David A. Williams, MA VetMB, PhD, MRCVS, DACVIM, DECVIM-CA
Many gastrointestinal disorders are acute and self-limiting but there are a number of patients in which chronic vomiting and/or diarrhea and/or weight loss are persistent problems. Specific diagnosis in these patients is desirable in order to give as accurate a prognosis as possible and also to allow appropriate therapy to be prescribed.
Specific diagnosis of many gastrointestinal disorders is frequently time-consuming, invasive and expensive, and clinicians are continually on the lookout for improvements. Flexible fiberoptic endoscopy and videoendoscopy have revolutionized our approach to diagnosis in that they permit direct observation of the lumen of the gastrointestinal tract, thorough non-invasive evaluation of potential lesions, and specific diagnosis by mucosal biopsy. Foreign body removal is also possible and follow-up evaluations permit assessment of the response to therapy.
Gastrointestinal endoscopy is indicated in any patient with signs of regurgitation, vomiting, hematemesis, dysphagia, melena, hematochezia, tenesmus or diarrhea. It is also indicated when there are abnormal but not diagnostic radiographic findings, or when screening tests such as serum cobalamin, folate or trypsin-like immunoreactivity indicate the presence of gastrointestinal disease. Prior to endoscopy conventional basic tests (such as a complete blood count and serum biochemical evaluation, urinalysis and fecal examination for evidence of parasitism) should be performed. Animals should be de-wormed and treated for possible occult giardiasis, and trial dietary modification may be warranted prior to more extensive investigations.
Finally, it is particularly important to remember that many intestinal biopsies from many canine patients with small intestinal disease do not exhibit morphologic abnormalities on routine light microscopic examination. In many cases the underlying problem is a functional one in the intestinal mucosa, and so tests of intestinal function may be required to confirm the presence of malabsorption. It is strongly recommended that prior to, or in association with, endoscopic examination, functional tests for malabsorption be performed, specifically, assays of serum trypsin-like immunoreactivity, cobalamin and folate. This combined functional and morphologic approach to diagnosis is most likely to lead to accurate diagnosis and effective therapy.
The fiberoptic endoscope is based on the principal of total internal reflection of light in tiny flexible glass fibers. Light entering one end of the fiber is transmitted by reflection along the walls of the fiber until it emerges at the other end. To minimize light loss and to prevent light from one fiber scattering into adjacent fibers, each fiber is wrapped with insulation, usually a glass coating of a different refractive index. Many of these insulated fibers are combined to form a bundle approximately 1/4" in diameter. There are 2 separate fiber bundles in flexible endoscopes, one for viewing and one for light transmission. The fibers in the visual bundle are spatially oriented so that the top of the object viewed at one end is oriented similarly at the other (a "coherent" fiber bundle). A lens system at the mucosal end focuses the image on that end of the bundle, while a lens system at the proximal, or viewing end, magnifies the image emerging from the bundle. Direct internal transmission of light through this flexible glass bundle enables the projection of images from one end to the other through the curves and coils that the endoscope adopts as it is passed into the gastrointestinal tract.
Light for illumination at the mucosal end of the endoscope is transmitted from an external light source through a non-coherently oriented bundle. In most light sources light intensity can be varied for observation or photography.
In addition to the fiberoptic bundles, endoscopes contain a suction channel that can be used to evacuate mucus, fluid and blood from the lumen of the gut. Suction is controlled by a button or valve at the head of the instrument. A similar and adjacent mechanism allows control of a flow of air from an external pump into the lumen to dilate it and allow better visualization. Without such distention the walls would collapse around the tip of the endoscope impairing visibility. Another proximally controlled mechanism allows control of a fine spray of water over the lens to clean it of blood, feces or mucus.
Most endoscopes also have a channel for the passage of instruments such as biopsy or grasping forceps, cytology brushes or irrigation catheters. The channel is usually the same as that used for suction. Biopsy catheters are passed through the channel so that the tip lies in the lumen and specimens are obtained under direct observation. Specimens are usually small (1-2 mm thick) and need both careful handling and an experienced pathologist for interpretation. Endoscopes also contain a pulley system whereby the tip end can be moved in 4 directions (right and left, and up and down) by control knobs on the handle. Complete 360 degree examination is achieved by slight rotation of the endoscope in combination with deflection of the tip.
