For every equine veterinarian who has ever had to deal with a colicky horse writhing on the ground in unrelenting pain, one of the foremost possibilities on a list of differential diagnoses is a strangulating lipoma. This is particularly relevant to horses from mid-teens to aged horses, with geldings especially vulnerable to this condition.
Small intestinal diseases, which are responsible for 20-30% of adult horse colic cases treated at a veterinary hospital, most often involve strangulation lesions that require surgery. Strangulating lipomas typically affect horses older than 14 years. Extreme pain associated with bowel strangulation due to an intestinal lipoma creates a serious colic situation often making it difficult to manage and treat a thrashing horse. Once the horse is stabilized sufficiently to load on a trailer, it is best to send it to a referral hospital as bowel strangulation and associated obstruction create a true surgical condition needing immediate attention.
How Does a Strangulating Obstruction Form?
How does a lipoma form and cause strangulation of loops of bowel? It starts with localized plaques of fat forming between two serosal layers of the mesentery. Over time, the serosal fat stretches to form a pedicle attached to the mesentery, in essence forming a benign “tumor.”
A small lipoma without much of a stalk can lie dormant in a horse’s intestines for long periods without causing any problem. The weight of a lipoma is critical to development of a strangulating lesion, with studies noting that “a lipoma heavier than 33 grams has the potential to cause an intestinal obstruction” [Edwards, G.B.; Proudman, C.J. Equine Veterinary Journal 1994, vol. 26 (1); pp. 18-21].
The length of the pedicle is also of significance to the potential for strangulation. On occasion in about 10% of cases, the small colon is involved in the strangulation obstruction.
Previously it was suspected that a lipoma caused small intestinal strangulation. It was thought that as a pedunculated lipoma (on a stalk) moved around the small intestine, it eventually looped through its own pedicle to entrap and strangulate loops of small intestine. A recent study could not corroborate this with surgical exploration, and the authors have refined the theory about the pathophysiology for development of a strangulating lipoma [Gandini, M.; Freeman, D.E.; Giusto, G. Hypothesis on the pathophysiology of small intestinal strangulation by a pedunculated lipoma. Equine Veterinary Education 2022, vol 34 (4); doi: 10.111/eve.13485].
The new theory is based on surgical findings in 11 cases. The researchers proposed that the lipoma stalk is pulled down by the weight of the lipoma or through downward traction from adjacent intestinal loops. Then, a slit-like aperture forms between the stalk and contiguous mesentery. Loops of intestine might “fall” into the aperture due to intestinal peristalsis and/or close confinement of the intestines within the available space of the abdominal cavity. Once intestinal loops enter the aperture and fill with ingesta, their weight traps them there. The pedicle “post” forms a constricting half-hitch knot that strangulates intestinal blood supply to cause death and necrosis of small intestine (or small colon).
The difference in the two hypotheses is that the current theory suggests that a more likely progression to strangulation is caused by intestinal peristalsis rather than the historical suggestion that the lipoma itself moves around a loop of intestine. The weight of the lipoma and/or the length of the stalk have considerable effect on increasing tension on the stalk and lipoma mass.
With consideration of peristalsis as a predisposing factor to strangulating lipoma lesions, the authors suggested that there might be value in using motility-inhibiting drugs such as hyoscine (Buscopan), alpha-2 agonists or opiates at the initial visit by a referring veterinarian. By limiting peristalsis, it might be possible to reduce the volume of entrapped intestines.
The authors proposed that resolution of the strangulation at surgery might be accomplished by reversing the direction followed by the lipoma. This is considerably safer than transecting the stalk blindly that has the potential to injure major mesenteric vessels and the mesentery.
Prevalence and Risk Factors for Strangulating Lipomas
There is no direct link correlating obesity with the development of lipomas. Horses of many different body types can develop this condition, including horses with normal body weight. Lipomas are often incidental findings during surgery or at necropsy. Many older horses develop lipomas that range in size and number yet without any ill effects within the abdomen.
