Colic (equine acute abdomen)

What it is

Colic is not a diagnosis but the name for abdominal pain in the horse. It is the most common medical emergency in the species. The horse's digestive anatomy explains much of it: a small stomach that can hardly vomit, a long small intestine on a very mobile mesentery, and a huge large intestine (caecum and large colon) that is only partly fixed to the abdominal wall and changes calibre at several points.

Causes are grouped by the damage to the intestinal wall:

• Simple (non-strangulating) obstructions: transit stops but blood supply is preserved. These are impactions (of the pelvic flexure of the large colon, the caecum, the small colon or the ileum), large colon displacements (right dorsal displacement and nephrosplenic entrapment or left dorsal displacement) and spasmodic or gas colic.
• Strangulating lesions: transit and blood supply are obstructed together, and the wall becomes necrotic within hours. The most typical are large colon volvulus, the pedunculated lipoma that wraps around the mesentery and strangulates a loop of small intestine, epiploic foramen entrapment (a loop of small intestine passes through the epiploic foramen and becomes trapped), small intestinal volvulus and inguinal hernias in stallions.
• Enteritis and other inflammatory causes, such as proximal enteritis or colitis.

Recognising early the colic that resolves with medical treatment and the one that needs surgery is the key decision.

Who it affects

Horses of any age and breed. The best-documented risk factors are management changes: in feeding, in the type or amount of forage, in housing and in exercise, as well as more hours stabled, travel, lack of deworming and a previous colic. Some risks are specific to each lesion:

• Pedunculated lipoma: older horses; lipomas grow over the years.
• Epiploic foramen entrapment: crib-biting or windsucking is very strongly associated.
• Large colon volvulus: broodmares, especially in the months after foaling, and taller horses.
• Pelvic flexure impaction: stabled horses with little exercise, poor-quality forage or little water, and dental problems.

Anatomy involved

• Small intestine: the duodenum is fixed and short, but the jejunum hangs from a long mesentery that makes it very mobile; this is why it easily wraps around the pedicle of a lipoma or slips into the epiploic foramen, the opening of the omental bursa between the caudate lobe of the liver, the caudal vena cava and the portal vein. The ileum opens into the caecum through the ileal orifice and can become impacted or intussuscept.
• Caecum: a comma-shaped sac, with its base in the right flank, under the paralumbar fossa, and its apex on the abdominal floor, towards the xiphoid cartilage. It communicates with the large colon through the caecocolic orifice. Its impactions can end in rupture with little preceding pain, so they are monitored closely.
• Large colon (Colon ascendens): forms a double horseshoe with four parts (right ventral, left ventral, left dorsal and right dorsal colon) joined by the sternal, pelvic and diaphragmatic flexures. Only the right dorsal colon and the base of the caecum are fixed to the dorsal wall; the left parts are free, which allows displacements and volvulus, which usually twists near the base of the caecum. At the pelvic flexure the lumen narrows abruptly and it is the usual site of impactions; it is palpable per rectum.
• Spleen: lies on the left, against the costal wall. Its dorsal end is joined to the left kidney by the nephrosplenic ligament; in nephrosplenic entrapment the left colon rises between the spleen and the left abdominal wall and becomes hooked over that ligament. It is recognised by rectal palpation or ultrasound of the left flank.

Clinical signs

• Pain: looking at the flank, pawing, kicking at the abdomen, lying down and getting up, rolling, stretching postures. The intensity of pain and its response to analgesics are among the most useful findings: severe pain that does not respond to analgesia points to a surgical lesion.
• Heart rate: rises with pain and hypovolaemia; persistent tachycardia is a warning sign.
• Congested mucous membranes or a toxic line, prolonged capillary refill time, weak pulse and cold extremities indicate shock and endotoxaemia.
• Reduced or absent gut sounds.
• Abdominal distension, no faeces and, in small intestinal obstructions, gastric reflux through the nasogastric tube.

Diagnosis

• Repeated general examination: heart rate, mucous membrane colour, capillary refill time, auscultation of the four quadrants and response to analgesia.
• Nasogastric intubation: both diagnostic and therapeutic. The horse can hardly vomit because of its powerful cardiac sphincter, so fluid accumulated in the stomach must be drained through the tube to prevent gastric rupture.
• Rectal palpation: can detect an impacted pelvic flexure, distended small intestinal loops, tight bands of the colon or caecum, or the colon in the nephrosplenic space.
• Abdominal ultrasound: distended, amotile small intestinal loops, thickening of the colon wall (suggestive of volvulus), colon in the nephrosplenic space, free fluid.
• Abdominocentesis: fluid appearance, protein and peritoneal lactate compared with blood lactate help recognise intestinal ischaemia. Packed cell volume and blood lactate indicate severity.
• Many lesions are confirmed at exploratory laparotomy.

