Diffusion of Radiodense Contrast Medium after Perineural Injection of the Palmar Digital Nerves

Reasons for performing study Limited information exists on distribution of local anaesthetic solution following palmar digital nerve blocks. Objectives To demonstrate potential distribution of local anaesthetic solution following perineural injection of the palmar digital nerves using 2 different volumes of contrast medium and 2 different injection sites. Study design Experimental. Methods Twelve mature horses were used. Perineural injection of the palmar digital nerves were performed at the level of or 2 cm proximal to the proximal aspect of the ipsilateral ungular cartilage, using 1.5 or 2.5 ml radiodense contrast medium. In total 96 injections were performed. Four standard radiographic views of the pastern were obtained immediately after injections and 10 and 20 min later. Images were analysed subjectively and objectively. Results After distal injections, the contrast medium was more localised around the injection site; after proximal injections the contrast patch had greater proximal-distal length. The greatest proximal diffusion was to 31.7% of the length of the proximal phalanx (from distal) after distal injections and to 70% after proximal injections. The larger volume resulted in significantly greater proximal diffusion than the smaller volume at the distal, but not at the proximal injection site (P<0.01). There was significant proximal diffusion with time after proximal and distal injections (P<0.01). In most limbs numerous radiopaque lines of various thickness extended proximally from the contrast patches; subjectively, their number and thickness were greater at the distal injection site. Conclusions Palmar digital nerve blocks at the level of the ungular cartilage using ≤2.5 ml local anaesthetic solution may improve proximal interphalangeal joint and pastern region pain. If using a more proximal site, distal fetlock region pain may be improved. Due to diffusion into lymphatic vessels, too small a volume at the distal injection site may not provide sufficient analgesia. Ethical animal research: Written consent had been obtained from a representative of the horses' owner prior to starting the study. Source of funding: None. Competing interests: None declared.