Ultrasonographic visualization of colonic mesenteric vasculature as an indicator of large colon right dorsal displacement or 180° volvulus (or both) in horses

Abstract

Visualization of colonic mesenteric vasculature during transabdominal ultrasonographic examination of horses with colic can be a predictor of right dorsal displacement of the large colon or 180° large colon volvulus, or both. Medical records of 82 horses having had surgical treatment of colic and having received a transabdominal ultrasonographic examination on admission were reviewed. Colonic mesenteric vessels were sonographically identified coursing laterally on the right side of the abdomen in 24 of the 82 cases. Horses with colonic vessels identified on ultrasound were 32.5 times more likely to be diagnosed at surgery with either large colon right dorsal displacement or 180° large colon volvulus than those in which vessels were not seen (P < 0.001). Visualization of colonic mesenteric vessels on ultrasound provided a sensitivity of 67.7%, specificity of 97.9%, positive predictive value of 95.8%, and negative predictive value of 81% for large colon right dorsal displacement or 180° large colon volvulus, or both.

Introduction

Transabdominal ultrasonographic examination of horses with colic is performed routinely in many equine hospitals. The appearance, location, and contents of abdominal organs provide clinicians with critical information to determine a diagnosis, recommend surgical or medical management, and later, monitor response to treatment (1–7). The use of ultrasound to diagnose left dorsal large colon displacement is well-described (3–5,8–10), and sonographic measurement of increased large colon wall thickness can be an accurate preoperative test for large colon volvulus (2). Additionally, identifying the left dorsal colon in the ventral abdomen using ultrasound can assist diagnosis of large colon volvulus (11). In 1 study, ultrasonography was more accurate than abdominal palpation per rectum for detecting abnormal small intestine in horses with colic (1). To the authors’ knowledge, little has been described regarding sonographic findings associated with right dorsal displacement of the large colon.

Large colon displacement and volvulus are common causes of colic in the horse, accounting for 33.7% of horses undergoing exploratory laparotomy (12). Despite their common occurrence, accurate preoperative diagnosis of large colon disorders can be challenging. Physical examination findings are often non-specific beyond general visceral pain. Percussion of the abdominal wall can aid in the identification of a large gas-distended viscus, but provides no further specificity as to the precise segment of colon affected or whether actual displacement has occurred. Thorough abdominal palpation per rectum may be impeded by considerable colonic tympany (13) and is limited to the caudal portion of the abdomen. Hematological analysis can be variable and is generally not specific for particular displacements, with the exception of gamma glutamyl transferase (GGT), which may become elevated in horses with right dorsal large colon displacements as a consequence of bile duct obstruction (14).

In the normal equine abdomen, the ascending colonic vasculature courses in the mesentery along the medial aspects of the colon (15) and should not be visible during transabdominal sonographic examination. If the colon is displaced or twists along its long axis such that the medial aspect of the colon becomes situated laterally, these vessels can be sonographically imaged against the body wall. The purpose of this retrospective study was to describe the visualization of ascending colonic mesenteric vasculature during transabdominal ultrasonographic examination as a predictor of right dorsal displacement of the large colon or 180° large colon volvulus (or both) in horses.

Materials and methods

Selection of cases and description of procedures

The case records of all horses that underwent exploratory celiotomies for colic between July 2008 and June 2009 at Littleton Equine Medical Center were reviewed. All the horses had presented for signs of colic of varying severity and duration. Transabdominal ultrasonographic examination was performed on each patient using a Biosound MyLab 30 ultrasound machine (Universal Ultrasound, Bedford Hills, New York, USA) equipped with a CA631 7.0–1.8 MHz convex array transducer. Isopropyl alcohol was used as an acoustic coupling agent, and when necessary regions of the hair coat were clipped to improve image quality. All examinations were performed in similar fashion; working from cranial to caudal over both the left and right sides of the thorax and abdomen. The large colon was identified by its semi-curved appearance and hyperechoic wall to gas lumen interface (3). Ventral colon segments were identified by their characteristic sacculations (3,4). Ascending colonic mesenteric vessels were defined as 2 or more hypoechoic circular structures directly adjacent to the colon wall, coursing horizontally along the right body wall between the 12th and 17th intercostal spaces, at approximately the level of the costochondral junctions with the probe oriented transversely to the spine (Figures 1 and 2).

Figure 1.

Ultrasound image obtained from a horse that presented for colic. Dorsal is to the right of the image. Colonic vessels are identified in the right 15th intercostal space just dorsal to the costochondral junctions, with the probe oriented transversely to the spine. Note large colon adjacent to the vessels, identified by a hyperechoic wall to gas lumen interface (arrows). A 180° large colon volvulus was confirmed at surgery.

Figure 2.

Ultrasound image obtained from a horse that presented for colic. Dorsal is to the right of the image. Colonic vessels are identified in the right 13th intercostal space just dorsal to the costochondral junctions with the probe oriented transversely to the spine. Note large colon adjacent to the vessels, identified by a semi-curved appearance and hyperechoic wall to gas lumen interface (arrow). Also note the echogenic mineralized costal cartilage casting an acoustic shadow ventrally (asterisk) and a small amount of free peritoneal fluid external to the mesocolon and vessels dorsally (double asterisk). Right dorsal displacement of the large colon was confirmed at surgery.

