Equine Colic: IV: Diagnosis: Determining the Need for Emergency Abdominal Surgery

Determining the need for surgery in a horse with colic is usually an emergency. Although a single distinguishing test, which is highly sensitive, has been sought through research and clinical examinations, there is no exact laboratory test, formula, or algorithm which accurately determines the need for emergency abdominal surgery in individual cases. Therefore, every case should be judged on its own merits based on a thorough history and physical examination. Through trial and error during the last 30 years and the honing of diagnostic skills with the addition of ultrasonographic examinations, the decision can be made based on specific physical signs (Table 1). Horses with colic can be categorized into three groups: those that need surgery, those that don’t need surgery, and those which fall in the "gray zone" when the veterinarian can’t be sure without further observation and monitoring changes of key clinical signs.

The complete examination must be performed prior to deciding to refer a horse for surgery because each parameter by itself has inherent error when used alone. In certain cases, a rapid decision can be made if an exact diagnosis is made. Alternatively, the history and presentation may indicate the need for immediate surgical intervention without processing all the information from a complete examination for colic. Horses with severe pain are more likely to require surgery that those with mild or no pain (Table 2). In a study examining the response to treatment after the initial examination, lack of response or recurrence of pain significantly increased the odds that surgery was necessary (Table 3) [1]. This is particularly true if there is no response to analgesic administration or if a second analgesic treatment is required (Table 4) [1].

Figure 1. A horse with small intestinal strangulation. Abrasions on the head are evidence of trauma from severe pain. Shock and administration of analgesics can mask the severity of the disease, and evidence of trauma should generally be interpreted as evidence of recent severe pain. To view click on figure

Figure 2. Duodenitis-proximal jejunitis (anterior enteritis) as it appears at surgery. Petechial hemorrhage, whitish streaks, and slightly thickened jejunum is characteristic of the disease. Decompression is indicated at surgery, but medical treatment is needed to support the horse while the intestinal inflammation resolves [5]. To view click on figure

With the advent of potent analgesics, veterinarians have used the response to treatment to effectively determine which horse needs surgery [2]. This is logical and, if used with other physical signs, monitoring pain after treatment is highly accurate in determining which horses will need surgery. The timing of the response still needs refinement, but most veterinarians have determined the appropriate response time for the analgesics they use routinely for colic. The key while monitoring signs of colic is adjusting one’s tolerance for any recurrence of pain, as horses may show pain after administration of an analgesic but the reactions can be markedly decreased. Though decreased, any recurrence of pain should be considered a failure of resolution of the problem and, therefore, an indicator of increased odds that surgery is necessary. Return of pain in horses that have a medical problem, which can normally be treated without surgery, is still important. Horses with impactions in the large colon or cecum or obstructions due to sand accumulation can have recurrent pain and usually respond to medical treatment. However, if the pain persists or returns and there is no evidence of bowel motility or change in the problem, surgery is indicated before the impaction causes bowel ischemia and subsequent mural infarction. In some cases, horses may appear depressed but have evidence of trauma to the head that suggests that they experienced severe pain in the recent past (Fig. 1). Because analgesics can mask pain, evidence of trauma should be considered as an indicator of severe disease which may still be present.

Figure 3. (A) Rectal palpation of an inguinal hernia. In this disease, the normal architecture of the inguinal ring cannot be distinguished by the examiner. In some cases, the tissue attached to the inguinal ring is easily identified as distended small intestine [9]. (B) Rectal palpation of small intestinal distention. Whether due to simple obstruction or strangulation, distention of the small intestine often indicates a need for surgery [9]. (C) Rectal palpation of a large colon volvulus. Marked distention with a thickened edematous wall is indicative of a large colon volvulus. As the colon distends, it becomes more difficult to palpate the abdominal contents due to the size of the colon [9]. (D) Rectal palpation of a cecal impaction. Marked distention of the cecum which eventually becomes edematous represents a cecal impaction. The cecum moves toward midline, and the ventral band on the caudal aspect of the cecum can be followed from the right dorsal to the left ventral abdomen [9]. (E) Rectal palpation of large colon entrapment in the renosplenic space. The colon may be felt trapped in the space or colon distension may prevent palpation of the spleen and renosplenic space. Ultrasound may help make a definitive diagnosis [9]. Figures A-E; reprinted from White NA, Rectal examination for colic, Chapter in Current Techniques in Equine Surgery and Lameness, Ed. by NA White and JN Moore, W. B. Saunders, Philadelphia, 1998; 262-270. To view click on figure

Figure 4. Ultrasonograms of distended small intestine. (A) Multiple fluid filled loops of intestine with no motility indicate an obstruction which, in the small intestine, most often indicates a need for surgery. (B) Distended small intestine with a thickened wall, which results from edema or blood during obstruction or strangulation. (Ultrasonograms courtesy of Dr. Anne Desrochers). To view click on figure

