Megacolon

Description

Megacolon is a state of permanently increased diameter of the large bowel. This change in intestinal structure leads to abnormal function, including reduced colonic motility and chronic constipation. The condition most commonly occurs in cats and dogs, but pigs can also be afffected. 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 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.

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 function³. Aquired megacolon can occur in both cats and dogs as a sequel to any disease or lesion that interferes with normal defecation: faecal retention caused dilatation of the colon and impairs colonic motility. Causes could include neuromuscular abnormalities (spinal cord disease, intervertebral disk disease, dysautonomia, trauma), metabolic disorders (severe dehydration, hypokalaemia), drug therapy (vincristine, anticholinergics, barium), mechanical obstruction (pelvic fracture malunion, foreign bodies, stricture, anal/rectal atresia) and conditions causeing dyschezia (anal sacculitus, perianal fistula, trauma preventing posturing, procititis). After megacolon has persisted for several months, it is unlikely that normal colonic motility will be restored after resolution of the underlying cause. In many cases, the aetiology of megacolon is not determined.

Signalment

Manx Cat. Source: Wikimedia Commons; Author: Jonik (2004)

Megacolon much more commonly affects cats than dogs, but any age, breed or sex of animal may develop aquired megacolon. Idiopathic megacolon is more common in middle-aged to older cats, and there is some evidence for an increased risk in Manx cats due to a sacral spinal cord deformity.

Diagnosis

Clinical Signs

Idiopathic megacolon is likely to be a long-term, recurring problem, with clinical sgns presenting over months to years. In acquired megacolon, signs can be acute or chronic in onset. The primary sign of megacolon is constpation or obstipation (severe constipation caused by intestinal obstruction). Animals suffer tenesmus when defaecating and produce little or no faeces. Faeces that is expelled is very hard and dry, and animals defaecate less frequently. After prolonged tenesmus, a small amount of often mucoid diarrhoea may be expelled. Vomiting, anorexia and weight loss are other key features. On clinical examination, an enlarged colon packed with hard faeces is palpated within the abdomen. The animal may be dehydrated, and the hair coat rough. Digital anorectal examination is necessary, and confirms faecal impaction as well as potentially revealing an underlying obstructive cause.

Megacolon must be differentiated from other causes of palpable colonic masses. These could include adenocarcinoma, lymphoma or intussusception and can be distinugished on the basis of texture, rectal examination and diagnostic imaging. Tenesmus can be caused by colitis as well as megacolon, and palpation, rectal examination and imaging can again be used to exclude this, Dysuria or stranguria may also look similar to the tenesmus caused by megacolon. Palpation of the bladder and colon and urinalyis will help eliminate this differential diagnosis.

Laboratory Tests

Haematology and biochemistry may give evidence of dehydration, such as raised packed cell volume, total protein and urea/creatinine. A stress leukogram may also be seen. The duration of constipation may result in changes to blood electrolytes, and dehydration can lead to pre-renal azotaemia. Urinalysis should always be perfored to rule out lower urinary tract disease as a differential diagnosis and to check renal function in dehydrated animals.

There are no laboratory tests specific for megacolon.

Diagnostic Imaging

Plain radiographs should be taken of the abdomen and pelvis in an attempt to confirm megacolon and to investigate underlying causes. The enlarged, faces-filled colon is easily seen on these x-rays. Subsequent contrast studies or abdominal ultrasound may help identify an underlying cause, such as mural or obstructive masses.

Endoscopy

Colonoscopy may be necessary to rule out obstructive masses in the large intestinal wall or lumen.

Pathology

The colon is found to be dilated and impacted with faeces, with the most sever dilatation occuring in the transverse and descending colon⁴. 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.

Treatment

The term subtotal colectomy describes the removal of the majority of the colon. There are two variations of the technique depending on whether the ileocaecocolic valve is preserved or removed. This structure prevents reflux of colonic contents into the small intestine; therefore, some surgeons consider that removing the valve will predispose to bacterial overgrowth in the small intestine, steatorrhoea and deconjugation of bile salts. The rationale for excision of the valve is that it minimises the recurrence of segmental megacolon and associated constipation. For the surgeon, an added complication is that differentiation between diseased and normal tissue cannot be reliably made by gross evaluation at surgery. Removing the majority of the colon will undoubtedly have certain metabolic effects on the animal. The colon normally acts to absorb water along an osmotic gradient created by active sodium absorption. Bicarbonate ions are secreted in exchange for chloride ions, and potassium is lost from extracellular fluid; in addition, mucus and cells are shed into the colon. Questions remain about water and electrolyte homeostasis, and absorption of specific nutrients such as vitamin K following subtotal colectomy. Enteric function following subtotal colectomy has been evaluated in both normal and clinically affected cats and no significant clinical or subclinical evidence of abnormal bowel function was found (Gregory and others 1990). Histology of the small intestinal wall following subtotal colectomy showed increases in villus height, enterocyte height and density, with an increased mucosal absorptive area. Serum cobalamin and faecal sodium levels were higher in cats treated surgically, and faecal potassium was low; however, the significance of these findings is unknown. Tenesmus and loose faeces are often seen immediately postsurgery, with faeces gradually progressing to soft but formed by about three months; resection of the ileocaecocolic valve is, however, associated with persistence of looser stools. The issue of exactly how much of the colon to remove will remain controversial until more information is published regarding longterm outcome and function. However, the author’s preference in the majority of cases is to resect the ileocaecocolic valve and perform enterocolostomy. If the ileocaecocolic valve is to be preserved, the colon is transected 3 cm from the caecum to ensure a tension-free colocolostomy (the short mesocolon restricts mobility). It is essential that there is no longitudinal tension on the anastomosis site postoperatively, as this is likely to lead to dehiscence, with potentially disastrous results. If the valve is resected and an enterocolostomy performed, the disparity in luminal widths can be dealt with by oblique incision of the ileal portion combined with a short antimesenteric incision to spatulate the intestine. A single layer of full-thickness appositional sutures using polydioxanone in a simple interrupted pattern is the author’s preference. An end-to-end or end-to-side anastomosis can also be created using surgical stapling equipment. It should be noted that older surgical procedures such as coloplasty and partial colectomy provide little or no long-term improvement and are not recommended. Complications associated with subtotal colectomy Operative complications are uncommon. Where reported, complications generally relate to the anastomotic procedure; for example, technical errors leading to dehiscence, or localised peritonitis resulting from contamination at surgery. The most commonly reported complication in the long term is recurrence of constipation. The majority of these individuals can be managed medically, but some cases will require repeat surgery.

References

1. 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.
2. Pratschke, K (2005) Surgical disease of the colon and rectum in small animals. In Practice, 27, 354-362.
3. 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.
4. YODER, J. T., DRAGSTEDT, L. R. 2nd & 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.
5. LY, J T (1977) Surgical correction of megacolon in a cat. Australian Veterinary Practitioner, 7, 210.