Diseases of the Anus and Rectum
World Small Animal Veterinary Association World Congress Proceedings, 2005
Michael Willard

Clinical signs of anal/rectal disease typically include constipation, dyschezia, and/or hematochezia. Diseases that commonly involve the anus and rectum include anal sacculitis, perianal fistulas, perineal hernia, and proctitis. Occasionally, one may find unusual granulomatous lesions or neuropathies/myopathies affecting the area.

The main causes of constipation include inappropriate diet, pain upon defecation (including musculoskeletal pain when trying to posture), colonic obstruction, and colonic weakness. Diets that include undigestible substances (e.g., plastic, sand) can be an important cause of constipation, especially in dogs with undiscriminating dietary habits. Pain during defecation can be caused by any number of anal/rectal lesions (e.g., anal sacculitis, proctitis, perianal fistulas, pelvic fractures, lumbosacral disease). Obstruction may be due to stricture (e.g., carcinoma, pythiosis, congenital, cicatrix) or deviation of the colonic lumen (e.g., perineal hernia). In dogs, hypothyroidism and hypercalcemia are well recognized causes of colonic weakness while in cats, idiopathic megacolon is the most common cause. A good physical examination including a digital rectal examination is the first step in making a diagnosis--pelvic fractures, constrictions, anal sac disease, and perineal hernia should all be exam room diagnoses. If it is necessary to sedate or anesthetize the patient in order to do an excellent examination, then the clinician should not hesitate to use drugs. This part of the physical examination is so important that doing a poor exam without chemical restraint is unacceptable. Trying to obtain a sample of feces to determine if it is hard, contains inappropriate substances, or is of normal consistency is part of the rectal examination in constipated dogs. It is important to note that it can be easy to miss partial rectal strictures in large dogs. A 45 kg dog may have a stricture that decreases the anal orifice by 75% but which is not noticed when the clinician inserts a single finger into the anus. If the history suggests a rectal obstruction in a large dog, then one may have to try to insert two (or in the case of people with small hands, three) fingers in order to see if the anus and rectum can expand to its normal diameter. Occasionally radiographs and endoscopy are required, but that is genuinely uncommon.

Dyschezia is caused by many of the same problems as constipation, and the approach is the same as for constipation.

Hematochezia is a relatively common sign of anal/rectal disease, as well as colonic disease. Anal sacculitis and neoplasms (including benign polyps and carcinomas involving the mucosa) are the two most common causes of anal/rectal disease causing hematochezia. Both should be diagnosed by rectal examination, although it can be very hard to find polyps in some dogs. Polyps are typically relatively soft, and a cursory digital examination can miss them or think that it is simply a fold of mucosa or some feces on the mucosal surface. If the history is suggestive of a proliferative rectal lesion, then one should not hesitate to keep performing the digital rectal examination for several minutes while carefully examining the entire mucosal surface over and over.

Perianal fistulas can be very easy to identify by simply looking under the tail--assuming the patient is not in so much pain that it tries to bite you as you lift the tail. Dogs with sloping tail heads are at increased risk, but any dog can get them. There are at least two situations in which diagnosis can be confusing. First, sometimes one will be examining a patient before the fistulas break open to the outside (i.e., they are still internal and will break open and form the fistulas tracts in the future), in which case it can be much harder to find them by digit examination and maybe proctoscopy. Second, there are a few other diseases that can look like exactly like perianal fistulas during a rectal exam, such as rectal pythiosis if it has formed fistulas. It is important to make the correct diagnosis because perianal fistulas have a much better prognosis than pythiosis. If you practice in an area without pythiosis, then physical examination is generally sufficient for a diagnosis (although sedation is often needed to get a good examination of the anal area). While surgery was once the treatment of choice, now immunosuppressive medications (e.g., cyclosporin or azathioprine) combined with antibacterial therapy (e.g., erythromycin, metronidazole) are clearly preferred. While quite effective, the biggest disadvantage of cyclosporin is its cost. Some dogs need to be treated for months and then may relapse and have to be treated again, and the only way to know if you are administering enough is to do measure blood levels of cyclosporin (which is also expensive). Therefore, cost can be a major impediment to therapy. In an effort to lower the cost, some people simultaneously administer ketoconazole in an effort to prevent metabolism of the cyclosporin and thereby achieve higher blood levels with lower dosing. However, therapeutic blood monitoring becomes critical as you cannot begin to anticipate what the blood levels will be. Therefore, some clinicians just treat with azathioprine which, although slower and not as reliable as cyclosporin, is considerably less expensive.

Rectal tumors are very easy to find once they are far advanced. However, in the early stages careful, methodical physical examination is necessary to find them. Submucosal carcinomas often do not disrupt the mucosa appreciably until very late in the course of the disease. Unfortunately, these tend to be scirrhous tumors which are very invasive and metastasize relatively early. It is important to diagnose them early so that the clients do not waste a lot of time and effort on tests and treatments that will not benefit the patient. It may be important to try to insert 2 or 3 fingers into the rectum of an anesthetized dog (if it is big) to find that a stricture exists. The most common mistake is superficial biopsies that only sample the mucosa and not the underlying submucosa (which is the most reliable site to identify the tumor).

Rectal polyps are also easy to find once they are far advanced, but in the early stages they are very easy to miss on physical examination because they are soft and feel like a fold of mucosa. It is impossible to distinguish a benign polyp from a malignant mucosal growth on physical examination or at endoscopy--histopathology is necessary. It is valuable to biopsy such lesions using a rigid biopsy forceps (as opposed to a flexible biopsy forceps) as this type allows a larger sample that can easily include copious amounts of submucosa. Being able to examine the submucosa can really help the pathologist decide if the mass is benign or malignant in difficult cases. Finding epithelial cells in the submucosa is prima facia evidence of malignancy. Fortunately, it is extremely rare that benign polyps undergo malignant transformation. If the polyp is removed entirely, it should not recur. Surgical removal after everting the rectal mucosa is the preferred way to remove such polyps. One may try electrocautery loops through a flexible endoscope, but this is probably not to be recommended unless the polyp is far enough inside the rectum that it cannot be everted during surgery.

Perineal hernias are seen frequently enough in dogs, but occasionally can also be found in cats. A cause of constipation, one may often (but not invariably) see distention of the perirectal area due to feces impacting in the hernia. Digit rectal examination should be diagnostic. Care must be taken whenever inserting anything into such a dog's rectum, lest one inadvertently perforate the hernia. If the urinary bladder or intestines find their way into the hernia, there can be urinary tract obstruction (post renal uremia) or incarcerated obstruction, both of which are emergencies.

Anal sacculitis is a relatively common disease, but severe abscessation is infrequent. It is important to distinguish an impacted or abscessed anal sac from an anal sac tumor (which can also cause constipation due to paraneoplastic hypercalcemia, thereby mimicking a more benign condition).


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Michael Willard, DVM

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