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Endoscopy of the gastrointestinal tract in dogs and cats
  1. Ed Hall


Flexible endoscopy systems are now affordable and available for small animal practice. Endoscopy forms part of the investigation of gastrointestinal (GI) disease, allowing visualisation and biopsy of the GI tract. This article is intended as a guide to the type of endoscope required and how to perform upper and lower GI endoscopy successfully.

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Ed Hall graduated from the University of Cambridge in 1979. He undertook postgraduate clinical and research training in Philadelphia and Liverpool, and is a diplomate of the ECVIM-CA. He is currently professor of small animal internal medicine and deputy head of school at the University of Bristol's School of Veterinary Sciences. He is a past president of the BSAVA, and was a member of the WSAVA GI Standardization Group.

FLEXIBLE endoscopy and gastrointestinal (GI) biopsy are only part of the investigation of the GI tract. Before undertaking endoscopy, routine investigations should be performed to confirm that the procedure is indicated. A complete history and full physical examination are essential, and faecal examination for parasites or empirical treatment should be performed first if there is diarrhoea. Routine laboratory tests will determine, firstly, whether there is systemic disease causing the GI signs and, secondly, if not, whether it is safe to anaesthetise the patient for endoscopy. Plain radiographs and ultrasound examination should also be performed before endoscopy to ensure there are no masses, lesions or foreign bodies beyond the reach of the endoscope. However, when available, endoscopy has virtually replaced contrast radiography with the exception of barium swallows to evaluate oesophageal function. Whether upper or lower GI endoscopy, or both, is performed depends on the clinician's suspicion, based on clinical signs, of where in the GI tract the disease is.

The advantages of endoscopy over surgical biopsy are obvious: there is no risk of wound dehiscence and GI perforation is very rare: there is no postoperative pain, and the patient can be discharged on the same day and even treated with steroids immediately if indicated. With the right endoscopic equipment and the necessary skill, most regions of the GI tract can be reached, examined and biopsied. However, surgical biopsies can provide a more reliable diagnosis as they are larger and full-thickness, and can access focal disease in the mid-GI tract that endoscopy cannot reach. Nevertheless, neither endoscopy nor surgery can identify functional GI prob-lems or infectious causes of GI signs.

Although flexible endoscopes were developed over 50 years ago, it is only in the past decade that their increased accessibility and decreased cost has made endoscopy readily available to small animal practitioners. Yet an endoscopist ‘needs to be proficient, undertaking procedures for good indications on patients who are fully prepared and protected, with skilled assistants, and using optimum equipment (Cotton and others 2008). Optimum equipment is an expensive investment, and proficiency only comes with practice once the endoscopist understands what they are doing as they manipulate the endoscope. While this article provides guidance on how to perform GI endoscopy, the reader is also referred to textbooks for the endoscopic appearance of specific conditions and lesions (see Further reading).


In a previous In Practice article, McGrotty and Dickson (2014) explained how to choose the right endoscope. They also highlighted the importance of leakage testing, cleaning and disinfection, and that message is reiterated here.

A light source with an integral air/water pump and a suc-tion pump are essential to be able to inflate and visualise the GI tract and to remove liquid and excess air. Video endoscopes are preferred over fibreoptic endoscopes, but it is the dimensions of the endoscope that are most important. Dedicated veterinary video endoscopes are now available at reasonable prices, and have dimensions that are the best compromise for the range of patient sizes encountered in small animal practice (Fig 1).

Fig 1:

An example of a video endoscopy system suitable for small animal practice. The insertion tube is 1.4 metres in length, 7.8 mm tip diameter, with a 2.8 mm biopsy channel. The light source (housing the air-water pump) is also compatible with rigid endoscopy instruments.

©2014 Photo Courtesy of KARL STORZ GmbH & Co. KG


Insertion tubes made for human gastroduodenoscopy are only 1 metre long as the distance from the mouth to the duodenum in adult patients is quite constant. Yet dogs and cats vary greatly in size, and a one metre endoscope may be too short to allow pyloric intubation in large and giant breeds of dog, and too wide in small dogs and cats. Therefore, veterinary endoscopes are typically 140 to 150 cm long, and as narrow as possible. While a long gastroscope facilitates duodenoscopy in large patients, it makes steering more difficult in small patients, as redundant lengths of the insertion tube may loop and drag. Ideally, a range of endoscopes of varying sizes would be available, but this is not realistic for most practices.

