IMAGING

5 Ways Ultrasonography Improves Foal Care

By Marie Rosenthal, MS

“This poses ambulatory veterinarians with a unique challenge where the situation may be dynamic, the patient might be quite ill, and information that you'd like to have diagnostically—like laboratory work—isn't readily available,” he said.

Portable ultrasonography can be a useful diagnostic tool in these situations, according to Dr. Gilsenan. “If you own a portable ultrasonographic machine, and if you treat foals and you're not using this regularly, I think you are missing out on an extremely helpful diagnostic tool,” he said.

The biggest drawback for many veterinarians is that they are uncomfortable with interpreting the image. However, there are so many advantages that it is a tool that is worth mastering, according to Dr. Gilsenan.

“The more you use it, the more you're going to be comfortable with it,” he said. Just like with physical examination done by practitioners, repetition of ultrasound examinations is going to enhance their skill and help them to discern between normal and abnormal.

“And as such, it should be employed as often as you can, if nothing else to give you the comfort level that's going to allow you to really use that information, to interpret it and help you guide treatment decisions and diagnostic decisions going forward,” said Dr. Gilsenan, who offered some tips for using the ultrasound equipment in foals at the 67th AAEP Annual Convention & Trade Show held in Tennessee.

Ultrasonography is not a one-person job. Someone must restrain the foal. He suggested the person who is restraining the foal put his or her hand over the shoulder of the foal, instead of the throat area, which will minimize stress and movement, and decrease the amount of chemical restraint that will be needed.

His preference for chemical restraint of foals that are younger than 2 weeks is diazepam by itself, or with butorphanol and xylazine for older foals.

Try to perform the ultrasound in a quiet, dimly lit environment. This allows the veterinarian to focus and see the screen better and the foal will be less stressed.

“We're going to want to get this examination done as rapidly as possible and the dim light is going to allow you to look at the screen as best as you can, and to interpret the image that you're getting from the ultrasound as rapidly as possible,” he said.

The pleural line seen on ultrasound of the thorax.

Image courtesy of Gilsenan W, AAEP Proceedings 2021 Vol. 67

There are advantages and disadvantages to each probe. A large-curve linear probe will generate signals to the lowest frequency, meaning it will penetrate quite deeply. Although that could be an advantage for certain structures, it compromises details.

A micro convex or small-curved linear probe will generate signals at a higher frequency, which enhances detail.

A rectal probe also will generate signals at a much higher frequency than the large-curve linear probe, which will give better detail, but will not penetrate that deeply. A rectal probe can be useful when looking at certain structures, however, such as a neonatal rib when a fracture is suspected.

Using the correct conductive medium, either isopropyl alcohol or ultrasound coupling gel, is critical. Dr. Gilsenan prefers isopropyl alcohol because it's readily available, dries rapidly and requires less cleanup.

He recommends that veterinarians develop a routine and always perform the ultrasonography in the same order, so nothing is missed. “There's no order that's right. It's just to help you figure out what works for you and helps you anticipate what to see and ensures that you don't miss any structures,” Dr. Gilsenan explained.

Evaluation of the Thorax

For instance, when evaluating the thorax, Dr. Gilsenan takes a dorsal-to-ventral direction from the intercostal spaces in the dorsum until he can visualize the diaphragm or abdominal viscera. Then he moves caudally to cranially to the lung, which usually occurs before the 17th intercostal space.

Common ultrasonographic findings of the lung include:

• Comet-tail artifacts, which represent irregularities on the pleural surface that indicate that some disease is present, but the lung is still receiving air.

• Pulmonary consolidation is another abnormality that can occur when the lung is not aerated. The consolidation appears isoechoic with the liver, which means it produces an echo similar to surrounding structures. This so-called "hepatization of the lung" makes the lung difficult to isolate, It is often a sign of pneumonia, possibly bacterial. “You can distinguish this from the liver because you can see some parts of the lung that are aerated through which the ultrasound signal is not passing through,” Dr. Gilsenan explained.

• A lung with a pulmonary abscess tends to be well aerated.

• Plural effusion is another common ultrasonographic finding on the thoracic exam in foals. When plural effusion is present, you can often see the diaphragm or liver, which are not present when the lung is well aerated. “And that's because there is some sort of liquid or effusion in the plural space,” Dr. Gilsenan explained.

To find out what is causing the effusion, the ultrasound would typically be followed up with a thoracocentesis.

