FRANK J. MILNE STATE-OF-THE-ART LECTURE


will be required. Although careful assessment of gait may be suggestive of the primary source of pain causing lameness, in my experience, many of the findings are non-specific and the same injury may be manifest differently among horses. Moreover, more than one source of pain may coexist in one limb, therefore accurate diagnosis can only be achieved with the use of diagnostic analgesia. The gait characteristics may change after abolition of one source of pain if another source coexists. For example, a horse had a low-grade bilateral forelimb lameness evident only on the lunge on a firm surface or ridden, as mild (grade 2/8) left forelimb lameness on the left rein and subtle (grade 1/8) right forelimb lameness on the right rein. After palmar digital nerve blocks of the left forelimb, right forelimb lame- ness was accentuated (grade 3) on the right rein, but only when ridden. Palmar digital nerve blocks of the right forelimb abolished this lameness, but then a more severe right forelimb lameness (grade 4) became apparent on the left rein when ridden. This was ultimately abolished by palmar metacar- pal (subcarpal) nerve blocks. Foot pain and proxi- mal suspensory desmitis coexisted. There are differences of opinion, reflecting per-


sonal experiences about whether a horse is lame enough to block, for example, is it likely that the observer would be able to detect an improvement if lameness was abolished? This can certainly be tough if a horse with subtle lameness is only as- sessed in-hand and on the lunge. However, it must always be borne in mind that subtle lameness may reflect bilateral lameness and if pain is abolished in one limb a much more obvious lameness may be- come apparent in the contralateral limb. If a horse is also assessed ridden and other aspects of perfor- mance are considered together with lameness, I believe many horses with subtle lameness can be nerve-blocked with meaningful results. This is likely to be much more rewarding than resorting to survey radiography or nuclear scintigraphic exami- nation, which often yield false-negative or false-pos- itive results. Nerve blocks must be performed in a systematic


way; there are few shortcuts, and this time-consum- ing procedure cannot be rushed without risks of misinterpretation of the results. However, on the basis of the findings of an initial clinical assessment, a logical decision can be made about where to start. For example, if there is a markedly positive response to distal limb flexion, then intra-articular analgesia of the fetlock may be performed, bearing in mind the potential to influence closely related anatomical structures, such as the suspensory ligament branches. In a hindlimb, in the absence of clinical signs related to the fetlock and more distal aspects of the limb, it would be reasonable to start by perineural analgesia of the plantar (at the junction of the proximal three-quarters and distal one-quar- ter of the metatarsus) and plantar metatarsal (distal to the “button” of the second and fourth metatarsal


bones) nerves (a “low four-point-block”). If a horse showed lack of hindlimb propulsion but no detect- able lameness, bilateral perineural analgesia of the deep branch of the lateral plantar nerve may result in substantial improvement in the horse’s perfor- mance when ridden, whereas a unilateral block may confusingly result in little change. However, the distal aspect of the limb should first be excluded as a potential source of pain by use of a “low four- point-block.” Nerve blocks can paradoxically result in an in-


crease in lameness severity. If the foot is desensi- tized and it is not the source of pain causing lameness, lameness may deteriorate. This is a non- specific finding but is often seen in association with suspensory ligament pain. I believe that the foot serves a proprioceptive function and the horse can modify its gait to reduce pain. Desensitization of the foot reduces its proprioceptive function, and the horse is less able to adapt its gait to reduce loading and therefore increased strain is placed on the sus- pensory apparatus, resulting in accentuation of lameness. Thus, interpretation of the response to local anal-


gesia is not always straightforward. How much improvement is expected after apparent desensiti- zation of a single source of pain? This depends on both the severity of the pain, the cause(s) of pain, and how that pain is mediated and whether there may be a mechanical component to the lameness. For example, severe foot pain associated with a sub- solar abscess, a fracture of the distal phalanx or navicular bone, laminitis, or adhesions of the DDFT may be unaffected or only partially improved by palmar nerve blocks performed at the base of the proximal sesamoid bones. Pain associated with a neuroma may be minimally influenced by perineural analgesia. However, it must also be borne in mind that more than one source of pain may coexist, and more proximal nerve blocks may be required. This is where the art and science of lameness diagnosis must be combined.


3. Some Further Observations Concerning Local Analgesia


Diagnostic analgesia is frequently required to local- ize the site(s) of pain causing lameness, but it is crucial to be aware of the limitations of the tech- niques used and how confusion may arise, a problem I first addressed in 1986.9 It is well-recognized that perineural analgesia may abolish pain distal to the sites of injection, but at some sites proximal diffusion of local anesthetic solution may result in abolition of pain at sites considerably proximal to the site of injection. Pain associated with the prox- imal interphalangeal (PIP) joint was induced by in- jection of bacterial lipopolysaccharide, and baseline lameness was recorded.10 Local anesthetic solu- tion (1.5 mL per site) was injected around the pal- mar digital nerves at sites 1, 2, and 3 cm proximal to the cartilages of the foot (ungular cartilages). The


AAEP PROCEEDINGS  Vol. 59  2013 99


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