Excluded from the study were horses with intractable pain or fractious behavior that prohibited complete sonographic examination. Also excluded were horses diagnosed at surgery with lesions not localized to the gastrointestinal tract. Data collected for each case included whether or not colonic mesenteric vasculature was identified during sonographic examination. The gold standard test to which ultrasound was compared was diagnosis at surgery.

Statistical analysis

The association between sonographic visualization of colonic vessels coursing laterally on the right side of the abdomen and surgical diagnosis was calculated as the risk ratio using the chi- squared test. The sensitivity, specificity, and positive and negative predictive values were calculated to assess the utility of ultrasound as a diagnostic test for detection of right dorsal displacement of the large colon or 180° large colon volvulus, or both. Sensitivity was defined as the percentage of horses with vessels seen on ultrasound coursing laterally in the right abdomen that were found at surgery to have either a right dorsal displacement or 180° volvulus of the large colon (true positives). Specificity was defined as the percentage of horses with vessels not seen in this same area on ultrasound that at surgery did not have either a right dorsal displacement or 180° volvulus of the large colon (true negatives). Positive predictive value was defined as the percentage of horses found to have either right dorsal displacement of the large colon or 180° large colon volvulus, as indicated by visualizing colonic vasculature on ultrasound, that actually did have either right dorsal displacement of the large colon or 180° large colon volvulus at surgery [true positives/(true positives + false positives)]. Negative predictive value was defined as the percentage of horses that were not found to have either right dorsal displacement of the large colon or 180° large colon volvulus, as indicated by lack of colonic vessels seen on ultrasound, that actually did not have either right dorsal displacement of the large colon or 180° large colon volvulus [true negatives/(true negatives + false negatives)]. A P-value of ≤ 0.05 was considered significant.

Results

Eighty-two horses met the criteria for inclusion in this study. Diagnoses made at surgery included right dorsal displacement of the large colon (n = 23), left dorsal displacement of the large colon (n = 11), 180° large colon volvulus (n = 11), 360° large colon volvulus (n = 12), 720° large colon volvulus (n = 1), and small intestinal lesion (including strangulation, obstruction, and volvulus) (n = 24). Colonic mesenteric vessels were identified on the right side of the abdomen during sonographic examination in 24 of the 82 cases. Of those 24 cases, diagnoses of right dorsal displacement of the large colon (n = 16), 180° large colon volvulus (n = 7), and 360° large colon volvulus (n = 1) were made at surgery. Vessel diameter measurement or pulsed-wave Doppler evaluation of blood flow (or both) was performed in some cases, but insufficient data were available for inclusion in this report.

One hundred percent of the horses with colonic vessels identified on ultrasound had surgical lesions localized to the large colon. Vessels were not visualized in any horse with left dorsal displacement of the large colon or any small intestinal lesion. Horses with colonic mesenteric vessels identified on ultrasound were 32.5 [95% confidence interval (CI) = 11.2; 94.4] times more likely to be diagnosed with either right dorsal displacement of the large colon or 180° large colon volvulus than those in which colonic vessels were not seen (P < 0.001). Visualization of colonic mesenteric vessels on the right side of the abdomen during ultrasound examination provided a sensitivity of 67.7% (95% CI = 51.9; 83.4), specificity of 97.9% (95% CI = 93.9; 100), positive predictive value of 95.8% (95% CI = 87.8; 100%), and negative predictive value of 81% (95% CI = 70.9; 91.9) for right dorsal displacement of the large colon or 180° large colon volvulus.

Discussion

In this retrospective study of horses undergoing exploratory celiotomies for colic, ultrasonographic visualization of colonic mesenteric vasculature on the right side of the abdomen was found to be highly specific and sensitive for the detection of right dorsal displacement of the large colon or 180° large colon volvulus, or both. Sonographic images of colonic blood vessels with right dorsal displacement or 180° colon volvulus (or both) are consistent with large colon and mesocolon anatomy. The equine large colon consists of 4 segments, the right dorsal and ventral colons and the left dorsal and ventral colons, that together occupy most of the abdominal cavity. Both dorsal colons are closely attached to their corresponding sections of ventral colon by a short mesentery (15,16). The only mesenteric attachment of the large colon to the body wall exists near the base of the cecum and attaches the right dorsal colon to the dorsal body wall (15). The remainder of the colon, particularly the left dorsal and ventral colon, is freely mobile within the abdomen, making the colon susceptible to displacement and volvulus (16–18).

The cranial mesenteric artery originates from the aorta and serves as the primary source of blood supply to the large colon. It has 2 main branches: the ileocolic artery and the right colic artery. The colic branch of the ileocecal artery courses in the mesocolon along the medial aspect of the right and left ventral colons, while the right colic artery courses in the mesocolon along the medial aspect of the right and left dorsal colons. The 2 vessels meet and anastomose at the pelvic flexure (15,16). There are no major blood vessels on the lateral aspect of the large colon — the surface that is normally against the body wall and visible during transabdominal ultrasound examination.

