Ultrasonographic evidence of colonic mesenteric vessels as an indicator of right dorsal displacement of the large colon in 13 horses
This report describes the use of ultrasound to diagnose right dorsal displacement of the large colon (RDDLC) in 13 horses prior to surgery. Horses had ultrasonographic examinations performed of the right lateroventral aspect of the abdomen upon admission to the hospital with a 2–5 MHz broadband curvilinear sector scanning transducer after alcohol was used to wet the hair. First, the caecal vessels were identified in the right flank and followed medially and cranially. Next, each intercostal space, from caudal to cranial, was scanned from dorsal to ventral evaluating for abnormally-located mesenteric vessels associated with the large colon. Abnormally-located mesenteric vessels associated with the large colon, distinct from the caecal vessels, were identified in 13 of 23 horses with a diagnosis of RDDLC obtained at exploratory laparotomy. In horses, ultrasonographic visualisation of mesenteric vessels along the right lateral abdomen, dorsal to the costochondral junction in at least 2 intercostal spaces, distinct from the caecal vessels, is consistent with a surgical diagnosis of RDDLC.
Diagnostic ultrasonography is gaining wide-spread use in the routine work-up of horses that present with colic as an increasing number of practitioners have access to ultrasound machines. Portable ultrasound machines with high imaging quality are becoming more affordable for field veterinarians and most referral clinics routinely use ultrasound. Diagnostic ultrasonography as part of a colic work-up is noninvasive and can be performed relatively quickly. Its usefulness has been documented in the diagnosis of nephrosplenic entrapment of the large colon (Santschi et al. 1993), large colon volvulus (Pease et al. 2004; Abutarbush 2006), right dorsal colitis (Jones et al. 2003), small intestinal strangulating lesions (Klohnen et al. 1996) and intussusceptions (Fontaine-Rodgerson and Rodgerson 2001).
Right dorsal displacement of the large colon (RDDLC) is a frequent cause of colic in horses, accounting for between 24.5% (Abutarbush et al. 2005) and 29% (Voigt et al. 2009) of surgical colics. At Steinbeck Country Equine Clinic, RDDLC accounted for 35 of 232 (15.1%) of the surgical colics during the study period. RDDLC is a nonstrangulating displacement of the large colon that functionally obstructs the large colon lumen, usually without compromising vascular integrity (Rakestraw and Hardy 2006). The definitive aetiology of RDDLC remains unknown. It has been proposed to occur due to retropulsion or aberrant motility of the pelvic flexure (Hardy 2008). Two main types of RDDLC may occur: most commonly, the pelvic flexure migrates anticlockwise from the left caudal abdomen cranially towards the diaphragm and to the right, moves caudally between the caecum and right body wall, then across and cranial to the pelvic brim to rest in the left cranial abdomen; less commonly, the pelvic flexure migrates clockwise between the caecum and right body wall and comes to rest in the right cranial abdomen (Rakestraw and Hardy 2006).
The typical presentation of a horse with RDDLC is mild to moderate abdominal pain that progressively increases in severity, sometimes over several days. The nonspecific clinical findings and variable pain levels in horses with RDDLC can make it difficult to diagnose prior to surgical exploratory laparotomy (Gardner et al. 2005). This report describes the ultrasonographic findings that correlate with the diagnosis of RDDLC in horses at the time of surgery.
Materials and methods
Horses that presented to Steinbeck Country Equine Clinic for abdominal pain were evaluated ultrasonographically as part of the diagnostic work-up. Alcohol was used to wet the hair over the area of interest and horses were only clipped if the hair coat precluded obtaining ultrasound images of diagnostic quality. The ultrasound evaluations were performed with a 2–5 MHz broadband curvilinear sector scanning transducer. The right ventrolateral aspect of the abdomen was evaluated ultrasonographically as part of a routine ultrasound performed in horses that present with colic to Steinbeck Country Equine Clinic. Initially, the lateral caecal vessels were identified in the right flank and followed axially and cranially. Next, each intercostal space, from caudal to cranial, was scanned from dorsal to ventral looking for abnormally-located mesenteric vessels associated with the large colon. The remainder of the abdomen was also routinely evaluated, including evaluation oflarge colon wall thickness, determination of location of colonic sacculations, duodenal motility and wall-thickness, evaluation for small intestinal motility, distention and wall-thickness, evaluation of peritoneal fluid, evaluation of gastric size and location, and evaluation of the nephrosplenic space.
Twenty-three client-owned horses with RDDLC confirmed via laparotomy were ultrasonographically evaluated at Steinbeck Country Equine Clinic between September 2006 and April 2010. There were 14 Quarter Horse type horses, 2 Warmbloods, 2 Icelandics, 2 Thoroughbreds and one each of Lipizzaner, Andalusian and other. There were 16 geldings, one stallion, and 6 mares. Age ranged from 5 months to 20 years, with an average of 8.8 years. All 23 horses had ultrasound procedures performed upon presentation for abdominal pain. Ultrasonography of the right lateral abdomen revealed mesenteric vessels along the right lateral abdomen in 13 of the 23 horses. Because the colonic vasculature lies within the mesentery on the medial surface of the colon, these structures are not seen under normal circumstances. The abnormally-located mesenteric vessels were identified in at least 2 intercostal spaces along the right side of the abdomen, between intercostal space 16 and 10, dorsal to the costochondral junction, adjacent to the large colon wall. They were distinct from the lateral caecal vessels and the bifurcation with the secondary caecal vessels sometimes visualized in the right flank.