Working channel

A working (biopsy/accessory) channel in the insertion tube is essential to facilitate GI biopsy and permit passage of instruments such as basket and grasping forceps for foreign body removal. The larger the channel, the larger and better quality the biopsies and certainly anything smaller than 2 mm will result in intestinal biopsies that do not retain their integrity during processing. Similarly, a channel greater than 2.8 mm will result in an endoscope tip that is too wide to pass the pylorus in most patients.

Tip diameter

The endoscope tip must be small enough to enter the patient and ideally be able to pass the pylorus and ileocolic valve: too large and intubation will not be possible, but too narrow and the size of the biopsy channel is compromised and inadequate biopsies are obtained. While gastroscopes with a tip diameter of 5 to 6 mm are available, a size of 7 to 8 mm is probably the best compromise enabling incorporation of a reasonably sized biopsy channel, but certainly nothing larger than 9.5 mm tip diameter should be contemplated.

Tip deflection

Four-way tip deflection and the ability to retroflex (180 to 210 degrees) are essential for successful gastroscopy.

Biopsy forceps

As a minimum, an endoscopist should have two pairs of biopsy forceps available in case one should break during a procedure. Disposable forceps are now available at a modest cost and can be charged to the client. Fenestrated, ellipsoid cups with serrated edges are likely to obtain the biggest and best quality biopsies, but the most important factors are the size of the biopsy channel, and hence the size of the cups, and how hard the endoscopist pushes the open cups against the tissue. So called ‘swing-jaw’ biopsy forceps can be useful when taking biopsies along a cylindrical length of intestine when it is not possible to turn the forceps perpendicular to the wall to exert pressure. The ‘swing-jaw’ hinges and turns the cups at almost right angles to the long axis of the forceps and into the mucosa (Fig 2).

Fig 2:

‘Swing-jaw’ biopsy forceps. These hinge and turn into the intestinal wall if the biopsy cups cannot be placed perpendicular to the mucosa. This modification allows greater pressure to be applied and a bigger biopsy to be obtained. These specific forceps cups also feature serrated edges, enabling a more secure grasp, and fenestration that allows the biopsy to bulge out and be less crushed

Steering the endoscope

The umbilical cord is plugged into the air/water pump and light source, and the operator holds the handpiece in their left hand, while guiding the insertion tube into the patient with their right hand, and then controlling the tip with the two (up-down and left-right) steering wheels (Fig 3).

Fig 3:

Holding the hand piece of the gastroscope. The operator holds the handpiece in their left hand, controlling the tip with the two (up-down and left-right) steering wheels

The top of the handpiece houses two valves (Fig 4). The rear button (normally colour-coded red) controls suction, and is operated by full depression. The forward button (normally colour-coded blue) controls air/water flow; coverage of the hole by light pressure from a finger deflects the air flow along the insertion tube and into the patient to enable insufflation of the viscus. Further depression of the button allows the flow of water to clean the lens when necessary.

Fig 4:

The two valves on the hand piece are operated by the fingers of the left hand. The rear button controls suction and the forward controls air/water installation. The thumb of the left hand manipulates the up/down steering wheel

Inexperienced endoscopists usually find it easier to use both right and left hands to steer, but single-handed steering is possible with practice and is necessary when the right hand is manipulating the forceps. It is achieved simply through deflecting the tip by the left thumb acting on the up/down wheel (Fig 4), and then flexing/extending/rotating the left wrist so that the endoscope tip looks left/right as the insertion tube rotates on its long axis, obviating the need to turn the right-left wheel at all. Only when fine control is needed (eg, traversing the pylorus) is it necessary to use the right hand to operate the right/left steering wheel.

The novice endoscopist often struggles to steer the endoscope, not simply because they are not proficient in turning the wheels but also because they dwell while thinking about which way to turn the endoscope tip. Holding the endoscope to the right side of the operator and inserting it in a straight line can make steering easier as looping is avoided. Although the absolute direction of the tip deflection is written on the steering wheels (Fig 4), that is of little help as it may be impossible to determine which way is up, down, left or right within a tubular length of the GI tract in a patient lying on its side. The experienced endoscopist does not think in these terms, but almost intuitively knows which way to turn by observing the direction the image moves as they start to move a steering wheel. Unfortunately, this skill is something that can only be acquired through practice.

