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The Use of Diagnostic Nerve Blocks in Lameness Evaluation
G.K. Carter
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Introduction
Examination of horses for lameness is one of the more important services provided by equine practitioners. Although the diagnosis of lameness is usually straight forward, determining the source of lameness can be difficult and frustrating. The importance of a thorough clinical examination in the evaluation of lame horses is well recognized.1 However, the results of the clinical examination often do not identify the source of pain and can be misleading. The use of diagnostic anesthesia to identify or verify the source of equine lameness is commonly employed by most equine practitioners. Diagnostic anesthesia often provides more useful information than is obtainable by other diagnostic modalities such as diagnostic imaging. There have been numerous examples of a lameness being alleviated by anesthetizing a specific area (for example the foot) without demonstratable radiographic abnormalities.
Diagnostic anesthesia can be either intraarticular, intrasynovial or perineural. Although intraarticular and intrasynovial injections have the advantages of being more specific, they also have the disadvantages of being more invasive and more time consuming in preparation and time for effect. Obviously there are certain situations where intraarticular anesthesia is preferential to perineural anesthesia and in many situations intraarticular anesthesia is used as an important complimentary procedure to perineural anesthesia.
Regional anesthesia is also not without limitations and although the majority of horses have a reliable distribution of nerves to the limbs, variations are occasionally encountered. Other limitations of perineural anesthesia that warrant consideration are the presence of mechanical lamenesses that do not respond to anesthesia, difficulty in accurately determining the extent of desensitization (for example the skin can be desensitized without deep structures being desensitized), and partial response to anesthesia.
Despite the limitations of performing and evaluating perineural anesthesia, it is in this author's opinion the quickest and most definitive method of determining or confirming the source of lameness in horses. The authors routinely utilize peripheral nerve blocks to localize the lameness to a given area of the limb. If more specific information is necessary, intraarticular or intrasynovial blocks are utilized.
Nerve Block Procedure
Many of the more commonly anesthetized nerves lie in close proximity to synovial structures which could be contaminated while performing the procedure, particularly if the animal is difficult to inject. For example, the deep digital flexor tendon sheath is in close proximity to where the palmar digital nerve is blocked. Although the probability of initiating a serious infection from performing perineural anesthesia is low, the possibility exists and the consequences could be severe. Therefore it is advisable to thoroughly cleanse the area prior to injection. The authors prefer to scrub the area with a povidone iodine surgical scrub1 and wipe the area with 70% isopropyl alcohol until all traces of dirt and soap are removed. It is rarely necessary to clip the hair over the injection site, but unusually long or soiled hair may be removed to facilitate cleaning the area and identifying anatomical landmarks.
Restraint required will vary with the animal. Most animals will allow perineural anesthesia with minimal restraint. However, others require significant restraint and superior athletic ability of the clinician to perform the injection. An experienced handler and common sense application of a variety of restraint procedures is helpful in achieving diagnostic anesthesia with minimal excitement or problems.
Sedation to perform the injections should be avoided in most cases, particularly in subtle lamenesses. Sedation can alter the horses gait and response to pain enough to affect interpretation of the response to a nerve block. Horses that are very difficult to inject and have a significant lameness may be tranquilized without seriously complicating interpretation of response to a nerve block. If tranquilization is indicated the authors routinely use 10 mg of Acepromazine2 in a standard size horse.
The most commonly used anesthetic solutions for diagnostic nerve blocks are mepivacaine hydrochloride 2%1 and lidocaine hydrochloride 2%.2 Both of these drugs have a rapid onset of action and are minimally reactive. Since mepivicane is reportedly less reactive and longer lasting than lidocaine,2 we commonly use it for all diagnostic purposes. Although infrequently utilized for diagnostic anesthesia solutions that contain epinephrine have a prolonged duration of action which is useful in some circumstances. These products should be used with care since white hair formation or tissue necrosis may occur at the injection site.2
Practitioners vary in their preference for performing nerve blocks while the animal is weight bearing or non-weight bearing. The authors feel that control of the limb is better achieved by holding the limb in a non weight-bearing position. However, the anatomy of the area is often more readily recognized by inexperienced people when the limb is in a weight-bearing position.
Diagnostic anesthesia is most commonly started at the most distal aspect of the limb and proceeds sequentially proximal in an organized fashion. Following this sequence will allow a more specific localization of pain. Occasionally it is more applicable to start at a more proximal point to expedite the examination, for example, performing anesthesia above the fetlock in the rear limb to rule out lower limb pain.
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1. Gabay C, Kushner I. Acute-phase proteins and other systemic response to inflammation. N Engl J Med 1999;340: 448-454.
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