Starting from the bottom of the limb and working up the limb with sequential diagnostic nerve blocks is common procedure for practitioners trying to localize a horse’s lameness. One study looked at potential proprioceptive deficits following equine perineural blocks that could contribute to motor dysfunction and risk of accidents [Lins, JLF.; Bernardo, JO.; Mariz, TMA.; Escodro, PB. Postural evaluation in horses submitted to perineural anesthetic blocks in the thoracic limbs. Seven Editora Mar 2023; doi: 10.56238/devopinterscie-030].
In the study of seven adult, sound horses, 0.5% bupivacaine was used for the nerve blocks in the left thoracic limb. Where possible, localization of each nerve was identified using a nerve stimulator. Once bupivacaine was administered into a specific nerve branch, the observers waited 30 minutes before evaluating the horse. Each horse received blocks in all the selected nerves, with seven days between each procedure. The nerves blocked included: a) medial and lateral palmar nerves; b) medial and lateral metacarpal nerves; c) ulnar nerve; d) median nerve; and e) musculocutaneous nerve.
The horses were trotted on asphalt in a straight line before and after a nerve block. Postural exams were conducted with three repetitions each test, with comparisons made of the horse executing these tests before and after the nerve block. The horse was timed to how long it took to restore to normal posture. The postural positions included flexing the left forelimb to rest on the soil; crossing the left forelimb over the right forelimb; placing the left forelimb on a wooden surface 6 inches high that slides laterally; and asking the horse to walk across a 6-inch-tall wooden plank to determine if he could cross without knocking it down.
The researchers concluded that equine perineural anesthetic blocks of the medial and lateral palmar nerves did not elicit proprioceptive deficits. Administration of a palmar metacarpal nerve block and proximal anesthetization of the ulnar, median and musculocutaneous nerves resulted in significant proprioceptive deficits such as stumbling or dragging the limb. They noted that the ulnar and median nerves have large myelinized nerve fibers to convey sensations to the spinal cord to integrate sensory information with locomotor reflexes.
The authors stressed that using perineural anesthetics in horses during sports activities is contraindicated. Care must also be taken when anesthetizing upper limb nerves for diagnostic evaluation under any circumstance.