The aim of steering is to maintain the lumen in the centre of the field of view. Logically, that will then be the direction in which the tip of the endoscope will go as it is advanced. However, as the endoscope is pushed forward, the GI tract wall is stretched and frequent readjustment of the tip is necessary. If the tip of the endoscope impinges on the mucosa, no clear image can be seen – a situation called ‘red-out’ (Fig 5). This is corrected by withdrawing the endoscope slightly, insufflating the lumen, and steering away from the mucosa. Any blood, mucus or debris is washed away by depressing the air/water button.

Fig 5:

Red-out. This occurs when the tip of the endoscope is too close to the mucosa and the image is lost

Patient preparation

For upper GI endoscopy, the patient should simply have food withheld overnight. However, for colonoscopy, preparation to remove all solid faecal matter is essential: food is withheld for at least 24 hours, and the colon is lavaged either with multiple, high, warm-water enemas, or with large volumes (25 to 30 ml/kg) of isotonic, polyethylene glycol/salt solution (eg, Kleanprep; Golytely) given three times within 12 hours by oral gavage (Fig 6).

Fig 6:

Administration of an oral lavage solution via a stomach tube in a conscious dog in preparation for colonoscopy. A thick-walled, foal feeding tube is directed behind the canines, swallowed and passed to the stomach, while holding the mouth semi-closed

General anaesthesia is required for all GI endoscopy except proctoscopy, and the best (safest) protocol is one the practitioner is familiar with. Induction with an intravenous agent (eg, propofol or alfaxalone) followed by an inhalation agent (eg, isoflurane or sevoflurane) is typically used. An endotracheal tube should be placed both for ventilation and to prevent inhalation of any refluxed liquid. A mouth gag should always be placed for gastroscopy to prevent damage to the endoscope if the patient becomes conscious prematurely.

For both upper and lower GI endoscopy, the patient is placed in left lateral recumbency. Insufflation of the stomach or colon, results in air distending the antrum and right ascending colon, allowing visualisation of the pyloric antrum and the ileocaecocolic regions respectively.

Upper GI endoscopy

Intubation of the oesophagus through the upper oesophageal sphincter is often done blindly, but once through, insufflation and steering the tip into the lumen allows visualisation and passage of the endoscope down the oesophagus, round the curvature at the thoracic inlet, then down to the lower oesophageal sphincter. Indentation by the trachea is seen cranially, and pulsation of the aorta/heart base through the thoracic oesophageal wall is expected (Fig 7a). The normal oesophagus is pale pink and smooth except for submucosal folds in the distal feline oesophagus (Fig 7b).

Fig 7:

Appearance of the oesophagus during oesophagoscopy. (a) Normal oesophagus of a dog: the mucosa is smooth and a pale pink and the indentation of the trachea is visible. (b) Normal distal oesophagus of a cat. The submucosal folds give a ridged appearance, and submucosal vessels are visible. (c) Normal lower oesophageal sphincter of a dog. (d) Entrance into the canine stomach showing rugal folds and a pool of bile-stained fluid which should be aspirated

The lower oesophageal sphincter is normally closed and its lumen passes at an approximate 30 degree angle (Fig 7c). Having steered through, the endoscopist recognises the endoscope has entered the stomach by the presence of rugal folds and a darker colour mucosa; there is often refluxed bile which should be aspirated (Fig 7d). Without significant insufflation, it may be possible to follow parallel rugal folds along the greater curvature into the antrum, which is recognised by its paucity of rugal folds and the presence of the pylorus at its apex (Figs 8a and 8b).

