To simplify acute colic as seen in the race horse, you will have either distention and/or impaction as the main mechanism which manifests as colic. Distention is just that, an abnormal enlargement of either the stomach or, further down, a specific region of the intestines by an accumulation of gas, fluids, and/or ingesta-causing a stretching of the walls. Impaction is a blocking of the inner space of either the stomach or the intestinal lumen by solid ingesta. Impaction can occur in degrees up to complete blockage. Gastric (stomach) impaction seems to be rare in the performance horse while stomach distention is much more common. The formation of gas in the stomach being the major cause of gastric distention. This can occur from a change in diet, the consumption of too much water while hot, or the ingestion of feed types that are easily fermentable. Last summer, my nephew fed a thoroughbred mare just off the track, freshly baled green hay. Within hours, she was experiencing continuous, severe colic. This can be a fairly common response to newly baled hay undergoing a heating fermenting process even before it was ingested into the animal's stomach. This external fermentation can be so intense that a person can experience extreme discomfort simply by placing his hand between one of the flakes in newly baled hay. The horse is unique in that its cardiac sphincter (round muscular valve) at the anterior opening of the stomach will not easily open, once the food has passed. This is why you will very seldom see a horse vomit or perform any type of reflux of the stomach's contents. Thus, fermenting gases will tend to stay in the stomach and not be burped back out the mouth or nostrils. This opens the horse up to painful gaseous distentions not seen in other species. You can see why large amounts of green hot hay is a no-no in feeding horses. My nephew learned a valuable lesson which will not be repeated. Tympanic colic in the large intestines is also sometimes observed. It is caused by highly fermentable ingesta coming into contact with the microbes in the large intestines resulting in a high volume of gas production.
Distention/impaction can occur anywhere from the stomach, the anterior small intestines backward to the descending colon of the large intestines.. Distention of the small intestines most probably occurs in conjunction with ingesta impaction. The gaseous distention is a result of the impaction. When there is an impaction (obstruction) you will have a simultaneous dilation just anterior of the obstruction. This is due to pooling fluids from intestinal wall secretions being stimulated from the blockage itself. Likewise, one often sees decreased absorption in that region which only adds to the dilation. With this obstruction there is a characteristic decrease in intestinal motility (movement) which further adds to accumulation of fluids and gas anterior to the blockage. So with this vicious cycle in full motion, the intestines continue to distend with increasing severe pain to the horse. This stretching causes colic. There can be various degrees of impaction and, thus, the severity of pain may be affected. Usually partial blockages allows some ingesta to pass causing intermittent colic symptoms. A complete simple blockage causes intensifying acute colic. Distention in the large intestines can occur from any circumstance which may alter the colon's peristaltic motion. The altering of the gas producing qualities of the colonic microbes can be another factor. Often one will not see any manure coming out during impactions, though it is not uncommon to see the horse pass small amounts. In the example of my nephew's mare experiencing gastric distention, she was periodically passing small clumps of normal looking manure. This would be another clue suggesting her distention was further up the line, in this case, the stomach. You may have the action of ingesta lodging in front of the blockage with the posterior part breaking off and being passed in the form of sporadic manure. Some manure is not necessarily a sign that your horse is out of the woods. Large intestinal impactions are often due to poorly digestible hay, a lack of adequate amounts of water or, for what ever reason, a decrease in the motility of the colon. One of the most common causes of colic, particularly during cold winter temperatures, is this lack of proper water consumption. Also, a change in feed can affect the movement (motility) of the gut.
The bottom line, a blockage of the intestines is due to a lack of intestinal motility or movement which normally carries the ingested food down the tubes. When this movement is altered one gets a blockage with accompanying distention. Pain as a result of this initial blockage causes more blockage due to a neural reflex action producing a vicious circle which must be broken.
What are the serious fatal sides to all of this? In the case of stomach distention, one can eventually have a rupture of the stomach with death. Likewise ruptures can occur in the small and large intestines, too. One can have what we call a "twisted gut". This is a strangulation of the intestine which causes a blockage of the blood flow. It is generally believed that if the colicky horse is allowed to violently roll and thrash on the stall floor, a strangulated intestine may result. This is why grooms memorial have walked affected horses incessantly-to keep the animal from rolling in discomfort. Strangulation of the intestines is fatal in most cases when left untreated.