LAMENESS EXAMINATION AND THERAPY


based on the findings of the clinical examination. The use of an automated lameness detection device, such as one that uses body-mounted wireless iner- tial sensors, removes subjective bias from assess- ment of the horse’s gait after diagnostic blocks have been performed.7


5. Gait Alteration by Sedation or Tranquilization


Applying a lip twitch or lip chain to the horse usu- ally provides sufficient restraint to allow diagnostic analgesia to be safely administered, but some horses must be sedated or tranquilized to administer diag- nostic analgesia, either because the owner resents having a twitch or lip chain applied to his or her horse or because diagnostic analgesia cannot be safely administered even after a twitch has been applied. The degree to which sedation may inter- fere with assessment of gait may depend on the severity of lameness and the skill of the clinician performing the examination. The effect of sedation or tranquilization on gait should be observed before the effect of diagnostic analgesia on gait is evalu- ated. A change in gait of a sedated horse can be erroneously attributed to the effects of diagnostic analgesia rather than to the effects of sedation if the gait was not evaluated after the horse was sedated but before diagnostic analgesia was administered. Xylazine (0.2 mg/kg, IV), detomidine (10 g/kg,


IV), or acepromazine (0.01–0.02 mg/kg) adminis- tered in the course of a lameness examination usu- ally does not substantially interfere with assessment of gait10–13 and lameness may even be- come more apparent after a horse is tranquilized.14 A tranquilizer, such as acepromazine, has no anal- gesic effect, whereas a sedative, such as xylazine or detomidine, provides some analgesia.15 Although a sedative may provide more restraint for administer- ing diagnostic analgesia than does a tranquilizer, we believe that a tranquilizer is less likely to adversely affect a lameness examination. If the clinician is concerned that the effects of sedation may confound the lameness evaluation, the horse can be examined after the effects of sedation dissipate, provided that analgesia imparted by the local anesthetic solution persists longer than the effects of the sedative. Some clinicians have advo- cated using bupivacaine hydrochloride when admin- istering diagnostic analgesia to intractable horses after sedating the horse with xylazine so that re- gional analgesia remains in effect when sedation has completely dissipated.16 Bupivacaine, how- ever, appears to be chondrotoxic and its use in dis- eased joints may be harmful.17 Waiting until the effects of the sedative dissipate may confuse the interpretation of the results of regional analgesia because the local anesthetic solution may diffuse with time from the site of injection, desensitizing unintended structures.3,18–22 After diagnostic analgesia is administered, the


effects of an 2-agonist, such as xylazine or detomi- dine, can be diminished by administering an adren-


ergic blocking agent such as yohimbine or tolazoline. Yohimbine, however, is not approved for use in the horse and administration of either drug to reverse


the sedative effects of an 2 agonist has been asso- ciated with adverse reactions in horses, including death.23


6. Inaccurate Testing of Regional Desensitization After Diagnostic Analgesia


The efficacy of a regional nerve block can be tested by observing the horse’s response to stimulation of skin in the area meant to be desensitized by the block. Testing efficacy by testing for lack of skin sensation is not infallible, however, because a lack of reaction to stimulation verifies only that the skin has been desensitized. When lameness is still pres- ent, distinguishing between effective and ineffective desensitization of the section of the limb supplied by the innervation targeted by the block is not always easy.


Pain sensation, skin sensation, and deep sensa-


tion are supplied by different afferent fibers, each of which may require a different amount of time to become fully anesthetized by the local anesthetic solution, depending on the degree of myelination of individual fibers.24,25 A local anesthetic injection can, therefore, remove pain without totally desensi- tizing the skin. For the same reason, there is fre- quently a time-lapse between removal of superficial sensation (e.g., skin sensation at the coronary band) and blockade of sensitivity to deep pressure (e.g., sensitivity to application of hoof testers or sensitiv- ity to a flexion test) by the local anesthetic solu- tion.25 The clinician cannot be certain, therefore, that after administering regional analgesia, deeper structures have been fully desensitized, even though skin sensation has been abolished. Local anesthetic solution deposited around a


nerve diffuses from the periphery of the nerve to- wards the center of the nerve.25 Sensation to the most distal portion of the limb is supplied by the core fibers of the main nerve trunk and, therefore, desen- sitization proceeds from the site of deposition of local anesthetic solution distally.25 After administering a tibial and peroneal (fibular) nerve block, for exam- ple, the time required for analgesia of the distal hock joints should be less than the time required for loss of skin sensation at the bulbs of the heel. After perineural injection of a local anesthetic so-


lution, sensations disappear in the following order: pain, cold, warmth, light touch, joint proprioception, and deep pressure.25 Re-examining a horse too soon after a nerve block, before all sensations have been abolished, may lead to erroneous interpreta- tion of results of the nerve block. A clinician may, after accurately administering a nerve block that would effectively eliminate pain causing lameness, re-administer the block without first trotting the horse if skin sensation is still present. By not trot- ting the horse, the clinician fails to realize that the first administered block was effective in ameliorat-


AAEP PROCEEDINGS  Vol. 60  2014 85


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