PUB MED Abstract System AM, Pleasant RS, Jacobson JD, et al.: Efficacy of an epidural combination of morphine and deoxidize in alleviating experimentally induced hind limb lameness in horses, Vet Sure 25: 511, 1996. PUB MED Abstract Howe JR, Yaks TL, Go VOW: The effect of unilateral dorsal root ganglionectomies or ventral rhizomes on alpha 2-adrenoceptor binding to, and the substance P, enkephalin, and Neurontin content of, the cat lumbar spinal cord, Neuroscience 21: 385, 1987.
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PUB MED Abstract Fielding CL, Limbaugh GW, Matthews NS, et al.: Pharmacokinetics and clinical effects of a subanesthetic continuous rate infusion of ketamine in awake horses, Am J Vet Res 67: 1484, 2006. PUB MED Abstract Peterbauer C, Learn PM, Knobloch M, et al.: Effects of a low dose infusion of race mic and S-ketamine on the nociceptive withdrawal reflex in standing ponies, Vet Anapest Analog 35: 1467, 2008.
PUB MED Abstract Field MJ, Cox PJ, Stott E, et al.: Identification of the alpha2-delta-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediation the analgesic actions of pregabalin, Pro Natl Acid Sci U S A 103: 17537, 2006. The horse’s type, age, and training regimen may give important clues to the lameness.
The length of time since the last shoeing will be noted, as well as any indication that the lameness improves with either rest or exercise. A detailed visual inspection is followed by a hands-on examination of the limbs in weight-bearing and non-weight-bearing positions to identify any heat, pain, swelling of joints, or abnormal tissue tension.
Your veterinarian will also study your horse’s reactions, look for any muscle loss, and measure the range of movement in the joints. In addition to the legs and feet, the back and neck should be thoroughly examined with the horse restrained and standing on a level surface.
If lameness is major and severe (for example, a suspected fracture), additional exercise could catastrophically injure the horse. These posture changes help to reduce that amount of pressure and weight that has to be held by the lame limb.
These tests help identify reduced response to stimuli, weakness, or spasms, as well as abnormalities in the motor function of the major muscle groups that flex and extend the limbs. Physiologic imaging methods, including scenography and thermographic, evaluate metabolism or circulation of the entire horse, completing the picture of the disorder.
Imaging techniques may help narrow the problem to a specific region when other diagnostic methods fail. Because it is a minimally invasive surgery, arthroscopy allows for rapid healing of soft tissues, less pain, and fewer surgical complications.
With diagnostic arthroscopes, an attached camera transmits images to a monitor allowing the veterinarian to see inside the joint. Corrective surgeries (such as removal of bone fragments or cartilage, or repair of fractures) can often be performed at the same time.
Endoscopes may also be used to explore and correct problems involving the tendons or bursae (small fluid-filled sacs that typically serve to protect other tissues), particularly in cases of infection. For a number of disorders, these techniques have revolutionized treatment and dramatically improved recovery rates.
In addition to diagnosing lameness, nerve blocks can allow some surgical procedures to be performed without the need for general anesthesia. It is used when physical examination fails to identify the affected area (for example, in cases such as superficial foot pain, inflammation of the particular bone and forefoot of the horse, traumatic joint disease, or inflammation of a ligament).
Suddenly removing the protective effect of pain can cause a horse to further damage an injured area. Pain reduction lessens the lameness, helping your veterinarian isolate the location of the injury.
Also, mechanical and neurologic lameness, which are not necessarily painful to the horse, may be undetectable with analgesia. Regional anesthesia is a valuable diagnostic aid used to localize lameness when, after a thorough clinical examination, the site of pain remains uncertain.
Localizing pain allows other diagnostic procedures, such as anesthesia of a joint, radiography, ultrasonography, CT, scenography, or MRI to be used more effectively and economically to identify the cause of lameness. The anesthetic effect of mepivacaine HCl, which lasts 90–120 min, makes this agent valuable for examining a horse with lameness in multiple limbs or if multiple sites of pain on a limb are suspected.
Spinal needles are flexible and more likely to bend than break and, thus, safer to use if there is a possibility the horse may move the limb. Using a flexible needle is especially important when the difference in range of movement between skin and deeper tissues is large, in case the horse moves during injection.
When the goal of regional anesthesia is to identify a site of pain below the carpus or hock, only the smallest effective volume of anesthetic solution should be administered to avoid inadvertent anesthesia of adjacent nerves. Before regional anesthesia is performed, the horse should be consistently and sufficiently lame so that any improvement in gait can be detected.
Relief of pain and resolution of lameness after local anesthetic solution is administered into the fascia surrounding a nerve in the distal portion of the limb usually occurs within 5 min, but anesthesia of larger nerves in the proximal portion of the limb may take 20–40 min. Results of a regional nerve block can be misinterpreted if the horse’s gait is assessed before the onset of pain relief.
When assessing the effects of anesthesia of nerves in the distal portion of the limb, the clinician should keep in mind that anesthetic solution might migrate up the nerve to anesthetize more proximal structures, thus confusing the results of the examination. To avoid this complication, the gait should be evaluated within 15 min after administering a regional nerve bock in the distal portion of the limb.
