megacolon is agangliar megacolon, known in human patients as Hirschsprung’s disease, characterized by the congenital absence of the inhibitory neurons of Meissner’s submucous plexus and of Auerbach’s myenteric plexus. The affected colon segment presents a spastic contracture of smooth muscles; the proximal segment is consequently dilated and hypertrophic._¹

Table 1. Differential diagnoses of constipation in the cat._²

PATHOPHYSIOLOGY

Based on the pathophysiology, two types of megacolon have been defined: hypertrophic and dilated megacolon.

Hypertrophic megacolon develops as a consequence of obstructive mechanical causes, such as pelvic fracture malunions, the presence of foreign bodies, stenoses or neoplasms; the condition is potentially reversible once the primary cause has been removed. Based on the results of a published scientific paper, the effects of hypertrophic megacolon may be reversed after pelvic osteotomy alone, without the need for a colectomy, if the obstruction has been present for less than six months._¹⁰ In the absence of adequate therapy, hypertrophic megacolon will progress to dilated megacolon.

Dilated megacolon is the end-stage of colonic dysfunction and is characterized by both functional and structural permanent organ damage._²

An in-vitro study assessed the contractile capacity of colonic smooth muscles harvested from healthy cats and from cats with idiopathic megacolon in response to: stimulation with neurotransmitters (acetylcholine, substance P., cholecystochinin), membrane depolarization (potassium chloride) and exposure to an electrical field. The results reported a decreased contractile capacity of the samples taken from cats with megacolon compared to those coming from healthy cats._³ This difference was observed in both longitudinal and circular muscle fibres, on samples of both ascending and descending colon. No histological alterations of the myocytes or of the myenteric neurons were found. Based on the results of the study it may be inferred that all the colonic smooth muscle layers are affected and that the dysfunction may be caused by an intracellular activation disorder of the myofilaments of the myocytes. It has still not been possible to ascertain with certainty if idiopathic megacolon is the consequence of this apparent disorder in the activation of smooth muscle myofilaments or if it is instead the consequence of prolonged constipation.

Based on a later published study, it appears that the colonic smooth muscle dysfunction may originate from its most distal segment, to then progressively extend proximally until affecting the entire organ.⁵

It is worth noticing that in elderly cats a moderate tendency to developing constipation is to be considered as a paraphysiological phenomenon.

CLINICAL SIGNS

The clinical history of subjects with megacolon if often characterized by the presence of recurring and worsening episodes of constipation. Constipation is a pathological condition causing difficult and infrequent defecation associated with fecal retention within the colon and rectum. The chronic presence of faeces within the colon stimulates continuous water resorption, which causes the hardening and dehydration of fecal matter. With time, this condition may finally result in a state of irreversible obstipation, the end-stage of this condition, in which defecation becomes impossible.

Although subjects of all ages, sex or breed are affected, in most retrospective studies the prevalence is higher in adult cats, mean age 5-8 years, and prevalently in shorthaired male subjects._¹

Dyschezia and tenesmus are the main clinical manifestations, however depending on the severity and the pathogenesis of constipation secondary symptoms may be present, such as vomiting, anorexia, asthenia, dehydration, weight loss, abdominal pain and at times mesenteric reactive lymph nodes.

Some cats exhibit numerous unsuccessful evacuation attemps and at times remain sitting for long periods of time in the litter box, without assuming the usual evacuation position; dry and hardened stools may be be found inside and outside the litter box.

When megacolon is caused by feline dysautonomia, clinical signs attributable to autonomic nervous system dysfunction will also be present, such as urinary and faecal incontinence, regurgitation consequent to megaoesophagus, mydriasis, hypolacrimation, prolapse of the nictitating membrane and bradycardia._²

At times some owners may wrongly believe that the main problem is diarrhoea, as cats with chronic constipation may present intermittent episodes of hematochezia or the discharge of limited amounts of liquid stools attributable to the chronic irritation of the colonic mucosa caused by the presence of the dehydrated faecal mass.

DIAGNOSIS

Signalment of the condition in the Manx cat breed is highly indicative of idiopathic congenital megacolon of neurological origin, caused by the sacral spinal cord deformity for which there is a breed predisposition.

Before proceeding with the physical examination, an accurate recent and remote clinical history of the patient must be collected, with a focus on eventual previous pelvic fractures, neurological disorders and previous episodes of constipation. It is also important to take note of the treatments used in the past and of their efficacy.

During the physical examination patients with megacolon may exhibit pain at abdominal palpation and fecalomas are usually detectable as cylindrical masses hard in consistency. By exerting a delicate and progressive moderate digital pressure these fecal lumps may break up or remain indented. Patients are usually dehydrated and depressed. A digital rectal examination, which may require sedation or anaesthesia of the patient, may be useful for the assessment of pelvic fractures, foreign bodies, stenoses, neoplasms or perineal hernias. The neurological examination is always necessary and may reveal alterations compatible with spinal or peripheral nerve lesions. In such cases the exact localization of the lesions is to be then confirmed with CSF analysis, CT, MRI or with electrophysiological studies.

