Oh, good afternoon. So in this webinar we're going to look at the topic of diagnostic anaesthesia for the assessment of lameness in horses and ask what what can it really, really tell us? Now we know that nerve blocks are, unfortunately, in some ways, the gold standard for localising lameness, but just like any other diagnostic test, they have a sensitivity and specificity, which will very much vary on a block to block basis and sometimes vary between horses and practitioners.
So ideally, we would like to have obviously a true positive or a true negative result, and it'd be as black and white as that. But unfortunately, we need to be aware of the potential for false negatives and false positive results. False negative results may occur as a result of incorrect placement of local anaesthetic.
We might use insufficient local anaesthetic to fully desensitise the area that we wish to. We might reassess the nerve block too early. Some horses may have abnormal anatomy that prevents us from fully desensitising them, and as everyone who's tried to block out a foot abscess will know, some sources of deep pain do not respond to perineural anaesthesia as we would expect them to.
False positive results may be due to fluctuation in lameness. Sometimes we might use excessive local anaesthetics, we're actually desensitising areas that we don't wish to. And sometimes we may leave insufficient time between blocks, and what I mean by that is that if we place some local anaesthetic, look at the horse after maybe 15 minutes, call that block a negative, and then do another one, we might find that when we're reassessing the second block, actually it's the first one that's having a delayed effect, leading to a false positive response of the of the second block.
The basic principles of blocking are to know the anatomy, obviously it's sometimes a while since you know, we've done some cadaver work, but, but really knowing what and where we're trying to block is, is essential. Get the basics right, so use the correct needle, volume and also type of local anaesthetics we'll touch on in a minute, you know, different local anaesthetics have different efficacies. We want to reassess these blocks at the right time, and that varies depending on the structure that we're blocking, and the volume of anaesthetic that we're using.
And then we also wish to employ a methodical blocking strategy as well. And that a lot of that will depend on personal preference, but also a degree of clinical experience as well developed over time. We're gonna run through the commonly performed nerve blocks and joint blocks now and, and discuss the individual sort of specifics to them.
If we start with the Palmer digital nerve block, which is obviously the most commonly performed block, certainly in the forelimb. There was a body of work done back in nearly 20 years ago now that, really nicely proved that palal nerve blocks are not as specific as first thought, even when we perform them very distally near the unguar cartilage. So as the right hand image would show, we'd like to place our local anaesthetic right down in the distal aspect of the past and just at the level of the ungular cartridges.
And the original dogma was that that was sensitive to the sort of palm a third of the foot as depicted by the, the, the sort of elliptical line. But unfortunately, we know that that's not, not true. Schumacher and colleagues, by using induced lameness through solar pain, induced distalline defalangeal joint lameness and induced navicular bursa lameness, really proved that palm digital analgesia was, was really not specific for any particular structure within the foot at all.
And then more recently, some contrast studies done by Russell Malton and Anna Maria Naji proved a similar thing. When we're talking about assessing nerve blocks, most commonly we would test skin sensation. If we touched briefly, as we are on the Palmer digital nerve block, there's a nice paper a couple of years ago now by Herderman and colleagues, and they looked at an experimental lameness model, and they did a Palmer digital nerve block using both mepivacaine and lidocaine, with, objective gait analysis.
And they actually found that mepivacaine was better than lidocaine for truly resolving the lameness, so it would suggest that we should be using mepivacaine for the majority of our nerve blocks. Loss of skin sensation occurred in all 8 horses that they did a pal nerve blocking, having used the pivicane, but they actually found that using the objective gait analysis, lameness took an awful lot longer to resolve. So, you know, that may explain why, you know, some of these horses with foot lameness only partially seem to.
Respond to a digital nerve block when we look at them 10 minutes later, they will be that actually, if we give them more time, and some of these horses took up to an hour to fully abolish lameness, actually, it may well be that we're reassessing them too early, and if they were given more time, then actually that we would have a a great response to a par digital nerve block. They also found that Mivcaine had a longer duration of action and a longer duration of response in terms of skin sensation as well. So, Interesting loss of skin sensation wasn't fully predictive of complete resolution of lameness, and also at the other end of the scale, skin sensation came back before lameness was completely returned as well.
