Analgesic effects of intraneural injection of ethyl alcohol or formaldehyde in the palmar digital nerves of horses.

This research paper investigates the impact of injecting ethyl alcohol or formaldehyde into the palmar digital nerves of horses to alleviate heel pain. The findings suggest that both substances provide lasting analgesia, although formaldehyde caused more soft tissue inflammation.

Research Methods

• The study involved six horses. In one forelimb, the medial palmar digital nerve of each horse was injected with ethyl alcohol. In the opposing limb, formaldehyde was injected into the equivalent nerve.
• The lateral palmar digital nerve in one forelimb was surgically exposed but not injected, providing a control against which the treatment could be compared. The opposite lateral palmar digital nerve was left untreated.
• Data around the extent of lameness due to induced heel pain, thermal reaction time and skin sensitivity scores for each horse’s heel were recorded. Heel pain was deliberately induced using a bolt pushed through a custom horse shoe to exert pressure on the hoof.
• Testing and collection of data continued for 112 days, at which point some nerve samples were taken for histological analysis, examining the cellular structure in the treated nerves.

Results and Conclusions

• The results indicated that the treatments significantly reduced all measures of heel pain and had a lasting analgesic effect throughout the 112-day study period.
• Formaldehyde treatment resulted in a significantly larger pastern circumference compared to alcohol treatment, indicating greater inflammation in these limbs.
• Microscopic examination showed preservation of nerve fibre alignment and an intact epineurium (outermost layer of the nerve) in both alcohol and formaldehyde-treated nerves. However, there were also signs of axon loss and degeneration, along with fibrosis and inflammation.
• The conclusion determined intraneural injection of either ethyl alcohol or formaldehyde in the palmar digital nerves of horses resulted in significant but partial alleviation of heel pain, which lasted for at least 112 days. However, no advantage of formaldehyde over alcohol was found, with formaldehyde resulting in greater soft tissue inflammation.