All 23 horses received exploratory laparotomy based on pain refractory to analgesics with deteriorating laboratory work, physical examination parameters or abnormal abdominocentesis findings. All 23 horses had a diagnosis of RDDLC during surgery. The diagnosis of RDDLC was correctly made in 13 horses prior to surgery, and 10 horses did not have abnormal vessels identified during abdominal ultrasound prior to surgery. Ultrasonographic visualisation of the abnormally-located vessels was consistent with a diagnosis of RDDLC via exploratory laparotomy. The type of RDDLC was unfortunately not recorded during surgery to compare whether or not the abnormally-located vessels were ultrasonographically visualised.
Right dorsal displacement of the large colon is a common cause of colic in the horse (Abutarbush et al. 2005; Voigt et al. 2009). Typical presentation of a horse with RDDLC is mild to moderate abdominal pain that progressively increases in severity, often over several days. The nonspecific clinical findings and variable pain levels in horses with RDDLC make it difficult to diagnose definitively prior to surgical exploratory laparotomy. Bloodwork and peritoneal fluid analysis are often within normal limits at the onset, although they may become abnormal as time progresses. Often these horses initially respond to analgesics, but the effects may wear off quickly. Horses with RDDLC may continue to pass small amounts of manure if occlusion of the large colon is not complete. Horses with RDDLC sometimes have nasogastric reflux that confounds the clinical assessment, as the distended colon compresses the duodenum or pulls on the duodenocolic ligament (Rakestraw and Hardy 2006). Transrectal palpation may reveal gas-distention of the large colon with tight tenial bands, sometimes in an abnormal location such as coursing laterally across the abdomen, although it is not often a specific finding. Abdominal palpation per rectum can be difficult in horses with severe gas distention or those that are small in stature. Evaluation of serum gamma glutamyl transferase (GGT) may be helpful, as 49% of horses with RRDLC have been shown to have GGT concentrations higher than the reference range (Gardner et al. 2005).
Abdominal ultrasound was very effective at making a presurgical diagnosis of RDDLC. Sensitivity and specificity cannot be reported due to the lack of a control group of ultrasound on horses without colic signs. Hundreds of abdominal ultrasound examinations were performed in horses with surgical colic diagnoses other than RDDLC during the study time period, and the abnormally-located vessels were not noted in any of these cases. These data were not included as this paper was intended to be a description of the ultrasonographic findings in a known group of horses with RDDLC, not a comparative analysis of transabdominal ultrasound findings in colic patients.
Correct diagnosis of RDDLC via ultrasound may depend on the expertise of the ultrasonographer, the type of right dorsal displacement and the thoroughness of the examination as allowed by the horse's comfort level. It is important to distinguish RDDLC from a large colon volvulus, which generally presents with more severe pain and abdominal distention than RDDLC. Although we have not observed the abnormally-located colonic mesenteric vessels in the right lower abdomen on ultrasound examination in horses with LCV, the possibility of this occurring cannot be dismissed. Some cases of RDDLC may resolve with medical therapy, so diagnosis on ultrasound is not a definitive indicator for exploratory laparotomy (Abutarbush et al. 2005; Voigt et al. 2009). However, it should be considered in the arsenal of diagnostics used to determine the need for surgery in horses that present with colic. It may help clinicians diagnose RDDLC earlier in the course of disease, when the horse is a better anaesthetic candidate and help formulate a more accurate prognosis for the owner.
Horses that undergo exploratory laparotomy with a diagnosis of RDDLC are significantly more likely to have future colic episodes necessitating veterinary intervention (Smith and Mair 2010). It is unlikely that early diagnosis and correction would reduce the risk of future colic episodes, since a nonstrangulating displacement does not disturb the vascular integrity of the colonic vessels. However, it is possible that in some cases of RDDLC there is stretching of the colonic vasculature and soft tissues that could possibly predispose the horse to future colic episodes as a result of subclinical damage. It would be useful to be able to tell owners of a suspicion of RDDLC prior to surgery to better develop a prognosis and inform them of the likelihood of post operative complications such as recurrent colic.
In conclusion, abdominal ultrasound is a rapid, noninvasive tool to use in the diagnosis of horses with RDDLC. Ultrasonographic visualisation of abnormally-located colonic vessels, distinct from caecal mesenteric vessels, along the right side of the abdomen may be a highly specific finding for RDDLC and was seen in approximately 50% of horses with this disorder.
Authors' declaration of interests
No conflicts of interest have been declared.