Methodsforassessing gastrointestinal motility

Methods available for evaluation of gastrointestinal motility include 1) radiography - survey, barium contrast, and radio­paque indigestible solids (e. g., barium-impregnated polyethy­lene spheres or BIPS), 2) quantitative videofluoroscopy, 3) ultrasonography, 4) nuclear scintigraphy, 5) tracer studies, 6) manometry, and 7) functional MRI (Table 1.17).

1.9.2.1 Surveyradiography

Survey abdominal radiography provides very little informa­tion about gastrointestinal motility, but is the imaging techni­que of choice in the initial assessment of any patient with a gastrointestinal disorder. Survey radiographs are useful in pro­viding information about gastrointestinal tract position and content that may help to delineate mechanical obstruction from functional motility disorders. Survey radiographs are also helpful in determining the size and shape of other abdominal organs (e. g., spleen, liver, biliary tract, and urogenital tract) and their relationship to the gastrointestinal tract.

1.9.2.2 Contrast radiography - liquid barium

Barium contrast radiography is often used in clinical practice to detect gross abnormalities of esophageal peristalsis, gastric emptying (Table 1.18), intestinal transit (Table 1.19), and colonic motility, but the technique does have some distinct limitations.⁴ In gastric emptying studies, for example, gastric emptying of a radionuclide-labeled solid meal was markedly delayed in a group of dogs with pyloric hypertrophy, although emptying of liquid barium was thought to be normal.⁵ Also, the barium swallow technique that is currently used to assess esophageal peristalsis provides only a qualitative assessment unless it can be coupled with quantitative videofluoroscopy. The latter technique requires sophisticated equipment and computer software that are generally not available in a clinical practice setting.

1.9.2.3 Contrast radiography - barium meal

Esophageal peristalsis, gastric emptying (Table 1.18), and in­testinal transit (Table 1.19) are affected by the physical proper­ties of the meal (solids vs. liquids), size of the ingested particles (large vs. small), and chemical composition (lipids vs. proteins vs. carbohydrates).^1,2 Barium mixed with food is thought to be a better contrast agent for the determination of gastrointestinal transit. Despite this, barium can dissociate from the food and re-distribute into the liquid phase of the ingested meal, which likely accounts for the wide variability in reported transit times when using this technique. For example, the gastric emptying time for ground kibble (8 g/kg) mixed with a bar­ium sulfate suspension (5-7 ml/kg) was reported in the range of 5-10 hours in mature Beagle dogs, while total gastric emp­tying time ranged from 7-15 hours in other studies.^6,7 Also, as with liquid barium studies, gastrointestinal motility disorders can be diagnosed only if the transit /emptying times are mark­edly prolonged.

1.9.2.4 Contrast radiography - BIPS

Small, indigestible radiopaque markers such as barium- impregnated polyethylene spheres (BIPS) have been used to quantify gastric emptying (Table 1.18) and intestinal transit (Table 1.19) times in dogs and cats.⁸ BIPS are administered in food as recommended by the manufacturer’s package insert, and two to four abdominal radiographs are taken over the next 13-24 hours.⁸ The percentage of BIPS that have been passed to the stomach and intestine is calculated and compared with standard emptying and transit curves (provided in the manufacturer’s package insert). Unfortunately, interpretation of BIPS emptying and transit data has some of the same limi­tations as liquid barium and barium meal studies.

1.9.2.5 Ultrasonography

Ultrasonographic equipment is now more widely available in veterinary practice, and recent studies suggest that ultrasound may be a useful non-invasive method for quantitative assess­ment of gastric emptying (Table 1.18) in dogs and cats.⁹ In healthy dogs fed a solid meal labeled with ¹³C-octanoic acid, there was a strong correlation between the rate of solid-phase gastric emptying assessed by use of gastric emptying ultra­sonography and the ¹³C-OBT (¹³carbon-labeled octanoic acid breath test) in dogs.⁹ Further research will be necessary to validate this method against nuclear scintigraphic imaging and to describe reference ranges for healthy and diseased animals.

1.9.2.6 Nuclearscintigraphy

Nuclear scintigraphic imaging is a very effective means of eva­luating gastrointestinal motility and is now considered to be the standard method of assessment.^{7,8,12 99m}Technetium (bound to sulfur, albumin colloid, disofenin, or mebrofenin) and ¹¹¹indium (bound to diethylene triamine penta-acetic acid

[DPTA]) are the radioisotopes most widely used because they are safe, simple to use, and non-absorbable. Two radionuclide markers can be tracked simultaneously, which allows solid and liquid emptying to be assessed during the same test period. Animals are fasted for 12-24 hours after which a test meal is fed incorporating one or two radioisotopes. Left lateral, right lateral, and ventral images are acquired with a gamma-camera and in­tegrated using a nuclear scintigraphy software package. Gastric, intestinal, and /or colonic regions of interest are identified, and the radioactive counts in these regions are recorded, usually at regular intervals for 6-9 hours (gastric emptying), 12-24 hours (intestinal transit), or 24-36 hours (colonic transit).

