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The condition most commonly occurs in '''cats and dogs''', but pigs can also be affected. White foals suffering congenital colonic agangliosis, an autosomal recessive trait, may develop secondary megacolon.

The condition most commonly occurs in '''cats and dogs''', but pigs can also be affected. White foals suffering congenital colonic agangliosis, an autosomal recessive trait, may develop secondary megacolon.

In small animals, megacolon may be '''congenital or acquired''', which may be '''idiopathic'''. Although well described in human medicine, congenital megacolon has not been convincingly described in animals despite first being reported in 1988¹. In man, Hirschsprung's disease manifests at a very young age is caused by an absence of inhibitory neurons in Meissner’s submucosal plexus

In small animals, megacolon may be '''congenital or acquired''', which may be '''idiopathic'''. Although well described in human medicine, congenital megacolon has not been convincingly described in animals despite first being reported in 1988<ref name="Rosin">Rosin, E et al (1988) '''Subtotal colectomy for treatment of chronic constipation associated with idiopathic megacolon in cats: 38 cases (1979-1895).''' ''Journal of the American Veterinary Medical Association'', '''193''', 850-853.</ref>. In man, Hirschsprung's disease manifests at a very young age is caused by an absence of inhibitory neurons in Meissner’s submucosal plexus

and Auerbach’s myenteric plexus in the distal colon or rectum². This gives persistent smooth muscle contraction in the affected region and proximal dilation of the colon. A similar pathogenesis is proposed in cats.

and Auerbach’s myenteric plexus in the distal colon or rectum<ref name="Pratschke">Pratschke, K (2005) '''Surgical disease of the colon and rectum in small animals.''' ''In Practice'', '''27''', 354-362.</ref>. This gives persistent smooth muscle contraction in the affected region and proximal dilation of the colon. A similar pathogenesis is proposed in cats.

Acquired megacolon is more common than the congenital form, and in cats this is most often idiopathic. The true cause of "idiopathic" megacolon is thought to be an '''intrinsic defect in colonic smooth muscle function'''³. Acquired megacolon can occur in both cats and dogs as a sequel to any disease or lesion that interferes with normal defecation: faecal retention causes dilatation of the colon and impairs colonic motility.  

Acquired megacolon is more common than the congenital form, and in cats this is most often idiopathic. The true cause of "idiopathic" megacolon is thought to be an '''intrinsic defect in colonic smooth muscle function'''<ref name="Washabau">Washabau, R J and Stalis, I H (1996) '''Effects of cisapride on feline colonic smooth muscle function.''' ''American Journal of Veterinary Research'', '''57''', 541-546.</ref>. Acquired megacolon can occur in both cats and dogs as a sequel to any disease or lesion that interferes with normal defecation: faecal retention causes dilatation of the colon and impairs colonic motility.  

Causes could include:

Causes could include:

===Pathology===

===Pathology===

The colon is found to be dilated and impacted with faeces, with the most sever dilatation occurring in the '''transverse and descending colon'''⁴. Histologically, the colon is usually normal: the only cases in which histological abnormalities have been reported were mature adult cats, which is an atypical signalment for a congenital condition^1,4,5. The histological findings in these cats were aganglionosis and absent myenteric ganglia.

The colon is found to be dilated and impacted with faeces, with the most sever dilatation occurring in the '''transverse and descending colon'''<ref name="Yoder">Yoder, J T, Dragstedt, L R 2 and Starch, C J (1968) '''Partial colectomy for correction of megacolon in a cat. (Report of a case).''' ''Veterinary Medicine: Small Animal Clinics'', '''63''', 1049-1052.</ref>. Histologically, the colon is usually normal: the only cases in which histological abnormalities have been reported were mature adult cats, which is an atypical signalment for a congenital condition<ref name="Rosin"/><ref name="Yoder"/><ref name="Ly">Ly, J T (1977) '''Surgical correction of megacolon in a cat.''' ''Australian Veterinary Practitioner'', '''7''', 210.

</ref>. The histological findings in these cats were aganglionosis and absent myenteric ganglia.

==Treatment==

==Treatment==

Dogs and cats must have adequate access to litter or the outdoors to encourage regular defaecation, and '''regular exercise''' may be beneficial. Diets containing bulk-forming fibre may contribute to the recurrence of impaction, but a '''high-fibre diet''' may occasionally be useful in prevention. '''Stool softeners''' such as lactulose, and '''prokinetic drugs''' such as cisapride can promote motility and defaecation. Medical management is usually an '''ongoing, life-long commitment''' which may be frustrating for the owner, and relapse is common.

Dogs and cats must have adequate access to litter or the outdoors to encourage regular defaecation, and '''regular exercise''' may be beneficial. Diets containing bulk-forming fibre may contribute to the recurrence of impaction, but a '''high-fibre diet''' may occasionally be useful in prevention. '''Stool softeners''' such as lactulose, and '''prokinetic drugs''' such as cisapride can promote motility and defaecation. Medical management is usually an '''ongoing, life-long commitment''' which may be frustrating for the owner, and relapse is common.

<u>'''Sub-total colectomy'''</u> can be used to correct megacolon in the '''cat'''. However, this technique is very poorly tolerated in the dog. The term refers to the removal of the majority of the colon, and the ileocaecocolic valve may be preserved or removed. The ileocaecocolic valve prevents reflux of colonic contents into the small intestine and so some surgeons believe that removing this structure can predispose to small intestinal bacterial overgrowth, steatorrhoea and deconjugation of bile salts². However, removing the ileocaecocolic valve minimises the recurrence of megacolon.  

