Diagnostic analgesia of the equine digit
Authors: Schumacher J. , Schramme M. C. , Schumacher J. , DeGraves F. J. 
Published: January 2013

Analgesia usually occurs within 5 min after administration of local anaesthetic solution into joints or around nerves in the distal portion of the limb. Gait should be assessed within 10 min after diagnostic regional analgesia of the distal portion of the limb because rapid diffusion of anaesthetic solution can result in anaesthesia of other nerve branches, thus confusing results of the examination. A palmar digital nerve block (PDNB) anaesthetises most of the foot, including the distal interphalangeal (DIP) joint (coffin joint), rather than just the palmar half of the foot, as was once commonly believed. To avoid partially anaesthetising the proximal interphalangeal joint (pastern joint), the palmar digital nerves should be anaesthetised near or distal to the proximal margin of the collateral cartilages. Clinicians should be aware that an abaxial sesamoid nerve block (ASNB) may ameliorate or abolish pain within the metacarpo/metatarso‐phalangeal joint (fetlock joint). Mepivacaine administered into theDIPjoint desensitises theDIPjoint and probably the palmar digital nerves to also cause anaesthesia of the navicular bursa, the navicular bone, the toe region of the sole, the digital portion of the deep digital flexor tendon (DDFT)and the distal portions of the collateral ligaments of theDIPjoint. When a large volume of mepivacaineHCl (e.g. 10 ml) is administered, the heel region of the sole may also be desensitised. Only a small percentage of horses with disease of the collateral ligament(s) of theDIPjoint show a significant improvement in lameness after intra‐articular analgesia of theDIPjoint, and no horse is likely to improve after intrabursal analgesia of the navicular bursa. APDNB, however, improves lameness substantially in most horses that are lame because of disease of the collateral ligament(s) of theDIPjoint, and all affected horses are likely to become sound after an abaxial sesamoid nerve block. The degree of improvement in lameness associated with injury to one or both collateral ligaments of theDIPjoint afterPDNBis determined by the extent of the injury and the level at which the palmar digital nerves are anaesthetised. The further proximal the level of the injury within the collateral ligament, the less likely that lameness is ameliorated by analgesia of theDIPjoint or aPDNB. Verschooten's technique appears to be the most accurate technique for centesis of the navicular bursa. Even though analgesia of theDIPjoint results in analgesia of the navicular bursa, analgesia of the navicular bursa does not result in analgesia of theDIPjoint. Pain arising from theDIPjoint can probably be excluded as a cause of lameness when lameness is attenuated by analgesia of the navicular bursa. Analgesia of the digital flexor tendon sheath (DFTS) is likely to desensitise only structures that are contained within or border on the sheath itself (i.e. the superficial and deep digital flexor tendons, the straight and oblique distal sesamoidean ligaments, the annular ligaments of the fetlock and pastern, and the portion of theDDFTthat lies within the foot). Because lameness caused by disease of theDDFTwithin the foot may fail to improve appreciably after analgesia of the palmar digital nerves, theDIPjoint, or the navicular bursa, a portion of theDDFTwithin the foot and distal to theDFTSprobably receives its sensory supply from more proximal deep branches of the medial and lateral palmar digital nerves that enter theDFTS. Performing intrathecal analgesia of theDFTSon horses with lameness that is unchanged after anaesthesia of the palmar digital nerves but resolves after anASNB, may be useful in localising lameness to that portion of theDDFTthat lies within the foot. Resolution of lameness after intrathecal analgesia of theDFTSjustifies suspicion of a lesion within the digital portion of theDDFTor within structures contained within theDFTS. The belief that concurrent or sequential intra‐articular administration of medication substantially increases the risk of joint infection or that inflammation caused by the local anaesthetic solution may dampen the therapeutic response to intra‐articular medication appears to be unfounded.