Every so often, equine surgeons take a look back at all the colic surgeries they have performed over the years in an effort to answer some important questions. If a horse undergoes colic surgery, for example, what are the chances that horse will survive? If a horse survives surgery, what are the odds of complications in the days and weeks afterward?
Complications are particularly frustrating to equine surgeons. It is very hard to help a horse come through a difficult colic surgery, only to see it succumb in the postoperative period. One of the worst of these complications that can kill horses is called postoperative ileus (lack of gut motility). Scientists aren't exactly sure what causes it, or why one horse will recover from surgery while another will develop ileus following the same surgical procedure. One thing is certain: A lot of research is being done in an effort to better understand and manage this deadly complication of colic surgery.
When a veterinarian places his stethoscope on a horse's flank at any time of the day or night, he hears a variety of gut sounds. These are the sounds of intestinal movement, which, unlike the case in humans, are unrelated to meal time in horses. In the natural state, horses graze almost continuously. The equine gut, therefore, is designed to process and digest food continually. Beneficial microbes are located throughout the intestine and aid in digestion. They allow the horse to gain the most nutritional benefit from a forage-based diet. These microbes also release gases, which are passed along the intestine and released along with the feces.
Normal motility of the intestines allows food material, or digesta, to move toward the later stages of digestion). This is called aboral movement, which simply means "away from the mouth." The outward signs of a healthy, well-functioning digestive tract in the horse are signs of this aboral movement: food going in, audible gut sounds in the intestine, and feces and gas coming out.
Ileus is defined as a failure of aboral movement of digesta, regardless of the cause. Many medical conditions can result in ileus, including shock, electrolyte imbalances, hypoalbuminemia (an abnormally low albumen content in the blood; albumen serves as a transport protein for many necessary body functions), peritonitis (inflammation of the abdominal cavity lining), ischemia (loss of blood supply), and trauma to the abdomen. Another cause is distention of the intestine, most often the small intestine. In fact, ileus that potentially is life-threatening most often affects the small intestine.
A Delicate Balance
While the outward signs of a healthy gut are fairly obvious, the inward signs of a healthy digestive tract are more complicated. Intestinal motility is under the control of a complex network of nerves, each one interacting with the other. The balance of this interaction determines how smooth muscle along the length of the intestine will contract and relax, and in what pattern between the small intestine, cecum, and large intestine. This allows the digesta to move along at the proper rate.
The Nervous System
Three components of the nervous system interact within the digestive tract. First, input from the nervous system, particularly the brain, provides voluntary control over digestive processes. Second, there's the autonomic, or involuntary, nervous system, including the sympathetic and parasympathetic branches. Third, there's the enteric nervous system, which is located within the gut itself, and functions independently of the other two components. While sympathetic and parasympathetic input can influence the enteric nervous system, the enteric system is also capable of keeping the intestine moving without any input from the other two components.
Just as we don't think about breathing, we normally don't think about digestion. Yet, certain emotional and physical states can interfere with this unconscious process. The sympathetic nervous system, for example, when stimulated by fear or intense exercise (the "fight or flight" response), causes the release of chemicals called catecholamines, including epinephrine and norepinephrine, which have been shown to decrease intestinal motility in horses. In humans, abdominal surgery has been shown to cause catecholamine release and subsequent decreased intestinal motility. Therefore, an imbalance in the autonomic nervous system--specifically, increased sympathetic nervous system activity--has been suggested as a possible cause of postoperative ileus.
Imbalance and Its Consequences
Autonomic nervous system imbalance is only one possible cause for postoperative ileus. If the interaction between the nerves and the intestinal muscles gets disrupted in any way, motility becomes disrupted, and the flow of ingesta is stopped. The result is ileus.
Obstructions of the intestine, whatever the cause, prevent digesta from moving along the intestinal tract. However, normal digestion continues throughout the rest of the digestive tract; microbes continue to digest food material and produce gases. These gases become trapped at the site of the obstruction, the intestine distends, and this distention causes pain for the horse. As the problem continues, fluid begins to move from the blood and enter the intestine, causing dehydration and increasing discomfort. If the obstruction is in the small intestine, that fluid backs into the stomach and causes it to expand. If the fluid is not removed with a nasogastric tube, the stomach will rupture. If the intestinal obstruction interferes with blood supply to that particular section of intestine, the tissue will begin to die, along with the microorganisms residing in that area. When large numbers of microbes die, they release endotoxins, the cause of endotoxemia. This condition can affect the entire digestive tract as well as the rest of the organ systems throughout the body.
The longer the digestive tract is held in check like this, the more the horse's condition will deteriorate.
