Ultrasonographic findings in colic cases (Proceedings)

Ultrasonography is invaluable in the diagnosis of the cause of colic in horses. The sonographic findings can aid the veterinarian in determining if the horse has a medical or surgical lesion. Diagnostic ultrasonography provides a window for noninvasive visualization of gastrointestinal viscera, which are otherwise difficult to examine. Transrectal ultrasonographic evaluation of abnormalities detected on rectal palpation can also be performed in adult horses to further clarify the rectal findings.

Normal Ultrasonographic findings in the equine gastrointestinal tract

Only large intestinal echoes are usually imaged in the intercostal spaces (ICS) and the flank in the adult horse. Occasionally small intestinal echoes are imaged between the stomach and spleen and in the caudal ventral abdomen of the adult horse. Both large and small intestinal echoes are usually imaged from the ventral abdomen in the foal, while primarily large intestinal echoes are usually imaged in the intercostal spaces (ICS) and the flank. The large intestinal echoes are recognized by their large semi-curved, sacculated appearance, except for the right dorsal colon. The right dorsal colon has a smoother nonsacculated appearance and is usually imaged from the right 14th – 10th intercostal spaces. The large intestinal wall is hypoechoic to echogenic with a hyperechoic gas echo from the mucosal surface and normally measures 3.5 mm or less in thickness. Peristaltic activity is normally visualized. The small intestinal echoes are recognized by their small tubular and circular appearance. The wall of the jejunum is hypoechoic to echogenic with a hyperechoic echo from the mucosal surface and is usually 3 mm or less in thickness. Some anechoic fluid and hyperechoic gas is often imaged in the lumen of the jejunum. Peristaltic waves are also normally visualized. The duodenum is imaged around the caudal pole of the right kidney and medial to the right liver lobe. It appears small circular (when sliced in its short axis) with a hypoechoic to echogenic wall, also < 3 mm in thickness, and has a fluid lumen. The duodenum usually appears partially collapsed and its peristaltic motion is easily visualized during real-time scanning. The gastric fundic echo is visualized in the left 9 - 12th ICS and is imaged as a large semi-circular structure medial to the spleen at the level of the splenic vein. In the neonate the stomach is also imaged from the ventral abdominal window, caudal to the liver. The gastric wall is hypoechoic to echogenic with a hyperechoic gas echo from the mucosal surface and normally measures up to 7.5 mm in thickness. Gastric rugal folds can be often be imaged in adult horses.

Surgical Colics

Herniation/Displacement

Abnormal positioning of the gastrointestinal viscera is difficult to diagnose ultrasonographically, unless the viscera are displaced into the scrotum, thoracic cavity or into an umbilical hernia. Displacement of the gastrointestinal viscera into the thoracic cavity through a diaphragmatic hernia can usually be diagnosed ultrasonographically by scanning the affected side of the thorax and cranial abdomen and looking for the rent in the diaphragm, as displacement of the overlying lung by the herniated viscera occurs. The approximate size of the diaphragmatic hernia can be estimated and the gastrointestinal viscera evaluated for the degree of bowel compromise. However, a diaphragmatic hernia could be missed ultrasonographically if it was located in the center of the diaphragm and the herniated viscera were not in contact with the thoracic wall. In horses with abdominal wall hernias or ruptures of the prepubic tendon, diagnostic ultrasonography can be used to measure the size of the defect, so an appropriately sized piece of mesh can be prepared preoperatively for implantation in horses with abdominal wall hernias. Furthermore the contents of the hernial sac and the presence of any adhesions can be identified and the hernial ring described.

Nephrosplenic ligament entrapment

Diagnosis of a nephrosplenic ligament entrapment is suspected ultrasonographically, based upon the inability to visualize the spleen or left kidney transabdominally and the visualization of ingesta and/or gas filled large bowel instead. The spleen is ventrally displaced. The most dorsal portion of the spleen that can be imaged has a straight horizontal dorsal border extending from the paralumbar fossa to the 10-12th intercostal space, at which point the colon is no longer visible due to the intervening lung. Dorsal to the spleen a bright hyperechoic reflection is imaged from the displaced or entrapped large colon. The sonogram can be used to see if treatment with phenylephrine, followed by lunging, or rolling the horse has successfully corrected the nephrosplenic ligament entrapment.

Sand colic/enterolithiasis

Sand colics, bezoars and enteroliths can all be diagnosed ultrasonographically, if the affected portion of bowel is scanned, as these disorders make the bowel much heavier than normal causing it to fall to the floor of the ventral abdomen. With an enterolith or bezoar, a large, hyperechoic mass casting a strong acoustic shadow will be imaged within the lumen of the intestine, if the affected portion of intestine is adjacent to the ventral body wall. Distention of the more proximal portion of the intestine by the obstructing bezoar or enterolith is also detectable. However, most enteroliths are not located in a position where they are imageable ultrasonographically. Small, pinpoint granular hyperechoic echoes, casting multiple acoustic shadows, will be imaged in the ventral most portion of the affected intestine in horses with sand colic, weighing down the intestine and limiting its peristaltic movement.