Figure 5. Ultrasonogram of thickened large colon. The arrow points to edema in the colon wall. (Courtesy of Dr. Anne Desrochers). To view click on figure

Figure 6. Ultrasonogram of a cecocecal intussusception. The intussusceptiens (arrow) can be seen within the outer wall of the intestine. (Courtesy of Dr. Anne Desrochers). To view click on figure

Temperature is usually not increased in horses with acute strangulation or obstruction. Horses with enteritis or colitis, which does not require surgery, frequently have an increased temperature suggesting that surgery is contraindicated. There is no exact cut-off temperature, but horses with a fever of >102° F often have a medical condition such as peritonitis, anterior enteritis, or colitis [3]. Strangulating disease can cause a fever if compromised intestine has been present long enough to cause endotoxemia. Clinicians should also remember that non-steroidal antiinflammatory drugs such as flunixin meglumine [1] can decrease or resolve a fever even with persistence of the problem.

Indicators of hydration and perfusion, such as heart rate, mucous membrane color, mucous membrane refill, packed cell volume, and plasma protein concentration are not specific for diseases that require surgery. In most horses, signs of shock are linked to complete obstruction, strangulation, or enteritis, so that these values by themselves may or may not indicate a need for surgery and more accurately predict the prognosis for survival [4]. For example, heart rate can be misleading. Low or near normal rates can be observed in the early stages of life-threatening diseases, even when concurrent with severe pain. In this instance, pain should be considered the most important sign. High heart rates, though associated with severe disease and poor prognosis, do not always indicate the need for surgery. Diseases such as enteritis and cecal or colonic tympany may result in high heart rates but do not normally require surgery [3].

Presence of nasogastric reflux increases the likelihood that the small intestine is obstructed by a disease that requires surgical treatment. Reflux can also be caused by ileus or anterior enteritis, which can most often be treated medically. Obstructions of the colon can also lead to nasogastric reflux due to stasis or obstruction of the duodenum caused by tension on the duodenocolic ligament, but again these problems do not always require surgery. Because of the lack of sensitivity for the need for surgery, other physical signs should be used in conjunction with gastric reflux to make a final determination. Conversely, the lack of gastric reflux does not rule out the need for surgery.

Anterior enteritis, which can cause large volumes of gastric reflux, can create a diagnostic dilemma. Anterior enteritis causes pain, gastric reflux, distended small intestine, and high peritoneal protein concentration, which together indicate a need for surgery [5]. However, fever and subsequent depression are often observed, suggesting the signs are due to enteritis. If in doubt, surgery to make a specific diagnosis is indicated to ensure that a strangulating or obstructing lesion is not present. If enteritis is diagnosed at surgery, bowel decompression is completed and the horse recovered from anesthesia (Fig. 2). Anesthesia and surgery used to make this diagnosis do not appear to decrease survival [5].

Horses with decreased or complete absence of borborygmi have significantly increased odds of requiring surgery compared to horses with normal intestinal sounds (Table 5) [1]. If borborygmi do not return after administration of an analgesic or other treatment, the disease should be considered more serious, possibly requiring surgery. Decreased intestinal sounds can occur with simple colic, and large intestinal sounds can be heard even when there is obstruction of the small intestine.

Finding an intestinal abnormality on rectal examination is not always indicative of a need for surgery. Nevertheless, any abnormal distention or abnormal positioning of intestine, which has no accompanying diagnosis, is possibly a surgical lesion [6-8]. Finding an abnormality on rectal examination does not significantly increase the odds of surgery [1]. Because distention from colon impactions and tympany can often be treated medically, a rectal examination may not be a sensitive test to indicate the need for surgery. Rectal findings which identify a disease requiring surgery include: inguinal hernia (Fig. 3A), tight multiple loops of small intestine (Fig. 3B), distended and edematous colon (Fig. 3C), tight cecum filled with fluid ingesta (Fig. 3D), and massive distention of any intestinal segment [9].

Figure 7. (A) Stained smear of peritoneal fluid from a horse with intestinal strangulation. Bacteria (arrows) have been engulfed by the neutrophils which are increased in number. Red blood cells are also increased in the fluid. (B) Gram stain of peritoneal fluid demonstrating gram positive and negative bacteria. Presence of bacteria can indicate intestinal rupture, peritonitis or intestinal infarction. To view click on figure

If the findings on rectal examination are not normal, assessment of other clinical signs is necessary to make the decision about the need for surgery. If there are no abdominal abnormalities during the first rectal examination, repeat examinations are indicated, particularly if other signs suggest a surgical disease. Distention not felt at the first examination may become evident in the near future.

