Efeito analgésico do tratamento com cloreto de amônio 2% e acetaminofeno associado à fenilbutazona em equinos

Abstract

The management of chronic pain is an important factor within veterinary medicine since it is related to animals’ welfare. Chronic hoof lameness is a frequent problem in the practitioners’ routine and it presents different success rates. Neurolytic compounds have a mechanism of action similar to the neurectomy, but they allow animals to maintain their physiological defense system. However, these drugs have lost credibility regarding their use due to their misuse. It was mistakenly believed that these have similar mechanism of action than local anesthetics. Thus, the first part of this work aimed to objectively assess the analgesic effect of ammonium chloride 2% (2%AC) in different cases of chronic hoof (podotrochlear apparatus, n= 5; podotrochlear apparatus associated with the distal interphalangeal joint, n= 6; distal interphalangeal joint, n= 2; laminitis, n= 1; and bruised sole, n= 1) for a period of 62 days. Perineural injection of 2%AC into the palmar digital nerve was able to induce partial to complete lameness improvement, with better results being observed on days 12 and 19 after treatment. Moderate to severe radiographic lesions of navicular bone and distal interphalangeal joint can negatively interfere on the 2%AC analgesic effect. Subsequently, in a second study was carried out to assess the neurotoxicity of 2%AC to better understand the mechanism of action of this drug. Thus, 18 and 6 digital palmar nerves were treated with 2%AC and saline solution, respectively, and later nerve samples were collect by neurectomy for histological evaluation. 2%AC induced moderate to severe Wallerian degeneration for up to 62 days after treatment, similar to those observed in nerves treated with saline. Perineural injection with 2%AC did not interfere in future neurectomy. The results of both studies reinforce that 2%AC can be a useful and safe therapeutic option for the management of chronic hoof lameness in horses. In turn, multimodal analgesia is also an important technique for managing chronic pain and has lower rates of side effects when compared to administration of one medication for the same purpose. Therefore, the third part of this study evaluated the analgesic effect and toxicity of acetaminophen (ACET, 20 mg Kg-1) associated with phenylbutazone (PBZ, 2.2 mg Kg-1) on lameness induced by hoof clamps. The association of ACET with PBZ demonstrated an overall analgesic effect, based on the change in the lameness, superior to the treatment with only ACET (p=0.008), PBZ (p=0.0117) and the control group (p<0.0001). The association of ACET with PBZ twice a day for 14 days did not show significant changes in hematological and biochemical parameters when compared to reference values. These data reaffirm the analgesic potential of acetaminophen associated with phenylbutazone for pain management in horses. Future studies to evaluate potential analgesic of different therapeutic protocols with ACET, associate or not with others drugs, in different experimental or clinical models, are necessary before the recommendation of its use in the clinical routine.