When a regional nerve block is administered in the proximal portion of the limb, the horse may develop a gait abnormality or stumble because of altered proprioception. When nerves above the hock or carpus are anesthetized, it may be prudent to assess the horse’s gait on a soft surface or after bandaging the distal portion of the limb so that abrasion to skin over the dorsum of the fetlock is avoided if the horse stumbles.
If the gait is unchanged after regional anesthesia, the effectiveness of the nerve block should be determined by checking for skin sensation within the dermatome expected to be desensitized. For a fractious horse, skin sensation is more safely checked with the limb held or from a distance using a blunt instrument taped to a 3-ft pole.
A well-behaved, Stoic horse may not react to stimulation of skin even though regional anesthesia was ineffective. When performing regional anesthesia, especially in the distal portion of the limb, local anesthetic solution can be administered inadvertently into a blood vessel, joint, tendon sheath, or bursa.
Opinions vary concerning the amount of skin preparation necessary before administering regional anesthesia. For short-haired horses, the site of injection is often prepared by wiping the site with cotton pledges or gauze sponges soaked in 70% isopropyl alcohol until a pledge or sponge appears clean.
Acepromazine has no analgesic effect and, therefore, is less likely to ameliorate lameness than is a sedative, such as magazine or deoxidize, which provides some analgesia. The degree to which sedation or tranquilization interferes with assessment of gait depends on the severity of lameness and the skill of the clinician performing the examination.
Restraining the horse in stocks to administer regional anesthesia of the distal portion of the limb increases the likelihood of injury to the clinician. When anesthetizing the foot of a forelimb, most clinicians prefer to hold the limb while facing in the opposite direction as the horse; however, some clinicians prefer to anesthetize the foot while facing the same direction as the horse.
The needle is inserted directly over the palpable neurovascular bundle ~1 cm proximal to the cartilage of the foot. The needle is directed distally, and 1.5 mL of local anesthetic solution is deposited near the junction of the nerve and the cartilage of the foot.
For a few horses, the PAN block may also cause at least partial anesthesia of the proximal interphalangeal (pastern) joint, especially if a large volume of local anesthetic solution (e.g., >3 mL) is injected. When performing an abaxial sesamoid nerve block, 2.5–3 mL of local anesthetic solution is deposited at the base of the proximal sesamoid bones over the neurovascular bundle, which is easily palpated at this location.
Needle, by depositing 2 mL of local anesthetic solution over each Palmer nerve where it lies subcutaneously at the dorsal border of the deep digital flexor tendon. To complete the 4-point block, 1–2 mL of local anesthetic solution is deposited SC at the distal end of each splint bone, where the Palmer metacarpal nerve lies next to the periosteum of the third metacarpal bone.
Anesthetizing the medial and lateral Palmer nerves alone desensitizes the flexor tendons and inferior check ligament. With the limb held or bearing weight, the Palmer metacarpal nerves are anesthetized slightly distal to the level of the carpometacarpal joint by inserting a 20- to 22-gauge, 1 ½-in.
An easier alternative to the high Palmer nerve block, when the site of pain causing lameness is suspected to be in the proximal portion of suspensory ligament, is the lateral Palmer nerve block, which is performed, with the limb bearing weight, by inserting a 25-gauge, 5/8-in. Needle over the lateral Palmer nerve where it courses over the medial aspect of the accessory carpal bone.
The needle is inserted in a medial to lateral direction at the distal third of a palpable groove, and 2 mL of local anesthetic solution is deposited. Because the medial and lateral Palmer metacarpal nerves arise from the deep branch of the lateral Palmer nerve distal to this site, the structures they innervate, such as the proximal aspect of the suspensory ligament, are desensitized.
The median and ulnar nerves are sometimes anesthetized simultaneously as part of a lameness evaluation to exclude pain below the elbow as the cause of lameness, but more commonly, they are anesthetized along with the medial cutaneous antebrachial nerve to allow surgery of the limb without the need for general anesthesia. After depositing local anesthetic solution for a low 4-point nerve block at the level of the distal aspect of the splint bones, the needle is redirected dorsolaterally or dorsomedially, parallel to the bearing surface of the foot, and an additional 2 mL of local anesthetic solution is deposited SC to anesthetize the medial or lateral dorsal metatarsal nerves.
Most lameness es of the pelvic limb, however, can be evaluated accurately without anesthetizing the dorsal metatarsal nerves. When the proximal aspect of the suspensory ligament is suspected to be the site of pain causing lameness, 3–4 mL of local anesthetic solution can be deposited through a 20- to 23-gauge, 1-in.
Needle, axial to the lateral splint bone and ~1 cm distal to the tarsometatarsal joint, between the tendon of the deep digital flexor muscle and the suspensory ligament. The solution diffuses to anesthetize the deep branch of the lateral plantar nerve, which branches into the medial and lateral plantar metatarsal nerves that supply the proximal aspect of the suspensory ligament.
The perineal and tibial nerves can be blocked simultaneously to exclude pain in the hock or regions distal to the hock as a cause of lameness, or the blocks can be performed separately to gain added insight as to the possible site of pain. The tibial nerve is blocked ~10 cm above the point of the hock on the medial aspect of the limb, where it lies in fascia on caudal surface of the deep flexor muscle, cranial to the Achilles tendon.