Functional Anatomy

The stomach acts as a reservoir to control the size and rate of passage of ingesta into the small intestine, to initiate the digestion of protein and fat, and to facilitate the absorption of vitamins and minerals (see Chapter 1)

The stomach is composed of five anatomic regions: cardia, fundus, corpus, antrum, and pylorus (Figure 56-1). The fundus and body expand greatly to accommodate ingesta and to regulate the emptying of liquids. The antrum grinds food into smaller particles (<2 mm) that are sieved into the duodenum. The gastroesophageal sphincter prevents reflux of gastric fluid into the esophagus, and the pyloric sphincter controls emptying into the small intestine.

Figure 56-1 Gastric anatomy of the empty (A) and full (B) stomach.

(From Guilford and Strombeck: Strombeck’s Small Animal Gastroenterology, St. Louis, Saunders, 1996, p 239.)

The gastric wall contains a mucosa, submucosa, muscularis externa, and serosa (see Figures 56-2 and 1-3, B). The mucosa has a superficial epithelium, gastric glands, and an innermost layer of smooth muscle (the muscularis mucosa), with fine structure and function varying depending on the gastric region. The mucosa in the cardia and pylorus is thinner and less glandular than in the fundus and body. The mucosa of the body contains mucous neck cells (producing mucous, pepsinogen A, and gastric lipase), parietal cells (producing H⁺, pepsinogen A, and intrinsic factor), and chief cells (producing pepsinogen A) (Figure 56-2).^1–4 A variety of neuroendocrine cells are involved in the regulation of acid secretion and are interspersed between the glands. The predominant cells are enterochromaffin-like and somatostatin-producing cells in the fundus, and gastrin and somatostatin-producing cells in the antrum (see Figure 56-2). Localized small aggregates of lymphoid tissues are frequently observed at the base of the gastric glands. A rich network of blood vessels, lymphatics, and nerves weaves between the gastric glands. Beneath the submucosa are two layers of smooth muscle (circular and longitudinal) that run perpendicular to one another. The serosa is the outermost layer.

Figure 56-2 Functional mucosal anatomy. Somatostatin-containing D cells contain cytoplasmic processes that terminate in the vicinity of acid-secreting parietal and histamine-secreting enterochromaffin-like cells in the oxyntic gland area (fundus and corpus), and gastrin-secreting G cells in the pyloric gland area (antrum). The functional correlate of this anatomic coupling is a tonic paracrine restraint exerted by somatostatin on acid secretion that is exerted directly on the parietal cell as well as indirectly by inhibiting histamine and gastrin secretion.

(From Schubert ML, Peura DA: Control of gastric acid secretion in health and disease. Gastroenterology 134:1842–1860, 2008.)

Regulation of Acid Secretion

Physiologically, luminal peptides and gastric distention are the primary stimuli for H⁺ secretion from parietal cells. Pharmacologically, parietal cell acid secretion is regulated by endocrine (gastrin), neurocrine (acetylcholine), and paracrine (histamine) mechanisms.^4,5 Somatostatin released in response to gastric pH levels below 3 provides negative feedback and decreases gastrin, histamine, and acid secretion.

Luminal peptides and gastric distention stimulate gastrin secretion from G cells and effect histamine release from enterochromaffin-like cells. Gastrin-releasing peptide and acetylcholine are the primary enteric neurotransmitters regulating gastrin release from antral G cells (Figure 56-3).

Figure 56-3 Regulation of acid secretion. Ach, Acetylcholine receptor; ECL, enterochromaffin-like cell; GRP, gastrin-releasing peptide; H₂, histamine H₂ receptor; PGE, prostaglandin E₂ receptor.

(From Simpson KW: Fluid and electrolyte disturbances in gastrointestinal, pancreatic, and liver disease. In: Dibartola Fluid Therapy in Small Animal Practice, ed 2, Saunders, 2006.)