So it does suggest if we do a palm digital nerve block early on in the day, and then we're looking to do, say, disal joint analgesia, at the end of the day, skin sensation predictor is skin sensation is not a great predictor of whether that lameness has truly recurred or not. The take home message from that, as I think most people would in the UK is that mepivacaine was certainly superior in that study for perineural anaesthesia. So we know that palmo digital nerve block should usually improve, and I, I, I, I don't say resolve, I mean, we don't expect a block to be 100% effective a lot of the time.
We certainly should see a significant improvement for all of these lesions, navicular pain. In the top right lesion, solar pain, such as, I guess this keratoma here in the, the bottom right, but also collateral ligament desmopathy and distal interphalangeal joint pain. So, so really, you know, any lesion within, within the foot would normally be improved following a palmmon disal nerve block.
But having said that, we would also commonly see that some horses will repeatedly block out to a palm digital nerve block, despite their primary lesion being within the Paston region. So on the left here, we've got quite marked proximal interphalangeal joint osteoarthritis that blocked out to a, a palmer digital nerve block. And also on the right hand side here, we can see a show jump with quite significant bone bruising in the proximal aspect of the proximal phalanx, and this was blocked out repeatedly to Palmer digital nerve block as well.
So, We know more and more that it's not specific to to the foot. And indeed, at Beaver Congress in 2015, Russell Morton and Anna Maria Nai presented some nice work looking at contrast studies after they performed a palm digital nerve block and a palm digital ring block, and they found quite significant proximal diffusion of local anaesthetic into the sort of mid or proximal Pastern region, which may explain why some of these pastor lesions are alleviated by that. Recent paper that came out by Jiling Eal demonstrated the use of a compression bandage after performing a part of a digital nerve block.
They used a vet wrap compression bandage around the the Patton region. You can see here the difference in contrast spread on the left. This is with the use of a compression bandage on the right without you can see it.
Greater proximal spread. So that's quite an interesting concept that with the use of compression bandaging, we might be able to limit the proximal spread of our local anaesthetic. Obviously, this was a, a radiographic study.
There was no, sort of clinical relevance to that. And certainly, I haven't used this, clinically yet to see if it, improves the specificity of a Palmer digital nerve. When we move up a block to the abaxial sesamoid nerve block, certainly in, in our hospital, we regularly get sent in horses for MRI scanning, for example, of, of the foot, having just had an abbaxial cessamoid nerve block performed.
And I think, you know, it's important to, to stress that, that, that, you know, an abbaxial sesamoid nerve block is not specific the foot. It should take out foot pain. But it will also take out past and pain in some, in some cases, fetlock pain as well.
So it's really not a specific, specific nerve block for foot pain. Obviously, a lot of horses that blocked an aversial sesamoid will have foot pain, and that's their primary lesion, but at the same time, there'll be plenty of them that, that, that don't. Again, Anna Maria Naji, the primary author on a nice paper back in 2009.
And they looked at contrast studies after performing in a axial sesamoid nerve block, and they performed them quite distally, you know, at the level of the, you know, distal aspect of the proximal sesamoid bones, and they found that there was quite significant, proximal diffusion of local anaesthetics. You see up to the level of the apex of the proximal sesamoid bones, occurred in, in quite a large number of horses, approximately 50% of horses, and it also occurred within 10 minutes as well. It happened quite rapidly.
And they found that er there was also no difference between standing these horses in a stable or walking them about. So, so restricting movement didn't, didn't restrict the spread of contrast material either. So, you know, these cases were all also positive to an abaxial sesamoid nerve block.
We can see see bowing of the Palmer aspect of the fetlock caused by annual ligament desmopathy. This horse here had an osteocyst-like lesion in its lateral proximal cesmoid bone, and obviously this horse here had quite significant proximal interphalangeal joint pain. So, We need to be aware that abaxial sesamoids do lack specificity certainly for the foot.
They they really only rule they miss out to the distal aspect of the, of the limb. Moving to one of the important synovial structures in the distal aspect of the limb, the digital flex tendon sheath. There are 4 approaches which are commonly applied.
