Small colon impaction (Proceedings)

Small colon impaction is a relatively infrequent cause of colic. However, when it does occur, it can be difficult to recognize and to manage. We have noticed over the last several years that many of these cases are initiated by diarrheal disease, making the eventual diagnosis of an obstructive condition unexpected by the veterinarian. In addition, the pathophysiology of post-diarrhea small colon impactions is poorly understood. Additional causes of small colon impaction include poor chewing of feed, with formation of fecaliths, noted most commonly in miniature horses. However, this presentation will focus on small colon impaction in adult full-sized horses.

Small colon impaction in the adult horse

This type of obstructive disease represents approximately 2% of all horses evaluated for colic at referral institutions according to previous studies. In a recent study underway at NC State University, 39 horses with small colon impaction were evaluated. Of these horses, all were 18-months to 4-years of age. Approximately 40% of these horses presented with diarrheal disease, whereas the others presented for evaluation of colic. On rectal palpation, small colon filled with fecal material was detected in almost all cases. In contrast to the normal small colon, a solid tube will usually be detected, without the presence of sacculations. Because of the smooth texture of the small colon once distended with fecal material, it can be difficult to differentiate from large colon impaction. However, careful palpation will usually reveal several loops of small colon, which frequently enter the pelvic inlet. Another complicating factor on rectal palpation may be the presence of large colon distension, which results from small colon obstruction. Gas distended large colon may be interpreted as the most important finding when palpating these horses, and may obscure the presence of impacted small colon.

Additional findings that can help with the diagnosis include the presence of a fever (noted in approximately 20% of the NC State cases), and a leukopenia with a left shift. These findings are more suggestive of infectious etiologies of gastrointestinal disease, so the veterinarian must be careful not to miss subtle signs of colic indicating the need for further workup for obstructive disease.

According to our recent study, horses with small colon impaction tend to present in the fall and winter, and appear to be more likely to occur on farms that have had horses with diarrheal disease or small colon impactions in other horses. In some farms, outbreaks of small colon impaction may be noted, further suggesting the possibility of infectious agents. Therefore, five serial fecal cultures for Salmonella taken approximately 24-hours apart should be initiated in adult horses with small colon impaction, particularly if they present with diarrheal disease. In the NC State cases, approximately 10% of those cultured were positive for Salmonella on at least one of five fecal cultures. Other studies have indicated higher rates of Salmonella-positive cultures. Further study at NC State on potential viral etiologies of this disease, such as corona virus, have not revealed the presence of any viral agents.

Management of small colon impaction

Horses with small colon impaction may be managed medically or surgically, largely based on the degree of colic and the degree of abdominal distension. For the NC State cases, the only significant differences between medical and surgical cases was the degree of abdominal distention and the degree of colic. Initial medical treatment should include intravenous fluid therapy and judicious use of analgesics so as not to obscure continued evidence of colic. Many surgeons believe that surgery is indicated in all cases because of the difficulty of treating long-standing cases surgically due to degeneration of the small colon wall. However, there has been considerable success with medical management. For example, in the NC State cases, approximately 60% of the cases were managed medically, with a 91% short-term survival rate (percentage of horses discharged from the hospital). Alternatively, of the horses treated surgically, there was a 94% short-term survival rate. Prior studies have shown varying results, with one study indicating similar survival with medical and surgical treatment, and another study showing significantly improved survival with surgical treatment. Based on this evidence, initial medical management of horses with small colon impaction is warranted, but if horses become excessively gas distended with accompanying signs of colic, they should be taken to surgery as soon as possible to avoid severe injury to the small colon.

Surgical management of small colon impaction involves evacuation of the small colon by either a high enema or an enterotomy. A high enema consists of the surgeon directing a stomach tube introduced into the rectum by an assistant. The tube is brought into the small colon until it reaches the impaction, at which time a stomach pump can be used to carefully pump in fluids to soften the impaction. The surgeon can use this fluid to massage the fecal contents, after which the tube can be removed and successive sections of the impaction evacuated via the rectum. Some surgeons will place DSS in the water as an additional adjunct to soften the fecal mass. This method is best reserved for relatively short impactions near the caudal-most extent of the small colon impaction. Extensive impactions, particularly those that involve the proximal small colon, will require excessive manipulation using the high enema method, and an enterotomy should be performed instead. Small colon enterotomies are performed in much the same way as large colon enterotomies, with hoses used to flush the small colon lumen. Once the small colon has been evacuated, it is advisable to additionally perform a large colon enterotomy in order to reduce the amount of feces that will reach the small colon in the immediate postoperative period.

Postoperative management includes continued used of anti-inflammatory agents, IV fluids, and antibiotics. Re-feeding horses following small colon impaction should include introduction of small volumes of low residue feed such as a complete pelleted ration, initiated approximately 36-48-hours following surgery. This will allow enough time for small colon inflammation to begin to subside. Veterinarians should be aware of the propensity of these cases to re-impact, presumably because full resolution of small colon inflammation likely takes 5-7-days. In cases presented to NC State, we have even noticed recurrence of small colon impaction 10-14-days following initial presentation, suggesting that very careful re-introduction of forage is warranted.

Pathophysiology of small colon impaction

The pathophysiology of this disease likely involves infectious agent-induced mucosal inflammation, which may initially manifest as diarrhea. However, as the lumen is compromised with thickened mucosa, small colon impaction occurs. Some horses will continue to have signs of diarrheal disease in the face of small colon impaction, as secreted fluid makes its way around the impaction. Although further study of biopsies from cases of small colon impaction at NC State is required, an initial neutrophilic infiltrate in response to invasion by infectious organisms may be noted. Diffuse edema throughout the mucosa and submucosa is typically noted, and in many cases hyperemia and edema are noted at the level of the serosa. These inflammatory changes contribute to partial obstruction of the lumen. However, it is also likely that inflammation interferes with normal motility patterns within the small colon. This may explain the recurrence of small colon impaction in a number of cases in which the thickness of the wall has returned to normal, but motility remains abnormal.

Conclusions

Small colon impaction should be a differential for horses that present with colic, particularly when associated with diarrhea, in young adult horses during the fall and winter months. Workup should include a full colic examination, abdominocentesis, and blood work. For horses with confirmed small colon impaction, fecal cultures should be initiated, and cases should be treated medically. However, horses should be very carefully observed for evidence of recurrent colic and abdominal distension, at which time surgery should be initiated. Judicious re-feeding is required as part of the post-operative treatment plan. Finally, for farms involving multiple cases, attempts to restrict access to affected horses on the farm should be made to prevent potential spread of infectious agents to other horses.