Unstimulated acid secretion in dogs and cats is minimal^6–8 (e.g., dogs <0.04 mmol/kg^0.75/h) and the unstimulated H⁺/K⁺-adenosine triphosphatase (ATPase), “the proton pump,” is localized within tubulovesicles in the cytoplasm of parietal cells.^4,9 The stimulated H⁺/K⁺-ATPase and KCl transporters are incorporated into the parietal cell canalicular membrane and hydrogen ions (derived from the ionization of water within the parietal cells) are transported into the gastric lumen in exchange for K⁺ by H⁺/K⁺-ATPase. Potassium and chloride transporters in the canalicular membrane enable luminal transfer of potassium (for recycling via H⁺/K⁺-ATPase) and chloride. Hydroxide combines with CO₂, catalyzed by carbonic anhydrase, to form HCO₃⁻, which diffuses into the blood giving rise to the “alkaline tide” phenomenon. Recent studies have expanded our knowledge of ion transport in the parietal cell by identifying KCNQ1 as the primary channel responsible for K⁺ recycling and establishing the contribution of CFTR (cystic fibrosis transmembrane regulator) and SLC26A9 to the process of chloride secretion.⁵ Figure 56-4 shows a schematic of ion transport in the parietal cell incorporating these findings. Stimulation results in a rapid increase in fluid and hydrogen ion secretion, with pH rapidly declining to around pH 1. The concentrations of K⁺ (10 to 20 mmol/L) and Cl⁻ (approximately 120 to 160 mmol/L) in gastric juice are higher than in plasma.

Figure 56-4 Ion transport in the parietal cell. Recent studies have expanded our understanding of ion transport in the parietal cell by identifying KCNQ1 as the primary channel responsible for K⁺ recycling and establishing the contribution of CFTR (cystic fibrosis transmembrane regulator) and SLC26A9 to the process of chloride secretion. A, Standard model of ion transport in the parietal cell. B, Revised model that incorporates additional ion channels.

(From Kopic S, Murek M, Geibel J: Revisiting the parietal cell. Am J Physiol Cell Physiol 298:C1–C10, 2010.)

The stomach is protected from gastric acid injury by a functional unit known as the gastric mucosal barrier.^10,11 The gastric mucosal barrier comprises tightly opposed epithelial cells coated with a layer of bicarbonate-rich mucus and an abundant mucosal blood supply that delivers bicarbonate, oxygen, and nutrients. Local production of prostaglandin (PGE₂) is important in modulating blood flow, bicarbonate secretion, and epithelial cell renewal. When damage occurs, epithelial cells rapidly migrate over superficial mucosal defects aided by the local production of growth factors such as epidermal growth factor.

Evaluating Gastric Secretory Function

Gastric secretory testing is primarily performed in patients with esophagitis, gastrointestinal (GI) ulceration, mucosal hypertrophy, and in patients suspected of having acid hypersecretion.

As a starting point, fasting gastric pH and serum gastrin can be measured to determine if acid hypersecretion is likely. Ideally, antisecretory therapy should be discontinued for 7 days prior to testing,¹² and renal and hepatic function should be monitored as these may be associated with decreased serum gastrin clearance and hypergastrinemia. The broad range of unstimulated (fasting) gastric pH in dogs and cats (from pH 1 to 8) makes definitive statements regarding basal acid production difficult. However, the presence of a gastric pH of less than 3 in the face of a high serum gastrin rules out the possibility of achlorhydria, or mast cell tumor, and raises the possibility of gastrinoma.^13,14 Dogs with mast cell tumors and hyperhistaminemia-induced acid hypersecretion have low serum gastrin concentrations,¹⁵ whereas dogs with achlorhydria likely have a high gastrin, but a gastric pH greater than 3.