On this slide, we can see the lateral proximal approach, which you can see in the right hand image here, which is performed in the proximal recess of the sheath immediately Palmer to the suspensory branch, which you can see here. There's also the basicsy sesamoid approach which is performed in a little triangle, see the neurovascular bundle here, and then we've got the level of the base with the proximal sesamoid bone and it's performed just in the little triangle there. We've also got two other approaches, the axial sesamoidian approach, which is depicted in these two cadaver pictures here.
So we try and slide the needle down the axial surface of the proximal sesamoid bone between the, the flex tendons within the sheath. And finally, there's the palm and midline approach as well, which with the leg up inflexion, we see this distal reflection of the sheath, like really quite distal within the within the past and region. There's often a little sort of bulging in the skin, which depicts it.
And my preference is to come in the midline like that at an angle of about 45 degrees to to the skin. Quite an elegant series of studies by Mirra Giorda and colleagues back in 2014, so five years ago now, looked at, limb desensitisation following, analgesia of the digital flex tendon sheath using these different approaches. And they found quite interestingly that in nearly a third of cases, complete desensitisation of a heel bulb occurred after digital flex tendon sheath block.
Usually, the lateral heel bulb was found, but actually in 7% of cases, they found bilateral loss of heel bulb sensation. So, again, if we go straight for a digital flex tendon sheath block, we need to be aware that we might be inadvertently taking out foot pain. They found the proximal lateral approach significantly increased the risk of heel bob desensitisation.
So certainly that approach is not recommended if it can be avoided. And interestingly, there was little risk of it occurring at less than 15 minutes after injection. So my approach.
When blocking a digital flex tendon sheath is to review the lameness after 10 minutes and then after about 30 to 40 minutes as well, because if you do see a significant improvement within 10 minutes of injection, it's very unlikely that that is as a result of inadvertently taking out heel bulb sensation. As I said, the axial sesamoidian and the Palmer midline in the past and approached were the least associated with heel bulb desensitisation. And I guess, the upshot of that is is that if we block the digital flex tendon sheath, we don't find that there are imaging findings that that explain that lameness.
If in doubt, do a Palmer digital nerve block just to rule out inadvertent desensitisation of foot pain. Again, moving one nerve block higher, if we look at the low 4 point nerve block. In this, we take out the palmar nerves and the palmar metacarpal nerves, both medially and laterally.
Interestingly, my concern was always that, you know, potentially I was taking out pain from the sort of proximal metacarpal region, but actually there have been a few studies, looking at that and, the conclusion of those, both from Anna Maria Naji in 2010, and also CBoetta, which is an American group, which published at the same time, was that actually significant proximal diffusion was considered unlikely. So, you know, they didn't in an in vivo and a next vivo study in horses, they injected contrast medium. After they performed a low 4 point nerve block and actually found that significant proximal diffusion didn't really occur in occasional radiographs, as you can see on the right hand side, there seemed to be a sort of streak of linear contrast material tracking proximate, which was thought to be within the lymphatic system.
But again, the clinical relevance of that was considered relatively minor. So there's a couple of studies within the literature which suggest that a low 4 point nerve block is relatively specific to the sort of at least the mid metacarpal region distally and likely to take up me pain. When we move into the proximal metacarpal region and the distal carpal region, we start to get into sort of quite an anatomically complex region.
And there are also multiple blocks to choose from as well. We can perform a high 4 point, which is essentially where we take out the medial and lateral palmar nerves and palmetacarpal nerves. We can block the mid-carpal joint.
Obviously, that communicates with the, carpal metacarpal joint as well. And then we can also, if we go slightly more proximately block out the lateral palmar nerve either at the level of the medial accessory carpal bone, or, or by performing a wheat block which we'll come onto in a minute, which is just slightly more, more distantly. If we look at the high 4 point, I, I very rarely perform a high 4 point nerve block now unless it's to perform standing surgery of the, the metacarpal region.
