GASTROINTESTINAL MOTILITY – CLINICAL ASSESSMENT IN SMALL ANIMALS
Frédéric P. Gaschen, Dr.med.vet., Dr.habil., DACVIM, DECVIM-CA
Baton Rouge, Louisiana, USA
INTRODUCTION
In people, disorders of gastrointestinal (GI) motility are common reasons for patients to visit their physician
1
. They
include a variety of problems such as heartburn, regurgitation, dysphagia, bloating, postprandial fullness, early satiety, and
constipation. In dogs, the prevalence of GI motility disorders cannot be documented precisely due to difficulty in obtaining a
definitive diagnosis
2,3
. Examples of such diseases include megaesophagus, disorders of gastric emptying, functional intestinal
obstruction (ileus), megacolon and constipation.
3,4
In dogs, abnormal gastric motility can lead to the feared and potentially fatal
gastric dilation-volvulus (GDV) syndrome. More generally, disorders of GI motility can cause discomfort in small animals and
their diagnosis and treatment is often challenging to general practitioners and specialists alike. A variety of primary and
secondary gastric defects have been identified as potential causes gastric atony.
5
To this day, the timely recognition and proper
diagnosis of GI motility disorders in small animals has been hampered by the lack of a practical, non-invasive ways to evaluate
GI motility.
PHYSIOLOGY OF GI MOTILITY - A BRIEF REFRESHER
6,7,8
GI motility is obviously a very complex series of events that aims at (1) storing and (2) grinding the ingested food into
particles of a determined size before they are ejected out of the stomach and (3) propel the ingesta through the small intestine
while allowing the digestive and absorptive processes to take place harmoniously and (4) optimize conditions for fermentation
and water reabsorption processes in the proximal colon and (5) store and coordinate evacuation of fecal material from the distal
colon and rectum. While GI motility is a function of smooth muscle contraction, a combination of myogenic, neural, and
hormonal factors regulate smooth muscle activity
6
.
The gastric antrum acts as a pump from which peristaltic waves originate while the gastric body acts as a high
compliance reservoir. Contractions only occur when excitatory neurotransmitters such as acetylcholine are released in response
to mechano- and chemoreceptors. The mechanical action of the antral pump is divided 3 phases: (1) propulsion, (2) emptying
of fine particles and mixing, and (3) retroplusion of particles > 2 mm and grinding. Gastric motility and emptying are
modulated by gastro-gastric reflexes: for instance filling and distention of the gastric reservoir elicits excitatory reflexes
stimulating antral contractions. Nitric oxide (NO) and vasoactive intestinal peptide (VIP) contribute to the regulation of gastric
motility. Gastric emptying is inhibited by nutrients entering the small intestine (feedback control) through entero-gastric
reflexes and release of intestinal hormones. Cholecystokinin (CCK) is released from I cells in the intestinal epithelium upon
presence of luminal HCl, amino acids and long-chain fatty acids. CCK reaches the stomach via bloodstream, causes relaxation
of the gastric reservoir and reinforces enterogastric neural feedback. Other hormones such as glucagon-like peptide 1 (GLP-1)
and peptide YY produced in the distal small intestine also exert a negative feedback on gastric emptying. Additionally, the rate
of gastric emptying in dogs is modulated by the composition of the diet (e.g. moisture and fat, protein and carbohydrate
content)
9
and other factors such as stress and body size.
A network of interstitial cells (interstitial cells of Cajal or ICC) exists the gastric and intestinal wall between the internal
circular and external longitudinal muscle layers. ICC produce pacesetter potentials and drive the electrical events in the smooth
muscle cells (slow waves). Pacesetter potentials determine the maximal frequency and propagation velocity of the peristaltic
waves. Different contractile patterns occur in the small intestine and include peristaltic waves, stationary segmenting
contractions, aboral giant contractions and stationary/migrating clusters of contractions. Phase III contractions also called
migrating motor complexes (MMC) occur in the interdigestive state and originate simultaneously in the stomach and in the
duodenum. They clean the corresponding gastrointestinal segments of residues and chime. Finally, canine colonic motility
consists of organized groups of contractions called colonic motor complexes (CMC) and giant contractions which are usually
associated with defecation.
MEASUREMENT OF GI MOTILITY IN DOGS AND CATS
The various methods available to investigate gastric emptying have been recently reviewed
8
. They aim at evaluating the
gastric emptying and/or intestinal transit time of solid food, and include scintigraphy, radiographic contrast studies, abdominal
ultrasound, gastric emptying breath test, and passive telemetry
14
. While the latter method is invasive and impractical for use in
privately owned pet dogs, the other methods are non-invasive, but all have potential pitfalls
8
. Additionally, some of them
require manual or chemical restraint of the animals, a potential source of stress.
Radioscintigraphy remains the gold standard
In human medicine, radioscintigraphic evaluation of gastric emptying is a standardized method and reference ranges
based on large study populations are available. The technique involves the use of
99m
Technetium-associated compounds in
most instances. It has also been used widely in small animals. Dispersion of radiolabeled food among the ingested test meal
may vary and influence the results. Although scintigraphy is a useful standard against which to compare newer methods, it is
not a widely used method to assess disorders of gastric emptying in dogs and cats. The availability of the equipment and the
special safety requirements for use of radionuclides are the most important limiting factors.
Radiographic contrast studies
Liquid barium has been widely used to assess GI transit times and is adequate to evaluate liquid phase gastric emptying.