1.9.2.7 Tracer studies

Several types oftracer studies, including gastric content, plasma, breath, and blood tracers, have been developed for the assess­ment of gastric emptying and /or intestinal transit (Tables 1.18 and 1.19).

Gastric tracer studies involve the serial aspiration of gastric contents after administration of a known concentration of a non-absorbable marker substance in food or by gastric intuba­tion. Chromium oxide, polyethylene glycol, and phenol red have all been used to assess solid (chromium oxide) or liquid phase (polyethylene glycol or phenol red) gastric emptying. The invasive nature ofthis method precludes its use in anything other than the research setting.

Plasma tracer studies take advantage of the site-specific ab­sorption of orally administered drugs following gastric em­ptying (acetaminophen) or orocecal transit (sulfasalazine). Acetaminophen is poorly absorbed in the stomach, but rapidly absorbed in the duodenum, and the appearance of acetamino­phen in plasma therefore reflects gastric emptying time of acetaminophen. Sulfasalazine is a compound molecule of sulfapyridine and 5-aminosalicylate linked by an azochemical bond. After oral dosing, most of the sulfasalazine is transported unmetabolized to the distal GI tract, where cecal and colonic bacteria metabolize the drug to its component parts. Sulfapy­ridine is largely absorbed intact by the colonic mucosa but much of the 5-aminosalicylate remains in the colonic lumen, where it inhibits mucosal cyclooxygenase and the inflam­matory cascade. The appearance of sulfapyridine in plasma therefore reflects the orocecal transit time of sulfasalazine. Acetaminophen and sulfasalazine plasma tracer studies have been validated in the dog, but there are no published studies comparing animals in health vs.

Breath tracer studies take advantage of the site-specific absorp­tion of orally administered compounds following gastric emp­tying (¹³C-octanoic acid), or of the site-specific fermentation (molecular hydrogen [H₂] generation) of orally ingested food or carbohydrate following orocecal transit. Both can be de­tected in expired breath, one reflecting gastric emptying time (¹³C), the other representing orocecal transit time (H₂). The ¹³C-OBT has been validated as a measure of solid phase gastric emptying in the dog, but there are no published studies com­paring animals in health vs. disease.⁹ The H₂ breath test has been validated as a measure of orocecal transit time in both dogs and cats.¹² Finally, gastric emptying can also be assessed by a ¹³C-octanoid blood test. However, only limited data are available about the clinical utility of this test.

1.9.2.8 Manometry

Manometry has limited application in the diagnosis of cri- coesophageal and gastroesophageal achalasia, gastroesophageal reflux, and aganglionic megacolon (Hirschsprung’s disease), but this technique is currently only performed at major refer­ral centers and university teaching hospitals.

1.9.2.9 Functional MRI

Functional MRI has been used to quantify gastric emptying in human beings, but this technique has not yet been validated in the dog or cat. Future MRI usage will likely be limited by expense and access to specialized equipment.

Table 1.17: Methods available for assessment of gastrointestinal transit in dogs and cats

* rarely performed

Table 1.18: Gastric emptying times of solids and liquids in dogs and cats

50% GET = 50% gastric emptying time, or the time it takes to empty 50% of the ingested / fed meal.

95% GET = 95% gastric emptying time;

100% GET = 100% gastric emptying time

BIPS - Barium impregnated polyethylene spheres

Table 1.19: Orocecal transit times in dogs and cats

OCTT - Orocecal transit time is the time taken from the oral administration of the test meal to the time when the first portion of the meal reaches the colon. BIPS - Barium impregnated polyethylene spheres

??9 Key Facts

■ Gastrointestinal motility disorders are important causes of gastrointestinal signs (e. g., nausea, vomiting, diarrhea, abdominal dis­comfort, and constipation) in dogs and cats.

■ Gastrointestinal motility disorders may involve the esophagus (e.g., idiopathic megaesophagus), stomach (e.g., delayed gastric emptying), intestine (e. g., ileus or pseudo-obstruction), or colon (e. g., constipation), independently, or as a more generalized and diffuse gastrointestinal motility disorder (e. g., dysautonomia).

■ Gastrointestinal motility may be assessed by a number of different methods including survey and contrast radiography, ultrasono­graphy, nuclear scintigraphy, tracer studies, manometry, and MRI (Table 1.17).