<u>'''Sub-total colectomy'''</u> can be used to correct megacolon in the '''cat'''. However, this technique is very poorly tolerated in the dog. The term refers to the removal of the majority of the colon, and the ileocaecocolic valve may be preserved or removed. The ileocaecocolic valve prevents reflux of colonic contents into the small intestine and so some surgeons believe that removing this structure can predispose to small intestinal bacterial overgrowth, steatorrhoea and deconjugation of bile salts<ref name="Pratschke"/>. However, removing the ileocaecocolic valve minimises the recurrence of megacolon.  

As yet, there is no established figure for how much colon to remove. If the ileocaecocolic valve is removed, an enterocolostomy is performed; if not, the colon is transected 3 cm from the caecum to ensure the following colocolostomy is not under tension from the short mesocolon. Any longitudinal tension on the anastomosis site is likely to lead to dehiscence postoperatively. In surgeries where the valve is resected, an oblique incision of the ileal portion and a short antimesenteric incision of the distal colon will allow for disparities in lumen diameter. The intestine is closed in a single layer using full-thickness appositional, simple interrupted suture in an absorbable material. Staples may similarly be used.

As yet, there is no established figure for how much colon to remove. If the ileocaecocolic valve is removed, an enterocolostomy is performed; if not, the colon is transected 3 cm from the caecum to ensure the following colocolostomy is not under tension from the short mesocolon. Any longitudinal tension on the anastomosis site is likely to lead to dehiscence postoperatively. In surgeries where the valve is resected, an oblique incision of the ileal portion and a short antimesenteric incision of the distal colon will allow for disparities in lumen diameter. The intestine is closed in a single layer using full-thickness appositional, simple interrupted suture in an absorbable material. Staples may similarly be used.

Although sub-total colectomy is an effective way of correcting megacolon, removal of majority of the colon has the potential to cause '''metabolic effects''' in the animal. For example, the colon normally functions to absorb water. This is achieved by the creation of an osmotic gradient by active sodium absorbtion. Additionally, extracellular potassium is lost and bicarbonate ions are exchanged for chloride ions in the colon. It could therefore be implied that sub-total colectomy '''disrupts water and electrolyte homeostasis''', but paradoxically cats undergoing sub-total colectomy have been found to show '''no clinical evidence of abnormal bowel function'''⁶. However, other, non-clinical changes were found: small intestinal enterocytes and villi were increased in height, the absorptive area of the small intestinal mucosa was increased, serum cobalamin and faecal sodium were increased and faecal potassium was low. The significance of these results is unknown.

Although sub-total colectomy is an effective way of correcting megacolon, removal of majority of the colon has the potential to cause '''metabolic effects''' in the animal. For example, the colon normally functions to absorb water. This is achieved by the creation of an osmotic gradient by active sodium absorbtion. Additionally, extracellular potassium is lost and bicarbonate ions are exchanged for chloride ions in the colon. It could therefore be implied that sub-total colectomy '''disrupts water and electrolyte homeostasis''', but paradoxically cats undergoing sub-total colectomy have been found to show '''no clinical evidence of abnormal bowel function'''<ref name="Gregory">Gregory, C R et al (1990) '''Enteric function in cats after subtotal colectomy for treatment of megacolon.''' ''Veterinary Surgery'', '''19''',216-220.</ref>. However, other, non-clinical changes were found: small intestinal enterocytes and villi were increased in height, the absorptive area of the small intestinal mucosa was increased, serum cobalamin and faecal sodium were increased and faecal potassium was low. The significance of these results is unknown.

After surgery faeces are '''loose''' and defaecation is associated with tenesemus, but faecal consistency improves over the following months. Complications associated with the surgery itself are rare and tend to be related to the anastomotic procedure². For example, the abdomen may become contaminated during surgery, leading to peritonitis, or the anastomosis site may break down. For this reason, the patient should be monitored for '''three to five days post-surgery'''.  

After surgery faeces are '''loose''' and defaecation is associated with tenesemus, but faecal consistency improves over the following months. Complications associated with the surgery itself are rare and tend to be related to the anastomotic procedure<ref name="Pratschke"/>. For example, the abdomen may become contaminated during surgery, leading to peritonitis, or the anastomosis site may break down. For this reason, the patient should be monitored for '''three to five days post-surgery'''.  

Long-term complications usually involve '''recurrence of constipation''', which can be managed medically in most cases. '''Stricture formation''' and persistent diarrhoea have also been reported.

Long-term complications usually involve '''recurrence of constipation''', which can be managed medically in most cases. '''Stricture formation''' and persistent diarrhoea have also been reported.

==References==

==References==

 

 

 

 

 

Petrus, D J, Nicholls, P K and Gregory, S P (2001) '''Megacolon secondary to autonomic ganglioneuritis in a dog.''' ''The Veterinary Record'', '''148(9)''', 276-277.

Petrus, D J, Nicholls, P K and Gregory, S P (2001) '''Megacolon secondary to autonomic ganglioneuritis in a dog.''' ''The Veterinary Record'', '''148(9)''', 276-277.