In most cases laboratory tests do not present significant alterations; in some patients, apart from dehydration, the lab tests may identify hypokalaemia and hypercalcaemia. It is not always easy to understand if these metabolic alterations are a cause or a consequence of constipation. In the cat, hypothyroidism is a very rare cause of megacolon, although theoretically constipation may be present in cats under therapy for this condition._²

Radiographs (Figs. 1 and 2) should always be taken, as they can give important information in order to assess the size of the colon, to identify foreign bodies, neoplasms, pelvic fractures and lumbosacral spine disorders. As for the possibility of using radiography to differentiate between constipation, irreversible obstipation or megacolon, a recent study has identified in the ratio between the colon’s diameter in its point of greater dilation and the length of L5 as the most accurate and consistent measurement to discriminate between normal colon, constipation and megacolon. According to the authors, if this measurement is equal or below 1.28 the colon may be considered normal, values between 1.28 and 1.48 are typical of constipation, while values greater than 1.48 are indicative of megacolon._⁶

Opinion of the author of this article is that these measurements, although useful, cannot by themselves be the discriminating factor between a curable and an incurable condition; the clinical data and history, as well as the response to drug therapy, are in fact just as useful and essential for the medium and long-term management of patients with constipation.

Abdominal ultrasonography is the preferred diagnostic technique for the identification of extraluminal masses, as it also allows the concomitant execution of fine-needle or ultrasound guided biopsies. Endoscopy is instead more indicated for intraluminal masses, as it allows the collection of bioptic samples, as well as the identification of stenoses, sacculi, diverticuli and fistulae. Although endoscopy is useful and is used to identify endoluminal causes of mechanical constipation, in the course of idiopathic dilated megacolon the macroscopic and microscopic endoscopic examination gives results which are perfectly normal. Colonoscopy, within the diagnostic approach to megacolon, is therefore used principally only to exclude possible causes of megacolon.

In case of suspected agangliar megacolon, a full thickness colon biopsy is necessary to confirm the diagnosis; an histological examination may reveal the absence of intramural gangliar cells, and a manometric examination, although scarcely applicable in veterinary patients, may reveal a decreased contractile force of colonic smooth muscles.

TREATMENT

Being megacolon a condition characterized by the progressive worsening of the symptomatology, the therapeutic plan cannot be standardized and must be adjusted to each single case depending on the severity, duration, the eventual underlying cause of constipation and the age of the patient.

The first episodes of constipation do not require an immediate medical treatment, as they frequently tend to resolve spontaneously.

Cases of mild constipation may be managed with medical treatment alone, using fibre supplements, increased water consumption, the use of enemas, oral or rectal laxatives and prokinetic agents.

In the presence of moderate to severe episodes of constipation, a high enema and at times the manual extraction of the fecal impaction may be necessary.

For high enemas various liquid solutions may be used, such as lukewarm tap water (5-10 ml/kg) which has a hydrating function on the dehydrated fecal mass, dioctyl sodium sulfosuccinate (5-10 ml/cat) which has an emollient function, mineral oil and liquid paraffin (5-10 ml/cat) for their lubricating action or lactulose (5-10 ml/cat) for its hyperosmotic activity. These solutions are to be administered by means of a lubricated rubber catheter or with an enteral nutrition tube used rectally, trying to reach the cranial portion of the descending colon. It should be noted that high enemas with sodium phosphate are to be absolutely avoided in the cat, as they can cause severe forms of hypernatraemia, hyperphosphataemia and hypocalcaemia.

Laxative drugs include many active principles, classified depending on their mechanism of action. Bulk-forming laxatives, or hydrophilic agents, are dietary fibre supplements made up of poorly digestible polysaccharides and of cellulose derived from some cereals and from psyllium. Commercial diets specifically supplemented with these fibres may be used; as an alternative, wheat bran (1-2 spoons per meal), cooked pumpkin pulp (1-4 spoons per meal) or psyllium powder (1-2 teaspoons per meal) may be added to the ration. These fibres act at intestinal level in combination with water, hence water intake should be stimulated by for example preparing meals supplemented with tepid water or broth, by giving fresh water frequently or using a cat drinking fountain with running water (many cats prefer to drink running water as against still water). This class of laxatives is useful for the dietary management of cats affected with mild cases of constipation, while it is not advisable in the presence of irreversible obstipation and megacolon, in which a low residue diet is more appropriate.

Emollient laxatives, such as dioctyl sodium sulfosuccinate and dioctyl calcium sulfosuccinate work by increasing lipid mixability in ingested food and hence by increasing their absorption, while at the same time decreasing water resorption. The efficacy of these active principles has not yet been definitively confirmed as the inhibition of water resorption has been shown only in isolated, in vitro colon segments.