A variety of endoscopes are manufactured for use in human gastroenterology, some of which can be used in the small animal patient. Endoscopes are also now manufactured specifically for the veterinary market. A 9 mm forward viewing 120-140 cm pediatric gastroscope with a 2.8mm biopsy channel is perhaps the best instrument for general clinical use in small animal practice. The instrument can be used to examine the esophagus, stomach, proximal small intestine and colon in all but the largest patient. The instrument can also be used as a bronchoscope.
A 180 cm x 13 mm colonoscope allows complete examination of larger dogs and exotic species. The extra length is useful because the distance from the mouth to the pylorus is much longer in the dog than in man. With practice this endoscope can be passed through the pylorus and can be used for duodenoscopy.
Video endoscopes contain a small camera chip at the tip of the endoscope which transmits via a processor to a TV monitor and other accessories. The image is therefore transmitted along the length of the endoscope in electrical form and there is no coherent fiber bundle to transmit the image, as in conventional fiber optic endoscopes. These endoscopes also have facilities for recording or still photography. The advantage of these endoscopes is that they do not contain delicate fiber optics and are thus much more resilient. They also provide a much better picture. The major disadvantage is cost. Miniature video cameras which attach to the viewing head of conventional endoscopes are also available, but do not provide the clarity of images obtained using a video endoscope.
Conventional rigid endoscopes are much less expensive than flexible models and permit adequate examination of the esophagus and descending colon. They are often superior to flexible models for removal of foreign bodies firmly wedged in the esophagus.
Canine and feline patients should be anesthetized for all endoscopic procedures. This reduces risk to both the patient and the endoscope. For upper gastrointestinal endoscopy an overnight fast is all that is required to ensure that the stomach is empty prior to the examination. For flexible colonoscopy the patient should be fasted for at least 24 hours, receive a bulk laxative such as magnesium citrate the night before, as well as plain water enemas the night before and the morning of the procedure. The enema should be repeated until the effluent is clear. Colonic lavage solutions used in human medicine have recently been used in dogs. Doses of 40 ml/kg given by stomach tube twice on the day before the procedure and early on the day of examination are usually effective. Metoclopramide given on the day of colonoscopy will help evacuate the stomach prior to induction of anesthesia. An initial enema to help stimulate gastric emptying and evacuation of large amounts of solid feces from the colon will also help. Complete cleansing of the colon often takes 2 or 3 days, and requires much effort. Lavage solutions help eliminate much of the frustration due to conventional enemas and laxatives evacuating only the distal part of the colon.
Esophagoscopy is indicated in the patient with signs of dysphagia, regurgitation, excessive salivation, anorexia and recurrent pneumonia (due to aspiration). Detectable lesions include foreign bodies, strictures, diverticuli, esophagitis, megaesophagus, tumors and gastric reflux (reflux esophagitis).
For the procedure the patient should be anesthetized and a cuffed endotracheal tube inserted to prevent aspiration. The patient should be placed in left lateral recumbency (right side up), a mouth gag inserted and the lubricated endoscope inserted under direct observation. Air should be insufflated to dilate the lumen and the endoscope passed distally. The mucosa should be examined systematically and any lesions biopsied. It is technically difficult to take biopsies from normal esophageal mucosa owing to its smooth and tough texture. Thus the esophagus is a rare exception where tissue is not taken if no lesions are seen.
Foreign bodies wedged in the esophagus may often be removed using a rigid endoscope. It is surprising how great a diameter a rigid scope can be passed into the normal esophagus--always use the widest rigid scope you can to facilitate identification and removal of the foreign body. If the foreign body cannot be retrieved through the mouth, it may sometimes be pushed distally into the stomach. In the case of bones they are often sufficiently digested by gastric acid to allow passage through the gastrointestinal tract. Other objects may require removal from the stomach at exploratory laparotomy, a much safer procedure than thoracotomy and direct removal from the esophagus.
Examination of the esophagus should always be followed by examination of the stomach, if the endoscope available permits this.
Endoscopic examination of the mucosal surface of the stomach is indicated when clinical signs or physical findings suggest gastric disease, or when there is a need for clarification or confirmation of radiographic findings. In the vomiting patient gastroscopy should usually be performed before contrast radiographic studies since it frequently permits a tissue diagnosis while radiology does not. Gastroscopy permits specific diagnosis of tumors, differentiation of the several types of chronic gastritis, identification of ulcers, and confirmation and frequently removal, of foreign bodies. Abnormal gastric emptying can also be identified by observation of food or fluid in the stomach after an overnight fast. An underlying cause of gastric outlet obstruction such as mucosal hypertrophy may also be tentatively identified in some cases.