Displacement of the large colon is classified according to the direction that the pelvic flexure traveled when viewed from the back of the horse (16). Right dorsal colon displacements occur when the left colons move laterally around the base of the cecum to lie between the cecum and the right body wall (9,17–19). The “counterclockwise” right dorsal displacement, named for the surgeon’s view when the horse is in dorsal recumbency, is characterized by lateral retroflexion of the pelvic flexure to course cranially along the left body wall before crossing over midline in the cranial abdomen and coming to rest lateral to the cecum. The “clockwise” right dorsal displacement occurs when the left colons migrate axially toward the right side of the abdomen, crossing the pelvic inlet to lie lateral to the cecum (16). Either of these 2 scenarios can result in the medial aspect of the colon becoming situated against the body wall. Venous drainage may or may not be affected, depending on the degree of the displacement and occlusion of the vessel lumen. Arterial supply generally remains intact (16–18). Large colon volvulus represents one of the most severe forms of large colon disorders, occurring when the colon rotates around its longitudinal axis, usually at the cecocolic junction (16–18,20). Depending on the degree of twisting, both venous and arterial circulation can be substantially compromised (17,18,20). If the colon twists along its long axis or becomes displaced such that the medial aspect becomes situated against the body wall, colonic vessels can become sonographically visible.

It is important to note that the cecum has blood and lymphatic vessels that course both medially and laterally (15,16). Unlike colon vessels, cecal vasculature can be visualized ultra-sonographically in the normal equine abdomen and should be differentiated from colonic vessels by their location in the ventral right paralumbar fossa (7), coursing ventrally along the curvature of the costal cartilages.

One horse in this study with colonic mesenteric vessels visualized during ultrasound examination had a 360° large colon volvulus diagnosed at surgery. In theory, 360° torsion should result in colonic vessels traveling 1 full rotation and ultimately resuming their original orientation at the medial aspect of the large colon — impeding them from being seen on ultrasound. In this particular case, it is possible that the vessels were imaged at the specific site of the twist, or the degree of volvulus shifted during the time between sonographic examination and surgery.

A limitation of this study is the inclusion of only horses that underwent exploratory laparotomy. The difficulty of definitively diagnosing right dorsal displacement of the large colon without laparotomy has been recognized (14,21). However, it is not possible to distinguish conclusively between various types of large colon displacement and volvulus with preoperative diagnostic methods alone. Therefore, only cases with a definitive diagnosis made at surgery were included in this report.

Standard evaluation performed on horses presenting for signs of colic admitted to Littleton Equine Medical Center typically includes sonographic examination of the thorax and abdomen in the course of obtaining a comprehensive body of information regarding the patient. Some horses that presented for colic during the sample time period had colonic mesenteric vasculature noted during sonographic examination yet did not require surgical intervention. Although large colon displacement is frequently considered to be a surgical condition, conservative medical management has been described (13), and these horses may have had early or partial displacements that responded to medical management alone. All of the horses responding to non-surgical treatment were sonographically re-examined and a return to normal colonic appearance was consistently observed in conjunction with resolution of the colic symptoms.

The presentation of horses with colonic vessels apparent on ultrasound examination that did not require surgical treatment makes it imperative that clinicians consider all possible factors in formulating a treatment plan. In addition to ultrasound findings, pain level, physical examination, and abdominal palpation per rectum should be incorporated into any decision to recommend surgery. The results of this study should be interpreted to support the use of ultrasonography in the diagnosis of right dorsal displacement of the large colon or 180° large colon volvulus (or both), and not necessarily as an indication for surgical intervention.

In this study, visualization of colonic mesenteric vasculature did not differentiate between right dorsal displacement and 180° volvulus of the large colon, but rather indicated a high probability that one or the other disorder was present. It is often similarly challenging to distinguish between the 2 conditions using standard preoperative diagnostic procedures for colic such as physical examination, rectal palpation, and hematological analysis. Large colon volvulus between 90° and 270° is generally considered a non-strangulating lesion and can be associated with signs very similar to a large colon displacement (16–19), including mild to moderate abdominal pain that is readily controlled with analgesic medications, vital signs that are normal or mildly deranged, and normal hydration and peripheral perfusion (16–18). Colonic tympany may be present with either condition and is generally palpable per rectum as colonic bands with distended haustra traversing the caudal abdomen (13,16–19).

In this study, ultrasonography was a highly specific and moderately sensitive test for the detection of right dorsal displacement of the large colon or 180° colon volvulus (or both). The positive predictive value found in this study indicates that when colonic vessels are observed on the right side of the abdomen during sonographic examination of horses with colic, there is a 95% likelihood that a right dorsal displacement or 180° degree large colon volvulus (or both) is present. However, the 81% negative predictive value indicates that approximately 19% of the horses that have right dorsal displacement or large colon volvulus (or both) will fail to display colonic vessels on abdominal ultrasound. The described method is noninvasive, easy to perform, and can be incorporated into routine diagnostic evaluations to facilitate rapid and more accurate diagnosis of horses presenting with colic. CVJ