Transabdominal ultrasound can also be helpful in finding intestinal abnormalities, which may not be felt on rectal examination. Specific indications for surgery include markedly distended small intestine, which has no motility and a thickened wall (Fig. 4).

Similarly, thickening of the large colon has been helpful in ruling out large colon volvulus [10]. When the large colon had ≥ 9 mm wall thickness, large colon volvulus was found in 8 out of 12 horses, although accurate prediction was not present in 100% of cases (Fig. 5) [10]. Ultrasound can also be used to confirm the diagnosis of inguinal hernia, entrapment of the colon in the renosplenic space, and, in some cases, jejunojejunal or cecocecal intussusception (Fig. 6).

Abdominal fluid analysis is helpful in determining the need for surgery [11]. Protein concentrations, WBC number and differential, and RBC number are helpful in determining the degree of intestinal injury [12]. Increased protein concentration in abdominal fluid with no change in cell numbers is often due to simple obstruction with bowel distention. If WBC numbers are increased and if the ratio of neutrophils to monocytes is increased (>70% neutrophils), bowel ischemia or degeneration with leakage of bacteria is likely. Excess numbers of RBCs in peritoneal fluid often results from diapedesis of cells from capillaries and is another indication of intestinal injury, in particular venous strangulation obstruction. Increased hemoglobin concentrations also increase the odds that surgery is needed and increases the sensitivity and specificity of the decision compared to visual assessment of peritoneal fluid [13]. Finding bacteria free in the abdominal fluid or engulfed by neutrophils during a cytologic examination is further indication of intestinal injury, including mucosal damage, which allows bacteria to move through the intestinal wall into the peritoneal cavity (Fig. 7).

Figure 8. A pony with colic has distended small intestine and frequent episodes of colic with frequent attempts to lie down (A) and assume a dog sitting position (B). After 10 mg of detomidine the pony is quiet for 30 minutes (C) and then intermittently paws (D) for a few seconds. Though markedly decreased in intensity, the pain, which breaks through a potent analgesic such as detomidine, increases the possibility that surgery is necessary. This pony had massive adhesions of the small intestines, which required surgery [3]. To view click on figure

Figure 9. Radiographs of the abdomens of foals with colic: (A) Radiograph of foal with atresia coli depicting gas build-up in the proximal colon and some small intestine. (B) Volvulus of the small intestine with markedly distended loops of small intestine. The amount of distension is indicative of a severe strangulation obstruction. (C) Enteritis of the small intestine with mild distension due to ileus. This foal did not require surgery. (D) Excess abdominal fluid, ileus, and sand in the ventral colon [c]. Reprinted from Rowe EL and White NA, Diagnosis of gastrointestinal diseases, Chapter in Equine Acute Abdomen, 2nd edition, edited by NA White, JN Moore, T Mair, Teton New Media, in press, 2006. To view click on figure

If abnormal abdominal fluid is present, bowel injury requiring surgery is usually present. If the fluid is normal but other physical signs indicate that surgery is necessary, bowel injury may not have occurred yet or it may be too early in the disease process to detect changes in abdominal fluid. Peritoneal fluid should not be used as the only determinant for surgery, since waiting for a change could delay surgery and decrease the chance for survival. An acute increase in protein concentration alone is sufficient to warrant surgery if other physical signs are also suggestive. If colic persists for days, peritoneal fluid should be monitored for increases in protein and cell concentrations. Both are indicators of bowel injury and may suggest exploration is needed earlier rather than later.

Frequently, physical signs such as heart rate and mucous membrane color and laboratory values will be normal at the onset of colic. The parts of the examination that are most helpful in the early period are observation of pain, rectal examination, abdominal auscultation, and the response to analgesic administration. If pain is constant or returns within 1 - 2 hours after administration of an analgesic such as flunixin meglumine [a] or detomidine [b], the horse is more likely to need surgery (Fig. 8). Normal values for heart rate, mucous membrane color and refill, and peritoneal fluid should be disregarded if pain, rectal findings, and lack of response to an analgesic indicate a surgical disease.

Figure 10. Abnormal color of the mucous membranes is indicative of shock and a possible surgical disease, but non-surgical disease can also cause endotoxemia and discolored mucous membranes. (A) Cyanotic mucous membranes with purple gingiva around the incisors. This is characteristic of poor perfusion due to endotoxemia, which can come from a strangulating lesion, enteritis, or peritonitis as in the case in this horse. (B) A horse under anesthesia for abdominal surgery for colic. The brick red mucous membranes are characteristic of the early stages of endotoxemia and can be seen with both surgical and non-surgical disease. To view click on figure

Figure 11. Spreadsheet example of an 18-year-old Morgan gelding with an abnormal rectal, intermittent pain, weak pulse, and absent abdominal sounds. In a hospital population where historically 50% of the horses need surgery, this formula predicts 87 horses out of 100 with these signs will need surgery (highlighted text). In a hospital with a much lower prevalence of surgery, the predicted need for this horse would be somewhat decreased. Though not an absolute predictor, the model gives the examiner confidence in their medical judgment [7,18]. To view click on figure

