Desensitisation of the distal forelimb following intrathecal anaesthesia of the carpal sheath in horses. (26th May 2020) - Miagkoff, Ludovic
Abstract: Background: The effect of intrathecal anaesthesia of the carpal sheath on distal forelimb sensitivity in horses remains unknown. Objectives: To assess the effect of carpal sheath anaesthesia on skin sensitivity of the distal forelimb and to determine potential locations for desensitisation of palmar nerves. Study design: In vivo experimental and descriptive anatomical studies. Methods: Mepivacaine hydrochloride 2% (0.6 mg/kg) was injected unilaterally in the carpal sheath of 8 horses. Mechanical nociception of the distal forelimb was measured with a dynamometer and compared with the control limb at t0, t15, t30, t60, t90, t120 and t180 minutes . Additionally, the carpal sheath of 10 pairs of cadaveric limbs was injected with latex and potential locations for anaesthetic diffusion to the neighbouring nerves were identified during longitudinal dissection (one limb) and in 3‐cm‐thick transverse cuts (opposite limb). Results: Six of 8 horses (75%) were completely desensitised at the level of both heel bulbs. Anaesthetic injection was not smooth in the 2 horses without desensitisation. Desensitisation started between 30 and 60 minutes in 67% of desensitised heel bulbs (8/12), and 50% (6/12) of them were still completely desensitised at 180 minutes. Cadaveric specimens revealed close proximity between the sheath and the medial palmar nerve as it travels inside the mesotenon of the digital flexor tendons in the carpal region and with both palmar nerves at the proximal metacarpal region. Main limitations: Skin mechanical nociception does not necessarily correlate with deep pain but remains the main clinical tool used by practitioners to assess distal limb anaesthesia. Conclusions: Intrathecal anaesthesia of the carpal sheath led to distal limb skin desensitisation through diffusion to the palmar nerves at 2 possible locations. Carpal sheath anaesthesia should be interpreted within 15 minutes following injection and anaesthetic blocks distal to the carpus should be delayed for more than 3 hours following carpal sheath anaesthesia.