Algometry to measure pain threshold in the horse's back - An in vivo and in vitro study

The aim of this study was to provide information on algometric transmission of pressure through the dorsal thoracolumbar tissues of the equine back. Using a commercially available algometer, measurements were carried out with six different tips (hemispheric and cylindrical surfaces, contact areas 0.5 cm², 1 cm², and 2 cm²). In nine live horses the threshold of pressure that lead to any reaction was documented. In postmortem specimens of five euthanized horses 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 respectively was measured.

Algometer tips with a contact area of 1 cm² led to widely similar results irrespective of the surface shape; these measurements also had the lowest variance. Contact areas of 0.5 cm² resulted in a lower pressure threshold, and those of 2 cm² resulted in a higher pressure threshold. The hemispheric shape of the contact area resulted in a higher pressure threshold, than the cylindrical contact area. Compared to the thoracic region, a significantly higher pressure threshold was found in the lumbar region in the live horses. This result corresponds to the increased tissue thickness in the lumbar region compared to the thoracic region, also documented as less pressure transmission in the lumbar region on the in vitro specimens.

Algometry is an easily practicable and well tolerated method to quantify pain but it is important to consider the many factors influencing the results obtained.