Discussion

Abdominal ultrasound has made large advances in technology over the past 10 years becoming more portable and affordable to the practitioner. Clinicians used to rely solely on blood work, abdominocentesis and physical examination parameters as a rapid assessment of horses with abdominal pain pre- and post operatively. The addition of ultrasonography provides a noninvasive, real-time method to directly assess the viability and contractility of the gastrointestinal tract as well as providing the ability to determine if an obstruction, peritonitis, hemoabdomen or uroabdomen is present.
Ultrasound images are extremely dependent on the probe selection, size of the horse and the nature of the abdominal disease. The higher the frequency of the probe used, the better the resolution that is obtained, at the cost of penetration. Ultrasound provides superb soft-tissue detail such that individual layers of small intestine and colon (mucosal, submucosal, muscularis and serosal) can be identified despite an intestinal wall that is <2 mm thick. Although a learning curve is present, the identification of abnormalities can generally be done if a thorough transabdominal ultrasound is performed.
The resolution of ultrasound is hampered by the fact that sounds travels very poorly in gas; therefore, only a small amount of the periphery of the equine abdomen can be evaluated with any regularity. This is because the colon lines the caudal and ventral abdominal periphery and the large amount of intraluminal gas prevents ultrasound penetration. Since the kidneys are in the retroperitoneal space and the spleen is generally lateral to the colon, these structures can also be evaluated with relative ease. Although this may seem like a limitation, the peripheral nature of the gastrointestinal tract allows for a high frequency probe to be used to optimise resolution.
One of the most problematic causes of acute colic in horses is large-colon torsion. Not only does this disease present a pre- operative dilemma for rapid diagnosis and immediate surgery, but it also causes a post operative metabolic disaster due to electrolyte abnormalities and reperfusion injury. Disseminated intravascular coagulopathies and thrombosis of infarcted bowel can also occur and be very difficult to diagnose using conventional methods of abdominocentesis and physical examination. Ultrasound examination provides the clinician with a direct evaluation of the bowel to help determine if abnormalities exist. This is done by imaging multiple areas of the abdomen including the right 10th intercostal space, the right dorsal aspect of the paralumbar fossa, the right inguinal region, the left dorsal paralumbar fossa, the left inguinal region, the left 10 - 12th intercostal space and ventrally just caudal to the xiphoid. These regions allow for the systematic imaging of the dorsal and ventral colon, as well as the nephrosplenic space, the small intestine (which generally are in the inguinal region) as well as the stomach. This form of FLASH imaging (fast localized abdominal sonography in horses) can allow for the rapid assessment of colon wall size, which if it is >9 mm means there is a colon torsion or colitis, as well as evaluating for obstructive lesions or small intestinal dilation.
Aside from identifying causes of colic pre-operatively, ultrasound is also extremely useful in predicting the risk of complications post operatively in the large-colon torsion patient. It has been shown that horses with a shorter time to involution of the colon wall to a thickness of 5 mm or less is associated with a decreased morbidity in the post operative period. In addition, longer involution times seem to correlate with horses at increased risk of developing multi-organ dysfunction syndrome.
Other diseases like functional ileus verses mechanical ileus can be determined. Using the basic premise that the small intestinal diameter should never be greater than 5 cm as a general guide in the average size horses, it is possible to separate medical management from emergency surgery. The cause for the mechanical ileus is usually not identified, although in the case of adhesion formation, the increased fluid in the abdomen and peritoneal fluid analysis can aid in determining if peritonitis is present. Enteritis has been described as small intestinal wall thickness greater than 3 mm; however, this would mean that the small intestinal wall of a horse is similar to a cat. Using a cut-point of 5 mm for small intestinal wall thickness allows for subtle enteritis or functional ileus to be missed, but it would not change the clinical course of action.
Ultrasound has also been used to try to assess motility and contractility. Multiple techniques from B-mode evaluation of the number of contractions per minute to spectral Doppler evaluation of gas movement within the bowel have been described. Contractility through measuring the number and strength of contractions per 30 seconds is the only reliable method to assess gastrointestinal motility and has only been described in normal animals. The main drawbacks to spectral Doppler are that the direction of the travel for gas and fluid within the gastrointestinal tract is perpendicular to the probe, so velocity is considered inaccurate. Also, since the ultrasound cannot penetrate gas, any readings identified are based on artifact and not real data. Lastly, gastrointestinal motility is severely changed by stress and since most horses with colic are undergoing some form of stress, the validity of the contractility assessment in colic animals compared to normal animals is still in question.
Ultrasonography is rarely a modality to provide the answer for the clinical cause of acute pain. However, it not only provides a method to determine what portion of the abdomen is involved (such as small intestine or colon), but it can provide information on the severity of the disease process as well. Also, the use of ultrasound in the post operative patient has been largely overlooked and can provide a wealth of insight into post operative complications. Ultrasound post operatively can help identify ileus, adhesion formation, as well as helping to determine prognosis for recovery and aid in treatment planning.