Page 692 - Adams and Stashak's Lameness in Horses, 7th Edition

658 Chapter 5

analgesia (predominantly by eliminating the distal limb nerves to demonstrate an improvement in lameness. In one
as the source of lameness) or by intra‐articular analgesia study evaluating horses with TC joint pain, intraarticu-
VetBooks.ir indicates the tarsal region to be the source of lameness, (70.6%), but <50% in 3/17 limbs (17.6%). Some peri-
lar analgesia improved lameness >50% in 12/17 limbs
If the clinical examination
of the distal hindlimb.
28,42,72
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then intra‐articular analgesia can be performed without
articular structures may need to be carefully evaluated
fear of blocking the lower limb. If the clinician has no with diagnostic imaging and require the use of more
indication as to the source of lameness, then regional specific analgesic techniques to confirm their signifi-
analgesia should be performed starting with a low plan- cance. For example intrasynovial injection of anesthetic
tar (six‐point) nerve block. Once the distal limb is elimi- into the TS is necessary to isolate a lateral deep digital
nated as the source of lameness (negative response to the flexor tendon (LDDFT) injury as the source of lameness.
low plantar), then intra‐articular analgesia of the tarso- Some clinicians use a subtarsal nerve block to eliminate
metatarsal (TMT) and distal intertarsal (DIT) joints the proximal plantar region. However, when subtarsal
should be performed. Communication between the DIT injection around the plantar metatarsal nerves (with/
and TMT joints is variable. Previous studies found com- without the plantar nerves) was evaluated utilizing con-
munication between of the DIT and TMT joints ranged trast medium, it was found that the TS was injected in
from 8% to 30% and even found variability between 40% of the limbs. No proximal diffusion was identi-
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both hocks of the same horse. 11,80 Surprisingly commu- fied when the perineural injection was place in the cor-
nication between the distal tarsal (DT) joints and the rect location outside the TS. Contrast medium was
tarsocrural (TC) joint was found to occur in about 3% identified in only one TMT joint, suggesting that the
of horses. Therefore, the author recommends that both likelihood of subtarsal analgesia influencing the tarsus is
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DT joints be injected for both diagnostic and treatment relatively small.
purposes. Anesthetizing the tibial and the deep and superficial
Intra‐articular analgesia is thought to be more spe- peroneal nerves above the point of the hock should
cific than perineural analgesia, but either technique may desensitize the entire distal portion of the limb. However,
be misleading particularly with respect to DT joint dis- the horse may not be able to utilize the limb appropri-
ease. Inaccuracies in the diagnostic analgesia techniques ately and may start to drag the toe of the desensitized
can and do occur. For example, a negative response to limb. The tibial nerve is a large nerve, and some clini-
DIT/TMT intra‐articular analgesia does not preclude cians have suggested that it may take up to 45–60 min-
DT joint pain. Narrowing of the DIT joint space or peri- utes before enough diffusion has occurred before it
articular new bone can impede or prohibit intraarticular provides complete analgesia. Blocking the stifle before
injection. In addition, the response to intra‐articular the tibial nerve is completely desensitized may lead to a
analgesia may be very poor or at best delayed in the false positive (the horse seems to improve to the stifle
presence of extensive subchondral bone damage. block, but the improvement is actually more complete
Isolating the source of pain to the proximal plantar analgesia of the tibial nerve). In addition, local diffusion
region requires that analgesia of the deep branch of the from the tibial injection site may result in resolution of
lateral plantar nerve (DBLPN) or perineural analgesia of lameness associated with gastrocnemius tendonitis.
the plantar metatarsal nerves at the subtarsal site should There is a close anatomical association of the TMT
be performed. This would indicate that proximal sus- joint and the proximal suspensory ligament (SL) and its
pensory desmitis (PSD) may be the source of lameness. innervations. The lateral plantar nerve gives off a deep
Lameness due to PSD should be substantially improved branch approximately 3 cm proximal to the head of the
(65%–70%) following these blocks. However, a recent Mt4 bone. The DBLPN branches into medial and lat-
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study showed proximal diffusion of contrast media after eral plantar metatarsal nerves, which provides sensory
DBLPN creating the potential for false‐positive results. and motor innervation to the proximal SL. A previous
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In addition analgesia of the DBLPN can influence DT study of injecting of contrast medium into the TMT
joint pain, and intraarticular analgesia of the TMT joint joint demonstrated significant amounts of contrast
can abolish lameness in some horses with PSD (8%). located plantar and distal to the TMT joint following
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Lameness of the proximal plantar region may occasion- injection. It was hypothesized that this may result in
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ally improve following a low plantar (six‐point) nerve perineural anesthesia of the plantar metatarsal nerves
block most likely due to proximal diffusion of the local and the proximal aspect of the SL. Perhaps more impor-
anesthetic. One study evaluating analgesia of the DBLPN tantly, these plantar outpouchings of the TMT joints
revealed that 37.5% of horses had contrast in the tarsal may permit inadvertent intra‐articular anesthesia of the
sheath (TS) and 25% had sufficient mepivacaine in the TMT joint following diagnostic anesthesia of the proximal
TMT joint to produce analgesia. Therefore, a compari- metatarsus similar to what occurs in the forelimb.
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son between the respective outcomes of intra‐articular Because it is considerably easier and safer to inject the
and perineural analgesia may be useful in those horses TMT joint, some clinicians prefer to block the TMT
without radiographic or ultrasonographic abnormali- joint first. If there is no or only partial improvement in
ties. If necessary, a tibial nerve block may prove useful to baseline lameness, the DIT joint should then be blocked.
distinguish between DT joint pain and PSD pain, as Lameness is often improved within 10 minutes, but due
analgesia of the tibial nerve will result in analgesia of to the extensive pathology in some horses, it may
PSD without significantly influencing DT joint pain. take longer to see complete clinical improvement. A
A wide range of conditions can cause TC joint pain in positive response to intra‐articular analgesia of DT joint
horses. Some of these conditions that affect the TC joint pain may be limited (<50% improvement from base-
may not respond to intra‐articular analgesia but in fact line lameness). When utilizing perineural analgesia of
require perineural analgesia of the tibial and fibular the peroneal and tibial nerves, it may take a minimum of