Distribution of radiodense contrast medium after perineural injection of the palmar and palmar metacarpal nerves (low 4-point nerve block): an in vivo and ex vivo study in horses.

This study investigates the dispersion of local anaesthetic solution in horses following a specific method of nerve block known as a ‘low 4-point nerve block.’ Using radiodense contrast medium and radiographs, the researchers visualized the spread of the injected solution in both live horses and cadaver limbs, noting a key finding that an inadvertent penetration of the digital flexor tendon sheath (DFTS) might occur during this procedure.

Objective and Methods

• The primary aim of this study was to observe the potential dispersion of local anaesthetic solution following a ‘low 4-point nerve block’ in horses, using a radiographic contrast model.
• This nerve block procedure targets the palmar (Pa) and palmar metacarpal (PaM) nerves in the distal aspect of the metacarpal (Mc) region. The researchers injected a radiodense contrast medium (which shows up on radiographs) into these areas in both forelimbs of 10 mature, lameness-free horses.
• Immediately after the injection and 10 and 20 minutes post-injection, radiographs were taken and analysed to examine the pattern of the contrast medium’s spread.
• This procedure was repeated with a mixture of methylene blue and the contrast medium in 20 cadaver horse limbs, which were then dissected for further analysis.

Results

• The outcomes revealed that in 77.5% of the live injections, the contrast medium appeared to spread within the neurovascular bundle. Over time, the contrast medium diffused proximally but never moved beyond the mid-Mc region. In two cases, there was an indication of the medium being present in the digital flexor tendon sheath (DFTS).
• Following the PaM injections, the contrast medium mainly scattered around the injection site. In the cadaver limbs, contrast medium and dye distribution within the neurovascular bundle was observed in 40% of instances, while penetration into the DFTS was noted in 30% of cases.
• After PaM injections into cadaver limbs, 45% showed diffusion around the injection site and 55% showed both diffuse and tubular distribution patterns.

Conclusions

• Based on these findings, the study suggests that local anaesthetic solution diffusion following a ‘low 4-point nerve block’ is unlikely to alleviate lameness caused by pain in the proximal Mc region, as the solution does not reach beyond the mid-Mc region.
• However, the research also demonstrated a potential risk of the DFTS being unintentionally penetrated during the nerve block procedure, a crucial insight for veterinarians during application.