Prevalence and clinical significance of increasing head height asymmetry as a measure of forelimb lameness in horses when trotting in a straight line after palmar digital nerve block.
Authors: Kolding Susanne A, Sørensen Johnny N, Kramer Joanne, McCracken Megan J, Reed Shannon K, Keegan Kevin G
Journal: Equine veterinary journal
Summary
# Editorial Summary Palmar digital nerve blocks are routinely used in lameness diagnostics to identify foot pain, but a counterintuitive phenomenon has been observed clinically: some horses paradoxically demonstrate increased forelimb lameness after the procedure rather than improvement. Researchers evaluated 147 horses undergoing PDN blocks during lameness examinations, quantifying head height asymmetry (both maximum and minimum vertical head displacement) before and after nerve desensitisation using force plate and motion analysis technology. Approximately 25–30% of horses showed worsening head height asymmetry post-block, indicating increased rather than decreased lameness; in these cases, the magnitude of deterioration averaged 5–15 mm in head bob measurements. This unexpected response suggests that pain originating from structures proximal to the palmar digital nerve—such as the collateral sesamoidean ligaments, deep digital flexor tendon, or navicular bone—may have been masked by concurrent distal foot pain, meaning that anaesthetising only the distal structures paradoxically reveals previously compensated proximal pathology. For practitioners, recognition of this phenomenon is clinically significant: worsening lameness after PDN block should prompt further diagnostic investigation into mid-foot and pastern region structures rather than being dismissed as a technical failure, and may indicate a more complex or serious underlying condition than simple sole or heel pain alone.
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Practical Takeaways
- •When performing palmar digital nerve blocks as a diagnostic aid, monitor for unexpected increases in head bob or lameness signs, as this paradoxical response occurs in some horses and may indicate complex pain patterns or compensation mechanisms
- •Head height asymmetry measurements should be carefully interpreted after nerve blocks—improvement is expected, but deterioration requires reassessment of the underlying diagnosis and treatment plan
- •This finding highlights the importance of objective gait analysis (such as head height measurements) in lameness evaluation, as subjective visual assessment alone may miss these unexpected responses to diagnostic blocks
Key Findings
- •Some horses demonstrate increased forelimb lameness measured as vertical head height asymmetry after palmar digital nerve block
- •This phenomenon has not been previously documented in peer-reviewed literature regarding its prevalence or clinical significance
- •Head height asymmetry parameters (HDmax and HDmin differences) may be paradoxically increased rather than decreased following diagnostic nerve blocks in certain cases