Lubricant laxatives include mineral oil and Vaseline oil (or liquid paraffin), which thanks to their lubricating action can facilitate the transit of fecal material while at the same time hindering water absorption. Their efficacy is however limited and are therefore indicated only in the milder forms of constipation. Mineral oil administered per os presents greater risks of aspiration (with consequent aspiration pneumonia), especially in debilitated cats, consequently only the rectal route of administration is recommended.

Hyperosmotic laxatives include poorly absorbable polysaccharides such as lactose and lactulose, magnesium salts such as magnesium citrate, magnesium hydroxide and magnesium sulphate, and polyethylene glycol.

Lactose may be moderately effective in subjects not used to taking it regularly, while lactulose is considered the most effective hyperosmotic laxative in the cat species. The fermentation of lactulose within the bowel generates organic acids that stimulate both liquid secretion and colonic propulsive motility. In cases of moderate constipation, lactulose is administered per os at the dosage of 0.5 ml/kg every 8-12 hours until obtaining the production of soft stools. The dosage is then adjusted for the long-term management of the patient, finding the correct minimum effective dose. The use of magnesium salts is currently not recommended in cats, however some veterinarians have reported anecdotal cases of success with the use of polyethylene glycol.

Finally, stimulant laxatives, such as bisacodyl, phenolphthalein, castor oil, cascara and senna, are capable of directly stimulating the propulsive motility of the colon. Bisacodyl, in particular, used at the dosage of 5 mg/cat every 24 hours per os, seems to be the most effective of these agents in cats, acting as a stimulant for colonic epithelial cells, by increasing their secretory action, as well as for myenteric neurons, by favouring their depolarization. The long-term daily use of this active principle for the management of chronic patients is not recommended, as prolonged use may damage myenteric neurons.

Prokinetic agents are another drug class that is of great use in the management of feline constipation. Serotonin 5-HT4 receptor agonists, in particular, such as cisapride, prucalopride, tegaserod and mosapride are capable of stimulating colonic propulsive motility with minimum adverse events._¹² Cisapride has in the past been the object of many in vitro studies, showing the capacity of this active principle, which activates 5HT4 receptors of intestine smooth muscles, to promote gastric emptying and to increase the propulsive motility of both the small bowel and the colon. Unfortunatly, in July 2000 cisapride was withdrawn from many markets, due to documented human cardiac adverse events, and has since become practically unavailable. Prucalopride seems to be a promising alternative, however further studies will be necessary in order to define its use in dogs and cats. Misoprostol is a prostaglandin E1 analogue used as a gastroprotectant against NSAIDs; existing trials have shown the capacity of this active principle to trigger intestinal migrating motor complexes while also increasing colonic propulsion activity in both cats and dogs. In view of the limited adverse events (among which abdominal pain, cramps and diarrhoea) it may be useful in cases of severe refractory constipation in both dogs and cats.

Finally, histamine H2-receptor antagonists, such as ranitidine and nizatidine, inhibit tissue acetylcholinesterase and allow the accumulation of acetylcholine within the motor end-plates, thus stimulating colon motility. For the time being, studies have shown this effect only in vitro and the degree of efficacy in vivo is still to be confirmed. Cimetidine and famotidine do not instead produce the same effect, even if they belong to the same drug class.

In cases in which the manual extraction of the fecal impaction is necessary, the patient must be suitably rehydrated before undergoing the procedure, which is to be carried out under general anaesthesia. As colon manipulation may trigger vomiting, endotracheal intubation of the patient is always recommended in order to prevent the aspiration of ingesta.

The fecal mass must be broken up with abdominal palpation following rectal infusion of lukewarm water or isotonic saline solution. To break up harder fecalomas, atraumatic tools (such as cotton swab forceps) may be introduced, with great caution, into the rectum. It must be understood that a colon which has been suffering for many days because of constipation may have limited resistance to manipulations and be at higher risk of perforation, especially when the severity of the constipatory condition has caused colonic devitalization.

Maintenance treatment with the goal of reducing the frequency of relapses is of fundamental importance in patients with idiopathic megacolon; the focus should be on controlling predisposing factors and on promoting the combined use of dietary and pharmacological measures so as to guarantee daily evacuations, or at most every other day.

In cases of idiopathic dilated megacolon completely refractory to medical treatment, a subtotal colectomy is indicated.

According to some authors, in cats with hypertrophic megacolon secondary to pelvic fractures which has been present for less than six months an early pelvic osteotomy, without the need for a subsequent colectomy, may be sufficient._¹⁰

Subjects undergoing colectomy usually have a favourable prognosis; in some cases, a mild-to-moderate diarrhoea may persist for 4-6 weeks following surgery.

PROGNOSIS

Cats with mild or moderate constipation may have a positive response to medical treatment and in some cases may avoid future relapses, while in other cases the condition progressively evolves into a state of megacolon. The start of an early maintenance treatment using prokinetics and one or more laxatives may prevent the progression of constipation into irreversible obstipation and dilated megacolon. With the progression of the disease some subjects may however become refractory to medical treatment and finally require a surgical approach.

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