CSU veterinary researchers examine new techniques to assess equine pain, back problems

Back problems are a common cause of poor performance in all horses — especially competitive horses, whose jumping ability may be diminished.

When back problems are perceived, clinical examination and manual palpation often are used to determine the presence and location of pain, but pain diagnosis can be subjective, especially for mild to moderate back pain.

"In veterinary medicine, animals cannot tell us whether they have superficial, sharp or deep burning pain, or if it only hurts when you sit on my back," says Kevin Haussler, DVM, DC, PhD, an assistant professor in the Orthopaedic Research Center, College of Veterinary Medicine and Biomedical Sciences at Colorado State University.

While measuring the presence of neck and back pain in horses and interpreting it objectively may be difficult, it is essential in the diagnosis and management of equine musculoskeletal injuries.

Pressure algometry (PA), a non-invasive clinical technique, has been shown to be useful in measuring mechanical nociceptive thresholds (MNTs) objectively within the equine axial skeleton, and to quantify both bony and soft-tissue pain.

PA has the potential to provide an objective assessment of nociception and aid in localizing pain to affected structures. Within the musculoskeletal system, persistent noxious stimuli often produce peripheral or central sensitization.

"PA is a tool that has been used in people to try to objectivity measure or put a number on pain thresholds," Haussler explains, "so we've extrapolated and brought that technology to the horses in an attempt to measure levels of pain, or nociception in horse's backs, objectively."

The technique measures the minimum amount of pressure necessary to produce a pain response.

The pressure algometer consists of a calibrated pressure gauge that is pressed against a predetermined landmark along the horse's back, measuring the horse's pain threshold. The mechanical nociceptive threshold is the quantitative unit of the response. Low MNTs are indicative of increased pain. MNTs vary in horses with differing ages, breeds and ridden exercise level.

The PA gauge registers MNT pressure measurements in units of kg force per square centimeter (kg/cm2 ). Pressure is applied perpendicularly to predetermined anatomical landmarks over two to three seconds until a local avoidance reaction (i.e., skin twitching, local muscular contractions, induced lordosis or stepping away) is noted, at which point the pressure is stopped immediately and the corresponding value recorded. While an in-depth clinical evaluation of PA and the establishment of accepted reference MNT values are still lacking for horses, the technique has the potential for improving diagnosis and therapeutic management of horses with back pain.

Equine pain studies

Recently, Haussler applied PA in several studies, conducted with colleagues at Cornell University's College of Veterinary Medicine and using horses from its Equine Research Park.

One study aimed to determine the usefulness of MNTs within the axial skeleton to evaluate healthy ridden and non-ridden horses. PA's ability to differentiate musculoskeletal pain sites from non-pain sites was unknown and needed to be assessed prior to its clinical application.

Another study used PA to evaluate horses with and without induced pain to establish upper and lower limits and ranges of MNTs at both non-pain and at surgically induced pain sites over time.

As part of a concurrent study evaluating spinal kinematics, pain was induced by placement of two fixation half-pins implanted in the dorsal spinous process of two adjacent vertebrae, under sedation and local anesthesia. Consistent responses to the applied pressure were readily identified at all surgical and non-pain sites.

At most non-pain sites, the median percentage change in MNTs pre- vs. post-pin placement was near zero (± 10 percent). The precise site of the implanted fixation half-pins could be recognized based on dramatic and localized decreases in MNTs at, and adjacent to, the pin-placement sites, indicating hyperalgesia. The median differences pre- vs. post-pin placement were within ±1.0 kg/cm2 at the non-pain sites, but > ±1.0 kg/cm2 at all surgical sites.

Three consecutive PA measurements at each site showed no change or consistent pattern in 68 percent of the measurements, an increase in 24 percent and decreases 8 percent of the time.

The median range of three consecutive measurements across all sites and all horses studied was 1 kg/cm2 , which was determined to be the measurement error of the instrument. MNTs varied more between than within horses. The median MNTs for all bony landmarks was 10 kg/cm2 , for the dorsal spinous processes 13 kg/cm2 and for all soft-tissue landmarks 12 kg/cm2 .

The regional MNTs gradually increased from head to tail and were cervical, 9; thoracic, 12; lumbar, 13; and pelvic, 16 kg/cm2 .

Reference MNTs of normal horses provide a standard to which horses with suspected or known axial skeletal pain can be compared. Pin placement into the dorsal spinous processes was an adequate method of inducing bone pain in the thoracolumbar region and reproduced the lower MNTs seen in clinically affected horses with documented back pain.

In horses with documented musculoskeletal injuries, MNTs in the affected areas often are <5 kg/cm2 .

PA is easy to use and is well tolerated at all musculoskeletal landmarks. Consistent, predictable responses to the applied pressure were identified readily, though there is a learning curve for examiners to use PA correctly and interpret responses. PA offers a more quantitative evaluation of musculoskeletal pain than normal palpation, differentiating it from adjacent non-painful landmarks.

But further studies are needed to evaluate PA clinically and to develop equine pain-management strategies.

PA as clinical tool

"Until now, we've mostly used PA as a diagnostic tool in the research setting," Haussler says, "but as veterinarians become more familiar with the technology it has potential as a therapeutic monitoring device."

It can be used in practice for subtle or hard-to-localize problems. "Affected horses often are stiff, but when PA was used as a diagnostic tool, we were able to localize the pain to one side and one vertebral segment. Without the tool, you couldn't really do that.

"There are a lot of horses that seem to be painful over their entire back, and if you take the time to map out the painful area, you find that the pain is localized maybe to one area or to specific muscles or bony landmarks," Haussler explains.

"It helps us as a clinical tool to identify whether a horse has more of a muscle injury that is causing the back pain, or a bony-related problem, such as impinged dorsal spinous processes or possible osteoarthritis of the synovial articulations."

The benefit of PA is in localizing the pain and grading its severity, but more accurately than the subjective grades of mild, moderate or severe.

There are other applications for PA to quantify limb pain, but more clinical research and case studies are needed.

"I talk about the potential clinical applications of PA in horses whenever I get the chance," Haussler says. "However, I do not know how common it is used by others."

PA is being studied by Willem Back at Utrecht University and the University of Queensland in Australia to evaluate sacroiliac problems and possible integration and assessment of physical-therapy techniques.

"With all things new and different, you need to see what its strengths and weaknesses are," Haussler says. At Utrecht University, researchers are evaluating repeatability between days and between examiners.

"It's probably not a tool that every practitioner has in the back of his or her truck, but if you're interested in looking at back problems, or if you're really serious about evaluating your treatment effectiveness, then I think that it is a tool that practitioners should start looking at."

Other methods studied

In addition, equine practitioners are looking at other methods to best diagnose and treat different types of back problems.

"We need to come up with better tools, similar to PA, to assess muscle spasms, back stiffness and poor performance objectively," Haussler concedes. "PA is one tool, but we're always trying to find new ones that can give us numbers or objective data rather than subjective assessments of back pain or poor performance."

Other methodologies being examined to improve diagnosis of equine back problems include improved radiography, diagnostic local anesthesia, thermography and diagnostic ultrasonography.

"Hopefully, in the near future, we will be able to do MRIs or CT scans on the entire horse, and not just on its head or its upper neck," Haussler says. "If we can approach equine back problems using several different diagnostic or therapeutic techniques, then I think we're going to be much more effective in helping horses with back pain."

Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.