How to Image the Adult Equine Abdomen and Thorax in Ambulatory Practice Using a 5-MHz Rectal Probe

1. Introduction

Ultrasonography of the reproductive tract and musculoskeletal system is a widely used diagnostic tool in equine ambulatory practice. The majority of equine practitioners have access to a portable ultrasound machine and a 5-MHz rectal probe, and many have a 7.5-MHz tendon probe. It is less common for the ambulatory practitioner to have a 3.5-MHz probe, which is typically used to image the adult equine abdomen. The ability to ultrasound the adult abdomen and thorax in the field can provide invaluable information for the diagnosis and treatment of the equine patient.

An extensive description of ultrasonographic examination of the adult equine thorax and abdomen can be found in several texts [1-3]. Reef [1,2] describes anatomy, scanning techniques, and normal and abnormal ultrasonographic findings of the thorax and abdomen. Although these techniques and findings have been well described in a multitude of texts and journal articles, they are not widely used by the ambulatory practitioner on a day-to-day basis.

The idea for this paper was initiated when the author returned to private practice and became aware of the lack of use of the ultrasound as a diagnostic tool for pathologies of the abdomen and thorax, especially in the adult horse. In the 1-yr period after the author joined the practice, there was a 198% increase in the number of non-reproductive ultrasonographic exams performed in the practice with the same number of doctors practicing as in the previous year. The increase in the total number of ultrasound exams was primarily caused by an increase in the number of abdominal and thoracic exams performed. The purpose of this paper is to illustrate the use of the 5-MHz probe to image the equine abdomen and thorax and its importance in diagnosis and treatment.

2. Materials and Methods

A portable ultrasound [a] equipped with a 5-MHz rectal probe was used to acquire images of the adult equine’s abdomen and thorax (Fig. 1). Isopropyl alcohol applied with a spray bottle dampened the hair to allow for adequate contact of the probe and to achieve the best image. It is important to prevent disturbance of the hair after it is saturated with alcohol. Disturbance introduces air between the hairs, which produces an unacceptable image. If the hair coat is long, clipping may be required to obtain an image. A systematic approach is followed for each ultrasonic exam. To image the abdomen, it is the author’s preference to start the exam on the left side in the paralumbar fossa. Next, move to the left ventral abdomen and left inguinal region and proceed to the right side. To examine the thorax, imaging is started in the seventeenth intercostal space, and the entirety of each space is scanned from the dorsal to ventral aspect. A systematic approach ensures that each organ and portion of the abdomen or thorax is scanned and that nothing is missed.

Figure 1. Ultrasonography of the equine abdomen and thorax provides invaluable information in the diagnosis and treatment of the equine patient. The 5-MHz rectal probe, which is readily available to most ambulatory practitioners, can be used to acquire these images. Positioning for imaging of the spleen and left abdomen (left top and bottom) and the liver, duodenum, and colon in the right abdomen (right top and bottom).

3. Results

The adult equine abdomen can be imaged with a 5-MHz rectal probe (Fig. 2). Although a 3.5-MHz probe provides the best depth penetration and image quality, the images acquired with a 5-MHz probe are adequate for identifying the majority of pathologies typically identified in the adult horse using a 3.5-MHz probe. There are limitations in depth penetration, especially when visualizing the kidneys in large horses.

On the left side of the abdomen, the spleen and left kidney are imaged in the paralumber fossa. The kidney, located in the seventeenth intercostal space, lies deep to the spleen, and in large horses, it is just visible at a depth of 15 cm. The spleen, found adjacent to the body wall from approximately the eighth intercostal space to the paralumbar fossa [1], can be followed ventrally and may extend to the midline. The left ventral colon and loops of the small intestine can be visualized deep to the spleen along the left ventral abdomen and into the inguinal region. The bladder can also be visualized in the inguinal region. There is a small window in the left cranio-ventral abdomen from approximately the sixth to the ninth intercostal spaces where the liver, spleen, large colon, diaphragm, and lung can be visualized together in most horses. On the right side, the duodenum, liver, right kidney, large colon, and small intestines are imaged. The right kidney can be imaged in the right dorsal paralumbar fossa in the fourteenth to the seventeenth intercostal spaces [1]. The liver is imaged from the fifteenth to approximately the sixth intercostal space with the large bowel (right colon and cecum) deep to the liver. The duodenum is found between the liver and large bowel from approximately the fourteenth to the seventeenth intercostal spaces. Along the ventral abdomen, loops of small intestine and the large colon can be imaged. The small intestine and bladder are located in the right inguinal region.