The dose of barium suspensions is 6 ml/kg in dogs and 10 ml/kg in cats and should be administered when the stomach is
empty. Barium sulfate should be present in the duodenum by 15 minutes in the dog and by 5 minutes in the cat. The stomach
should be free of barium after 1 to 4 hours in the dog and after 20 minutes in the cat. However, assessment of gastric emptying
of liquid is an insensitive method for the detection of abnormalities in gastric emptying, with the exception of mechanical
obstructions due to foreign bodies or other space-occupying lesions obstructing the gastric or intestinal lumen. Mixing barium
with food may better evaluate the solid phase of gastric emptying, however barium can easily separate from the test meal and
cause the study to be unreliable.
Barium-impregnated polyethylene spheres (BIPS
TM
) have been used for evaluation of GI transit times in dogs and cats.
They come in various sizes (from 1.5 to 5 mm diameter) and can easily be used in practice. However, correlation between
gastric emptying of BIPS and radioscintigraphy has been disappointing in dogs and in cats.
15,16
This probably reflects the facts
that BIPS > 2 mm are only emptied after all solid food has left the stomach during the interdigestive MMC. At this time, it is
not clear if evaluation of the time between ingestion of indigestible markers and onset of gastric MMC is useful in the
evaluation of gastric motility disorders of small animals or humans.
Ultrasound evaluation of gastric emptying time
Qualitative evaluation of GI motility using ultrasonography is regularly performed during abdominal scans. Recently,
ultrasound was shown to be a reliable and accurate quantitative method to measure emptying of liquids and solids from the
stomach when compared to standard methods.
10-12
Dogs are gently restrained while standing and the ultrasound transducer is
placed in a longitudinal orientation on to the ventral midline, caudal to the xyphoid. Electronic calipers are used to measure the
cranio-caudal and ventro-dorsal diameters of the gastric antrum. The antral area is calculated using the software incorporated
in the ultrasound machine. Measurements are made at each of the following each time points: 30 min. prior to feeding, then
every 20 min. post-prandially for 6 hours. Gastric half-emptying time (50% of AUC), time of maximal antral area, and time at
which antral area was reduced by 50% can be calculated.
11
This method is easily applicable to clinical patients but requires the
availability of an experienced ultrasonographer.
Tracer studies
The assessment of gastric emptying by stable isotope breath tests relies on the ingestion of a
13
C-labeled substrate that is
rapidly absorbed and metabolized to
13
CO2 after gastric emptying. This produces an increase in
13
CO2 in the exhaled air that
can be relatively easily measured.
8
Recently, the
13
C-octanoic breath test has been validated for use in both dogs and cats.
13,17
Octanoic acid is a medium chain fatty acid that is rapidly absorbed in the duodenum, oxidized in the liver where it produces
CO2 which is exhaled in the breath.
13
Exhaled air is collected at several time points after the test meal added with
13
C octanoic
acid has been ingested. The method is non-invasive and the whole procedure can be done outside the veterinary hospital.
Analysis of exhaled air for
13
C is automated and provides useful data about the dynamics of gastric emptying.
Non-invasive monitoring pH, pressure and temperature during GI transit
A new method for non-invasive evaluation of GI motility has been recently approved for use in human patients. Patients
ingest a large 13x26 mm non-digestible capsule with pH, pressure, and temperature sensors (SmartPill pHp
TM
). It immediately
transmits measurements to a receiver/recorder located on the dogs back. The information can then be uploaded in a dedicated
computer and be processed in detail. Typically, gastric emptying time, small intestinal/colonic transit time, and total GI transit
times are calculated. However, detailed data about pH, pressure and temperature can be collected for specific time points using
proprietary software for data interpretation. Studies from Lousiana State University and from another institution validating the
use of this capsule in the dog and evaluating its sensitivity in reporting changes due to dietary modifications and various drugs
influencing GI motility yielded promising results. They will be reported in the scientific abstract session of this meeting. Due
to its size, the SmartPill capsule only leaves the stomach at the onset of the interdigestive MMC when all liquids and solids
have already been propelled into the small intestine. Additionally, the pill size also limits its use exclusively to dogs larger than
approximately 15 kg body weight.
REFERENCES
1. Parkman and Doma. Practical Gastroenterology 2006; 30: 23. 2. Hall and Washabau. Veterinary Clinics of North America - Small Animal Practice 1999;
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Gastroenterology, 3rd ed: 532. 5. Woosley. Clinical Techniques in Small Animal Practice 2004; 19: 43. 6. Strombeck 1996; Strombeck's Small Animal
Gastroenterology, 3rd ed: 1. 7. Ehrlein and Schemann, 2007, Technical University Munich: http://www.wzw.tum.de/humanbiology/data/motility?alt=english;
8. Wyse et al. J.Vet.Int.Med. 2003; 17: 609. 9. Davenport et al. 2000 Small Animal Clinical Nutrition, Mark Morris Associates; 725. 10. Choi et al. J.Vet
Med.Sci. 2002; 64: 17. 11. McLellan et al. AJVR 2004; 65: 1557. 12. Chalmers et al. Veterinary Record 2005; 157: 649. 13. Wyse et al. AJVR 2001; 62:
1939. 14. Burger et al. Journal of Veterinary Medicine Series A: Physiology Pathology Clinical Medicine 2006; 53: 85. 15. Goggin et al. Vet Radiol
Ultrasound 1999; 40: 89. 16. Lester et al. Veterinary Radiology and Ultrasound 1999; 40: 465. 17. Peachey, Dawson, and Harper. Comp Biochem.Physiol A
Mol.Integr.Physiol 2000; 126: 85.