Fig 8:

Appearance of the normal canine stomach during gastroscopy. (a) Parallel rugal folds on the greater curvature lead the endoscope towards the antrum. (b) The antrum has relatively few rugal folds and the pylorus is visible at its apex. (c) Retroflexion of the endoscope tip within the stomach shows the insertion tube entering through the lower oesophageal junction at the cardia. (d) Gradual release of the retroflexed endoscope tip reveals the angle of the lesser curvature below the insertion tube seen entering at the cardia, with the antrum and pylorus below that

Often, initial orientation in the stomach is difficult, and further insufflation is necessary. However, overinflation should be avoided as it stretches the greater curvature excessively and causes the angle of the lesser curvature to fold, such that the pylorus moves out of view and potentially out of reach. Full retroflexion with the stomach inflated allows visualisation of the endoscope as it enters through the cardia (Fig 8c). Gradual release of the retroflexion brings the angle of the lesser curvature into view, and below it the entrance to the antrum (Fig 8d). Sometimes, excessive foam in the stomach obscures the view. Instillation of simethicone into the stomach before duodenal intubation will clear this by the time duodenal examination and biopsy is complete and full examination of the stomach is undertaken.

Once it is clear that there is no major gastric pathology (mass, ulcer) causing the signs, pyloric intubation is attempted. The principle is still to keep the lumen in the centre of the field of view, but pressure of the insertion tube as it slides along the greater curvature often moves the pylorus to one side and continual readjustment is needed. Again, overinflation should be avoided as it folds the stomach and stretches the greater curvature such that it is hard to visualise and reach the pylorus; it also stimulates peristaltic contractions which are a nuisance. Once engaged with the pyloric canal, the tip can either be pushed through, or suction of air from the stomach causes the stomach to collapse over the endoscope tip and force it through into the proximal duodenum.

On entering the duodenum, there is often ‘red-out’ as the tip passes round the proximal flexure. Eventually insufflation and manoeuvring should allow visualisation of the descending duodenum as far as the distal duodenal flexure (Fig 9a). In small patients, it may be possible to pass further into the jejunum, but steering becomes more difficult as the endoscope is forced through multiple twists and loops. The major duodenal papilla is often seen in dogs (Fig 9b) and sometimes in cats; a minor papilla is seen in some but not all dogs. Also in dogs, paler depressions representing Peyer's patches are seen on the antimesenteric border of the descending duodenum (Fig 9c).

Fig 9:

Endoscopic appearance of normal canine duodenum during duodenoscopy. (a) Distal duodenal flexure, which is a good site at which to collect several endoscopic biopsies. (b) The major duodenal papilla, on the left side of this image, is where the common bile duct and a pancreatic duct enter; this structure should be recognised and not biopsied. (c) Descending duodenum showing a row of pale depressions, representing lymphoid tissue (Peyer's patches), which should not be biopsied as they are not representative of the rest of the duodenal mucosa

Having taken duodenal biopsies, the endoscope is gradually withdrawn back into the stomach, all the time observing the duodenal mucosa to ensure the region that was obscured by ‘red-out’ on entering the descending duodenum is examined fully; it is easy to miss duodenal ulcers and masses if this is not done carefully.

On returning to the stomach, the whole surface is examined. Any liquid should be sucked out and the greater curvature examined by withdrawing the endoscope towards the cardia, while observing how normal rugal folds flatten and reform as air is insufflated/removed. Retroflexion and withdrawal of the insertion tube brings the tip closer to the cardia (Fig 8c), and rotation of the insertion tube on its long axis at this point allows visualisation of the shallow, blind-ending fundus beyond the cardia. Submucosal blood vessels can normally be seen in the fundus upon retroflexion, but their presence elsewhere in the stomach is an indication of atrophic gastritis.

After gastric examination and biopsy, all air is sucked out and the endoscope drawn back up the oesophagus, sucking out any refluxed liquid.

Lower GI endoscopy

Before commencing colonoscopy, a thorough digital rectal examination should be performed to check for perineal hernia, anal sac disease, and anal furunculosis. If rectal polyps or masses are palpated, they may be better visualised with a rigid hollow proctoscope, as insufflation of the terminal rectum is difficult with flexible endoscopy.

Intubation of the anal sphincter and distal rectum with a flexible endoscope is often done blind and may even need to be facilitated by insertion of the endoscope alongside a gloved finger. Once the endoscope is in the descending colon, the lumen is visualised and centred, and any free fluid sucked out. Insufflation is better maintained if an assistant digitally clamps the anus closed to stop air escaping. The normal colonic mucosa is pale and smooth with visible submucosal vessels (Fig 10a). Suction of liquid may allow the mucosa to also be sucked up the channel, resulting in a small red spot on the mucosa (Fig 10b); such suction artefacts should not be confused with lesions.

