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Ultrasound, Palpation per Rectum to Diagnose Colic

When it comes to diagnosing a colicky horse in the field, veterinarians typically work quickly and thoroughly to identify the problem and either start treatment or refer the horse to a clinic. Two modalities veterinarians often employ to narrow down the source of a horse’s abdominal pain are palpation per rectum and percutaneous (through the skin) ultrasound.

At the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah, Tracy E. Norman, VMD, Dipl. ACVIM, reviewed how veterinarians can use these techniques while examining a colicky horse.

“The main goals when performing diagnostics in a horse with abdominal pain are to determine a diagnosis, choose an appropriate therapeutic plan (which might involve surgery), and provide the owner with a prognosis,” said Norman, now a practitioner at the Blue Ridge Equine Clinic, in Staunton, Virginia.

She said palpating the horse’s abdomen per rectum has long been considered a mainstay of a colic exam, but the “ready availability of portable sonographic equipment and better protocols for rapid transabdominal sonography have made percutaneous (through the skin) ultrasound an increasingly utilized modality in the diagnosis of acute colic.”

And, like many other diagnostic modalities, these two often better when used together.

“Rather than viewing these diagnostic modalities strictly independently of each other, it is useful to think about how they contribute to answering questions about the state of the anatomy within the abdomen,” Norman said. “Each one is a powerful diagnostic tool, but together they could provide information that neither could alone.”

One reason these modalities are best used in concert in many cases is that each has its own strengths and weaknesses, which Norman reviewed with veterinary attendees.

Palpation per rectum is most useful for evaluating structures located in the caudal (closest to the rear) abdominal cavity, she said. A veterinarian might use rectal palpation on horses with acute or chronic colic, fever of unknown origin, urogenital problems, weight loss, and inappetence, among other issues. She cautioned that a late-term fetus, bladder distension, and colonic or cecal gas distension can all obstruct a practitioner’s ability to palpate other structures in the abdomen.

Norman advised veterinarians against using palpation per rectum in very small or very fractious horses and noted that veterinarians who are small in stature might have difficulty palpating very large horses successfully. While practitioners can examine about a third of the abdomen by rectal palpation, she said this percentage can drop to about a quarter when a veterinarian with short arms is working with a large patient.

“The most dreaded complication of rectal palpation is a rectal tear,” Norman said. Horses of some specific breeds (including Arabians and Miniature Horses), those older than 9 years, and mares have all been shown to be at increased risk of rectal tear following palpation, she added.

Additionally, she said, palpating a horse’s abdomen from behind can place the veterinarian in a precarious position.

“Although we as veterinarians are often more concerned about potential concerns to our patients’ well-being, another potentially serious consequence of rectal palpation of a horse is injury to the veterinarian through kick or crush injury,” Norman said. “Care must be taken in providing the safest environment for both patient and practitioner.”

Percutaneous ultrasound generally allows veterinarians to visualize the superficial (on the outside near the skin) two-thirds of a horse’s abdomen. Additionally, Norman said, scanning per rectum can also augment information veterinarians collect on rectal exam.

“Sonographic evaluation is especially helpful in evaluating colicky horses in which rectal palpation is impossible or unacceptably risky,” she said.

As with rectal palpation, however, abdominal ultrasound has its limitations.

First, Norman said, a horse’s hydration status, perfusion, ambient temperatures, skin thickness, fat coverage, and hair coat thickness can all negatively affect an ultrasound’s image quality.

To that end, most patients must be groomed and clipped, and the area to be ultrasounded must be coated in either ultrasound gel or isopropyl alcohol.

Another potential drawback, she said, is that ultrasound units require power, and while some can use battery power, others cannot. Thus, this modality might not be available for use on farms without safe electric wiring.

“In addition, although more affordable units and probe options are becoming increasingly available, there is cost involved in obtaining optimum equipment,” Norman said. “However, even using a standard rectal probe, some useful information may be obtained.”

Take-Home Message

“Ultimately, in evaluating a horse with colic, the results of all diagnostics … fit together to form a data set that gives the practitioner diagnostic and prognostic information that hopefully leads to a successful therapeutic plan,” Norman concluded. “Performed together, palpation per rectum and sonographic examination provide complementary information about lesion location, etiology, and severity.”