Again, Anna Maria Nai in 2012, found that carpal metacarpal joint penetration occurred in about 50% of cases when we use the, lateral and medial approaches. Now they did remove, reduce that down to about 12% just by using a lateral approach alone. So by that, I mean, essentially they approached laterally and they directed the needle diagonally across the limb to inject the medial palma metacarpal nerves, then withdrew slightly and then, it went in perpendicular to the kind of bone to inject the lateral palma metacarpal nerves.
But they did find that there was some carpal sheath penetration in, in 12% of cases. Well. So given the proximity of the carpal sheath and the distal, the palm round pouching of the carpal metacarpal joint, I think we have to assume that if we're performing a high 4 point nerve block, there is a significant risk of inadvertent synovial penetration.
So it's important we form these blocks using aseptic technique, but also that we're aware we may inadvertently be blocking out those, those structures. If we look at a wheat block, this is performed, where the red dot is, so between the distal aspect of the accessory carpal bone and the head of the, lateral splint bone, through the accessoryio metacarpal ligament. There is little risk of joint penetration, so we're not likely to get into the carpal joints with this approach, but back in the late 80s, actually, Ford and colleagues showed a very significant risk of carpal sheath penetration.
So, you know, for that reason, again, I, I don't tend to perform this block particularly commonly, and if we do, we should obviously be using aseptic technique. My preference would be to perform the lateral Palmer block at the level of the accessory carpal bone. So the landmarks for this block would be in the distal third of the accessory carpal bone.
You see the proximal and distal extents are marked by the red arrows. We, access the medial aspect of the bone, and if, when you palpate it with your finger, there's a little ridge there. Which essentially, sits as a useful anatomical lar mark.
There is quite a lot of dense soft tissue in that area, so sometimes there can be quite a lot of resistance to injection. I tend to just sort of fan, sort of 3 or 4 mLs of local anaesthetic into that area. And that's a very useful block for blocking out the proximal aspect of the metacarpal region.
It's also nice as well, because unlike with the high 4 point, there's no sort of gas injected around the Palmer soft tissues in the proximal metacarpal region as well. So you can then perform an ultrasound examination straight away afterwards. Having said that, the important thing to stress is that actually, this block is not specific for the Palmer proximal metacarpal region alone.
It will also take out the proximal metacarpal region and also the distal carpal region too. And certainly on the right is a nice example of a horse that was referred in with a suspected, proximal suspensory lesion, but, but actually had a third carpal bone slab fracture which had repeatedly blocked out to this block here at the level of accessory carpal bone. Midcarpal joint, obviously, when we inject the mid carpal joint, we're also blocking the carpal metacarpal joint, but also the soft tissues as well.
The Palmer out pouchings of those two joints can extend into the proximal metacarpal region. So we can get desensitisation of local anaesthetic, which in Advertently can desensitise certainly horses with, for example, proximal suspendoryopy. And certainly, if the imaging findings do not correlate with, you know, carpal joint pain, ultrasound scan of the palm of soft tissues is warranted as well.
So if we look at a case study that was in at the hospital, that that quite neatly emphasises how difficult it can be in some of those situations. This was a, a sort of early teenage warm-blood girling dressage horse who was really quite significantly lame, on the left forelimb, positive to carpal flexion. And the referring veterinary surgeon had seen a significant improvement to a mid-carpal joint block that they performed.
Radiography detailed mild degenerative changes within the mid-carpal joint itself. The joint was medicated, but the horse hadn't responded, and at that stage, the horse was sent into the hospital for, for carpal arthroscopy. But I guess the concern in this case was that, you know, the radiographic changes within the midcarpal joint were relatively mild, and obviously that would be quite an unusual disease process for a horse of this type, not out of the question, but certainly unusual.
So, the horse was referred in for further investigation pending arthroscopy. When we looked at the horse, we found that on, on, on, on this occasion, actually inarticular anaesthesia of the mid-carpal joint only produced a very mild positive response, actually. And then we performed a lateral Palmer, nerve block at the medial aspect of carpal bone, and that did produce a significant improvement, a sort of 75% improvement in, in lameness.
