Horses tended to have lower pressure thresholds in areas of their back with less tissue thickness, especially so where there is less muscle, a study has found.
The researchers in the Austrian study based their findings on the results of testing using an algometer − a device for measuring sensitivity to pressure − on the backs of horses.
Researchers Una Pongratz and Theresia Licka, from the University of Veterinary Medicine in Vienna, focused their attention on the dorsal thoracolumbar tissues – that’s broadly the area where the saddle sits.
They noted that even though a large number of diagnostic aids were available to investigate thoracolumbar pain, palpation remained one of the most important methods.
The pair conducted their measurements using six different algometer tips, with surface areas of half a square centimeter, one square centimeter and two square centimeters.
The pair used the algometer on nine live horses – seven geldings and two mares – noting the threshold of pressure that led to any reaction. Signs that the discomfort threshold had been reached included turning the ears, looking back at the examiner, hollowing the back away from the pressure, stamping the feet, and kicking.
They also conducted an experiment to measure the transfer of algometer pressure through skin and tissue to the bone, using five horses that had been euthanized at the veterinary hospital for reasons unrelated to the study.
They found that the algometer tips with a contact area of one square centimetre led to widely similar results irrespective of the surface shape and delivered the lowest variance.
Among the nine horses, a significantly higher pressure threshold was found in the lumbar region, where tissues were thicker, than in the thoracic region. The pair also documented less pressure transmission in the lumbar region in their work involving the euthanized horses. Thus, the increase of pressure threshold from front to back was clearly documented in both phases of the study.
The researchers said the increase in tissue thickness in the lumbar region was due mainly due to an increase in thickness of the long back muscles.
“The effect of an increase in the pressure threshold in the present study was most obvious in the lumbar region,” the pair noted in the journal BMC Veterinary Research, noting that this finding was similar in other studies.
“This marked increase in pressure threshold in the lumbar region is not the consequence of a marked increase in tissue thickness at this level,” they said. For humans, where a similar increase was found, a variation in the density of neurovascular structures at this level had been discussed in scientific studies.
“The tissue quality in addition to the tissue thickness is of relevance for the transmission of pressure exerted by an algometer,” they said.
Pressure thresholds over bony areas have been found to be much lower in humans, they noted, as well as in a study on the pressure threshold in horses over their dorsal spinous processes − the bony projections off the back of each vertebra.
This, they said, was reflected in the results of the research involving the euthanized horses, where the skin and related subcutaneous tissue samples did little to reduce the pressure transmission.
“In this context it is important though, that the reduction in muscle mass commonly present in horses with prolonged thoracolumbar pain and the reduction of the pressure pain threshold due to the reduction of muscle volume are not interpreted as two independent indications of thoracolumbar pain and dysfunction.”
Based on the results of this study, they concluded that tissue thickness and tissue character, as well as anatomical location, should be considered for the interpretation of algometry results from the equine back. There were lower pressure threshold in areas with less tissue thickness, especially less muscle, even in horses without any clinical signs of back pain.
The said the algometer tip used in such studies should also be carefully chosen regarding its shape and surface area, with smaller tips creating lower pressure thresholds, and rounded tips creating higher pressure thresholds.
Algometry to measure pain threshold in the horse’s back – An in vivo and in vitro study
Una Pongratz and Theresia Licka
BMC Veterinary Research 2017 13:80 DOI: 10.1186/s12917-017-1002-y