It is absolutely essential that the patient be in left lateral recumbency (right side up) for successful examination of the entire lining of the stomach, particularly if the pylorus and upper duodenum are to be evaluated. After the gastroscope is passed into the stomach, air should be insufflated to slightly distend the lumen.
The longitudinal glandular rugae of the body of the stomach are usually readily identified. The tip of the endoscope is then maneuvered along the greater curvature parallel to the rugae and in a dorsal direction until the incisure (fold in the lesser curvature) is identified as a band of tissue stretching across the lumen with apparent dorsal and ventral lumens. The ventral lumen leads to the antrum and pylorus while the dorsal lumen leads back to the cardia.
Gastric lesions are identified and biopsied, or foreign bodies are grasped and removed. Suspect gastric neoplasms should be deeply biopsied to maximize the chance of a tissue diagnosis (repeated samples from the same site are often useful in these cases, and cytologic as well as histologic examination may more reliably demonstrate the presence of neoplastic cells). The tip of the endoscope is then withdrawn and retroflexed (bent back on itself) to examine the fundus and cardia.
A detailed and systematic examination of the whole stomach is essential. Abnormal mucosa may be reddened, ulcerated, thickened and abnormally fibrosed, or abnormally friable such that it bleeds abnormally following contact with the tip of the scope or after biopsies have been taken. Whether or not lesions are seen, biopsies should be taken since microscopic lesions may be present even though the gross appearance is normal.
Gastroscopy may also be used to facilitate placement of a percutaneous gastrostomy tube. This type of feeding tube is particularly valuable for nutritional support of patients with mandibular and maxillary fractures, neurologic disease, megaesophagus, oral neoplasia, oronasal fistula, and hepatic lipidosis or other diseases associated with anorexia. These tubes are tolerated well by both cats and dogs and their owners, and can function as the sole route of delivery of food and water into the gastrointestinal tract for many weeks or even months if necessary.
Duodenoscopy is indicated in animals with diarrhea, melena or a palpably thickened intestine. The procedure facilitates the diagnosis of ulcers, foreign bodies, diffuse chronic enteropathies of any type, intestinal lymphosarcoma, fungal infections and giardiasis. Before the advent of flexible fiberoptic endoscopy, most if not all of these lesions required exploratory laparotomy and full thickness biopsy. Endoscopy therefore reduces both patient morbidity and the cost of diagnosis.
To enter the duodenum the tip of the endoscope should be brought up against the pylorus. As the pylorus relaxes the tip should be passed through with gentle pressure. Repeated insufflation an/or flushing with water often facilitates passage of the scope into the small intestine. The tip usually comes up against the wall of the pylorus or duodenum causing a temporary 'red out'. To visualize the lumen a small quantity of air is insufflated and the tip withdrawn a few millimeters. The mucosa of the duodenum then becomes readily apparent with the villi giving a velvet-like appearance to the mucosa. The light on the tip of the endoscope can be seen reflected on the villous tips, often producing a glistening appearance. Gentle pressure is applied and the tip of the endoscope is passed down the lumen as far as convenient with minimal insufflation under direct observation. Normal peristalsis usually facilitates this movement, and it often takes two or three minutes for the scope to pass as far as desired. Sometimes there is a temporary hold up as the scope negotiates the bend between the descending and ascending duodenum.
As with gastric mucosa, abnormal duodenal mucosa may be reddened, ulcerated, thickened and abnormally fibrosed, or abnormally friable such that it bleeds abnormally following contact with the tip of the scope or after biopsies have been taken. The most common abnormality is a granular or cobblestone appearance that is readily appreciated after a little experience with observing normal mucosa. Whether or not lesions are seen, biopsies should be taken since in the small intestine as in the stomach, microscopic lesions may be present even though the gross appearance is normal. Biopsies are taken from representative areas as the tip is withdrawn. Care should be taken to distinguish lesions seen on withdrawal of the endoscope that merely reflect normal trauma due to passage of the scope, from those that were truly present as the scope was first introduced.
