Page 191 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 157

PERINEURAL AND INTRASYNOVIAL ANESTHESIA

VetBooks.ir Gary M. Baxter

Local anesthesia is commonly used during a lameness SKIN PREPARATION AND RESTRAINT
examination to confirm or identify the site or sites of
pain where obvious pathology may not exist. 9,17,63,66 It The only skin preparation necessary for most sites of
can also be useful to prove the location of a lameness to regional anesthesia is scrubbing/wiping the area with
a client who may be suspicious of another site. Local gauze soaked in alcohol until clean. However, there are
anesthesia may be accomplished by perineural infiltra­ a few sites for perineural anesthesia that are adjacent to
tion (local nerve block), ring block, direct infiltration of synovial structures where during injection the needle
a painful region, or intrasynovial injection (joints, ten­ may inadvertently enter a synovial cavity or the solution
don sheaths, and bursae). The reader is referred to the may diffuse into these structures. These exceptions
accompanying website for individual video clips that include the low four‐point block (digital tendon sheath
demonstrate many of the various techniques. or fetlock joint), the high palmar/plantar blocks (carpo­
Perineural infiltration and ring blocks are used to metacarpal or tarsometatarsal [TMT] joints, carpal or
localize the source of pain to a specific region and, there­ tarsal sheaths), and the lateral palmar block (middle car­
fore, should be performed in a systematic manner start­ pal joint or carpal sheath). A more thorough skin prepa­
ing with the distal extremity and progressing proximally. ration of these sites is recommended to avoid potential
Direct infiltration and intrasynovial anesthesia are used complications. A minimum 5‐minute scrubbing of the
to identify the involvement of a specific structure or skin and hair with an antiseptic and alcohol is recom­
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synovial cavity and do not necessarily have to be per­ mended for intrasynovial injections. The horse may be
formed in a systematic manner. It is not uncommon to clipped if the hair is unusually long or soiled, but this is
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perform several different types of local anesthesia in the not necessary unless utilizing spinal needles with sty­
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same horse (perineural together with intrasynovial), lets. In all cases, the least amount of local anesthetic
depending on the specific clinical signs and problem. possible should be used to reduce tissue irritation and to
With lameness, local anesthesia is important to docu­ prevent local diffusion of anesthetic that may compli­
ment the specific site of the problem so that diagnostic cate the interpretation of the block.
imaging (radiography, ultrasonography, or magnetic When performing local anesthesia, the horse should
resonance imaging) can be used to determine the cause be haltered and restrained by an attendant who is stand­
of the problem. Other uses of local anesthesia include ing on the same side of the horse. For intrasynovial anes­
providing analgesia during and after surgery and pain thesia, a twitch is usually applied so that there is minimal
control for other painful conditions. limb movement during the insertion of the needle and
injection of the anesthetic. When using local anesthesia
in the hindlimb, the practitioner should always be in a
TYPES OF LOCAL ANESTHETICS position so that minimal bodily harm will result if rapid
movement occurs. In most cases the needle is inserted
The local anesthetics most frequently used are 2% rapidly, and the syringe applied just tight enough to pre­
lidocaine hydrochloride (Xylocaine hydrochloride) and vent loss of local anesthetic solution during injection.
2% mepivacaine hydrochloride (Carbocaine). These
solutions are potent and rapidly effective, but can be PERINEURAL ANESTHESIA
locally irritating. Because mepivacaine is longer lasting,
less irritating, and more reliable than lidocaine, it is used Perineural anesthesia is used when the lame limb has
most frequently. 5,33,48,66 Lidocaine is thought to last only been identified, but the exact region affected cannot be
60 minutes with the maximum effect at 15 minutes. It determined by other methods. Even if a suspicious
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was also recently reported to not resolve lameness region is identified, it is often useful to anesthetize the
despite the loss of skin sensation. One study using region to confirm that the lameness is emanating from
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force plate evaluations indicated that mepivacaine was a single location. It is not uncommon to find several
also only fully effective for 15–60 minutes after a pal­ regions on one limb or find that other limbs may be
mar digital (PD) nerve block was performed. The effect contributing to the overall lameness problem. In these
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of the block began to subside between 1 and 2 hours, cases local anesthesia allows one to interpret the per­
but gait characteristics persisted beyond 2 hours. A more centage that each region is contributing to the lame­
recent crossover study indicated that the mean duration ness. In addition, the degree of improvement with
of action was 366 minutes with skin sensation returning perineural blocks may aid in the interpretation of the
before the return of lameness. These variables in skin findings on imaging.
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desensitization and duration of action are very impor­ Perineural anesthesia is typically performed in a step­
tant to remember when performing multiple nerve wise manner starting from the distal limb and progress­
blocks on any given horse over a prolonged period of ing proximally. In general, the accuracy of desensitizing
time. Bupivacaine hydrochloride (Marcaine) may be a nerve is greater in the distal limb (distal to the carpus
used if the goal is to provide a longer duration of anal­ and tarsus) than more proximally where the nerves are
gesia (4–6 hours), such as following surgery. 48 deeper and covered with soft tissue. Also, the more distal