Colonic anatomy and physiology
The large intestine serves numerous physiological roles: maintenance of fluid and electrolyte balance, storage site for faecal material, and reservoir for a complex microbial ecosystem that performs diverse functions. Normal colonic physiology may be disrupted by disease that causes diarrhoea or constipation.
The average length of the colon in dogs and cats is 60-75 cm, and it is divided into three anatomically distinct regions: the ascending, transverse and descending colonic segments (). The ascending colon is the shortest segment and is attached to the ileum and caecum at a juncture termed the ileocolic orifice. The caecum is a diverticulum of the ascending colon that ranges in length up to 15 cm in dogs and lies midway between the right flank and median plane and is ventral to the duodenum. The ileocolic orifice is usually a button-like structure in dogs but may be slit-like in cats. It functions to separate the small and large intestine but also serves to prevent oral movement of faeces and bacteria. The ascending colon becomes the transverse segment at the right colic (hepatic) flexure where it lies cranial to the duodenum, near the pyloric region of the stomach and left lobe of the pancreas. The transverse colon terminates at the left colic or splenic flexture in the cranial abdomen. The descending colon is the longest segment and passes caudally in the sublumbar region along the media/ ventral border of the left kidney, and then gradually inclines to the pelvic region where it becomes the rectum. The rectum is generally considered to begin at the pelvic inlet and continues through the pelvic canal terminating at the anus.
There are four histologically distinct layers in the colon and rectum: the mucosa, submucosa, muscularis and serosa. In contrast to small intestinal mucosa, the colon has a more compact microstructure which lacks villi, is composed of columnar epithelial cells arranged in parallel crypts, and is richly supplied with mucus secreting goblet cells (). The muscular layers of the colon are similar to those in the remainder of the gastrointestinal (Gl) tract and consist of an inner circular and outer longitudinal layer. The colon has a well known and important role in conservation of water, sodium and chloride ions, and it is the site of storage of intestinal waste products until they are expelled as faeces.
There is increasing evidence that the microbial population of the colon plays a unique and important role in digestion and metabolism in the Gl tract. Fermentation of dietary fibre and other poorly digestible carbohydrate (CHO) sources by colonic microbes produce short chain fatty acids (SCFAs), carbon dioxide, water, methane and hydrogen gas, and these byproducts have many effects on the luminal ecosystem and colonocytes. Enzymes produced by the colonic microflora convert primary bile acids into secondary bile acids, which are known colonic irritants and tumour promoters and may contribute to development of inflammatory bowel disease (JBD) or neoplasia.
The proximal colon, which functionally corresponds to the ascending and transverse segments of the colon, is the primary site of fermentation of undigested CHOs and proteins by the bacterial flora. Dietary fibre is the primary substrate for bacterial fermentation but not all dietary fibres are fermented equally. Fibres have been categorized (Table: Summary of the physiological effects of fibres) based on their fermentability.
- Highly fermentable fibres
- Moderately fermentable or mixed fibres
- Poorly fermentable fibres.
The highly fermentable fibres produce the largest quantities of SCFAs but the fermentation process also reduces faecal bulk. Conversely, the less fermentable a fibre is, the more it retains its structural characteristics and faecal buik but a lower concentration of SCFAs are produced. The production of SCFAs (e.g. acetate, propionate and especially butyrate) is important because these products are utilized by the colonic epithelial cells as a source of metabolic energy and are a substrate for cellular lipid synthesis. Furthermore, the production of SCFAs also serves several other important purposes:
- They are the primary cations in the colonic lumen
- They help to maintain a more acidic pH and are involved in sodium and water fluxes in the colon
- The acidic luminal pH reduces the ionization of long chain fatty acids and bile acids (both known colonic irritants)
- They increase the concentration of ammonium ions so that they are removed in the faeces.
The distal colon is the major site of faecal storage and is important in determining the final water content of faeces. The colon has a large capacity for absorption of fluid which is dependent on active sodium absorption coupled with the absorption of SCFAs. Diarrhoea occurs when either the reserve capacity of the colon to absorb water and electrolytes is exceeded, or when there is abnormal colonic function.
The motor activity of the colon includes both segmentation and propulsive movements. Segmentation, controlled by the slow-wave activity of colonic smooth muscle, is responsible for mixing colonic contents which slows the passage of ingesta. Rhythmic segmentation within the colon is mediated by the intrinsic nervous system. Propulsive motor activity propels faeces by three patterns; peristaltic activity, reverse peristalsis and mass peristalsis. Peristalsis is the aboral movement of ingesta through the colon by tonic contractions of smooth muscle. These contractions are stimulated by luminal distension. Reverse peristalsis is unique to the proximal colon and is characterized by contractions that move orally. The motility pattern in the proximal colon is a combination of both segmentation and reverse peristalsis, which serves to thoroughly mix food and allows for complete absorption of nutrients. Mass peristalsis is responsible for motility that propels the colonic contents caudally. In the distal colon, the primary motor pattern is segmentation and propulsion with mass peristaltic activity triggering the act of defecation. Decreased segmentation or propulsion leads to the development of diarrhoea while increased segmentation may cause constipation.