Measurement of serum gastrin following the intravenous infusion of secretin or calcium is used to further investigate the possibility of exogenous gastrin production by pancreatic gastrinomas (Zollinger-Ellison syndrome).^12–14 Basenji dogs with gastroenteropathy and diarrhea are reported to have elevated gastrin release in response to secretin stimulation, without evidence of gastrinoma.¹⁶ Provocative testing of gastric acid secretion with pentagastrin or bombesin stimulation may be performed to detect achlorhydria in patients with atrophic gastritis, or elevated serum gastrin and gastric pH levels higher than 3, and in those with putative idiopathic small intestinal bacterial overgrowth to determine if achlorhydria is a contributing factor. Pentagastrin-stimulated acid secretion in dogs reaches a peak of 28 mL/kg^0.75/h, 4.1 mmol HCl/kg^0.75/h, 0.34 mmol K⁺/kg^0.75/h, and 0.09 Na⁺ mmol/kg^0.75/h.⁶ Sedation with oxymorphone and acepromazine may be a suitable alternative to anesthesia for secretion studies in dogs.¹⁶ In cats, acid output (mean ± standard deviation) in response to pentagastrin (8 µg/kg/h) ranges from pH 0.9 to 1.1, with secretion rates (median values) of 1.2 mmol/15 min to 1.4 ± 0.5 mmol/15 min in conscious cats and 1.2 (0.6 to 2.7) mmol/kg^0.75/h in anesthetized cats.⁷

Gastric Motility

Normal gastric motility is the result of the organized interaction of smooth muscle with neural and hormonal stimuli.^17,18 The rate of gastric emptying is directly proportional to the difference in pressure between the stomach and the duodenum, and inversely proportional to the resistance to flow across the pylorus. Liquids are emptied more rapidly than solids and the rate of emptying of liquids increases with volume. The emptying rate for digestible solids depends on caloric density. In dogs, digestible solids smaller than 2 mm are emptied into the duodenum, and gastric emptying is modulated via intestinal osmo-, chemo-, and tryptophan receptors. Lipids particularly retard gastric emptying. The release of cholecystokinin in response to fatty and amino acids such as tryptophan, is one factor that slows gastric emptying. Large, undigestible solids are expelled from the stomach in the fasted state by phase III of the migrating motility complex (MMC) in response to the release of motilin.

There is wide variation in the rate of gastric emptying reported in healthy dogs and cats that is largely a function of the method used to evaluate it (Table 56-1; see also Table 26-2),¹⁹ and in clinical practice delayed gastric emptying is usually suspected when food is retained in the stomach for more than 8 hours. The advent of noninvasive, nonradioactive methods of measuring gastric emptying, such as telemetry capsules,^18,20,21 holds the promise of improving our basic understanding of gastric motility in health and disease.

Table 56-1 Measurements of Gastric Emptying

Digestion and Assimilation of Nutrients

The stomach has a limited role in the digestion of proteins, fats, and micronutrients. Pepsin, which hydrolyzes peptide bonds, is secreted as pepsinogen in response to acetylcholine and histamine in parallel with gastric acid secretion. Dog gastric lipase, which digests fat, is secreted in response to gastrin, histamine, prostaglandin E₂, and secretin, and lipase secretion parallels the secretion of gastric mucus. Although pepsin is active only at acidic pH, dog gastric lipase remains active in the small intestine and constitutes up to 30% of total lipase secreted over a 3-hour period.²² Although gastric lipase and pepsin are not essential for the assimilation of dietary fat and protein, the entry of peptides and fatty acids into the small intestine likely helps to coordinate gastric emptying and pancreatic secretion.

Intrinsic factor, which is necessary for cobalamin (vitamin B₁₂) absorption, is produced by parietal cells and cells at the base of antral glands in the dog but not the cat.^1,23 The importance of gastric intrinsic factor secretion in dogs is questionable as the pancreas is the major site of secretion in both dogs and cats. Gastric acidity may also have an effect on the availability of minerals such as iron and calcium.

