Until recently, there was little scientific basis for interpreting response to various techniques of diagnostic analgesia of the digit. As a result, some diseases, such as navicular disease, were often over-diagnosed, and others over-looked. Recently, some clinicians began to question long-held beliefs concerning analgesic techniques of the foot, especially that analgesia of the distal interphalangeal (DIP) joint localized pain to that structure or that a palmar digital nerve block (PDNB) localized pain to the palmar half of the foot. Within recent years, many studies have examined the response to diagnostic analgesia of horses lame because of pain in a particular structure or region of the foot. Clinical observations, anatomical studies, magnetic resonance images of feet of horses with lameness caused by foot pain, and results of clinical trials that created pain in certain structures of the foot have helped to clarify interpretation of the results of regional, intra-articular, and intra-bursal analgesia of the foot of horses. These studies have also highlighted the limitations of diagnostic analgesia of the horse's digit. In this paper, we present a summary of current knowledge of the use of analgesia to localize sites of pain within the digit of lame horses.

Assessment of lameness before application of local anesthetic techniques

Before performing regional anesthesia, the horse should be consistently and sufficiently lame so that any improvement in gait can be detected. Lunging or riding the horse may exacerbate a subtle lameness. The lameness of some horses improves or resolves during exercise, and so, for these horses, a false positive response to regional anesthesia may result if the horse has not been sufficiently exercised before it is examined for lameness. If a horse is subtly lame, independent observation and grading of its gait before and after regional anesthesia by two or more clinicians skilled at lameness examination may increase the accuracy of interpretation. Video-recordings of gait before and after diagnostic analgesia enable blinded assessment of response to local anesthetic techniques and remove bias from the subjective interpretation of results, especially when lameness is mild. Similarly, the use of computerized methods to detect and quantify gait asymmetry before and after the use of diagnostic analgesia can also result in more objective interpretation of the effect of regional analgesia.

Cleaning the site of injection with 70% isopropyl alcohol usually is sufficient for perineural administration of local anesthetic solution. When the distal portion of the limb is particularly dirty, however, the injection site should be scrubbed with antiseptic soap. As long as local anesthetic solution is deposited subcutaneously, complications from poor aseptic technique are unlikely. The clinician should be aware, however, of the potential for disastrous results when non-sterile technique is used, and a needle is misdirected into an adjacent synovial structure, such as the digital flexor tendon sheath (DFTS) or the proximal interphalangeal (PIP) joint. The injection site of a synovial structure should be aseptically prepared, but one study showed that clipping of hair over the injection site is not necessary for aseptic arthrocentesis, as long the site is scrubbed for at least seven minutes. Drawing the drug for intra-synovial deposition from an unused bottle may decrease the likelihood of introducing bacterial contaminates into the synovial structure.

Some clinicians consider the addition of an antibiotic to the local anesthetic solution to be contra-indicated because of the risk of intrasynovial precipitation of the antibiotic. Amikacin is a good choice of antibiotic, however, for clinicians that prefer to administer an antibiotic along with local anesthetic solution because 500 mg of amikacin administered into a normal joint provides a concentration above the reported MIC for most equine pathogenic bacteria for 72 hours with minimal inflammatory effects.