Medical Management of the Acute Abdomen in the Field: Laxatives, Motility Agents, and Nutritional Management

Author’s Note: Please see the appendix on page 56 that list drugs, doses indications, and author's opinion. 

Take Home Message  

When surgical intervention is not required, the mainstays of therapy for the acute abdomen are medical interventions and dietary management. Many treatments are available, with varying anecdotal or research based support. Drugs in common usage have varying degrees of potency,  duration and clinical familiarity.  

Introduction  

Motility of the gastrointestinal tract is a complex interplay between central stimuli, the  autonomic nervous system, the enteric nervous system, amount and physical characteristics of  intestinal content, and integrity of the resident microbial flora. Disturbances in motility are  common causes of colic and may result from diseases of the gastrointestinal tract itself, such as ischemia, inflammation, endotoxemia or metabolic derangements, or occur secondary to disease  processes affecting the peritoneum or systemic health. To re-establish movement of ingesta, laxatives/cathartics or fecal bulking agents may be tried. Depending on the surmised cause of  pain, prokinetics or alternatively anti-spasmodic agents will be prescribed. Nutritional  management should include the restoration of electrolyte and acid-base derangements that may  affect myoelectric potentials and thus intestinal motility, as well as considerations of nutritive  value, fecal bulk and consistency.  

Laxatives  

Due to the extensive nature of the equine gastrointestinal tract, the efficacy of any laxative will  be compromised as it is often not possible to provide the volume of laxative required for optimal  efficacy. Many laxatives/fecal bulking agents in use are extrapolated from human usage. Dose  rates are pro-rated not on a bodyweight basis, but on a colonic weight basis, therefore there is not  a linear relationship between human and equine dose rates. Prior to nasogastric administration of  any laxative, it is necessary to ensure that gastric dilation and therefore the potential for reflux is  not present. Following their use, preferably at 12 hour intervals, the administration of water is  recommended to restore volume deficits that may result from the action of osmotic and cathartic  agents.  

Mineral oil serves as a laxative and marker for gastrointestinal transit. Passage within 12-18  hours of administration is taken as evidence of patency of the gastrointestinal tract. However, it  is considered by some that oil can pass around some obstructions without softening them. It is a  relatively safe non-toxic stool softener, however, it is necessary to ensure that gastric dilation and  therefore the potential for reflux is not present before administration. Fatal aspiration pneumonia  can result.¹ 

Magnesium sulfate (Epsom salts) is a saline laxative thought to exert an osmotic effect increasing fecal bulk and water.² However, the exact mechanism of action is not known. When compared  with equal volumes of other substances administered via nasogastric intubation, magnesium  sulfate was more effective at increasing fecal water and bulk. All agents stimulated defecation via the gastrocolic reflex.  

Dioctyl sodium sulfosuccinate (DSS) is a surfactant agent that can act as an irritant laxative and  fecal softener. Toxicity results from mucosal damage to the stomach and small intestinal lining.³ Reports of magnesium toxicity due to enhanced absorption when administered concurrently with DSS exist.⁴ 

Psyllium hydrophilic mucilloid acts as a bulk laxative and is widely used for impactions.^5,6 Most widely known as Metamucil® (Procter and Gamble). There is little evidence that usage increases  passage of sand impactions for which it is often recommended.⁷ 

Bran has been used extensively for many years due to its purported laxative effects; however, no change in fecal water has been demonstrated with the feeding of wet or dry bran mashes  compared to other diets.⁸ 

Vegetable oils may act as laxatives. Castor oil acts as an irritant laxative. In higher doses severe colitis resulting from epithelial disruption has resulted.^9,10 It is not recommended for usage in the  horse. Linseed oil (raw) similarly at higher doses may cause watery diarrhea, colic, depression  and neutropenia.¹¹ Although successfully used, therapeutic index is narrow.  

Enteral fluids are cost-effective and efficacious at assisting in the hydration and passage of intestinal impactions. While having a considerable cost advantage over parenteral fluids, they also avoid the complications inherent with intravenous catheterization and permit interval  treatment. Indwelling small-bore nasogastric tubes (Mila International, Florence KY) are  available for continuous enteral fluid therapy in controlled circumstances. In one study enteral  fluids were found more effective at hydrating colonic contents than intravenous fluids combined  with oral magnesium sulfate.¹² 

Motility Agents  

Adynamic ileus is a well recognized condition that complicates case management; however, the mechanism by which it occurs is subject to controversy. Research into intestinal motility has included in vitro and in vivo mechanical studies, measurement of myoelectrical activity, transit  time of nonabsorbable markers, and surgical harvesting of intestinal musculature. Difficulties have arisen in the extrapolation of findings in non-diseased to diseased horses.  [...]