Since most diseases of the small intestine are diffuse, biopsies are usually representative of the underlying problem. However, on occasion a focal or diffuse lesion may be present distal to the furthest point to which the endoscope can be passed. Nonspecific changes may be present proximal to such a lesion, for example lymphocytic-plasmacytic enteritis has been seen in the duodenum of a dog with diffuse alimentary lymphosarcoma affecting the jejunum.
Colonoscopy is indicated in the patient with chronic large bowel diarrhea (increased frequency of defecation, small volume feces, blood and/or mucus) and can be used to discriminate between the various types of colitis, colonic tumors and cecal inversion. Since most colonic lesions are diffuse, rigid colonoscopy can be used to diagnose the majority of lesions. This is faster and cheaper as the patient needs less preparation and no anesthesia. None-the-less flexible endoscopy is preferred in most cases, if available since a more thorough examination of the entire large bowel is generally possible.
For flexible colonoscopy, after adequate preparation the patient should be anesthetized and placed in left lateral recumbency (right side up). This positioning allows passage of the scope around the flexures into the transverse and ascending parts of the colon. This is usually difficult or impossible if the other abdominal viscera compress these bends.
For rigid colonoscopy the patient may be positioned in right lateral recumbency so that the contents of the descending colon (if any) drain back into the upper parts of the colon and do not obscure the view.
The perineum should be examined further and a rectal examination performed; next, the tip of the lubricated endoscope should be inserted and passed cranially with gentle pressure and minimal insufflation under direct observation. Rotation and redirection using a 'slide by' technique are frequently required to transverse the splenic and hepatic flexures.
The ileocolic junction is in the caudal abdomen so the tip of the colonoscope is pointing towards the operator by the time the mushroom-like ileocolic junction and adjacent cecal orifice are identified. The normal colonic mucosa reflects light evenly and submucosal vessels can be readily identified. Disappearance of these vessels is the first sign of edema and inflammation and this, coupled with excessive amounts of mucus in the lumen, are reliable indicators of abnormality. Bleeding lesions are rare in canine colonic disease, but ulceration is sometimes observed. Sequential representative biopsies should be taken as the endoscope is withdrawn. Stenotic areas are usually associated with tumors and, as in the case of gastric neoplasms, repeated samples from the same site are often useful in these cases since superficial tissue may only reveal nonspecific inflammatory change. Cytologic as well as histologic examination may more reliably demonstrate the presence of neoplastic cells.
Flexible endoscopy facilitates the diagnosis of many gastrointestinal disorders in the small animal patient. The equipment is cost effective, time saving, and most importantly, reduces patient morbidity in that it often negates the need for exploratory surgery. Most practitioners recoup the initial investment in the equipment within a 12 to 18 month period. When purchased the equipment should not be locked away, but should be left set up and available for instant use. This encourages regular use and ensures that the practitioner becomes familiar with the equipment and expert in its use.
The fibers are sensitive and the instrument should be handled delicately. Regular and thorough cleaning is also essential. With care most endoscopes last for 10-15 years without the need for service or repair.
It should be appreciated that endoscopy is not always the complete answer. Many gastrointestinal tumors are submucosal, and endoscopic biopsies are often non-representative. Endoscopists and pathologists also do not always agree, and there is about a 30% discrepancy rate between what the endoscopist sees and what the pathologist diagnoses. Furthermore, there is huge variation between what individual pathologists report when examining the same biopsy specimens. Biopsies should always be taken, however, (except in the normal appearing esophagus) since even the most normal appearing tissue may be extensively diseased. On the other hand, if obvious lesions are present the clinician should immediately institute appropriate therapy. The rule of thumb is if it looks abnormal treat what you think you see, even if the pathologist reports the appearance of biopsies to be normal. In contrast, it looks normal wait for the pathology report which often reveals histologic change.
Finally, it should be emphasized that while endoscopic examination is one of the most important recent advances in the diagnosis of gastrointestinal disease, it is no substitute for an initial thorough evaluation of the patient, particularly careful abdominal palpation, and a logical diagnostic approach. Many dogs with diffuse small intestinal diseases have minimal morphological abnormalities, and so endoscopic findings should be considered in combination with results of function tests such as serum cobalamin and folate concentrations and fecal jα1-proteinase inhibitor concentrations. Endoscopic examination complements other approaches to diagnosis and management of gastrointestinal disorders.