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.
Authors: Nagy A, Bodò G, Dyson S J, Compostella F, Barr A R S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Low 4-Point Nerve Block Distribution in Horses Nagy and colleagues used radiographic contrast medium to map the actual spread of local anaesthetic during low 4-point nerve blocks (targeting the palmar and palmar metacarpal nerves in the distal metacarpal region) in both live horses and cadaver limbs, addressing a significant gap in evidence-based understanding of this commonly performed technique. In vivo radiographs showed that palmar nerve injections achieved perineurovascular bundle distribution in 77.5% of cases, though contrast medium diffused proximally over time without progressing beyond the mid-metacarpal region; palmar metacarpal injections produced diffuse periaxial spread rather than discrete perineural uptake. Cadaver dissections revealed a lower success rate for accurate perineural placement (40% for palmar, with concerning inadvertent digital flexor tendon sheath penetration occurring in 30% of palmar injections and 45-55% variability in palmar metacarpal distribution patterns). The findings suggest that whilst proximal diffusion is unlikely to anesthetise proximal metacarpal pain, there is substantial risk of unintended intra-synovial injection of local anaesthetic, with important implications for technique refinement and client communication regarding potential complications such as tendon sheath irritation or chemical synovitis.
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Practical Takeaways
- •The low 4-point nerve block distributes local anaesthetic primarily in the distal and mid-metacarpal regions; do not rely on this block for proximal metacarpal pain sources
- •Inadvertent penetration of the digital flexor tendon sheath is a risk with this technique—use appropriate needle depth and caution to avoid intra-synovial injection
- •Palmar nerve injection sites show better organised distribution along the neurovascular bundle than palmar metacarpal sites, which tend to diffuse broadly
Key Findings
- •77.5% (31/40) of palmar nerve injections showed contrast distribution in the neurovascular bundle with proximal diffusion over time, but never progressed beyond mid-metacarpal region
- •Contrast medium was present in the digital flexor tendon sheath in 2/10 in vivo cases and 6/20 cadaver limbs after palmar injections
- •Palmar metacarpal injections resulted in diffuse distribution around the injection site in majority of limbs
- •Proximal diffusion from low 4-point block is unlikely to relieve pain originating in the proximal metacarpal region