Treatment and prevention

• Medical treatment: analgesia (non-steroidal anti-inflammatories, α2 agonists, butorphanol), gastric decompression, oral (by tube) or intravenous fluid therapy to rehydrate the contents of impactions, laxatives, withholding feed and frequent reassessment. Nephrosplenic entrapment can resolve without surgery, with exercise and phenylephrine or by rolling the horse under anaesthesia; a meta-analysis of 19 retrospective studies found no difference in resolution between medical and surgical treatment, or with phenylephrine, although there are no randomised trials.
• Surgical treatment: ventral midline laparotomy with the horse in dorsal recumbency. The displacement or volvulus is corrected, the colon is emptied through an enterotomy at the pelvic flexure and non-viable intestine is resected with an anastomosis. Strangulating lesions carry a worse prognosis than simple obstructions, and small intestinal lesions a worse one than large colon lesions. The most frequent postoperative complications are ileus, recurrent colic and wound complications.
• The decision to operate and its cost should be discussed early with the owner: delay worsens the prognosis of strangulating lesions.
• Prevention: gradual changes in feeding and management, good-quality forage available for many hours, water always available, regular exercise or turnout, parasite control and dental checks.
• The horse is a food-producing species in many countries. In the EU its identification document (passport) states whether it is excluded from the food chain; only then can it receive medicines not authorised for food-producing animals, and some substances considered essential for equids can be used with a six-month withdrawal period recorded in the passport. Otherwise national regulations and their withdrawal periods apply.

Key studies

• Curtis et al. (2015), prospective study of 1016 first examinations of horses with colic by 167 practitioners in the United Kingdom. Of the 822 classifiable cases, 23.6% were critical (needed intensive treatment or surgery, or died). In the final model, pain score (OR 1.19 per point), heart rate (OR 1.06 per beat/min), capillary refill time > 2.5 s (OR 3.21), weak pulse (OR 2.90) and absent gut sounds in one or more quadrants (OR 3.65) were associated with a critical case. Cases were classified as critical retrospectively, by outcome.
• Curtis et al. (2019), scoping review of 58 studies on risk factors for colic and systematic review of management changes. Only 14 studies (1 cohort, 8 case-control and 5 cross-sectional, all from 1990–2008) met the criteria; changes in feed (5 of 14) and housing (3 of 14) were the factors with most evidence. Eight of the 14 did not control confounding well and heterogeneity prevented a meta-analysis.
• Archer et al. (2008), matched case-control study in hospitals in the UK, Ireland and the USA with 109 cases of epiploic foramen entrapment and 310 controls. Crib-biting or windsucking was associated with a much higher risk (OR 67.3; 95% CI 15.3–296.5), as were colic in the previous 12 months (OR 4.4) and greater height (OR 1.05 per cm). The very wide confidence interval reflects few exposed cases, and the data come from telephone questionnaires.
• Suthers et al. (2013), prospective case-control study in 4 UK hospitals with 69 horses with large colon volvulus and 204 controls. Risk increased with greater height, several colic episodes in the last year, being a mare (more so if she had foaled), more hours stabled in the previous 2 weeks, being fed hay or sugar beet, and recent changes of pasture or of the amount of forage. Data were obtained by telephone and may carry recall bias.
• Suthers et al. (2013), cohort of 116 horses operated on for strangulating large colon volvulus (≥ 360°). 76.7% survived anaesthesia; of these, 70.7% reached discharge, 48.3% one year and 33.7% two years. Higher preoperative packed cell volume (HR 1.08 per point) and abnormal serosal colour at surgery (HR 3.61) were associated with shorter survival. It is a retrospective study of referral hospitals.
• Proudman et al. (2002), cohort of 341 horses that recovered from colic surgery at a UK hospital, followed long term. The probability of survival was 0.87 at 10 days, 0.82 at 100 days and 0.75 at 600 days, with the highest mortality in the first 10 days. Epiploic foramen entrapment had worse survival (RR 2.1) and 29% of horses had at least one episode of postoperative colic. Techniques and care have changed since.