Recognizing and Treating Ileus
The clinical signs of postoperative ileus are related to the disruption of intestinal motility, along with the buildup of gases and fluid within the digestive tract. Indications of ileus will progress as long as motility remains disrupted. Intensive management is required, including both nursing care and sometimes specific prokinetic drugs designed to stimulate the intestine to move again (see table below).
|These are the drugs veterinarians use to decrease the duration and severity of ileus. Because prokinetic drugs are non-specific in their mechanism of action, they can be associated with side effects. Metaclopramide is the most clinically effective drug in treating ileus, but it is also associated with significant side effects.|
|Name of Drug||Basic Mechanism of Action||Clinical Effectiveness||Side Effects|
Blocks dopaminergic activity
Stimulates parasympathetic activity Blocks sympathetic activity
|Promotes forward motility||Restlessness, sweating, colic|
|Bethanecol||Stimulates parasympathetic activity||
Promotes gastric and cecal emptying
Best used in combination with Yohimbine
|Cramps, diarrhea, salivation|
|Yohimbine||Blocks sympathetic activity||Best used in combination with Bethanecol|
|Neostigmine||Prolongs parasympathetic activity||Best used to stimulate large colon motility||Abdominal pain|
Stimulates parasympathetic activitiy
Stimulates enteric smooth muscle activity
|Promotes gastric and cecal emptying||Abdominal pain|
The most common surgical site associated with ileus in horses is the small intestine. The proximity of the small intestine to the stomach allows for gastric distention to occur fairly rapidly. Therefore, gastric decompression, or removal of this fluid, is a vital component of nursing care for horses with postoperative ileus. Gastric distention can occur quite rapidly in afflicted horses. The only outward sign in the horse might be depression from the pain. Therefore, the nasogastric tube is routinely pumped to relieve any fluid accumulation. As motility in the intestine begins to return, less fluid will move into the stomach and the nasogastric tube can be removed.
Dehydration is a problem in horses with ileus not only because fluids are being drawn into the intestine, but because these horses are not allowed to eat or drink until there is evidence of motility in the gut. They are, therefore, maintained on intravenous fluids to support the cardiovascular system and to maintain fluid and electrolyte balance. Veterinarians frequently monitor gut sounds in horses with ileus as one way of watching for the return of intestinal motility. Defecating is another indicator; once a horse begins to move feces, it won't be long before complete motility is restored.
It is necessary to closely monitor body temperature in horses with ileus, because the surgical incision that is healing can become infected. There is also a chance for infection within the abdomen. Digital pulses are monitored in case laminitis develops. This disease can occur in any horse postoperatively, but it is particularly worrisome in horses with ileus. Mild exercise in the form of hand walking is therapeutic for the condition, because it might help stimulate intestinal motility, and it is generally beneficial for the horse's well-being.
Pain relief for ileus is problematic. Unfortunately, some drugs used to treat pain in the horse, including xylazine and detomidine, have their effect by stimulating sympathetic nerve receptors located in the intestine, which suppresses intestinal motility. These drugs might be helpful during an episode of mild colic, but not during the recovery phase of ileus. The better choice for pain relief in these horses is anti-inflammatory drugs such as phenylbutazone, which provides good pain relief, prevents some of the effects of endotoxins, and might help fight the root cause of postoperative ileus.
What Causes Postoperative Ileus?
Many causes of postoperative ileus have been suggested and are currently being researched. It is unlikely that the condition has a simple, straightforward cause. It is much more likely that the cause is as complex as the interaction of nerves that govern motility within the intestine.
As mentioned previously, an imbalance in the autonomic nervous system has been proposed as part of the problem. Increased activity of the sympathetic nervous system, with increased release of the catecholamines epinephrine and norepinephrine, has been shown to be associated with suppression of both human and equine intestinal motility. Similar studies have determined that decreased activity of the parasympathetic nervous system, associated with decreased concentrations of the chemical acetylcholine (which is responsible for carrying information across the spaces between nerve cells), are also associated with suppression of motility.
Dopaminergic nerves, which release the chemical dopamine (a precursor of epinephrine and norepinephrine), might also be involved in the etiology of postoperative ileus. When humans are given dopamine as a drug therapy, ileus is a side effect. It appears that antidopamine medications can block this. In a small study of horses with the condition, the drug metaclopramide, which blocks the effects of dopamine, was shown to be effective in the treatment of ileus.
Researchers have recently begun to suspect that intestinal inflammation plays a role in the development of the condition. It was suggested some years ago that endotoxemia also plays a part. Endotoxemia has been shown to inhibit intestinal motility, but it doesn't seem to contribute to the development of gastric distension. It has been determined, however, that some of the body's own chemicals, called inflammatory mediators, help initiate inflammation and account for the effects of endotoxin. While it seems that endotoxin plays some role in triggering postoperative ileus in its early stages, inflammation of the intestinal smooth muscle might have the larger role.
In a study involving rats, intestinal surgery triggered both ileus and white blood cell migration, which is an inflammatory response. If the researchers gave the rats a drug that prevented white blood cell migration, ileus didn't develop. This led to the conclusion that white blood cell migration, or inflammation, was at least partially responsible for postoperative ileus. There are other theories about the causes of the condition, and scientists continue researching the complexity of this condition.
Depending on the research study you read, postoperative ileus is believed to account for 20-80% of surgical complications in horses. Fortunately, improvements in understanding why it happens and how to treat it have helped to decrease the mortality associated with the condition from an early reported rate of 86% to only 13% more recently. However, ileus is still responsible for 40% of deaths due to all postoperative complications in horses treated for colic. Obviously, postoperative ileus is killing too many horses, and there is work yet to be done.
- Loenig, J.; Cote, N. Equine gastrointestinal motility--ileus and pharmacological modification. Can Vet J, 47, 551-559, 2006.
- Nathaniel A. White II, James N. Moore, eds. Current Practice of Equine Surgery. Philadelphia: J.B. Lippincott Co., 1990.
- N. Edward Robinson, ed. Current Therapy in Equine Medicine 5. Philadelphia: W.B. Saunders, 2003.
About the Author
Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.
POLL: University Equine Hospitals