One of the most common causes of surgical colic in a mature horse is the pedunculated lipoma, which accounts for more than 40% of the lesions found in the small intestine.
A lipoma is a benign fatty growth, or tumour, that grows in the mesentery — a thin, veil-like tissue that carries blood vessels and nerves to the intestines and attaches the intestines to the abdominal wall. Lipomas typically develop in the mesentery of the small intestine, but can also occur in the small colon mesentery.
After starting life as a small, discrete lump of fat, a lipoma slowly enlarges over months to years and typically pulls away from the mesentery on a stalk of tissue called a pedicle. The pedicle can vary in length from a few centimetres to 50cm or more, while the lipoma itself can range from the size of a golf ball to a grapefruit.
A lipoma that grows close to where the mesentery attaches to the intestine may press against the intestine walls, causing a partial obstruction. This narrowing of the intestine can subsequently result in repeated bouts of colic that usually respond to medical treatment with either painkillers or dietary changes, or both.
A lipoma that is pedunculated — attached to a pedicle — can wrap itself around a portion of the intestine and create a physical obstruction.
The stalk acts like a ligature, cutting off the blood supply to the affected segment and causing rapid death of that part of the intestine. This “strangulating obstruction” usually results in signs of severe colic.
Who’s at risk?
Intestinal strangulation by a pedunculated lipoma is typically a disease of older horses with an average age of 14 to 19 years at diagnosis; horses under 10 rarely suffer from the condition.
All types and breeds can be affected, although some studies have suggested that ponies, Arabs, American saddlebreds and quarter horses are at a higher risk and thoroughbreds are at a lower risk. Affected horses are usually in good condition, but not necessarily obese. Geldings seem to be significantly more susceptible to the condition than mares.
Diagnosis is usually made at the time of surgery. The signs associated with the condition are usually similar to those of other causes of small intestinal strangulation. This is a form of moderate to severe colic pain that does not respond or responds only temporarily to painkillers.
Although small intestinal strangulations most commonly result in severe colic pain, some stoic horses will show little sign of discomfort.
When the affected portion of intestine has died, the pain may subside. Affected horses may therefore appear dull and depressed rather than showing classical signs of colic pain.
In evaluating a horse with colic, a vet will be looking for other signs that may indicate a small intestinal strangulating obstruction. These include: congested mucous membranes; a high heart rate; the presence of gastric reflux when a stomach tube is passed into the stomach; a distended small intestine that can be felt on rectal examination, and abnormal peritoneal fluid, a sample of which can be obtained by popping a needle into the abdomen along the midline of the belly.
Ultrasound examination of the abdomen can be helpful in identifying small intestinal obstructions and will occasionally reveal the presence of a lipoma.
Surgery is the only effective treatment. Once the affected portion of intestine is identified, the lipoma pedicle is cut or broken to relieve the constricting effect on the intestine and its blood supply.
Depending on how long the strangulation has been present, and whether or not irreparable damage has occurred, the affected intestine may need to be cut out (resected) and the remainder re-plumbed (anastomosed) to re-establish a healthy and functional intestinal tract.
Intestine that has completely lost its blood supply and has died as a result — a process known as infarction — is usually dark red or black in colour, whereas intestine that has been damaged but can survive once the blood supply is re-established may appear bruised and swollen.
The decision as to whether or not intestine needs removing is not always straightforward, even for an experienced colic surgeon. If just a small segment of dead intestine is left behind, the horse will not recover and may require a second surgery to remove that piece of gut.
The amount of intestine that needs to be resected will depend upon the length of the strangulated portion. Most commonly, a loop of intestine is involved, which may vary in length from 30-50cm to many metres. The small intestine of the average adult horse is around 25m long — up to half of this length can, if necessary, be removed.
A proportion of horses that undergo surgical correction will not survive.
Short-term survival rates of 48%-78% have been reported. Sometimes a horse is found in the morning after colicking all night, resulting in treatment at a later stage in the course of the disease. The resulting shock and toxaemia (blood poisoning) associated with the strangulating obstruction can result in damage to many other organs and may prove fatal.
In addition, a significant number of horses undergoing surgical treatment will develop post-operative ileus. This complication results in a failure of the intestine to move the gut contents along the intestinal tract, so that gas and fluid accumulate in the stomach and small intestine.
The lining of the intestine then suffers further damage as the pressure increases. Since horses are unable to vomit, the fluid that accumulates in the stomach must be removed by repeatedly passing a stomach tube to prevent the pressure increasing to the point where the stomach ruptures.
There is no specific effective treatment for post-operative ileus; management is primarily supportive in the hope that intestinal function will return. If this happens within a day or two of surgery, the horse can make a full recovery.
Unfortunately, the longer it persists, the less likely it is that the horse will survive. A second surgery is sometimes undertaken after a few days of post-operative ileus to allow decompression of the distended intestine and to check no irreparably damaged intestine is still present.
The underlying cause of lipomas is not yet fully understood, so there are no proven preventative measures. Since this is a disease of the fat, however, it is sensible to maintain an older horse or pony in an appropriate body condition instead of allowing him to become obese.
Ref Horse & Hound; 2 August 2018