With more than 200 muscles in the equine body, it’s no surprise muscle injuries and trauma are significant causes of lameness and performance limitations. Yet, according to Jean-Marie Denoix, DVM, PhD, DECVSMR, founder of ISELP and ENVA’s Centre d’Imagerie et de Recherche sur les Affections Locomotrices Equines in France, primary muscle injury is only rarely considered in the differential diagnosis of lameness in horses. And, while magnetic resonance imaging (MRI) is a commonly used technique for diagnosing muscle injuries in humans, ultrasound is an invaluable tool in equine practice for diagnosing and monitoring muscle injuries, he said during his presentation at the National Alliance of Equine Practitioners’ (NAEP) 2024 Saratoga Equine Practitioners Conference, held Sept. 25-28 in Saratoga Springs, New York.
To begin his presentation, Denoix reminded attendees that the ultrasonographic appearance of muscle is related to the muscle fibers, connective tissue, vessels, and nerves.
“There is a lot of connective tissue in muscle, which is echogenic on ultrasound,” he said.
Using a series of case examples with accompanying ultrasound images, Denoix described common lesions affecting muscle of the limbs, neck, and back.
Muscle Disorders Affecting Horses’ Limbs
- Blunt trauma
Denoix showed two examples of muscle trauma. The first was a kick to the brachiocephalicus (at the point of the shoulder), and the second was blunt trauma to the quadriceps femoris after a kick from another horse.
In both examples, ultrasound examination showed echogenic fluid consistent with blood in the muscle (hematoma). Follow-up ultrasound on Day 3 showed anechoic (dark) fluid with fibrin clots. By six weeks, the horses were back at work, but follow-up ultrasound examination showed persistent disruption of the muscle and connective tissue fibers.
“This is a very common finding for muscle injuries, that clinical improvement is faster than complete healing,” said Denoix. “If you are waiting for complete healing, then you’re wasting time keeping the horse out of competition. But you need careful clinical reassessment!”
- Internal tears/rupture of muscle body
A 4-year-old Thoroughbred presented with severe enlargement of the caudolateral aspect of the crus. Transverse ultrasound of the unaffected limb showed normal architecture with many intramuscular aponeuroses in the lateral digital flexor muscle, whereas the affected limb showed extensive hematoma due to complete rupture of the muscle. The aponeuroses were no longer visible.
Denoix also presented the case of a Western horse with a semitendinosus tear after turning in a tight circle. Radiographs of the stifle and tibia were normal.
“On ultrasound, the muscle body was present, and the tendon was also present, but nothing was in the middle except fluid,” he explained. “There was a complete dislocation between the muscle and tendon.”
Five days later, fibrin was present in the space between the muscle body and tendon, which is how the tendon will be reconstructed. But even after healing, the muscle body will end shorter than normal and the tendon will end longer than normal, said Denoix.
- Fibrotic myopathy of the semitendinosus muscle
Clinical examination of a horse revealed muscle atrophy and a reduced cranial phase of stride with the foot landing caudally (when it usually lands cranially). An ultrasound examination showed increased echogenicity both around and within the muscle. This increased echogenicity is particularly noticeable compared to the adjacent muscles (semimembranosus and biceps femoris) and the contralateral limb.
Pathology of the Neck and Thoracic Muscles
Denoix presented two cases showing how veterinarians can use ultrasound to diagnose muscle injuries in the neck and thorax. First, he described a horse with enlargement in the caudal neck on one side that was having trouble jumping. Ultrasound identified deformation of the serratus ventralis cervicis due to an (old) complete tear.
“The muscle was heterogenous in appearance compared to the brachiocephalicus that was also visible in the same image,” Denoix said. “Show jumpers are predisposed to this injury because of the biomechanics of jumping and reception (contact with ground). It is a chronic disease.”
The second case was a horse with rupture of the serratus thoracis ventralis muscle, which is a broad muscle with the thoracic longus nerve running through it.
“This is seen as an overtraining injury in endurance horses,” he explained. “The shoulder presents higher than the withers, and there is edema of the ventral aspect of the thorax. It is easy to diagnose clinically via ultrasound because the muscle is thicker than normal with a heterogenous echogenicity.”
Expert Advice
“Ultrasound is a valuable technique for the diagnosis and follow-up of equine muscle injuries, providing clear information on the muscle affected and type of injury,” Denoix summarized. “This information can then be used to design physical therapy protocols and establish specific rehabilitation programs.”
He added, “Be certain to always image the homologous muscle on the contralateral side to improve accuracy!”
Related Reading
- How To Turn Your Veterinary Equipment Into a Revenue Stream
- Bad Behavior and Possible Neck Pain in Mares
- Association Between Turnout and Musculoskeletal Disease in Young Thoroughbreds
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