Motility disorders

Gastric emptying is a highly coordinated physiological re­sponse to the presence of food in the stomach and can be impaired during a range of different disease processes. In gen­eral, there are three separate gastric motility disorders: acceler­ated gastric emptying, retrograde transit, and delayed gastric emptying.

In normal monogastric animals, the pylorus serves as a sieve during the postprandial period. Liquids easily pass through the

Figure 4.15:

Radiographic view of a gastric dilation-volvulus. This figure shows a radiographic view of a 10-year-old male German Shepherd dog. The radiographic image shows a large amount of air in the gastric fundus (dorsal “bubble”) and the pyloric an­trum (ventral “bubble”; “double-bubble is trouble”) with gas-filled and dilated intestinal loops.

pylorus and empty relatively rapidly from the stomach by first- order kinetics. The rate of liquid expulsion from the stomach is proportional to its volume and the greater the gastric fluid volume, the more rapidly it is expelled.

Figure 4.16:

Graph of a ¹³C-sodium acetate breath test in a healthy dog. The test was performed on two consecutive days in the same dog. Note the repeatability of the curve for both experiments. There is an initial plateau phase at about 90 minutes and a more-or-less linear emptying thereafter. Gastric emptying is complete after 5 hours.

in cats, up to 3 hours in dogs). Studies using barium mixed with food have shown gastric emptying times varying from 4 to 16 hours in the dog and 4 to 17 hours in the cat, depend­ing on the composition of the food, thus making it difficult to diagnose an emptying disorder unless gastric emptying times are markedly prolonged.⁵⁴ Furthermore, when solid meals are

Table 4.6: Methods for assessing gastric emptying

BIPS = barium-impregnated polyethylene spheres

mixed with barium granules or suspension, the barium can dissociate from the food and redistribute into the liquid phase of the gastric contents.

Recently, ultrasonography has been used as an alternative method for assessing gastric emptying times.⁵⁶ Finding more than just a small amount of fluid in the stomach 18 hours after feeding provides evidence for delayed gastric emptying in the dog. Gastric emptying can also be evaluated by means of ¹³C- breath or blood testing.⁵² The main advantages of ¹³C-based

tests are that no radiation is required, the tests are non-invasive and non-operator dependent, and they can be performed sev­eral times in the same subject without any biological hazard. ¹³C-based breath or blood tests involve the detection of an increased fraction of ¹³CO₂ in comparison to ¹²CO₂ after the ingestion of a labeled meal or administration of a labeled sub­strate. The substrate or meal is rapidly digested and absorbed at the site of interest by enzymatic or microbial digestion and the rate and appearance of the isotope in the breath or blood is a direct reflection of the GI transit of the substrate.

Patients with abnormalities of gastric motility appear normal except for intermittent postprandial vomiting. The vomitus is characteristically undigested or only partly digested, and oc­casionally contains mucus. It may also have an acidic pH, but bile is absent. The precise interrelationship between pyloric stenosis and pylorospasm is unclear. Signs of gastric outlet obstruction vary with the degree of obstruction. Vomiting is the predominant clinical sign and may occur at any time after a meal. The time for complete emptying of a normal meal from the stomach is 7 to 8 hours in dogs fed once daily. Vomiting of all or part of a meal at periods more than 10 hours after ingestion suggests delayed gastric emptying and the prob­ability of a gastric, pancreatic, or proximal duodenal lesion.

Treatment

In addition to a dietary trial, prokinetic therapy is often needed. A low-fat, highly digestible blended or liquid diet fed over multiple feedings may be beneficial. Dopaminergic agonists (e.g., metoclopramide) have GI prokinetic and antiemetic properties because they inhibit peripheral and/or central dopamine receptors.

other pharmacological properties (e.g., 5-HT₃-receptor or 5- HT₄-receptor antagonism).⁵⁷ Metoclopramide increases the amplitude and frequency of antral contractions, inhibits fundic receptive relaxation, and coordinates gastric, pyloric, and duo­denal motility, all of which result in accelerated gastric empty­ing. Erythromycin, an antibiotic, has motilin-like action and at low doses increases the pressure of the gastroesophageal sphincter and accelerates gastric emptying by inducing antral contractions similar to phase III of the migrating motility complex.⁵⁷ The prokinetic dose (1 mg/kg PO q 12 h) is much lower than the antimicrobial dose. Some histamine H₂-recep- tor antagonists (e.g., ranitidine and nizatidine) also have ace­tylcholinesterase-inhibiting properties, thereby stimulating gastric emptying and small intestinal and colonic motility.

Cisapride was used to treat several GI motility disorders in dogs and cats throughout the 1990s. Because of unexplained deaths in humans, cisapride has been taken off the market and is currently only available through compounding pharmacies. There has been little clinical experience with the newer pro- kinetic agents, such as tegaserod (Zelnorm [USA], Zelmac [Europe], Novartis) and prucalopride (R093877, Janssen). Te- gaserod is a potent partial nonbenzamide agonist of 5-HT₄ receptors and a weak agonist of 5-HT_1d receptors with known prokinetic effects in the canine colon. In vitro studies suggest that tegaserod does not delay cardiac repolarization or prolong the QT interval of the electrocardiogram as had been occa­sionally reported with cisapride. Its clinical efficacy has been demonstrated in human motility disorders. Gastric and intes­tinal effects of tegaserod have not been reported in the dog and this drug may not prove as useful as cisapride in stimulat­ing proximal GI motility in small animal patients.

The only effective treatment for pyloric hypertrophy is py­loroplasty. In some patients, the outlet obstruction may be suf­ficiently severe that nutritional homeostasis cannot be sus­tained on any kind of diet, and enteral or parenteral nutrition will become necessary.

Key Facts

■ Dolasetron and maropitant are highly efficacious as antiemetic agents.

■ A diagnosis of gastritis can only be achieved after a gastric biopsy has been analyzed histopathologically.

■ Gastric ulceration is an uncommon albeit serious complication of NSAID treatment and should be aggressively treated with sucral­fate and either a histamine₂-receptor antagonist or a proton pump inhibitor.

■ In dogs with gastric dilation-volvulus (GDV), aggressive intravenous fluid therapy should be initiated before the stomach is decom­pressed.

■ Gastric motility disorders are best diagnosed by scintigraphy or ¹³C-based breath or blood tests.