Clinical laboratory tests are usually not helpful in deciding if surgery is needed for colic [3]. Leukopenia (<3000 WBC/μL) may indicate an endotoxic crisis, which is more often caused by colitis or peritonitis rather than an obstruction or strangulation, at least in the initial stages of the disease (Table 1). Alternatively, leukocytosis may indicate duodenitis-proximal jejunitis [5] or an abdominal abscess [14] which may not be helped with surgery. Decreased serum calcium concentrations have been reported in horses requiring surgery, but this has not been used as a predictor of the need for surgery [15]. Serum electrolyte concentrations are often normal in horses with acute intestinal obstruction. Serum chloride concentration can be abnormally low in horses with gastric reflux. Gastric reflux can also create an alkalosis due to loss of acid from the stomach. Hematology and blood chemical values are of most value for predicting survival and guiding treatment to correct metabolic abnormalities.

Deciding on the need for surgery in foals with colic is more difficult due to an inability to perform a rectal examination. However, radiographs and ultrasound can help distinguish between obstructions and strangulation requiring surgery and enteritis (Fig. 9). Chronic or recurrent distention of the stomach may indicate pyloric stenosis or duodenal ulceration and stenosis, which warrants surgical exploration if there is no response to medical therapy.

Determining the need for surgery based on mathematical analysis of the clinical signs has been proposed [16-18]. This has not been successful or widely accepted in clinical practice because most veterinarians deal with a low prevalence of surgical disease, making the prediction difficult and unreliable. Parry [19] developed a scoring system which grouped signs together to more accurately predict the need for surgery, rather than individual signs. The signs including moderate to marked colic signs; tacky to dry oral mucosa which is discolored (especially when brick-red or blue) (Fig. 10); decreased to absent borborygmi; nasogastric reflux; tachypnea, tachycardia, increased hematocrit, hyperglycemia, and uremia are considered signs indicating the need for surgery and also a poor prognosis.

Reeves developed a model using multivariable analysis to examine rectal findings, abdominal pain, peripheral pulse, and abdominal sounds [7]. The mathematical equations developed a likelihood ratio using a logistic regression model which included disease probability. The formula is dependent on the prevalence of surgery in a specific population to calculate the odds of an individual horse needing surgery. This method has high accuracy, but the formula works on the concept of comparing the horse being examined to other horses in a database providing odds that horses with similar signs would require surgery based on previous cases. This does not give an absolute prediction, but the method does provide confidence for making decisions. In the model used to predict the odds that surgery is needed, rectal examination, frequency of pain, pulse strength, and decreased abdominal sounds or absent abdominal sounds are used together. The result provided a post-test odds ratio and a probability of surgery which can be stated as the number of horses out of 100 that would need surgery given the values and the prevalence of surgery in the population at that hospital (Fig. 11).

When combining all the signs mathematically, constant pain, the need for a second analgesic treatment, and absent abdominal sounds are significant in indicating a need for surgery (Table 6) [1]. Though the rectal examination can provide evidence of a surgical disease, when considering all cases during a horse’s first examination for colic, an abnormal rectal examination did not increase the odds that surgery was needed [1].

When veterinarians are undecided about the need for surgery in a horse with signs of abdominal pain because signs are confusing or suggestive but not convincing about the need for surgery, surgery will most likely be needed, and the horse should be referred for a second opinion at a surgical facility where monitoring and surgery can be completed.

The decision to refer a horse for surgery is accompanied by a responsibility to provide support for the period of transport, if needed. Specific recommendations for referral of horses with colic are listed in Table 7. It is important to initiate treatment such as administration of antibiotics and flunixin meglumine prior to transport if shock, sepsis, or a strangulating lesion is present. Intravenous fluid therapy can also be initiated, but this is done rapidly so as not to delay delivery of the horse to a surgical facility as soon as possible.

The decision to recommend surgery is a medical challenge. The owner of the horse which is very painful and has distended loops of small intestine with physical characteristics that suggest surgery may not be able to afford surgery but can request medical treatment. Some of these horses respond to medical treatment without surgery, demonstrating the error in the diagnosis criteria used for this decision. Nevertheless, the criteria set forth here will more accurately select horses that need surgery and, therefore, a delay in the decision in hopes of a medical cure should be avoided.

Footnotes

[a] Banamine, Schering Plough Animal Health, 1095 Morris Ave., Union, NJ 07083.
[b] Dormosedan, Pfizer Animal Health, 812 Springdale Dr., Exton, PA 19341.
[c] The Equine Acute Abdomen, Teton New Media, in press.