EQUINE VETERINARY EDUCATION / AE / SEPTEMBER 2017
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Critically Appraised Topic What does a positive palmar digital nerve block mean?
E. J. O. O’Brien Royal Veterinary College, Hawkshead Lane, Hatfield, Hertfordshire, UK. Corresponding author email:
eobrien@rvc.ac.uk
Summary
Clinical scenario During a lameness investigation, the horse becomes sound after a palmar digital nerve block is performed on the affected limb. Could any structures, in addition to those of the heel, be contributing to the lameness in this case?
Search strategy The PubMed database (
https://www.ncbi.nlm.nih.gov/ pubmed/) was searched using the string ‘horse AND ((palmar digital nerve) OR (palmar digital nerves))’. Proceedings of the Annual Convention of the American Association of Equine Practitioners were searched using the International Veterinary Information Service database (
http://www.ivis.org/search.asp) and the strings ‘palmar digital nerve aaep’ and ‘palmar digital nerves aaep’. Thirdly, the journal Equine Veterinary Education was searched using the Wiley online library (http://
onlinelibrary.wiley.com/) and the string ‘palmar digital nerve’. An additional study that the author was aware of was also included (Dyson and Murray 2007). Results were screened first by title and second by
abstract. Where similar reports by the same authors were available as conference proceedings and journal articles, the former were excluded. Similarly, smaller case series were excluded where data collected over a longer period (same start time) by the same authors were also available. Review articles were excluded.
Results
Quantity of evidence A total of three journal articles (Schumacher et al. 2000, 2004; Nagy and Malton 2015) and four conference proceedings were identified (Ross 1998; Easter et al. 2000; Dyson and Murray 2007; Contino et al. 2012).
Quality of evidence Experimental studies Schumacher et al. (2000) demonstrated that palmar digital nerve blocks (PDNBs; 2 ml local anaesthetic each medial and lateral site) could improve lameness arising from solar pain. These authors used set screws, fitted to the inside of customised shoes, to apply pressure to the solar surface just dorsal to the apex of the frog to induce lameness. Median lameness scores were significantly improved after PNDBs were performed just proximal to the edge of the heel cartilage. Easter et al. (2000) (3 ml LA each site) and Schumacher et al. (2004) (1.5 ml LA each site) showed that PDNBs could ameliorate pain arising from the distal interphalangeal (DIP) and proximal interphalangeal (PIP)
joints, respectively. Both studies used intra-articular endotoxin to induce lameness and intra-articular analgesia was performed at the end of data collection to confirm that the pain arose from the treated joints. Strengths of these three studies included the use of blinded lameness assessors. In the PIP joint study (Schumacher et al. 2004), PDNBs
were performed separately 1, 2 and 3 cm above the heel cartilage. The effects of local anaesthetic were allowed to dissipate between each block. Median lameness scores were significantly improved for the higher two levels only but 1/6 horses improved substantially to blocking at the 1 cm level. In the study by Easter et al. (2000) the PDNB was performed just proximal to the heel bulb and a significant improvement in median lameness scores resulted. In the fourth study, Nagy and Malton (2015) compared
distribution of radiographic contrast medium (contrast) injected in the fashion of a PDNB using two injection volumes (1.5 or 2.5 ml) at the heel cartilage or 2 cm proximal in live horses. The animals were examined after 0, 10 and 20 min. These authors showed that significantly more proximal spread occurred when the larger volume was used at the distal but not proximal site. Significantly greater contrast diffusion occurred with time. Limitations of this study include that the spread and uptake of contrast may not be the same as commonly used local anaesthetic agents.
Case series Contino et al. (2012) reported a series of 15 horses in which lameness was improved by at least 90% by a PDNB (≤ 2.5 ml LA each site) and that underwent magnetic resonance imaging (MRI) examination of the front feet. The most significant MRI finding in these cases was a metacarpophalangeal (MCP) joint lesion and lameness in these animals subsequently improved with intra-articular analgesia of the MCP joint. Limitations of this study are that the level at which the PDNBs was performed was not specified, nor was the approach for intra-articular analgesia specified (e.g. dorsal or palmar). In a group of horses which were diagnosed with ‘primary
[sic]’ injuries of a collateral ligament of the DIP joint based on ultrasonography or MRI, lameness improved by 50% in 45/101 cases and 36/101 were sound (Dyson and Murray 2007). Limitations of this observational study include that it cannot be known with certainty that the pain was arising from the collateral ligament. Ross (1998) reported a series of 164 horses in which
lameness was localised by PDNB and diagnosis reached by a combination of clinical, scintigraphic and radiographic examination. These diagnoses included proximal phalanx fracture and PIP, third phalanx and DIP pathology. This report lacked detail on how the PDNBs were performed.
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