LAMENESS EXAMINATION AND THERAPY


of the distal sesamoidean ligaments by intra-articu- lar analgesia of the metacarpophalangeal or meta- tarsophalangeal joint.51 Recent imaging studies using magnetic resonance imaging have identified these ligamentous injuries as the cause of lameness in horses with significant improvement or elimina- tion of lameness after intra-articular analgesia.51–54 Leakage of local anesthetic solution at the injec-


tion site or its diffusion from the digital flexor ten- don sheath may result in desensitization of the skin at the heel region of the foot by anesthetizing the palmar digital nerves of some horses receiving in- trasynovial analgesia of the digital flexor tendon sheath and may, perhaps, result in desensitization of the distal portion of the limb.55 The effect of leakage of local anesthetic solution from the digital flexor tendon sheath on the palmar digital nerves varies, however, according to the site of synoviocen- tesis. One study found that intrasynovial analge- sia of the digital flexor tendon sheath using the palmar axial sesamoidean approach56 did not ame- liorate lameness caused by pain from the sole, navic- ular bursa, or distal interphalangeal joint, indicating that the palmar digital nerves were not anesthetized by direct analgesia of the digital flexor tendon sheath performed at this site.57 Nevertheless, to avoid misinterpreting the results, skin sensitivity at the heel bulbs should always be tested after intrasyno- vial analgesia of the digital flexor tendon sheath to determine if leakage of local anesthetic solution at the injection site has caused inadvertent anesthesia of one or both palmar digital nerves.55 Intrasynovial injection of a large volume of anes-


thetic solution or repeated punctures of a synovial membrane should logically result in leakage of local anesthetic solution.58 Even when a small volume of local anesthetic solution is injected into the scapu- lohumeral joint after numerous attempts at cente- sis, leakage of that solution through the multiple punctures of that joint can temporarily paralyze the surpraspinatus and infraspinatus muscles by anes- thetizing the suprascapular nerve or part of the brachial plexus.58 When a large volume of local anesthetic solution


(i.e., 10 mL) is injected into the tarsometatarsal joint, the joint capsule may rupture, causing leakage of local anesthetic solution, which may desensitize the insertions of the tibialis cranialis and peroneus tertius muscles and the tarsal sheath and anesthe- tize the medial and lateral dorsal metatarsal nerves or plantar metatarsal nerves.32,59 Presumably, overdistention of other synovial structures with lo- cal anesthetic solution could also cause leakage of solution and inadvertent analgesia of peripheral nerves in close vicinity to the site of injection or synovial rupture.


10. Time of Assessment After an Analgesic Technique is Inappropriate


The results of diagnostic analgesia can also be mis- interpreted if the horse’s gait is assessed before the


onset of relief of pain. Relief of pain and resolution of lameness after local anesthetic solution is admin- istered near a nerve in the distal portion of the limb usually occurs within 5 min, but anesthesia of larger nerves in the proximal portion of the limb may take 20 to 40 min.3,13,25 In a study concerning the extent of proximal migration after perineural injection of positive radiocontrast solution along the palmar nerves at the base of the proximal sesamoid bones, 11% of injections appeared to be outside the neuro- vascular bundle.18 The amount of time required for local anesthetic solution to anesthetize a nerve when the anesthetic solution is deposited outside the perineural fascia is not known, but presumably, more time would be required for anesthesia of that nerve. Some clinicians prefer to confine a horse after a


nerve block because they fear that walking the horse may increase diffusion of local anesthetic solution causing desensitization of more proximal structures, thereby complicating interpretation of the nerve block. Walking, however, does not seem to influ- ence the extent of proximal or distal migration of local anesthetic solution after perineural injection.18 The results of intrasynovial analgesia can be mis- interpreted if the expectation for timing of onset of analgesia is incorrect. Lameness has been ob- served to resolve within 5 min after injecting mepi- vacaine into a painful intercarpal joint60 and, therefore, administering local anesthetic solution into a synovial structure of the distal portion of the limb also probably results in synovial analgesia within 5 min. Onset of analgesia is delayed when local anesthetic solution is administered into a sy- novial structure larger than those of the distal por- tion of the limb. For instance, analgesia of the coxofemoral joint may not occur for 30 min after local anesthetic solution is administered into this joint (authors’ observation).


11. The Clinician Does not Understand What Structures are Desensitized by the Diagnostic Block


The misinterpretation of regional analgesia can arise if clinicians do not realize that a diagnostic block may desensitize more than the target region or that the diagnostic block may not completely desen- sitize the target region. For example, the results of a low palmar nerve block, as part of a low 4-point nerve block, can be misinterpreted if one palmar nerve is anesthetized proximal to the ramus commu- nicans and the other is anesthetized distal to the ramus communicans, because sensory fibers travel in both directions in the ramus communicans to connect the medial and lateral palmar nerves.61 When administering a low palmar nerve block, both palmar nerves should be anesthetized distal or prox- imal to the ramus communicans to avoid leaving nondesensitized sensory nerve fibers traveling through this neural connection. Alternatively, lo- cal anesthetic solution could also be deposited adja- cent to the ramus communicans when anesthetizing the palmar nerves. Because the ramus communi-


AAEP PROCEEDINGS  Vol. 60  2014 89


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