Gastric Flora and Immune Surveillance

The concept of the stomach as a sterile place was completely dispelled by the isolation of the gastric bacterium Helicobacter pylori from people in 1983.²⁴ A mixed flora of aerobes and anaerobes (approximately 10⁶ to 10⁷ cfu/mL) is rapidly established soon after birth in dogs,²⁵ and colonization with Helicobacter spp., which are likely acquired from the dam, has been documented as early as 6 weeks of age. The stomach of healthy dogs and cats harbors a diverse spectrum of large, spiral, acid-tolerant Helicobacter species²⁶ that stimulate local and systemic immune responses characterized by lymphoid follicular hyperplasia and seroconversion,^27–29 and may play a role in the development of gastritis^30,31 and cancer. These host–bacterial interactions are discussed further in Chapter 2 and in “Inflammation” section of Chapter 56. Helicobacter spp. are adapted to life in an acid environment and produce urease, which catalyzes the formation of ammonia from urea to buffer gastric acidity. Other bacterial species, such as Proteus, Streptococcus, and Lactobacillus, cultured from the canine stomach, may transiently increase after a meal or coprophagia. Acid secretion and gastric emptying likely regulate much of this transient flora and bacteria may proliferate in the event of gastric acid hyposecretion because of glandular atrophy or pharmacologic inhibition. From a diagnostic standpoint it is important to recognize that bacteria such as Escherichia coli and Proteus spp. produce urease that can lead to a false-positive test result for Helicobacter spp.

History and Physical Examination

The diagnosis of diseases of the stomach starts with a thorough history and physical examination. The history should include consideration of signalment (age, gender, breed), description and duration of clinical signs, frequency and severity of vomiting, vaccination status, diet, past medical history, anthelmintic usage, travel history, and previous or current medications.

Special attention should be made if the vomiting animal has been treated with nonsteroidal antiinflammatory drugs (NSAIDs), as serious GI ulceration is a well-known side effect of this treatment.^1,2 Concurrent weight loss could indicate a low calorie intake, small intestinal involvement (e.g., inflammatory bowel disease [IBD]), or neoplasia. Access to garbage, plants, and cleaners all may be important causes of vomiting. The age and breed is important as young animals are more likely to ingest foreign bodies and older animals more frequently experience gastric cancer. Gastric cancer is more frequently seen in the Belgian Shepherd, Rough Collie, Staffordshire Bull Terrier, Beagle, and Lundehund breeds. The large and giant breeds with deep chest conformation, such as the Great Dane, are at increased risk for gastric dilation volvulus syndrome.

Vomiting is one of the major clinical signs of gastric disorders (see Chapter 23). Descriptions of the vomitus should include the amount, color, consistency, odor, and presence of bile or blood. The relationship of vomiting to food and water consumption should also be noted. Vomiting should also be differentiated from gagging, coughing, dysphagia, and regurgitation, all of which may appear the same for the pet owner, before beginning a medical workup for gastric disease. Nonproductive retching and vomiting in deep-chested dogs may require immediate medical attention for gastric dilation or gastric dilation and volvulus (GDV) syndrome. Persistent vomiting of large volumes of fluid is highly suggestive of GI obstruction, whereas vomiting of digested food 12 hours after feeding suggests delayed gastric emptying because of gastric outflow obstruction or a gastric motility disorder. Associated clinical signs, such as anorexia (nausea caused by gastritis), diarrhea (intestinal involvement), and melena (gastric bleeding), may occur in animals with gastric disease. Box 56-1 lists the most common causes of vomiting, and Table 56-2 lists the diagnostic procedures that are used in the differentiation of the vomiting disorders.

Table 56-2 Diagnostic Procedures of Value for the Differential Diagnosis of Vomiting

Complete physical examination is needed with special attention to abdominal palpation for distention and tympany (e.g., gastric dilation), masses or organomegaly (e.g., neoplasia, intussusception, and foreign body), pain (e.g., perforation, peritonitis, pancreatitis, intestinal obstruction), and effusion (e.g., peritonitis). The occurrence of fever suggests infection or inflammation.

Based on the history and duration of clinical signs, the animal is classified as having either acute or chronic vomiting. The diagnostic approach depends on the duration of the clinical signs and whether the animal has other systemic signs (Figures 56-5 and 56-6). Chronic vomiting is often defined as vomiting that has persisted for 7 to 10 days or when there is no response to initial treatment.

