Your horse’s back hurts, but exactly where and how much? Your veterinarian gets a reaction when she pushes down with two fingers here, your horse flinches when your trainer prods with one finger there, and you feel like his entire back is sensitive.
Austrian scientists say quantifying back pain with objective measurements can lead to more targeted pain-relieving therapy as well as provide a basis for comparison as the horse improves (or worsens). And while this hasn’t been possible in the past, they said they can now gather objective back pain measurements using algometers (pressure measurement devices).
“Algometers allow results of back examinations to be better comparable over different examiners than digital (finger) pressure, and they offer numerical results,” said Theresia Licka, PhD, DVM, Dipl. ACVSMR (Equine), equine orthopedic surgeon and researcher at the University of Veterinary Medicine (UVM) in Vienna, Austria; a member of the university’s Movement Science Group (MSG); and an Honorary Fellow at the Royal (Dick) School of Veterinary Studies, in Scotland.
“We wanted to know how the different areas of the back of the horse are reflected in their sensitivity to algometer pressure, to which depth an algometer actually exerts pressure in the equine back, and which of the different tips of the algometer are most reliable over several horses,” she said.
In collaboration with fellow UVM researcher and MSG member Una Pongratz, DrMedVet, Licka examined nine horses’ reactions to various pressures coming through different algometer tips. They performed this procedure in several places across the length of the horses’ backs and recorded the algometer readings when the horses reacted to the pressure. Then, in five euthanized horses, the researchers took post-mortem measurements, evaluating the transmission of algometer pressure onto a pressure sensor placed underneath the dorsal thoracolumbar tissues at the level of the ribs or the transverse lumbar processes.
Tips of Different Shapes and Sizes
Licka’s algometers were fitted with custom-made tips of different shapes and diameters, ranging from 0.5 cm to 2.0 cm. The laws of physics dictate that the same amount of pressure applied to a smaller contact area is going to feel like a greater force—think of how pushing your finger against the tip of a nail hurts more than pushing it against the head of the nail. So, the smaller tips should naturally elicit greater reactions from the horse. But the researchers wanted to know which tip would be most effective as a standard for measuring back pressure sensitivity across a wide range of horses, Licka said.
They found that the rounded, 1-centimeter-diameter algometer tip was the most reliable for consistent results, they said, noting that it was a nice “middle ground” for detecting pain—not too small and not too large. Also, they determined that their custom-made aluminum tips were probably more reliable than commonly used rubber tips, because the rubber’s elasticity could affect how the horse perceives the pressure.
Regarding back sensitivities, the researchers found that the horses’ lumbar (lower back) region was less sensitive to pressure than the thoracic (mid-back) area.
“In the present study we could correlate this to the thickness of the soft tissue cover over the bony structures of the ribs and the transverse processes of the lumbar vertebrae, both in the live horses as well as in the (post-mortem) sections, where more pressure was required to create a reading at the bone surface in the lumbar region than in the thoracic region,” Licka said.
Essentially, this means the horse’s reaction comes when the algometer tip squeezes structures against a bony structure, causing pain, rather than when the tip dents the soft tissues’ surface, she explained.
“This will most likely be the same when palpating the horse back with your fingers, if you do increase the pressure slowly as we did with the algometer,” Licka continued. “Factors such as size of fingertips, etc., will of course be relevant, too.”
She cautioned that their results cannot yet be considered a basic data set for all pain-free horses. Rather, it’s a “guide” for better understanding how horses handle pressure.
“You have to take all the individual traits into account, like skin thickness, thickness of the subcutaneous fat, muscle volume, and the individual sensitivity of the horse,” Licka said. Ideally, owners or veterinarians should measure a horse’s individual thresholds when it’s not in pain to be able to compare to times when it is.
A Good Starting Point
Overall, Licka said, the study can help caretakers and riders recognize general principles about horses’ pressure sensitivity across the back. This can be particularly important when trying to prevent back pain from developing.
“If a horse has only a thin muscle/fat/skin cover on the back, it will experience discomfort already at lower pressures on the tissue, even without any true back pain,” she said. “Of course, such a situation can easily develop into genuine back pain if such a horse is ridden in that poor development state as the pressure of the rider and the saddle can create true and longer-lasting pain. This scenario may be avoided if ridden exercise is only started after training has led to an increase in muscle volume.”
The study, “Algometry to measure pain threshold in the horse's back - An in vivo and in vitro study,” was published in BMC Veterinary Research.