Figure 2. These images (clockwise from top left) of the normal spleen, normal lung, and normal liver were all acquired with a 5-MHz probe.

The thorax is easy to ultrasound through the intercostal spaces, which allows for the examination of the pleural surface, pleural space, diaphragm, and heart.

Additional structures that can be examined ultrasonographically are the fetus, placenta, and uterus in the pregnant mare.

Pathologies identified during ultrasonographic examination of the equine abdomen include distended loops of small intestine, thickened and edematous small intestinal and large colon wall, excessive peritoneal fluid, abdominal abscesses, entrapment of the large colon in the nephrosplenic space, lung abscesses, and pleural effusion (Fig. 3).

Figure 3. These images show thickened loops of small intestine in a horse with suspected duodenal ulcers (top left) and a weanling with Lawsonia (top right). Images of a lung abscess in a horse with pleuropneumonia (bottom left) and a dilated hypomotile duodenum deep to the liver in a horse with enteritis (bottom right).

4. Discussion

Ultrasonography of the equine abdomen and thorax are well-described procedures that are used routinely by specialists at referral facilities for the diagnosis and management of a multitude of disease processes. However, in the ambulatory practice, the ultrasound is used to a much lesser degree for examination of the adult equine abdomen and thorax. It is the perception of many practitioners that a 3.5-MHz probe is required for these examinations. Although a 5-MHz rectal probe does not provide the same depth of penetration as a 3.5-MHz probe, it is adequate for examination of a large portion of the adult abdomen and thorax and can provide invaluable information. In addition, it is a safe, non-invasive procedure that is well tolerated by most horses.

During a normal exam, the spleen, liver, kidneys, duodenum, large colon, small intestines, bladder, lungs, heart, and diaphragm can be imaged. Because of the large size of the equine adult, it is impossible to image the entire abdomen; however, the information gained by ultrasonography can be very useful in the diagnosis of disease processes of the abdomen and thorax. Cases in which the ultrasound is routinely used include colic, enteritis, colitis, weight loss, diarrhea, Lawsonia, abdominal abscesses, peritonitis, cholangiohepatitis, splenic masses, high-risk pregnancies, pneumonia, and pleuropneumonia. During a left-dorsal displacement, it is often not possible to visualize the left kidney deep to the spleen because of the presence of the large colon in the nephrosplenic space; ultrasonographic examination can be used to confirm this. Diagnosis of enteritis and colitis are aided by ultrasonographic imaging of the intestinal-wall thickness and intestinal motility. The normal intestinal-wall thickness is ≤4 mm. Presence of intestinal-wall thickness may be indicative of edema, infection, or proliferative or inflammatory bowel disease. Small intestinal motility can also be evaluated. Loops of small intestine may be mildly distended and hypomotile to extremely distended and exhibiting no signs of motility. Imaging of the duodenum, deep to the liver in approximately the right fourteenth intercostal space, can provide information regarding the motility and intestinal-wall thickness of this portion of the small intestine. In cases of proximal enteritis, the duodenum may be distended, hypomotile, and have increased wall thickness.

The large colon wall, which can be easily measured during ultrasonographic examination, may become edematous and thickened during cases of colitis. When these intestinal pathologies are detected, repeat ultrasonographic examinations are extremely beneficial for monitoring the response of the disease to treatment. Reef [1] provides an extensive description of a large number of abnormalities that may be identified in the equine abdomen. A complete understanding of the ultrasonographic images obtained from the normal equine abdomen allows the practitioner to better recognize any abnormalities that are present.

Ultrasonographic examination of the high-risk pregnancy is critical for monitoring the health of the fetus. The 5-MHz probe can be used to obtain a fetal heartbeat as well as evaluate the placenta and the amnionic and allantoic fluids.

The adult equine thorax is amenable to examination by ultrasound, and the 5-MHz probe provides an excellent image of the pleural surface and space. Use of the ultrasound is extremely helpful in diagnosis and treatment of cases of pneumonia and pleuropneumonia.

When a practitioner becomes comfortable interpreting ultrasonographic images of the adult equine abdomen and thorax, use of the ultrasound for the types of cases listed above becomes as routine as examination of the musculoskeletal system and reproductive tract. Although the 5-MHz probe does not have the depth penetration of a 3.5-MHz probe, it produces an adequate image that provides invaluable information in the diagnosis and treatment of the equine patient.

Footnote

[a] Pie Medical Scanner 200, Pie Medical, 6227 AT Maastricht, The Netherlands.