14 Mar 2016
Rob Pilsworth reviews key points from a peer-reviewed journal paper by him and Sue Dyson that discussed why nerve block results can sometimes be misleading.
Rob Pilsworth
Job Title
Figure 3. The diagnostic nerve block continues to form the backbone of lameness investigation in the horse. Image: © Fotolia/kateleigh.
Diagnostic nerve blocks are the mainstay of lameness investigation in practice, but temptation should be resisted to dive into using them before a thorough physical and clinical examination of a horse has been made.
Nerve blocks should be used to support and confirm a clinical judgement, which can often be formulated by a combination of assessment of a horse at walk and trot, during ridden exercise and by detailed physical examination of the limbs.
Many diagnostic nerve blocks used, however, are much less specific than previously thought and, as experience of the use of each has grown and clinicians have “pooled” information, the reasons why this may be the case have become increasingly clear.
The aim of a perineural block is to deposit local anaesthetic in the immediate vicinity of the nerve.
The normal anatomical arrangement in the limb is for the nerve to lie in a neurovascular bundle, containing artery, vein and nerve, giving a potential ease of passage along the fascial planes containing the neurovascular bundle for injected fluid.
Local anaesthetic can progress both proximally and distally for some considerable distance following injection and desensitise structures that were not the intended target (Figure 1).
This has been well documented – for instance, by Dyson and Romero (1993) with regard to the hock, the carpus (Nagy et al, 2012) and the structures of the fetlock joint with regard to nerve blocks carried out at the level of the base of the sesamoid (abaxial sesamoid block; Daniel et al 2013). All of these studies, discussed in more detail in the author and Sue Dyson’s review paper, highlight the dangers of proximal diffusion, giving an erroneous outcome to the intended block.
In 1993, Dyson and Kidd published a series of 59 horses in which lameness had been abolished or significantly improved by palmar digital nerve block. These horses were later subjected to intrathecal analgesia of the navicular bursa and intra-
articular analgesia of the distal interphalangeal joint – all on separate occasions.
The majority of horses (41 out of 59) responded equally well to analgesia in all three sites, highlighting the potential for cross-diffusion of local anaesthetic solution placed in adjacent synovial structures (Dyson and Kidd, 1993).
Similarly, Schumacher (2001a) showed even solar pain can be eliminated by injection of local anaesthetic solution into the distal interphalangeal (DIP) joint. This was, to some extent, volume-dependent – 6ml of local anaesthetic produced desensitisation of the toe only, whereas injection of 10ml into the DIP joint abolished sensation to both the toe area and the palmar heels.
Similarly, intra-articular analgesia of the metacarpophalangeal joint will, in many cases, result in pain relief from lesions in the distal suspensory ligament branches and the proximal aspect of the straight and oblique sesamoidean ligaments (Daniel et al, 2013; Marneris and Dyson, 2014).
Another source of both false-negative and false-positive results of nerve blocks is the inadvertent penetration of a joint, tendon sheath or lymphatic vessel.
This can result in the local anaesthetic solution never reaching the intended nerve (false-negative) or blockade of the actual structure that is penetrated, such as the carpus, producing a false-positive result, where interpretation of a reduction in lameness would be the local anaesthetic had been deposited around the target nerve, not into the joint.
Inadvertent injection of local anaesthetic solution into the lymphatics has been recorded by Nagy et al (2009) and the same work has shown the possibility of inadvertent penetration of the digital flexor tendon sheath when performing a low four-point block. Direct penetration of the carpal joints has also been documented following sub-carpal injection by Ford et al (1989) and, once again, Nagy et al (2012).
When one looks at the anatomy of the outpouchings of the middle carpal joint when injected with contrast media, it becomes apparent why this sometimes happens (Figure 2).
As well as the implication for false-negative and false-positive results, a risk of synovial sepsis is also apparent when inadvertent entry is made to the joint through an area of skin that has been insufficiently cleansed. This is a small, but real, risk and the clinician has to weigh up the risk-benefit equation of carrying out a full four-and-a-half
minute surgical scrub for every perineural nerve block on the basis inadvertent joint or sheath entry may possibly be made in certain sites.
Most clinicians take the pragmatic view of trying to operate in a clean, but not sterile, manner and in the author and Dr Dyson’s paper, they commented in their combined busy careers totalling more than 60 years of practice, they had, between them, seen only two cases of synovial sepsis following routine perineural analgesia.
In complex joints, such as the carpus and tarsus, local anaesthetic solution deposited in one joint can diffuse into and affect lameness originating from the other joints.
Direct measurement of mepivacaine concentration in the middle carpal and antebrachiocarpal joint by Gough et al (2002a) indicated the potential for “cross-blocking” both joints by deposition of local anaesthetic solution into only one.
In the antebrachiocarpal joint, there is the added complication it has been established some lesions involving the distal radius do not show an improvement in lameness following desensitisation of the antebrachiocarpal joint (Shepherd and Pilsworth, 1993). This is presumably because the pain involved in some of these lesions is transmitted by nerves proximal to this joint, which are unaffected by the intra-articular block.
Dr Dyson had also had experience of horses in which the pain causing lameness originated within the carpal sheath, but this pain and consequent lameness was significantly reduced by intra-articular analgesia of the carpal joints themselves (Pilsworth and Dyson, 2015).
In the tarsus, Gough et al (2002b) showed a similar situation, in that there was cross-diffusion in 100% of the examined horses between the tarsometatarsal joint and the centrodistal joint and approaching 90% of horses showed diffusion of local anaesthetic solution into the tarsocrural joint following deposition of the same into the centrodistal and tarsometatarsal joints.
These figures were much higher than had been predicted from in vitro studies using injected dye or latex, and reflected the fact the low molecular weight of local anaesthetic solution allowed far greater potential diffusion between adjacent synovial structures than was previously thought.
In several clinical situations, it is apparent horses with marked pathology of the subchondral bone show incomplete resolution of lameness when local anaesthetic solution is deposited in the adjacent joints.
This appears, in some cases, to be because the pain from this bone is conveyed by nerves that exit the bone in a more proximal position. In the case of the fetlock joint this pain is carried by nerves that exit through the nutrient foramen in the middle third of the metacarpal bone (Todhunter, 1996).
In cases of advanced subchondral bone change affecting the middle phalanx, as part of the degenerative joint syndrome in the DIP joint, a similar situation occurred where intra-articular analgesia of the DIP joint does not abolish lameness associated with the pain from diseased subchondral bone, which is carried by nerves, which, again, exits through the more proximal nutrient foramen (Dyson, 1986; Niv et al, 2003).
Advanced subchondral bone changes in the small bones of the hock will similarly, on occasions, lead to lack of response to intra-articular analgesia of the small hock joints, despite these being associated with the cause of lameness (Dyson, 2012).
The diagnostic nerve block continues to form the backbone of lameness investigation in the horse (Figure 3) and it was certainly not our intention to devalue these techniques by summarising the pitfalls in interpretation with which they can be associated. However, the clinician should be aware of these phenomena to take a balanced and logical approach to the use of these diagnostic techniques in practice.
Constraints of space here do not allow full discussion of the contents of the review paper, to which the reader is directed if further information is required.
