Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.

This research investigated how a local anaesthetic solution distributes in a horse’s pastern after a mid-pastern ring block—a common method for numbing pain in a specific area. The researchers injected a radiodense contrast medium, a substance that shows up on radiographs, to track the diffusion of the anaesthetic. The results indicate that the size of the injected volume affects the extent of distribution. These findings could influence pain management strategies in equine veterinary medicine.

Methodology

• The study was experimental and involved twelve mature horses. These horses went through a procedure involving the injection of a radiodense contrast medium, which simulates the action of the anaesthetic during a mid-pastern ring block. This contrast medium allows for clear visibility in radiographic imaging.
• The contrast solution was injected at specific points around the pastern, essentially mirroring the procedure for a mid-pastern ring block. Different volumes – 2 ml and 5 ml – were used to observe if the amount has any effect on the diffusion.
• This process was carried out on both forelimbs of the horses over two days, resulting in a total of 48 injections.
• Radiographic images were taken immediately, 10 minutes and 20 minutes after the injections to track and analyze the diffusion progress.

Results

• The results showed a specific pattern during dorsal injections – the contrast medium created a diffuse patch across the half length of the affected horse’s proximal phalanx, with the extent of spread being significantly influenced by the volume of injected substance.
• The study found that the contrast medium diffused significantly in the first 10 minutes after injection, and continued to spread at all measured time points.
• A positive relationship was noted between the injected volume and the proximal extension of the dorsal contrast patch – a larger volume resulted in further spread.
• The probe also found that the dorsal and the palmar contrast patches did not meet, indicating distinct diffusion routes.

Conclusion

• The study concluded that even small volumes of contrast medium (and by analogy, anaesthetic solution) are able to reach the proximal aspect of the first phalanx (P1) in a horse’s leg.
• As such, it’s possible that pain in the fetlock region could be influenced by a mid-pastern ring block. This means the mid-pastern ring block not only has a local effect, but may also affect regions proximal to the injection site.

Research Ethics

• Before the start of the study, written consent had been obtained from a representative of the horses’ owner. There were no reported conflicts of interest.