Page 201 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 167

that the superficial peroneal nerve is blocked. The depth Intrasynovial anesthesia is thought to be more spe­
of the superficial peroneal nerve can vary, so the more cific than perineural anesthesia because if the lameness
VetBooks.ir 2.5 cm deep. 75 problem. The three major exceptions to the specificity of
improves, the synovial cavity is considered the site of the
superficial injection should include a region from 0.6 to
An alternative to blocking the superficial and deep
intrasynovial blocks are the distal interphalangeal (DIP)
peroneal nerves individually is to block the common joint, middle carpal joint, and the TMT joint. These
peroneal nerve proximal to its division. This can be exceptions are usually related to regional nerves being
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accomplished by blocking the nerve near the origin of close to synovial outpouchings of the synovial cavities
the long digital extensor tendon. The nerve can be pal­ or passive diffusion of local anesthetic from the synovial
pated at this point, and it is anesthetized using a 1.5‐ cavity. 25,61,63,64,66,67 Intrasynovial anesthesia of the
inch (3.8‐cm), 20‐gauge needle to inject 15–20 mL of DIP joint can anesthetize the nerves innervating
anesthetic. the foot, 6–8,17,18 and intrasynovial anesthesia of the
middle carpal joint or TMT joint can provide analgesia
Direct Infiltration of Anesthetic of the proximal palmar metacarpal/plantar metatarsal
regions, respectively. 14,23,24 In general, the more anesthetic
Direct infiltration of anesthesia can be used anywhere that is used, the greater the likelihood of inadvertent
a sensitive area is identified. However, it is used most analgesia of surrounding structures. 62,66 Less commonly,
often at sites of insertions of ligaments and tendons (e.g. nerves that lie close to the site of intrasynovial injection
the proximal interosseous muscle) or at bony promi­ also may be inadvertently desensitized. For example, the
nences (i.e. splints or swellings). The region is infused lateral plantar nerve may be desensitized when injecting
directly with local anesthetic instead of performing peri­ local anesthetic into the TMT joint, and the lateral pal­
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neural anesthesia. This approach often permits the clini­ mar/plantar nerve above the fetlock may be desensitized
cian to be more definitive regarding whether a painful when injecting anesthesia into the digital tendon sheath
region is contributing to the lameness. The amount of using the proximal approach. 37
local anesthetic administered depends on the location
and dimensions of the area involved. General Technique: Site Preparation, Restraint,
The origin of the suspensory ligament can be desensi­ and Interpretation
tized by direct infiltration (Figure 2.153c and c ), but
1
2
perineural anesthesia using the lateral palmar nerve There are several different anatomic approaches that
block in the forelimb and the DBLPN block in the can be used for intrasynovial injections in most synovial
hindlimb is preferred. Direct infiltration of the proximal cavities. Knowledge of the anatomic landmarks for
suspensory is best performed with the limb held with the intrasynovial injections is imperative to be able to com­
opposite hand. A 1‐inch (2.5‐cm), 20‐ to 22‐gauge nee­ petently perform these injections. Practice on cadaver
dle is inserted between the attachments of the suspensory limbs can be very beneficial to improve the proficiency
ligament and the inferior check ligament in the forelimb in performing the injection techniques because repeated
or between the fourth metatarsus and the SDFT in the attempts to locate the synovial cavity are not well toler­
hindlimb. The needle is directed toward the origin of ated by most horses.
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the suspensory ligament and 4–6 mL of anesthetic is Proper preparation of the site for injection is necessary
injected. Both the lateral and medial sides can be blocked to prevent subsequent infection within the synovial cavity.
in the same manner, but this is usually unnecessary. Iatrogenic infection appears very rare following anesthe­
Inadvertent injection into the carpometacarpal joint in sia of a synovial cavity but is more likely to occur when
the forelimb and the TMT and tarsal sheath in the synovial cavities are treated with medications, especially
hindlimb can occur with these techniques. 2,3,75 corticosteroids. However, this risk is still quite low with a
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reported occurrence of 1/1,279 injections (0.08%).
INTRASYNOVIAL ANESTHESIA Clipping the hair is usually unnecessary because there is
no appreciable difference in bacteria‐forming units in
The use of intrasynovial anesthesia plays an impor­ clipped and haired skin after 5 minutes of preparation
tant role in the diagnosis of equine lameness. 28,34,43,47,79 with povidone iodine scrub followed by an alcohol
In most cases, it is more specific and efficient to anesthe­ rinse. However, if the hair is very long and soiled, it is
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tize the specific synovial structure (joint capsule, tendon best to clip the hair overlying the injection site, and
sheath, or bursa) that is thought to be the cause of the clipping is recommended when using spinal needles with
lameness than performing local perineural anesthesia. stylets. In general, the authors tend to clip a very small
1
This is especially true in horses that tend to have joint area (1–2 cm square) over the site unless the owners/train­
problems (racehorses) or if the clinical findings suggest ers request otherwise. Clipping the hair also has the
involvement of a synovial structure. In addition, intra­ advantage of marking the site of injection so that a helper
synovial anesthesia is commonly performed above the knows exactly where to prep the skin. A 5‐ minute sterile
carpus and tarsus where perineural anesthesia becomes skin preparation is recommended using either povidone
more difficult. If intrasynovial anesthesia needs to be iodine or chlorhexidine and alcohol.
performed after perineural anesthesia, it is best to wait An experienced helper makes performing intrasyno­
at least 3–4 hours for sensation to return. 5,33,75 There is vial injections much easier and safer. Proper restraint
also evidence to suggest that the return of skin sensation of the horse is also required to prevent injury to per­
does not coincide with the actual return of lameness in sonnel and damage to the articular cartilage and to
some horses. In addition, gait changes may persist reduce the risk of needle breakage. Be sure to always
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beyond 3–4 hours, and it is probably safest to perform keep in mind that the handler and any person observ­
intrasynovial anesthesia on a different day. 5,33 ing becomes your assumed responsibility. Twitch
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