We ultrasound scanned the palmar proximal metacarpal region, found enlargement in the lateral lobe with the proximal suspensory ligament. But I guess the question is at this stage, you know, how do blocks explain all of this and, and really give us a, a way forward, and I, and I think this is one of those situations where actually diagnostic anaesthesia probably can't tell us a huge amount more in this case, actually, and therefore we elected to perform advanced imaging in the form of low field MRI scanning, . And that did detail, you know, some changes within the lateral lobe with the proximal suspensory ligament, but also we found in the proximal metacarpal bone, 3rd metacarpal bone, there was some bone densification and some fissuring as well in the introsseous region.
So, you know, I, I think this just highlights that actually, you know, There are some cases where actually, you know, nerve blocks or joint blocks can only tell you so much. And if conventional imaging doesn't give you an answer, unfortunately, then we have to look at other options, of which advanced imaging is probably the most useful if, if that is. We move up into the, into the level of the anti-braium, the median and ulnar nerve block, is usually performed, ideally to sort of, you know, rule out lameness from the carpus down.
I guess once we get up to this level, these are deep, and they're large nerves, and it's quite hard to test this block accurately. Even if we do have a successful, desensitisation, particularly if we've already performed distal limb blocks as well, which will clearly affect cutaneous sensation. Obviously, because they are deep and large nerves, a false negative is a concern, so we may just be, not putting our local anaesthetic within the correct place.
As I said, how do we truly know whether we've, blocked those nerves or not? Some question in my mind always, is whether or not we've got a false positive too with regards to, potentially elbow pain. This has been looked at in 2019 by McGlinchyetau.
They looked at horses with naturally occurring, or, or, experimentally induced, elbow lameness. And they found that in the 9 horses that they injected the median nerves, not a single one of those horses with experimentally induced elbow lay this was improved after that. So that is encouraging with regards to false positives.
Having said that, we at the hospital have had certainly one clinical case that I can remember, which essentially blocked positively to a median nerve. Block, we imaged the carpus, we didn't find anything at all. And then actually, we went back and then performed intraarticular anaesthesia at the elbow joint and then alleviated the lameness with that as well.
And and that horse responded to intraarticular medication. So, I do in the back of my mind have a concern that a perineural anaesthesia of the median nerve can particularly Take out elbow pain sometimes. In terms of improving the efficacy of our nerve blocking.
Again last year, a nice paper by Bonneau, they looked at experimentally induced foot lameness, and they performed a median on a nerve block, either using mepivacaine but either with or without buffering with sodium bicarbonate. They used a small amount of sodium bicarbonate, so 1 mL, made up with 9 mLs of mepivacaine. And they actually found that horses that have been buffered with sodium bicarbonate showed a, a swifter and, and also greater response to the median and ulnar nerve block, so.
I, I haven't yet used that clinically, but it's certainly something to think about potentially when we're blocking larger nerves such as median and ulnar nerves and also in the hind limb, the tibial perineal nerves, you know, the use of sodium bicarbonate buffering might improve our outcome in those situations. Moving on to the hind limb, obviously in the distal limb, the principles, apply as in the forimb with regards to the palm digital and the axial sesamoid nerve block. I guess because foot lane is, is less frequently seen within the distal limb, we tend to perform those blocks, less frequently.
The dorsal branches need acknowledgement. Obviously, we might find sometimes that, You know, our Palmer digitalbaxial sesamoid nerve box might not, completely desensitise, 360 skin sensation as they would perhaps in the falling because of the presence of the dorsal branches. When we move up to the level of the, the low 6 points, or indeed the low 4 point, obviously, a low 6 point incorporates the two dorsal branches medially and laterally to the, digital extensor tendon.
My personal preference is not to perform those. I only perform a, a low 4 point in the hind limb, and, and personally, I don't feel it gives me any, poorer response to, to a low 4 point nerve block, by emitting the dorsal branches. But obviously, that's, clinician preference.
When we move up into the proximal metatarsal and distal tarsal region, there are again, just in the proximal metacarpal region, a number of nerve blocks to, to, to look at. We can perform a high 6 points. Again, my personal preference really is now only to perform that when I'm performing standing surgery of the, the metatarsal region.
