Horse Back Pain, the Multifidus Muscle, and Ultrasound
With the increased recognition of back problems in horses, evaluating the spine and associated musculature now plays as important a role in the “whole horse” evaluation of lameness as the limbs. Nonetheless, back pain remains an “insidious performance problem,” continuing to present significant diagnostic challenges, said Philippe Benoit, DVM, MS, Dipl. ACVSMR, from PHB Consulting in Cardiff, California, and Cooper Williams, VMD, Dipl. ACVSMR, from Equine Sport Medicine of Maryland, in Hampstead.

“Back problems occur across all disciplines and can negatively affect a horse’s quality of life and harm the health of the equine industry as a whole,” the authors stated in their abstract presented at the 2019 Annual Convention of the American Association of Equine Practitioners (AAEP), held Dec. 7-11 in Denver.

With the widespread availability of X ray and ultrasound units with sufficient power to image the equine back, practitioners are no longer limited in their ability to diagnose specific injuries isolated to the back. In fact, they can collect a complete survey of the muscles, tendons, ligaments, bones, and joint surfaces.

Benoit described in detail how equine practitioners can ultrasonographically evaluate a specific muscle, the multifidus muscle (MM) in the thoracolumbar region of the spine.

Basic MM Anatomy

The MM is not a single muscle but rather a series of overlapping muscle bundles grouped into five distinct bands. Each bundle of muscle, or fascicle, originates from the outer aspect of the tail end of a spinal vertebra and extends the length of two to four vertebrae. The overlapping pattern of the MMs provide support and stabilization to the back. Stretch receptors in the MMs send information to the brain regarding spinal position and motion.

“The MM serves as one of the most important core muscles in the horse’s body, and injuries to the MM are likely severely underdiagnosed in daily practice,” Benoit and Williams noted.

While atrophy (wasting) and dysfunction of the MMs occur secondary to back pain, the anatomic location of the MM close the vertebrae and deep to other soft-tissue structures precludes diagnosing MM pathology (disease or damage) via palpation or even radiographically. Bone scanning (nuclear scintigraphy) also fails to point to a problem with the MM. Often, asymmetry of the back muscles and back spasms are good indicators of an MM issue.

“Instead, ultrasound is the only tool available that helps gauge the size and function of the MMs,” explained Benoit and Williams.

Luckily, most ultrasound units with a 7.5 mHz linear transducer can provide quality images for lighter horses, while 2.5/3.5 MHz macroconvex transducers are necessary in larger horses. Veterinarians should obtain both cross-sectional and longitudinal images and adopt a methodical pattern to fully assess both the right and left MMs from the 11th thoracic vertebra to the sixth lumbar vertebra.

“A complete thoracolumbar ultrasound examination requires multiple images and measurements and is, as one might expect, time consuming,” said Benoit and Williams. “It is, however, worth the time and effort, as the results will provide a great deal of information about how to treat, manage, and prognose patients with MM lesions.”

Major findings on MM ultrasound include:

  • Changes in size and shape of the MMs;
  • Decrease in muscle echogenicity (blacker on the ultrasound), suggesting a tear or abnormal fiber pattern, particularly at their insertion points on the vertebrae; and
  • Increased echogenicity (whiter on the ultrasound), particularly at the attachment sites to the spinous processes and vertebrae body, can indicate avulsions or the production of enthesophytes—abnormal projections of bone into the muscle fibers that develop at sites of stress.

In summary, Williams and Benoit encouraged the use of ultrasound to assess the MM, identifying this technique as “… noninvasive and easy to realize for equine practitioners already skilled in soft-tissue ultrasonography.”