Fig 10:

Appearance of the canine colon during colonoscopy. (a) Smooth, quite clean colonic mucosa, showing visible submucosal vessels. (b) Suction artefact; a small haemorrhagic blister caused by inadvertently sucking up mucosa while trying to remove liquid from the colon

Forward passage is continued around the left (splenic) and right (hepatic) flexures until the ileocaecocolic junction is reached. Sometimes the ileocolic valve is obscured by faecal liquid (Fig 11a), and in dogs it may not be realised that the caecum has been entered until it is recognised that the lumen is spiralling and narrowing. Slow withdrawal and suction of any faecal liquid will allow observation of the ileocolic valve with the adjacent caecocolic orifice. The valve is mushroom-shaped in dogs (Fig 11b), but is a small, relatively insignificant slit in the feline colon, where the caecum is also too small to be intubated routinely.

Fig 11:

(a) The caecocolic orifice is visible but the adjacent ileocolic valve is obscured by a pool of faecal liquid. (b) Removal of the faecal liquid allows visualisation of the mushroom-like ileocolic valve, with the caecocolic orifice visible below it. (c) Blind intubation of the ileocolic valve with biopsy forceps allows either a few blind ileal biopsies to be obtained, or the endoscope to be passed into the ileum. (d) Normal ileal mucosa after an endoscopic biopsy; note the degree of bleeding, which is often more severe than after duodenal biopsy because the ileal mucosa is thinner

In larger dogs it may be possible to enter the ileum by direct steering of the endoscope. But in smaller patients, passage of biopsy forceps through the valve may allow the endoscope to be manoeuvred into the ileum using a Seldinger-type technique with the forceps acting as a guide wire; if this is not possible, at least a few ileal biopsies can be obtained blindly (Fig 11c). The normal ileal mucosa is thinner than the duodenum and larger biopsies with more bleeding are expected (Fig 11d). Although ileal intubation is difficult, it is often indicated, especially if there is evidence of low serum cobalamin. Casamian-Sorrosal and others (2010) showed that histopathological changes were seen more often in the ileum than in the duodenum in paired samples from the same patient.

Thorough examination of the colonic mucosa is achieved as the endoscope is withdrawn, and red streaks at flexures, caused as the endoscope passed round them, must be ignored. The colonic mucosa is quite thin and submucosal vessels can be expected to be visible; their absence suggests the mucosa is infiltrated and thickened (Fig 10a). Multiple biopsies are obtained at different levels of the colon, but this may be difficult in the rectum as air continually escapes and the lumen collapses. Indeed discovery of rectal lesions is often best achieved by digital rectal examination and/or proctoscopy using a hollow rigid proctoscope. In large dogs, the insertion tube of a flexible endoscope can be retroflexed and the rectum observed aborally (Fig 12). Finally, care should be taken as the endoscope is withdrawn as the weight of the insertion tube may drag the tip of the endoscope out unexpectedly, and it may be damaged as it then falls to the floor.

Fig 12:

View of the rectum with the endoscope entering through the anus, achievable in larger dogs by retroflexing the tip while in the descending colon and then withdrawing the endoscope through the anus


In all regions examined, except the oesophagus, biopsies should be taken routinely even if the mucosa appears grossly normal, as microscopic histopathological changes may be present. There is rarely value in taking biopsies of the oesophagus: the mucosa is very tough and if it looks grossly normal any biopsies will be very small and superficial. If it subjectively looks inflamed, it is inflamed; only if there is a suspicion of neoplasia is biopsy indicated.

A minimum of six and preferably 10 to 12 good quality biopsies should be collected from each region of the GI tract examined. They can be placed directly into 10 per cent formalin, or collected in a foam-lined tissue cassette before fixation (Fig 13). Multiple biopsies are needed to compensate for the fact that some are inadequate or will be sectioned in the wrong plane. It is recommended that the pathologist uses the WSAVA GI Standardization Group histopathological template for their report (Day and others 2008)

Fig 13:

Tissue cassette with foam insert suitable for processing endoscopic GI biopsies. After orientation on the foam, the cassette is closed and immersed in a formalin pot, ready for submission


Finally, a record of the procedure should be made. Capture of still images and/or video onto DVD is possible and recording forms can be downloaded free from the WSAVA website


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