We can perform perineural anaesthesia of the deep branch of the lateral plant nerve, which is now very commonly performed nerve block. We can perform intra-articular anaesthesia of the tarso metatarsal joint, and we can also perform intrathecal anaesthesia of the tarsal sheath. If we focus specifically on the deep branch lateral plantar nerve to start with, I think one thing that a number of papers have shown is that it is not specific, for the proximal suspensory ligament itself, and that is a common misconception.
Raffler Benz and colleagues back in 2010 performed high field MRI of 46 horses, all of which had responded to perineural anaesthesia of the deep branch of the lateral plant nerve, and they found that in only 21 out of those 46, so less than half of those horses was a proximal suspensory desopathy, the primary diagnosis. Now, that was a population of a lot of sort of western horses and horses that may have a high propensity for our bone lesions, for example, in that region. But I think the point still stands that we really can't look at the deep branch as a block that is specific for proximal suspensionary desmopathy.
There's lots of other structures in that region that can be affected. Contino Atal in 2015 looked at spread of contrast material after performing a a deep branch, as did launch in 2017, and they found that essentially, you know, inadvertently we, in about a quarter of cases, local anaesthetic finds its way into the tarsal metatarsal joint, and also in about a similar percentage, or I think it was 3 out of 8 horses, there was significant local anaesthetic in the Tarsel sheath as well. So we can inadvertently desensitise synovial structures following a deep branch block, which does also emphasise the need for aseptic technique.
Guy Hennigan, in 2014 performed a nice study where they did some, cadaver work following performing a deep branch with methylene blue. And they found that in about 90% of cases, there was methylene blue around the lateral plant and nerve, as well as the deep branch. And then they followed that up with an in vivo study as well, and they found that in about 90% of cases where they performed a deep branch block, those horses then had some loss of sensitization, skin sensitization of the lateral heel bulb.
We just need to be careful, particularly when we're aware that For example, a lot of racehorses, for example, may have pain in their lateral planter condyles of the fetlock region, and obviously that could be alleviated by deep branch blocks. So, you know, if in doubt, we should ideally rule out the distal limb with a low 4 point nerve block and potentially cross reference our deep branch response with intra-articular anaesthesia of the tarsome metatarsal joint as well. If we look at the tarsal metatarsal joint, obviously, we know that that should desensitise distal tarsal joint pain.
But also we know that just with the carpal metacarpal joint, the proximal out pouchings of the plant out pouchings of the joint are in very close proximity to the proximal suspense ligament as well. So we may get inadvertent desensitisation of the proximal suspensory ligament when we inject that joint as well. And, and so my preference, particularly if we've got imaging findings that suggest a little bit of osteoarthritic changes and some changes in the suspensory ligament would perform both blocks and compare and contrast response to, to both, a TMT block and a deep branch block as well.
In terms of communication with other joints, but unfortunately that's really very variable, and that very much depends on the, the substance which is injected into the task, metatarsal joint. In the early studies which looked at latex, which is obviously quite a large molecule, Belletal found about 25% of cases there was communication between the TMT and the distal interarsal joint. Using arthrotrophy, so, you know, again, quite large molecules that increased to about 38% in the early 90s.
But actually, when we look at very small molecules such as mepivacaine, Goff and colleagues found that about 2/3 of horses had a sort of therapeutically relevant concentration of mepivacaine within the distance. Tarsal joint after it been injected into the arsome metatarsal joint. And Serena Atal in 2005 found 100% of cases where methyl prednisolone acetate was injected into the TMT had found its way into the DIT.
Now, the only word of caution I would have with that is that actually You've got quite marked distal into tassel joint pathology, as we do sometimes see, not all of those will block out to a task metatarsal joint. And in fact, they don't always block out brilliantly to a distal interarsal joint anaesthesia as well. So, you know, I think we have to take these studies with a slight pinch of salt when we're looking at clinical cases.
And I think this case study is quite a nice example of that, actually. This is a 10 year old Connemara pony, that went acutely right behind him lame and improved by about 50% after perineural anaesthesia, the deep branch, the lateral plant nerve. The referring vet quite rightly blocked the, tarsal metasal joint and found that block to be negative.
