Is anaesthesia of the deep branch of the lateral plantar nerve specific for the diagnosis of proximal metatarsal pain in the horse?

The study explores the specificity of anaesthesia in the deep branch of the lateral plantar nerve (DB-LPN) for diagnosing proximal metatarsal pain in horses. The researchers found that a single injection method often results in concurrent anaesthesia of the lateral plantar nerve (LPN).

Methodology

• The study was conducted on twenty horses that had DB-LPN anaesthesia administered via a single injection as part of a lameness investigation.
• Researchers measured the mechanical nociceptive threshold (pain sensitivity to pressure) using a handheld force meter at six points on the side of the limb before the anaesthesia (T0), and also 15 (T15) and 30 (T30) minutes after the administration of the anaesthesia.
• To evaluate the extent of medication diffusion within the proximal metatarsal region, ten cadaveric horse limbs were injected with 2.5 ml of new methylene blue solution using a similar single injection technique.
• Statistical analysis was performed using paired t-tests and a p-value of less than 0.05 was considered to indicate statistical significance.

Results

• There was a significant decrease in the nociceptive threshold for all test points combined both at T15 and T30 when compared to T0. This implies that the anaesthesia reduced the horses’ sensitivity to pressure across the board.
• The decrease in nociceptive threshold was particularly notable at the lateral third metatarsal bone at T15, and also at the lateral sesamoid, lateral third metatarsal bone, and mid metatarsus at T30.
• Of the 20 horses, four had a nociceptive threshold greater than 10 N at the lateral heel bulb 30 minutes after the anaesthesia was administered.
• In the cadaveric limbs, the methylene blue solution diffused to an average distance of 70.4 ± 20.5 mm. The dye encapsulated the DB-LPN in all the limbs and the LPN in nine out of ten limbs.

Conclusion

• The findings indicate that when DB-LPN anaesthesia is administered using a single injection technique, it is likely that the LPN will also be numb concurrently.
• This observation is key for equine practitioners as it could potentially affect the interpretation of pain or sensitivity in an equine lameness investigation.