Evaluating the Abdomen

When evaluating the abdomen, Dr. Gilsenan looks at the left flank, then the right flank. He prefers to start his examination in the caudal abdomen, starting at the caudal aspect of the paralumbar fossa and working dorsally to ventrally along the paralumbar fossa until he reaches the ribs. At that point, he works on each intercostal space ventrally to the thoracic cavity, then cranially until the elbow, to finish the exam.

On the left flank of the abdomen, a veterinarian will see the spleen, left kidney, stomach, large colon and liver. The size of the stomach will vary depending on the amount the foal has ingested.

“Ultrasonographic evaluation of the stomach is an extremely important part of the foal abdominal ultrasonic examination, especially in foals that are showing signs of colic,” Dr. Gilsenan said. A thick- ened, turgid stomach wall, or a stomach that is abnormally enlarged are important signs.

He showed an ultrasound where the stomach was markedly round and could be imaged all the way to the 14th intercostal space. To him, this suggested that something was impeding gastric flow, such as a mechanical obstruction or a functional obstruction like ileus, indicating the need to pass a gastric tube “because the foal is certainly going to be at risk of gastric rupture,” he explained.

On the right flank, one can see the right side of the abdomen, the right kidney, the duodenum, the liver and the large colon. When he evaluates the right flank, again, he starts caudally in the paralumbar fossa and works his way in increasingly cranial planes until he reaches the ribs, then he works ventrally to the lung fields. He evaluates every part of the abdominal cavity until the elbow.

“Ultrasonographic evaluation of the duodenum is another extremely important component of the abdominal ultrasound exam in the foal,” he said.

“When you're looking at the duodenum, you want to look at wall thickness. So again, this is the duodenal wall—and for a foal—normal wall thickness is about 2 or 3 millimeters,” he said.

If it’s turgid or distended, that will signal a problem, possibly an obstruction.

“There's no hard sciences about how distended is ‘too distended,’ to signal a surgical intervention,” heexplained, so serial images might be useful.

“Personally, if I see an internal diameter in a young foal greater than 2 or 3 centimeters, I get very concerned about the potential for a mechanical obstruction, but even with the severe functional obstruction like ileus, you may see some pretty marked distention of the duodenum,” he said.

Evaluation of the Ventrum

One typically will see the ventrum, large colon, the bladder, the internal umbilical remnant, the small intestine and peritoneal effusion. Dr. Gilsenan prefers to start in the most medial plane cranially and then work his way caudally and into progressively lateral planes working outward toward the flanks.

It’s helpful to look at colonic ingestions. A normal large colon will show shadowing that indicates gaseous and solid feed ingest that probably has some liquid in it. If the foal has colitis or diarrhea, the image will likely have no gas or solid feed, and instead be filled with liquid and echogenic material.

The small intestines are a little tricky to visualize because they can be anywhere in the abdominal cavity. Even in the normal foal there may be "a certain amount gas or solid feed in the large colon. If a foal is colicky, fluid-filled small intestinal loops tend to sink eventually. They are often quite turgidly distended, and the walls tend to be thick. Normal wall thickness in a small intestine should be less than 3 mm in a foal.

Evaluation of the Internal Umbilicus

Evaluation of the internal umbilical remnant should be performed in any foal with fever of unknown origin. It should be performed on farms, or if the horse has a history of an internal umbilical remnant.

“It's extremely important to remember that the presence of a normal, external umbilical remnant does not preclude the presence of severe internal remnant infection,” he said.

When evaluating the internal remnant, Dr. Gilsenan typically identifies the stalk--the external umbilical remnant--and then uses the ultrasound probe to come up from it caudally and then looks cleanly onto the internal remnant itself, and then moves in a transverse plane, fanning caudally.

By the time the foal is about 5 or 6 weeks of age, the internal remnant should mostly be gone. In a newborn, the internal remnant can be up to 2.5 cm in diameter. The diameter of each umbilical artery should be less than 1 cm, and the umbilical vein should be less than 7 mm. Larger remnants may be an indication for antimicrobial therapy.

As the saying goes, practice makes perfect, and the more the ultrasonography is used during a foal examination, the more it can improve one’s diagnosis. M

HOW DOES ULTRASONOGRAPHY WORK?

Diagnostic ultrasonography is a non-invasive diagnostic technique used to image soft tissue inside the body. Ultrasound probes, called transducers, produce sound waves that echo off tissues and are returned to the transducer. When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner, which displays an image. The sound waves are reflected back to the transducer by boundaries between tissues in the path of the beam, such as the boundary between fluid and soft tissue or tissue and bone. Areas that appear dark or black mean that the waves go through them without emitting a return echo.