Table: Summary of the physiological effects of fibres
|Type of fibre||Physiological effect|
|Highly fermentable fibre sources||Good source of short chain fatty acids
High incidence of diarrhoea and excess gas production
|Moderately fermentable fibre sources||Good source of short chain fatty acids
Provide ideal faecal dry matter and defecation frequency
Increase colonic weight
Increase colonic absorptive surface area
Produce colonic hypertrophy
Have a low incidence of cryptitis and mucus distension
|Non-fermentable fibre sources||Reduced short chain fatty acid production (in vitro)
Increase the dry matter content of the faeces and may actually exacerbate constipation
May increase retention time
Decrease colonic weight, surface area, crypt depth
May (ead to cryptitis and mucus distension
The storage and voluntary evacuation of faeces is under the control of the muscles and nerves serving the pelvic canal, rectum and anus. The ability to retain faeces, to perceive that the rectum is full and to determine appropriate defecation is termed faecal continence. Abnormal function of these muscles or nerves results in loss of faecal continence or constipation but rarely leads to haematochezia or mucoid diarrhoea. Alternatively, inflammatory disorders of the rectal mucosa result in tenesmus, increased faecal mucus, haematochezia or frequent defecation. Faecal incontinence may occur secondary to reservoir function failure in dogs with severe rectal inflammatory disease.
Anatomy and physiology
The origin of the rectum is the pelvic inlet and it ends at the anal canal. The involuntary smooth muscle of the internal anal sphincter and striated muscle of the voluntary external anal sphincter surround the anal canal and determine its function. Paired anal sacs in this region may become impacted with secretions leading to infection orabscessation. Circumanal glands (non-secretory, subcutaneous sebaceous glands located in the anal subcutaneous zone) may grow in intact male dogs throughout life and form adenomas due to constant androgen exposure. Anal glands are tubular sweat glands located near circumanal glands, which produce a fatty secretion of unknown function ().
The innervation of the anorectum consists of a well defined myenteric and a submucosal plexus. The innervation to the anus includes branches of the pelvic nerve that provide parasympathetic fibres, which are excitatory to the rectum and inhibitory to the internal anal sphincter. Sympathetic fibres arise from the hypo-gastric nerve and are inhibitory to the rectum (i.e. causing relaxation) and excitatory to the internal anal sphincter (i.e. causing contraction), thus allowing appropriate storage of faeces. The function of the external anal sphincter allows maximum distention and storage of faeces in the rectum while maintaining anal control. Faecal incontinence can develop in animals with pudendal nerve damage since external anal sphincter function is impaired.
History and clinical signs
Most colorectal diseases cause overt signs of large bowel diarrhoea, tenesmus and/or dyschezia (e.g. painful defecation) (Table: Causes of tenesmus and dyschezia). Large bowel diarrhoea is characterized by frequent (3-6 times normal) urges to defecate, with each bowel movement producing small volumes of faeces that contain excessive mucus and sometimes fresh blood (i.e. haematochezia). Urgency may be seen with coionic or rectal diseases while dyschezia occurs most often with recta! disorders. Since the colon principally functions to absorb water and electrolytes, systemic signs attributable to nutrient malabsorptron (e.g. voluminous faeces, significant weight loss) are uncommon. Animals are generally alert, active and well fleshed with normal appetites on presentation. Occasionally, dogs may exhibit abdominal pain or discomfort with colitis. Furthermore, some diseases (i.e. IBD, alimentary lymphoma, mycotic enterocolitis) may cause concurrent small intestinal signs.
Table: Causes of tenesmus and dyschezia
|Colorectal disease||Perlneal-perianal disease||Miscellaneous|
Colitis-procffis (e.g. inflammatory bowel disease, histoplasmosis, enterotoxicosis, protothecosis)
Neoplasia or polyps
Irritable bowel syndrome
|Anal sac neoplasia
Anal sac abscess or impacfon
|Neoplasia of urethra, bladder, vagina or prostate
Pregnancy and parturition
Prostatic enlargement (benign hypertrophy, paraprostatic cyst, neoplasia); prostatic inflammation (prostatitis, prostatic abscess)
Urinary calculi (cystic or urethral)
Pelvic trauma or disease
Caudal abdominal mass or iymphadenopathy
The following historical concerns are of significance in patients suspected of having colorectal disorders:
- Is the diarrhoea acute or chronic? Acute, self-limiting large bowel diarrhoea is common and rarely requires an in-depth diagnostic evaluation
- Are clinical signs static, progressive or cyclical? Colorectal neoplasia may cause progressive signs while coionic IBD is characterized by a waxing/waning clinical course
- Is there evidence of dietary, environmental, parasitic or infectious causes for large bowel diarrhoea? Dietary and parasitic causes may constitute up to 50% of clinical cases
- What type of diet is the patient being fed? Note recent dietary changes (this might incriminate responsible nutrients), the amount and frequency of feeding, and the administration of medications (e.g. antibiotics, narcotics, motility modifiers, laxatives) that might alter coionic function
- Do clinical signs resolve when the animal is fed a hypoallergenic or elimination diet? This might suggest the presence of food hypersensitivity or intolerance
- A positive response to glucocorticoid therapy may indicate the presence of inflammatory or immune-mediated disease, such as IBD or perianal fistula
- Does the animal roam freely? If so, parasitic, toxic and infectious causes for colorectal diseases should be considered more likely
- Does the animal’s trave! history suggest an increased risk of disease with a regional incidence, such as histoplasmosis (Midwest USA) or intestinal parasites (Southern USA)?
