Since its introduction into equine medicine in the latter part of the 20th century, ultrasound has been enthusiastically adopted by practitioners—both hospital-based and in the field—as a noninvasive way to glean information about tissues lying beneath the skin.
Almost undoubtedly, the most common uses of ultrasound are in evaluation of the female reproductive tract and for evaluation of the tendons and ligaments of a horse’s lower limb. Other, less common uses for ultrasound may be learned relatively fast.
The mechanics of ultrasound are straightforward. Ultrasonic waves are generated by a probe directed by the veterinarian. When an interface between different ultrasound densities is encountered by the waves, some reflect back. These reflected waves then are detected by the probe and assimilated into an image by the machine’s computer.
Different tissues have different densities; bone is bright white, fluid is black and everything else falls into innumerable shades of grey. Higher frequency probes provide better structural detail but lack the depth of penetration of lower frequency probes, so relatively superficial structures such as tendons may be evaluated with a 7.5 MHz probe while deeper structures require frequencies as low as 1.5 to 3.5 MHz.
Ultrasonography of Equine Eye
Ultrasonography is a useful diagnostic technique for evaluation of the eye and adnexa in many equine ocular diseases. Ultrasonography can help practitioners obtain critical information about the horse’s eye when a routine ophthalmologic exam is not possible due to conditions such as opacities of the eye or severe eyelid swelling.
Ultrasound also can help practitioners distinguish the causes of changes in the size or position of the globe, such as helping to distinguish between glaucoma and retrobulbar masses.
High-frequency linear transducers (7.5 to 10 MHz) at a depth of 4 to 5 centimeters usually allow for the best visualization of ocular structures. Ultrasound images may be obtained either through the eyelids (transpalpebral approach) or directly through the cornea (transcorneal). While the transpalpebral approach is more readily tolerated, as the horse does not see the ultrasound probe, an increased risk of artifacts is possible with this approach.
Auriculopalpebral nerve blocks are most commonly used to help facilitate examination. An excellent review for ultrasound examination of the equine eye is “Practical Ultrasonography of the Equine Eye.”1
Though ultrasound does not penetrate the surface of cartilage or bone, much useful information may be gleaned by viewing things such as joint contours, the character of the synovial fluid, menisci, or associated ligaments and tendons. Here are a few examples of where ultrasound may assist in evaluating and treating joints.
Stifle: While the three joints that make up the equine stifle are most commonly evaluated radiographically, X-rays give little information about the soft tissue structures of the stifle, including the medial meniscus, a common source of stifle problems. The meniscus is evaluated in both weight-bearing and non-weight-bearing positions.
The shape of the meniscus is concave, following the contour of the femoral condyle. One retrospective study concluded that ultrasound was superior to arthroscopy in assessing damage to the medial meniscus.2 Probes used to examine the stifle include 10 to 12 MHz linear, or 8 MHz curvilinear, transducers.
While cartilage is invisible on radiographs, some cartilage defects may be readily seen on ultrasound. However, due to the complexity of the stifle joint, accurate diagnosis of stifle problems often requires a combination of radiography, ultrasonography and arthroscopy.
Tarsus: The collateral ligaments of the tarsus, as well as various tendons, including the superficial digital flexor tendon, long digital extensor tendon, gastrocnemius tendon and peroneus tertius, are most commonly assessed with ultrasound. But due to the large, superficial tarsal pouches on the dorsal aspect of the joint, even relatively minor wounds may induce septic arthritis, tenosynovitis and bursitis. In such cases, ultrasound can be used to show involvement of synovial structures.3
Ultrasound-guided joint injections: Joints such as the shoulder, hip and vertebral facets are complex and not easily accessible. An ultrasound-guided technique may make injections of these areas easier and more accurate, and help ensure an accurate assessment of diagnostic and therapeutic procedures.
Ultrasound for Colic Evaluation
In cases of severe colic, ultrasound may be useful in helping veterinarians distinguish between surgical and nonsurgical cases. For example, ultrasound may help identify increases in peritoneal fluid, decreased or absent small intestinal motility, or small intestinal enlargement that can accompany conditions such as strangulating obstructions of the small bowel. These problems may avoid detection by rectal palpation.
Other ultrasound findings are almost pathognomonic. For example, large colon enlargement and the inability to image the left kidney is significantly associated with renosplenic entrapment. Large colon torsions also may be precisely identified with ultrasonography. For these reasons, most equine referral hospitals will routinely examine the equine abdomen by ultrasound as part of the colic examination, and it may be done in the field.
While extremely useful, ultrasound does not obviate the need for a rectal examination. Research has shown this is a superior technique for the diagnosis of intestinal dislocations and fecal impactions.4
A good clinical examination is still critical in evaluating cases of colic, and the results of the clinical examination and ultrasound should be used together in making the decision for colic surgery.
The applications of diagnostic ultrasonography seem almost limitless as a variety of structures may be imaged. Information obtained by ultrasonography may be used to follow a case’s progression and develop a prognosis. If you’re wondering whether ultrasound might provide you with valuable diagnostic information, it probably can.
- Hallowell, GD and Bowen, IM. “Practical Ultrasonography of the Equine Eye” Equine Veterinary Education 2007, 19(11):600-605.
- Adrian, AM, Barrett, MF, Werpy, NM, et al. “A Comparison of Arthroscopy to Ultrasonography for Identification of Pathology of the Equine Stifle” Equine Veterinary Journal, 25 Feb 2016, DOI:10.1111/evj.12541.
- Whitcomb, MB. “Ultrasonography of the Equine Tarsus” Proc 2nd AAEP. 2006:13-30.
- Beccati F, Pepe M, Gialletti R. “Is There a Statistical Correlation Between Ultrasonographic Findings and Definitive Diagnosis in Horses With Acute Abdominal Pain?” Equine Vet J Suppl. 2011 Aug;(39):98-105.
Originally published in the October 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!