Intussusceptions

Intussusceptions have a characteristic target or bull's eye sign in the affected portion of intestine. There are many different possible sonographic appearances for the intussusception, depending upon which portion of the intussusception is being imaged. Often fibrin is imaged between the 2 loops of affected intestine. In foals the intussusception is usually jejunal and is imaged from the ventral most portion of the abdomen. In yearlings and young horses, ileal intussusceptions are more common and may be imaged rectally or transcutaneously. Intussusceptions in adult horses usually involve the ileum and/ or large bowel. The majority of intussusceptions imaged in adult horses are imaged from the right side of the abdomen because the cecum or right ventral colon is involved.

Strangulating small intestinal lesions

Distended, fluid-filled small intestine is usually imaged proximal to a strangulated portion of small intestine. The strangulated small intestine usually has thickened, edematous, hypoechoic walls with little or no peristaltic activity. Complete volvulus of the small intestine may also occur, similarly affecting the entire small intestine. Distended small intestine with thickened walls is most frequently detected in the ventral portion of the abdomen, as the increased weight of these loops brings them in contact with the ventral portion of the abdomen. Amotile edematous loops of small intestine have been imaged in the right side of the abdomen in horses with epiploic foramen entrapment of the small intestine. Mucosal thickening of the wall of the strangulated loops of small intestine is usually less echoic than wall thickening associated with a cellular infiltration, fibrosis or hypertrophy of the intestinal wall, usually seen in nonstrangulating lesions. Surgical intervention was indicated in one study of horses with colic when edematous small intestine was imaged in conjunction with decreased small intestinal motility. Sloughing of the intestinal mucosa may occur in association with an anechoic fluid line in the underlying layer. Gas echoes may also be imaged within the wall of the intestine in horses with intestinal necrosis.

Strangulating lesions of the small colon

Increased wall thickness of the small colon and intestinal distention have been reported in several horses with strangulating lesions in the small colon caused by a pedunculated lipoma using transrectal ultrasonography. Transabdominal ultrasonography has been useful in diagnosing strangulating lesions in the small colon in miniature horses.

Small intestinal masses

Masses within the intestinal wall are thickened areas, often compromising the lumen of the affected portion of intestine, which may be anechoic to echogenic, depending upon their etiology. Intestinal carcinoids, leiomyomas, granulomas, hematomas and fibrosis have all been reported to cause small intestinal obstruction. Hemorrhage in the lumen of the intestine often appears as echogenic clots or echoic swirling fluid. Areas of mural stricture have been imaged in several horses with chronic colic that were ultimately surgically resected. Thickening of the wall of the ileum in yearlings and horses with ileal hypertrophy can also be detected sonographically both transrectally and transcutaneously.

Meconium impaction

Meconium impactions are imaged in the caudal portion of the gastrointestinal tract of foals and can appear as hypoechoic, echoic or hyperechoic masses in the lumen of the large colon, small colon or rectum. If the meconium is obstructing the flow of ingesta, the small colon proximal to the obstruction will be fluid distended. In older foals, Ascarid impactions can be imaged in the bowel lumen, usually in the ileum. Paralyzed Ascarid worms can be imaged in the lumen of the large colon in many affected foals within fluidy ingesta.

Impaction

An impaction can often be imaged from the flank or side of the abdomen in horses with cecal or right dorsal colon impactions. Small colon impactions may be imaged transrectally, when scanning the caudal abdomen, as echogenic intraluminal masses. Distension of the more proximal portion of intestine, proximal to an impaction, is usually present, making ultrasonographic visualization of the impaction easier. The impaction appears as a round to oval distended viscus, often measuring 20 - 30 cm or more, lacking any visible sacculations. The bowel wall may be normal thickness or may be thicker than normal and there is a large acoustic shadow cast from the impacted ingesta adjacent to the colonic mucosa. Small colon impactions have also been imaged from the flank in miniature horses. Impactions can only be imaged sonographically when the impacted portion of the large colon or cecum is adjacent to the body wall or fluid is interposed between the affected portion of the intestine and the body wall. In larger horses, small or large colon impactions can be imaged transrectally if palpable. Little to no motility of the affected portion of the intestinal tract will also be imaged in horses with impactions.

Medical colics

Enteritis/duodenitis

Fluid distension of the intestinal tract with increased peristalsis indicates developing enteritis. The wall of the affected portion of the intestine may be thickened, edematous and more hypoechoic than normal, particularly with severe inflammatory bowel disease. Shreds of intestinal mucosa may be imaged in the lumen of the intestinal tract. Marked thickening of the intestinal wall has been imaged in foals with Lawsonia infection. Fluid distention of the duodenum can also be imaged with anterior enteritis and other more distal obstructions. The lack of motility in these intestinal segments is consistent with an ileus and the thickness and echogenicity of the bowel wall are an indication of the degree of involvement of the bowel wall.