Figure 56-5 Diagnostic approach to the acute vomiting patient.

Figure 56-6 Diagnostic approach to the chronic vomiting patient.

Laboratory Data

Clinicopathologic testing is often warranted when differentiating primary gastric disorders from nongastric disorders. Complete blood cell count, serum chemistry, urinalysis, and fecal flotation should be considered when the patient is persistently vomiting or if there are signs of systemic diseases. Blood and urine samples should be obtained before treatment, which gives the clinician the chance to evaluate the metabolic consequences of the GI disease. Biochemical abnormalities in primary gastric diseases are usually consistent with mild to moderate loss of chloride, potassium, sodium, and bicarbonate. Prerenal azotemia is a result of dehydration. Elevated blood urea nitrogen without concurrently elevated serum creatinine may indicate gastric bleeding. Electrolytes and acid–base status are important parameters if symptomatic therapy is to be optimized in the vomiting animal.^3,4

Laboratory data should be reviewed for evidence of systemic diseases; for example, kidney and liver insufficiency, toxemias (pyometra, peritonitis), anemia (chronic disease or GI bleeding), Addison disease, and diabetes mellitus. If the database suggests any of these disease entities, additional supportive evidence should be sought; for example, pre- and postadrenocorticotropic hormone (ACTH) cortisol, pre- and postprandial serum bile acids, pancreatic-specific lipase, and trypsin-like immunoreactivity. In cats with chronic vomiting, feline leukemia virus (FeLV) and feline immunodeficiency virus testing is warranted. In older cats, serum T₄ (thyroxine) should be measured as standard for the test for hyperthyroidism in which vomiting is a common clinical sign.

In patients with extensive GI bleeding (melena or hematemesis), an assessment of coagulation is essential. Severe GI bleeding as a consequence of coagulopathy caused by infection with Angiostrongylus vasorum has been reported in dogs. When gastric acid hypersecretion is suspected, as with gastrinoma, gastric secretory testing (e.g., gastrin and fasting gastric pH measurement) should be performed.⁵ The reader is referred to Chapter 25 for further detail.

Fecal examination is important in the detection of helminths, Giardia spp., Salmonella spp., Campylobacter spp., and parvovirus (as detected by enzyme-linked immunosorbent assay [ELISA]) as a cause of vomiting and diarrhea.

The pH of the vomitus can be used to differentiate vomiting from regurgitation. Additional laboratory evaluation for volume and content such as food, bile, foreign ingested material, digested or fresh blood, and parasites (ascarids, Physaloptera spp., cestodes, and Capillaria spp.) may be needed. Small clusters of fresh blood may be secondary to capillary microtrauma during the forceful vomiting process, but large amounts of digested blood usually indicate significant upper GI bleeding. Microscopic evaluations of the vomitus may sometimes reveal inflammatory or neoplastic cells, eggs of Physaloptera spp., and larvae or adult stages of Ollulanus tricuspis.

Gastric Imaging Techniques

Survey abdominal radiographs are often the initial diagnostic test when evaluating vomiting, abdominal pain, or abdominal distention. Radiographs provide information about gastric position, size, and content, which may help to diagnose GI obstruction, foreign bodies, gastric masses, ileus, intussusception, and gastric dilation or GDV. Survey radiographs are recommended in all animals in which GI foreign body such as carpet, socks, toys, bones, stones, or trash is suspected.

Contrast radiographs with barium sulfate liquid are also useful when inspecting size, shape, position, and motility of the stomach, as well as when nonradiographic foreign bodies, for example, socks, are missed on survey radiographs. Over time, the combination of ultrasonography and endoscopy have minimized the use of barium-contrast studies when evaluating mucosal conditions such as inflammation, neoplasia, and foreign bodies in the stomach.