But somewhat concerningly or confusingly, the ultrasound scanned the proximal suspensory ligament and found that to be normal. So the horse was essentially referred in me for suspension neurectomy and fasciotomy, but with some concerns from the veterinary surgeon that they were worried that their imaging findings just didn't really sort of fit with their blocking pattern. When we reviewed the diagnostic imaging, we found that actually, although the tarsal metatarsal joint was relatively normal, they were quite marked osteoarthritic changes in the distal interarsal joint, and, Just because the owner was concerned with this, we went ahead and performed diagnostic imaging in the form of standing low field MRI.
We actually found the suspensory ligament was relatively normal in appearance, but actually we had really quite marked osteoarthritic changes within the distal distal interarsal joint. And actually when we went back and blocked the distal interarsal joint, we, we found a significant response to, to that. .
So, in this case, the horse did have surgery, but actually it had arthrodesis of the distal tarsal joints and went on and did very well. But it just goes to show how really in that region where you've got some clinically very important structures very close to each other, you know, diagnostic anaesthesia really isn't, very specific to any one particular structure. If you look at the tibial perineal block, again, just within the medial, median and ulnar nerve block, these are very large nerves, tend to use large volumes of local anaesthetic, 15 to 20 mLs, and I think it's really important that we leave these for long enough before we reassess them.
So I tend to Leave them for certainly half an hour at the very least, and I I really don't perform any other nerve block, particularly a stifle block, for example, for at least an hour and a half or ideally sort of 2 or 3 hours after I've done these nerve blocks, because they can have a really sort of latent onset of action. I think in these situations, false negative is the, obviously the primary concern when the nerves are so large and obviously it's difficult to sort of actually palpate them. My preference when performing the tibial block is to actually have someone, actually toe touch the limb, and when the limb is.
On weight bearing, you can actually palpate the nerves sometimes just immediately dorsal to the gastrocnemius, which can make blocking it a little bit easier. It, it's normally slightly more medially placed and laterally placed, so I actually approached the horse across the contralateral limb to to perform that injection. Again, as far as I'm aware, it's not been reported in the literature, but the use of sodium bicarbonate buffering is, is interesting, as has obviously been reported for the median and ulnar nerve block.
If we look at the stifle, obviously you see quite a lot of stifle lameness, particularly in our population at the, at the hospital. It's a very large joint and sometimes, For that reason, we only get a relatively modest improvement in lameness after intra-articular anaesthesia. I think it's important in all of these cases actually to relate the clinical findings to the response to blocking as well.
As you can see here, this stifle is not been blocked yet, and we've got really quite marked diffusion of the femoraltibial joint, the lateral femoraltibial joint, medial. Fetu joint and also the femoral patellar joints as well. So, you know, obviously, we have to be aware of, of sort of red herrings and aberrant clinical findings, but actually relating the clinical findings and the imaging findings to the response to diagnostic anaesthesia is really key to a successful, you know, nerve block and lay investigation.
Just as seen in the distal tarsal region, we'd see a variable communication between the joints. I think for me, for the purposes of diagnostic anaesthesia, if I'm asking you the question, is this the stifle yes or no, then I would always preferentially block the three joints, together. .
There has been some work done looking at diffusion of local anaesthetic, which we'll come on to in a moment, but essentially because the response to local anaesthetic is sometimes not 100%, and because they are very large joints, my preference would be to to block joints. We need to make sure that we're using sufficient volumes of local anaesthetic as well. So in a large, you know, sort of thoroughbred type horse, that would be 20 to 30 mLs of mepivacaine, in each synovial compartment.
If we look at communication between the three joints, there's a couple of studies that have looked at that. You know, the original dogma from the Gough study in 2002, and from Vatchek in 1992, it gives slightly conflicting results, to be perfectly honest. When you look at latex communication, as was performed by Vatchek, and they essentially found that there was absolutely no communication through the midline septum, as you'd expect.
So, you know, the dogma was essentially that there was no communication between the medial and the lateral femoral. Tibial joints, but there was, in about 2/3 of cases, communication between the media femitibial joint and the femoral patella joint, and in about 1/3 of cases, so 20 to 30% of cases, there was communication between the femoral patella joint and the latter a fematibial joint. As I said, all for me, that really suggests is that actually, you know, if you're going to truly block out the stifle, then ultimately, we need to inject all three joints.
