Have you ever assisted with an equine lameness exam? If so, you are probably well aware of how complex these exams can be! In many cases, a physical exam alone is insufficient to localize the cause of a horse’s lameness.
An exam may allow you to localize the lameness to a particular limb but, unlike in small animal medicine, you can’t image an entire limb in a single radiographic view.
Once lameness has been isolated to a particular limb, diagnostic nerve blocks are typically used to further narrow down the affected area. These blocks are administered in a specific order, beginning distally and gradually working proximally up the limb. When you find the most distal block that results in the resolution of the patient’s lameness, you can identify the area of the limb that is responsible for the horse’s discomfort.
Which Local Anesthetics Are Used in Diagnostic Nerve Blocks?
Although there are a variety of local anesthetics available, most equine vets use lidocaine or mepivacaine. Both agents act quickly, showing effects within about 15 minutes. However, lidocaine’s effects typically wear off within 60-120 minutes, while mepivacaine’s effects persist for 90-180 minutes. This makes mepivacaine a more practical choice for many vets, because lidocaine may wear off before you finish your lameness exam.
How Is a Nerve Block Administered?
Most diagnostic nerve blocks are administered with a 25-g needle, although a 22-g needle may be used in some cases. The needle is often inserted without the syringe attached, to reduce the chance of breaking the needle off in the horse’s limb, and the syringe is connected once the needle has been placed. Therefore, you will want to use a non-Luer-lock syringe when drawing up your injection.
Begin by preparing the site before injecting. If the horse is very dirty, scrub your injection site with antiseptic soap. In most cases, however, several wipes with an alcohol-soaked gauze (until you are no longer removing visible dirt) is adequate.
When injecting a local block, direct the needle slightly distally. This minimizes the likelihood of accidentally affecting more proximal tissues. Use small volumes when possible, to limit migration of the local anesthetic.
Re-evaluate the horse within 15 minutes of administering each block. (If you wait too long, local anesthetic can migrate to more proximal sites, confusing your results.) If the lameness does not resolve, it’s important to determine whether your block was effective before jumping to conclusions about the source of the lameness. The horse’s skin in the area that you blocked should lack sensation; use the tip of a pen or a key to press on the skin and see whether the horse responds. If the response is equivocal, compare sensation in the blocked limb to the contralateral limb (some horses are very well-behaved and stoic). If your block did not take effect, wait several more minutes or repeat your administration of the nerve block. If your block was effective and had no effect on the lameness, progress proximally up the leg.
Which Nerve Blocks Are Used To Diagnose Distal Limb Lameness in Horses?
The following four nerve blocks are commonly used to assess the distal limb:
- Palmar/Plantar Digital Nerve (PDN) Block
This block, sometimes erroneously called the “heel block,” anesthetizes the entire foot, including the coffin (distal interphalangeal) joint, navicular bursa, and sometimes the pastern (proximal interphalangeal joint). The only tissues of the foot that are not affected are the dorsal coronary band and the sensitive lamina of the dorsal wall of the toe.
Administer this block by palpating the palmar/plantar digital nerves along the lateral and medial borders of the deep digital flexor tendon, palmar/plantar to the accompanying artery. Inject along the palmar/plantar border of this neurovascular bundle.
- Abaxial Sesamoid Nerve Block
This block anesthetizes the palmar nerves at the level of the base of the proximal sesamoid bones. Affected tissues include the pastern, distal sesamoidean ligaments, as well as partial desensitization of the fetlock in some cases.
Administer this block by palpating the neurovascular bundle that includes the dorsal and palmar/plantar digital nerves, over the abaxial surface of the proximal sesamoid bones. Insert the needle distally, just palmar/plantar to the neurovascular bundle near the base of the proximal sesamoid bone. Inject on each side of this bundle.
- Low Palmar/Plantar or Low 4-Point Block
This block anesthetizes the fetlock and distal cannon bone, as well as all tissues distal to that point. If lameness fails to respond to the abaxial sesamoid block but responds to the low palmar nerve block, the source of the lameness can be isolated to the fetlock or distal cannon bone.
Administer this block by injecting subcutaneously on either side of the limb, between the suspensory ligament and deep digital flexor tendon at the distal buttons of the splint bones.
- High Palmar/Plantar or High 4-Point Block
This block anesthetizes tissues just above the low palmar nerve block, as well as all tissues distal to that point. If lameness fails to respond to the low palmar nerve block but responds to the high palmar nerve block, the source of the lameness can be isolated to cannon bone, suspensory area, or distal carpus.
Administer this block by injecting between the suspensory ligament and deep digital flexor tendon, deep to the heavy fascia of the flexor retinaculum, approximately 2-3 cm distal to carpometacarpal joint.
Are There Other Diagnostic Blocks Available?
A number of nerve blocks can be used in the diagnosis of equine lameness. While these are four of the most common examples, veterinarians may incorporate a number of other blocks, depending on their prior experience and clinical suspicions.
What Complications Are Associated With Diagnostic Nerve Blocks?
Potential complications include broken needle shafts and infection of the subcutaneous tissues or other adjacent structures. Local anesthetics may also be detectable in the horse’s serum if the horse is drug tested for competition.
Can Joints Be Blocked During an Equine Lameness Exam?
In addition to the above blocks, local anesthetic can be injected directly into joints, including the coffin joint, navicular bursa, pastern, fetlock, carpus, or more proximal joints. Doing so can aid in isolating lameness to a specific joint. In some cases, lameness caused by intra-articular diseases will not respond very well to the nerve blocks discussed above.
When blocking a joint, special care must be taken to prevent infection. The skin should receive a thorough sterile scrub, although opinions differ on whether the hair should be clipped. (Some veterinarians believe that clipped hair is more likely to enter the joint and cause problems.) Additionally, a fresh, unopened bottle of local anesthetic should be used for joint blocks. Finally, a larger needle should be used to prevent the needle from breaking off within the joint.