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An experimental study using 6 normal horses was conducted to determine how much local anesthetic agents used for diagnostic nerve blocks might diffuse away from a point of injection on the lower limb. Also examined was the influence that hand walking on injection diffusion. X-ray dye was injected under the skin and along the palmar nerve over the base of the proximal sesamoid bone and follow-up X-rays showed where the dye migrated at 5-minute intervals up to 30 minutes.
In 90% of the horses, the dye diffused away from the point of injection up or down the limb in a pattern described as a “patch” following the path of the neurovascular bundle. In addition about half the horses showed a fine line of dye extending up the limb (25% showed the line extending down the limb) along the expected path of the lymphatic ducts. Most diffusion occurred within the first 10 minutes, but walking the horses had no significant influence on the result.
Diffusion of local anesthetic beyond the point of injection can delay or decrease the effect of a diagnostic nerve block. This may also cause areas other than those intended to become desensitized. These results highlight the fact that while diagnostic nerve blocks may be a critical part of a lameness workup, they are not necessarily an exact science.