Goff in 2002, about the time that they were looking at the distal task or joint communication as well, they measured concentration of local anaesthetic, having injected just one or of three joints, and they actually found or documented much higher rates of communication, reflecting, I guess, that local anaesthetic, the pivocate in particular is quite a small molecule, quite readily diffuses across, or across, joint, joint capsules. But again, you know, I wouldn't really want to, just inject one joint and, and, and say that I'd blocked out the whole stifle for that. The one thing I will do is sometimes if I block a stifle or three compartments and the lame then switches to the other limb, I might then just drop block out 1.
Of those three joints, such as the medial feti joint to see if I then improve that that that bilateral lameness, which might give me a little bit of a point on perhaps where I might want to focus in particularular imaging, but also medication, for example. One thing that is, I guess, a little bit interesting and a bit concerning, is the potential for false positive results following, following intra-articular anaesthesia of the, the stifle joint. In last year in the EVJ a convertory Journal, really nice study by Radkar Eal, looked at experimentally induced foot lameness, so they put a foot clamp on the hoof capsule, and induced, lameness in 9 horses.
And then they went ahead and performed intra-articular anaesthesia of the stifle again in all three compartments. And then they, objectively looked at lameness, for up to an hour and a half after, performing intra-articular anaesthesia. Again, using objective gait analysis, and this wasn't subjective at all.
The theory behind this is obviously the peripheral nerves that innervate the distal limb are very closely associated with the caudal aspect of the the stifle joint with the actual joint capsule itself, and they were worried that the diffusion of local anaesthetic out across the joint capsule may inadvertently perform perineural anaesthesia. Somewhat worrying, they found that a third of their horses improved by about 50%, so the lame is improved by about 50% at 30 minutes post stifle joint injection. 2 out of 9 horses improved by about 3, and in about half of them, 4 out of 9, they saw really no improvement in lameness at all.
I guess take home points from this would obviously be we have to equate our imaging findings with with our responses to diagnostic anaesthesia. So, you know, if, for example, you know, we inject a stifle joint and the horse improves by 50%, which sometimes may be, you know, the only response we get even in sti with stifled lameness, if we go back. Image that stifling, we find absolutely nothing really that is relevant.
We need to perhaps consider the possibility that we might inadvertently have taken out some distal limb pathology, and it might be worth going back and performing a temptibial perineal block, for example, before, before moving forward with, with stifled treatment or or further imaging or perhaps surgery. I think the other take home message was is they didn't really see a significant improvement before 30 minutes. It took quite a lot of time for the local anaesthetic to diffuse across the joint capsule, so.
As we're talking about with the digital flex attendant she, you know, my preference, having injected a stifle is to, review these horses at 10 minutes and then sort of 30 to 40 minutes after injection as well. Purely because, you know, if we've seen no improvement at 10 minutes, but then we start to get a subtle improvement by, half an hour, we need to consider that we might again be inadvertently desensitising some distal limb pathology. So in summary, I think, you know, it would be lovely that if nerve blocks were sort of black and white in terms of localising lameness.
In reality, they would love to tell the truth, but they can't help but lie, and we need to be aware of that. They have limitations like any diagnostic tests, and this will very much vary, you know, depending on the animal, depending on the person doing the nerve block, and a whole other host of factors. None of this is our fault, actually, you know, it is what it is, and I think it's only now that we are actually a little bit more enlightened, and the peer reviewed literature shows us as to the limitations of diagnostic anaesthesia, means that actually we can educate owners and make them aware that, although it is a very useful way of localising lameness, it's not entirely infallible.
And I think if you have any doubts that your imaging findings, Don't entirely tee up with, the response to diag diagnostic anaesthesia, then, then my advice would be to, repeat the diagnostic anaesthesia, potentially, block, synovial structures within that area if we've performed perineural anaesthesia, but also perhaps consider, either conventional or advanced diagnostic anaesthesia, to try and, provide a specific diagnosis. And I hope this webinar has been of help, and obviously the supplementary notes that accompany it.
