Regional nerve blocks
Hoof diseases are a frequent disorder of horses requiring diagnostic investigations such as nerve blocks.
Based on the results of numerous experimental trials in which pain was temporarily induced in different regions of the forefoot, there is no diagnostic technique of local or synovial anesthesia that desensitizes only the heel region of a horse. Furthermore, the results of those studies indicate that the various techniques of diagnostic analgesia of the foot are not specific in localizing pain-causing lameness. However, because each of these diagnostic techniques anesthetizes the palmar digital nerves in an area above or below branches that provide sensation to a particular region of the foot, clinicians can use these techniques sequentially to more closely localize the source of pain in the foot. Response to hoof testers or flexion tests, coupled with response to different diagnostic techniques of regional or synovial anesthesia of the foot, may also be helpful in localizing pain to a particular region of the foot.
Most comparisons of different anesthetic techniques of the horse's foot have focused on the forefoot. Results of these techniques can probably also be applied to the hindfoot because the medial and lateral dorsal metatarsal nerves, which are branches of the deep peroneal (fibular) nerve, supply sensation to only the laminar corium within the hindfoot.
- Abaxial sesamoid nerve block
Anesthesia of the palmar digital nerves at the level of the base of the proximal sesamoid bones is the least specific technique used to localize the source of pain in the foot. This block anesthetizes not only the entire foot, including the portion of the deep digital flexor tendon within the confines of the foot, but also the pastern joint and possibly a portion of the fetlock joint.
- Palmar digital nerve block
Although the palmar digital nerve block is often referred to as a heel block, it anesthetizes the entire foot (i.e., the distal interphalangeal joint, entire sole, distal phalanx, and navicular apparatus), except for the dorsal aspect of the coronary band and the dorsal laminae. To perform this block, a small amount of local anesthetic solution (1.5 ml) should be deposited subcutaneously over each palmar digital nerve near the margin of the cartilage of the foot. For some horses, branches of the palmar digital nerves that supply a portion of the proximal interphalangeal joint are anesthetized with this block.5 When a larger volume of local anesthetic solution is deposited or local anesthetic solution is deposited more proximally than at the margin of the cartilage of the foot, the likelihood of anesthetizing a portion of the proximal interphalangeal joint increases.
- Pastern semiring block
If there is no change in a horse's degree of lameness after the palmar digital nerve block, a semiring pastern block can be used to anesthetize the dorsal branches of the palmar digital nerves. This block is unlikely to ameliorate lameness for two reasons: First, the dorsal branches of the palmar digital nerves contribute little to sensation within the foot. Second, the palmar digital nerve block will already have anesthetized the entire foot, except for the dorsal aspect of the coronary band and the dorsal laminae of the foot.
Analgesia of distal interphalangeal joint
When a large volume of local anesthetic solution (i.e., 10 ml) is administered into the distal interphalangeal joint, the structures in the foot that are anesthetized are similar to those anesthetized with a palmar digital nerve block.
Local anesthetic solution diffuses from the joint to the navicular bone and bursa to desensitize those structures, and because of their close proximity to the synovium of the distal interphalangeal joint, the palmar digital nerves are also anesthetized.
Furthermore, the distended joint contacts the proximal branches of the palmar digital nerves that supply the heel region of the sole, causing these branches to be anesthetized. Administering a smaller volume of local anesthetic solution (i.e., 6 ml) avoids anesthesia of these branches, possibly by avoiding distention of the palmar pouch of the distal interphalangeal joint. However, clinicians should be aware that in a large clinical study, 20% of horses with lesions involving the navicular bone and its related structures failed to respond to intraarticular analgesia of the distal interphalangeal joint.
Analgesia of the navicular bursa, by itself, is the most localizing diagnostic analgesic technique of the foot because administering local anesthetic solution into this structure anesthetizes the palmar digital nerves at a more distal site than do other techniques. Administering 3 to 4 ml of local anesthetic solution into the navicular bursa desensitizes the navicular bone and its related structures as well as the toe region of the sole. Although analgesia of the distal interphalangeal joint desensitizes the navicular bone and related structures, analgesia of the navicular bursa does not desensitize the distal interphalangeal joint. This is because analgesia of the bursa probably anesthetizes the palmar digital nerves distal to the branches that supply the distal interphalangeal joint and because local anesthetic solution is slow to diffuse from the bursa to the distal interphalangeal joint. To further localize the source of foot pain, careful use of a hoof tester before anesthetizing the navicular bursa can help rule out solar pain.
Analgesia of the digital portion of the deep digital flexor tendon
Reports of magnetic resonance imaging for diagnosing disease within the foot indicate that lesions involving the digital portion of the deep digital flexor tendon within the foot often cause chronic lameness. In one study, slightly less than two-thirds of horses that were lame as a result of abnormalities involving the digital portion of the deep digital flexor tendon had temporary resolution of the lameness after a palmar digital nerve block was performed at the level of the cartilage of the foot or after anesthesia of either the distal interphalangeal joint or the navicular bursa. However, all horses became sound after an abaxial sesamoid nerve block was administered, indicating that branches from the palmar digital nerves to the deep digital flexor tendon arise proximal to the site most commonly recommended for a palmar digital nerve block (i.e., at the level of or slightly below the proximal margin of the cartilage of the foot). Thus administering local anesthetic solution into the digital flexor tendon sheath of horses with lameness that is unchanged after a palmar digital nerve block but resolves after an abaxial sesamoid nerve block may be useful in localizing the source of a horse's pain. Resolution of lameness after intrathecal analgesia of the flexor tendon sheath justifies suspicion of a lesion in the digital portion of the deep digital flexor tendon.
Accurately localizing pain within the foot using regional or synovial anesthesia is time-consuming and often confusing. Options regarding medical treatment of horses with chronic lameness caused by foot pain are currently limited to administration of NSAIDs, corrective trimming or shoeing, intrasynovial administration of corticosteroids, or a change in the type of work performed by the horse. Thus precise localization of pain may be unnecessary, except when prognosis for return to soundness is required.
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