When delayed gastric emptying is suspected, evaluation of gastric emptying is carried out using barium contrast (liquid or mixed with food), barium-impregnated polyethylene spheres, fluoroscopy, nuclear scintigraphy, or ¹³C-octanoate breath test (see Chapter 26 and Table 56-1).^6–8

The effects of sedation or other medications on gastric emptying times should always be taken into account when interpreting abdominal radiographs.

Ultrasonography is widely used when evaluating GI clinical signs. The gastric fundus is evaluated in left lateral recumbency, and the pylorus is evaluated in right lateral recumbency. Longitudinal and transversal views of stomach are required to measure the size of the organ and thickness of the gastric wall. Ultrasound may be used to assess wall thickness, wall layer identification, wall symmetry, extent of lesions, GI content, and motility. Diseases such as gastric cancer, chronic gastropathy, uremic gastritis, and delayed gastric emptying disorders have been diagnosed by ultrasonography in cats and dogs.^9–11 The test is noninvasive and requires little in the way of patient preparation. Ascites may enhance the image of abdominal organs in those patients with ascites. Aerophagia, on the other hand, may compromise identification of gastric or other organ pathology.

Endoscopy

Endoscopic evaluation and biopsy of the stomach is a commonly used procedure to evaluate the gastric mucosa in cats and dogs.¹² Endoscopy permits direct observation of the lumen and provides a noninvasive method of obtaining biopsy specimens for histopathologic evaluation. Brush cytology, fluid aspirations with culture, and foreign-body retrieval are all possible during routine endoscopy.

Endoscopy provides direct observation of the lumen and mucosa, but may miss extramural diseases. Endoscopy has the additional disadvantages of equipment expense, the need for general anaesthesia, and the inability to obtain full-thickness biopsies. Furthermore, the procedure does not permit the diagnosis of motility disorder or partial obstructions of the intestine. Endoscopy procedures are described in further details in Chapter 27.

Histopathologic Evaluation for Inflammation

Gastritis is a common finding in chronic vomiting dogs and cats. A diagnosis of gastritis requires histopathologic examination of gastric biopsy specimens. The histopathologic evaluation of the biopsy specimens includes cell types and number of infiltrating cells (lymphocytes, plasma cells, mononuclear cells, eosinophils, and neutrophils) and the presence of atrophy or hypertrophy, metaplasia, fibrosis, edema, or lymphoid follicles. Disagreements do exist among classification systems used by different pathologists when evaluating gastric biopsy specimens, making it important for the clinician, together with the pathologist, to define the criteria upon which the diagnosis is made. Figure 56-7 illustrates a visual scale grading system from 0 to 3 based on a photographic scale of cellular infiltrate, atrophy, and fibroses. Grade 0 is normal gastric mucosa and grade 3 is severe gastritis.¹³ A standardized system of characterizing gastric cellularity and morphology has been developed by the World Small Animal Veterinary Association Gastrointestinal Standardization Group.¹⁴

Figure 56-7 Photographic scale used to standardize the evaluation of atrophy, fibrosis, cellular infiltrate in fundic biopsies. A grade normal [0], mild gastritis [1], moderate gastritis [2], severe gastritis [3]. Magnification ×100; staining method: Masson trichrome (fibrosis) and hematoxylin and eosin (atrophy and cellular infiltrate).

(From Wiinberg B, Spohr A, Dietz HH, et al: Quantitative analysis of inflammatory and immune response in dogs with gastritis and their relationship to Helicobacter spp. infection. J Vet Intern Med 19:4–14, 2005.)

There is an increasing interest in characterizing mucosal proinflammatory and immunomodulatory cytokines in cats and dogs with gastritis. Perhaps this will help the clinician’s understanding of the immunologic response in the stomach and help the clinician’s understanding of the mechanisms of gastritis.¹³ Gastric biopsies can also reveal infections with parasites (O. tricuspis) or bacteria (Helicobacter spp.).^13,15,16 The reader is referred to other sections (Gastric Inflammation, Gastric Infection) in this chapter for more specific detail.

Acute Gastritis

Chronic Gastritis