The physical examination in most animals with colonic disease is normal. Abdominal palpation is important in assessing for prostatomegaly, genitourinary disorders, faecal impaction and intussusception, which can cause tenesmus or dyschezia (). A carefully performed digital examination of the rectum should be undertaken tosearch for rectal masses, pain, diverticulum (perineal hernia), stricture (rare), increased mucosal granularity and perianal disease. Additionally, rectal evaluation affords the opportunity to collect fresh faeces for diagnostic evaluation (e.g. presence of haematochezia, faecal mucus) and provides valuable exfoliative specimens for cytological review.
Diagnostic strategies for colorectal disorders vary considerably depending on the severity, chronicity of signs, presence of systemic illness and historical or likely responses to therapy (). Full haematology, biochemistry profile and urinalysis should be obtained from all animals with colorectal disease on admission.
This serves to rule out metabolic or systemic disease that may result in large bowel diarrhoea,
An adequate faecal examination for intestinal parasites is required in all patients. Some parasites (e.g. Trichuris vulpis and Giardia spp.) are notoriously difficult to diagnose necessitating performance of serial faecal flotations. In order to identify Giardia cysts, a zinc sulphate (ZnSO4) flotation solution is recommended. Direct saline faecal smears are used to detect motile trophozoites of Giardia, Trichomonas, Balantidium and Entamoeba ().
Faecal cytological examination involves evaluation of stained rectal/colonic mucosal scrapings under high power or oil immersion to identify aetiological agents and inflammatory cells. A flat conjunctival spatula (carefully advanced digitally) procures specimens of excellent quality, which should be placed on a microscope slide, air-dried and stained with Diff-Quik or Wright’s stain. Increased numbers of leucocytes indicate a possible inflammatory or infectious aetiology. The presence of fungal (Histoplasma) organisms, neoplastic cells, ora predominance of spirochaetes or clostridial spores also suggests a possible cause for large bowel signs.
Faecal cultures are useful when faecal cytology shows evidence of an inflammatory diarrhoea or when infectious diarrhoea is suspected. The major bacterial pathogens in dogs and cats include Campylobacter jejuni, Salmonella spp. and Clostridium. Note that faecal specimens for culture must be fresh, of adequate quantity (e.g. small pea size amount ot stool) and transported rapidly for inoculation into enrichment media ().
Imaging is infrequently diagnostic in animals with colorectal disease. However, survey abdominal radiography may identify radiopaque foreign objects, faecal impaction or extraluminal obstruction (e.g. pelvic canal stenosis, prostatomegaly, regional lymphadenopathy). Pneumocolon can be performed as a simple alternative to barium enema tohighlightluminal masses or when colonic displacement (via mass, organomegaly, adhesions) cannot be determined by survey abdominal radiography.
Colonoscopy with mucosal biopsy often helps provide a definitive diagnosis of colonic mucosal disease. Either flexible colonoscopy or rigid proctoscopy may be performed. Rigid proctoscopes are inexpensive and easy to use but only permit endoscopic evaluation of the distal colon and rectum. Flexible endoscopy is generally preferred as it enables evaluation of the entire colon, caecum and potentially the distal ileumvia retrograde ileoscopy. Proper patient preparation is critical for successful colonoscopy. It is begun by withholding food for 18-24 hours before the endoscopic procedure. In dogs, the administration of two or three doses of a colonic electrolyte lavage solution (e.g. polyethylene glyco/ electrolyte solution at a dose of 20 ml/kg given 4-6 hours apart orally) the afternoon before endoscopy will adequately cleanse the colon. This is followed by the administration of a warm-water enema the morning of the procedure to optimize mucosal visualization. Cats are typically prepared for colonoscopy by administering several tepid water enemas (20 ml/kg rectally for each enema procedure), although oral lavage solutions can be given via a nasooesophageal tube using a syringe pump.
Abnormal endoscopic observations may include erythema, increased mucosal granularity, increased friability and erosions. Mass lesions (e.g. granuloma-tous, inflammatory or neoplastic) are predominately observed in the descending colon and rectum (). Colonicmucosal biopsy specimens (10-12 total from all three colonic regions) are always obtained for histopathological interpretation regardless of the endoscopic appearance.