Foals with proximal duodenitis may have a history of colic and have elevated biliary enzymes and an associated cholangiohepatitis. Foals with cholangiohepatitis or biliary obstruction usually have larger than normal livers, with increased echogenicity of the hepatic parenchyma. Biliary distension, sludging of bile within the ducts and thickening of the bile ducts may be imaged in the biliary tree. Adults with proximal duodenitis/ anterior enteritis may also have an associated cholangiohepatitis with elevated biliary enzymes. Adults with cholangiohepatitis or biliary obstruction also usually have larger than normal livers, with increased echogenicity of the hepatic parenchyma. Biliary distension, sludging of bile within the ducts and thickening of the bile ducts may also be imaged in the biliary tree.

Right dorsal colitis

Right dorsal colitis associated with nonsteroidal anti-inflammatory drug toxicity can be diagnosed ultrasonographically by detecting a thickened right dorsal colon ventral to the liver in the right 10th – 14th intercostal spaces. The right dorsal colon was imaged in the right 11th , 12th and 13th intercostal spaces in all horses and in the 10th and 14th intercostal spaces in some of the horses. The wall of the right dorsal colon is usually thickened with an abnormal pattern of echogenicity of the bowel wall detected sonographically when compared to control horses. In affected horses, a hypoechoic layer was detected in the thickened right dorsal colon, surrounded on each side by a hyperechoic mucosal and serosal surface. The hypoechoic layer was less echoic than the adjacent liver. In this study, the hypoechoic layer detected in the wall of the right dorsal colon in all horses with right dorsal colitis appeared to correspond to submucosal edema, inflammatory cell infiltrates and granulation tissue that were subsequently observed on post mortem examination. The thickness of the right dorsal colon was also reported to be significantly greater than the thickness of the right ventral colon in horses with right dorsal colitis. Thinning of the wall of the right dorsal colon was reported in one horse treated successfully for right dorsal colitis, as well as in one horse that experienced a rupture of the right dorsal colon. In the treated horse that ruptured its right dorsal colon, the hypoechoic layer persisted in spite of the decrease in wall thickness to normal.

Verminous arteritis

Verminous arteritis can be imaged ultrasonographically if the affected vessel is imageable transrectally. The affected vessel wall is thickened and large plaque-like or mass lesions can be imaged along the intimal surface of the vessel, invading the arterial lumen.

Gastric distention

Gastric emptying problems may be identified sonographically if large amounts of milk or ingesta persist unchanged in the stomach in a fasted, anorexic or refluxing foal on repeat examinations. The distended stomach is usually slightly less circular than normal, with anechoic to hypoechoic fluid in the lumen of the stomach. Gastric emptying problems may be identified sonographically in the adult horse if large amounts of ingesta persist unchanged in the stomach in a fasted, anorexic or refluxing horse on repeat examinations. The sonographic appearance of a gastric impaction is a markedly enlarged gastric echo extending over 7 or more intercostal spaces on the left side of the abdomen. The stomach is usually slightly less circular than normal, with hyperechoic material casting an acoustic shadow in the lumen of the stomach. In one horse the enlarged stomach was also imaged for several intercostal spaces on the right side of the horse's abdomen. Gastric squamous cell carcinomas are not uncommon in older horses and a complex pattern of echogenicity can be seen in the wall of the stomach in affected horses, often with invasion into the adjacent spleen or liver parenchyma.

Intestinal neoplasia

Masses within the intestinal wall are most likely to occur in adult horses with lymphosarcoma or granulomatous enteritis. Neoplasms affecting the wall of the gastrointestinal tract may be visualized upon transabdominal ultrasonographic examination of the abdomen. If abnormal loops of bowel are palpable rectally, rectal ultrasonographic examination would enable further characterization of the mass invading the intestinal wall. Lymphosarcoma usually has a relatively homogeneous soft tissue density appearance when imaged in areas outside the spleen, unless severe tissue necrosis has occurred or the tumor is very aggressive.

Abdominal abscess

Abdominal abscesses in foals are usually found in the ventral portion of the abdomen associated with the mesentery of the gastrointestinal tract. These abscesses may be anechoic, hypoechoic or filled with echoic material and are often multiloculated. Hyperechoic echoes representing free gas may be detected suggesting concurrent anaerobic infection. Large and/or small intestine may be adhered to the wall of the abscess and its motion restricted. Abdominal abscesses in the adult horse may be detected in the ventral abdomen, but are also frequently found dorsally associated with the root of the mesentery, cecum and large colon. Abdominal abscesses have also frequently been reported in the adult horse associated with the liver. The sonographic appearance of the abscess and its relationship to the surrounding intestine and abdominal organs can usually be determined in the adult, as in the foal.