However, it is prudent to advise clients to keep their horse in a good body condition along with giving plenty of exercise. Overfeeding results in laying down of fat, which could predispose to lipoma development. Some breeds are at higher risk—Arabians, Morgan horses, Saddlebreds, Quarter Horses and ponies, for example—as they tend to develop obesity when overfed. The syndrome is most prevalent in horses over 14 years of age.
A Tennessee study identified an association of insulin-resistance (IR) with higher body condition scores (BCS of obesity), hyperplasia of the pituitary, and the presence of a significantly greater overall area of mesenteric lipomas found at necropsy compared to non-IR horses.
The study examined 11 IR horses and 19 insulin-sensitive controls. The authors stated that: “The current study fails to identify any relationships between BCS and lipoma frequency or area. Our findings suggest that IR horses and those with higher pituitary scores, regardless of BCS, may be more likely to have lipomas, and therefore may be more likely to develop lipoma-associated colic” [Newkirk, K.M.; Chameroy, K.A.; Tadros, E.M.; Rohrbach, B.W.; and Frank, N. Pituitary Lesions, Obesity, and Mesenteric Lipomas in Insulin-Resistant Horses. Open Journal of Veterinary Medicine 2014, vol. 4, pp. 190-196. doi.org/10.4236/ojvm.2014.49022].
Clinical Signs of a Strangulating Lipoma
In initial stages of a strangulating lipoma episode, a horse might only demonstrate depression and mild to moderate abdominal pain that responds reasonably well to analgesic medication. Gut sounds might be reduced or absent. A horse with an obstruction from a strangulating lipoma typically has gastric reflux when stomach tubed due to small intestinal bowel obstruction.
A rectal exam might determine the presence of distended loops of small intestine that have been likened to the feel of a bicycle tire filled with air, suggesting an inciting cause such as bowel strangulation.
An abdominocentesis is a useful diagnostic tool to characterize the abdominal fluid: Serosanguinous fluid, an elevated white blood cell count (>10,000 cells/ul) and elevated protein (>2.5 mg/dl) are all surgical indicators.
Transabdominal ultrasound might not yield an exact diagnosis due to abdominal fat and gas in the bowel making it difficult to identify that specific lesion. However, abdominal ultrasound can differentiate between strangulating and non-strangulating obstructive conditions.
Rapid referral for a suspected strangulating small intestinal lesion improves chances of survival, particularly if achieved prior to cardiovascular compromise and/or ischemic injury or infarction of the bowel. Aged horses reportedly fare just as well with surgery as younger individuals, so age should not limit a decision for an owner to send the horse to surgery.
Of colic cases undergoing surgery, pedunculated strangulating lipomas were reported to occur in 10–18% of cases in several studies. Short-term survival rate (up to a week post-op) for pedunculated strangulating lipomas averages around 43% compared to long-term survival rate (up to one-year post-op) of 38%.
In contrast to large colon obstructions that tend to have a more gradual onset with the horse alerting an owner to clinical signs often before a lesion becomes irresolvable, a strangulating lipoma tends to occur acutely, rapidly and completely.
By the time a horse shows overt clinical signs, a good deal of intestinal death and necrosis has occurred, coupled with release of endotoxins and sepsis into the circulation.
Strangulated bowel is associated with circulating endotoxemia, which can be fatal. Also, postoperative ileus can lead to euthanasia following failed attempts at treatment to restore bowel motility.
In the year following surgical correction of a strangulating lipoma, other complications can arise, including intra-abdominal adhesions, mesenteric stump abscesses, or stenosis of the intestinal anastomosis. Any of those can lead to euthanasia and a lessened long-term survival rate. Post-operative complications following surgery for strangulating lipoma are reported to occur in 72% of cases.
Once a horse survives a year following surgery, there is a good chance for return to previous riding pursuits. The best outcomes result in horses not needing intestinal resection and anastomosis, when that is even possible with this kind of lesion. Rapid recognition and diagnosis are key to the most favorable outcomes.