Chapter 10 Diagnostic Analgesia
Local Anesthetics: Pharmacology and Tissue Interactions
Strategy, Methodology, and Other Considerations
A few basic principles must be followed to ensure success. A thorough working knowledge of regional anatomy is required. Even for seasoned veterans a review of anatomy may be required before less common techniques are performed. A most important principle when performing perineural analgesia is to start distally in the limb and work proximally (Figures 10-1 to 10-4). If possible, sequential blocks from distal to proximal should always be used, but in certain circumstances a different strategy can be successful. Sequential blocking requires a fair amount of time, and in certain horses, selective intraarticular or local blocks can be performed without following this “golden rule.” However, in most situations, blocking a large portion of the distal limb at a proximally located site may preclude accurate determination of the source of pain causing lameness and may require an additional visit to perform additional diagnostic procedures.
Perineural Analgesia in the Forelimb
Palmar Digital Analgesia
Classically, most horses that responded positively to palmar digital analgesia were thought to have navicular syndrome, but this block desensitizes many lameness conditions within and outside the hoof capsule (Table 10-1). This is an important and common misconception. Lameness in horses with proximal interphalangeal joint pain, midsagittal fracture of the proximal phalanx, or other conditions involving the fetlock joints can be abolished using palmar digital analgesia.7,23 Although using small volumes of local anesthetic solution and performing the block just above the cartilages of the foot may help to minimize the area of analgesia, these procedures do not prevent inadvertent diagnosis in some horses. Diffusion of local anesthetic solution is the most likely explanation, and even a small volume can readily spread in a proximal direction, but the normal anatomy of the digit prevents distal placement of local anesthetic solution (Figure 10-6).
High Palmar Block
To provide analgesia to the metacarpal region, the high palmar block (high four-point, subcarpal block) is the most common technique, but a modified block (lateral palmar or Wheat block) can be performed. Inadvertent penetration of the carpometacarpal joint is a potential complication with the high palmar block. A similar complication can occur in the hindlimb but is less frequent (see the following discussion). Inadvertent penetration of the carpometacarpal joint occurred in 17% of specimens, in which a conventional high palmar block was performed, because of extensive distopalmar outpouchings (Figures 10-8 and 10-9). However, when the high palmar block was performed within 2.5 cm of the carpometacarpal joint, inadvertent penetration of this joint occurred in 67% of specimens. The carpometacarpal joint always communicates with the middle carpal joint, and therefore penetration of the carpometacarpal joint during high palmar analgesia would lead the clinician to diagnose a metacarpal problem, when in reality the authentic lameness condition exists in the carpus. Moving the injection site in a distal direction decreases the possibility of entering the carpometacarpal joint but also narrows the scope of the technique and may result in a false-negative response in a horse with proximal suspensory desmitis. Two ways around this likely complication are these: first, the clinician could perform middle carpal analgesia before performing high palmar analgesia; second, the clinician could perform a lateral palmar block in lieu of the conventional high palmar technique. In an experimental study, the carpal joints were unlikely to be entered inadvertently during performance of the lateral palmar block, although in every specimen, local anesthetic solution would have entered the carpal canal.25 Unless the clinician is familiar with the lateral palmar block, the most straightforward approach to reduce the possibility of misdiagnosis in this region is to perform middle carpal analgesia before proceeding to the high palmar block. When local anesthetic solution is placed in the middle carpal joint, not only is the carpometacarpal joint blocked, but also the possibility exists of providing local analgesia to the proximal palmar metacarpal region. With this approach, abolishing pain associated with proximal suspensory attachment avulsion injury (desmitis, fracture), stress remodeling, and longitudinal fracture is possible (see Chapter 37). The palmar metacarpal nerves and suspensory branches from the lateral palmar nerve are closely associated with the distopalmar outpouchings of the carpometacarpal joint, and diffusion of local anesthetic solution from this area could explain in part this clinical finding (Figure 10-10).