Diagnostic Anesthesia for Lameness Localization in Horses
Principles and Objectives:
Primary Goal: To localize the origin of pain when physical examination alone is insufficient.
Strategic Benefit: Localizing pain allows for more effective and economical use of imaging (radiography, MRI, CT, scintigraphy).
Prerequisites: The horse must be consistently and sufficiently lame to recognize improvement; assessment includes trotting in straight lines and lunging.
Interpretation: While pain is usually distal to the block, clinicians must remain aware that lesions can occasionally exist above the desensitized region.
Pharmacologic Agents and Supplies:
Mepivacaine HCl (2%): The agent of choice due to less tissue reaction and more reliable, longer-lasting desensitization (90–120 minutes).
Lidocaine: Less expensive but more reactive and shorter-acting; has a poor correlation between skin sensation loss and deep desensitization.
Needle Selection:
Distal Limb: 25-gauge, 1.6-cm (5/8-inch) needles.
Proximal/Intra-articular: 22- or 20-gauge, 3.8-cm (1.5-inch) needles.
Restraint and Sedation:
Acepromazine: Preferred as it has no analgesic effect; may even accentuate subtle lameness by relaxing the horse.
Xylazine/Detomidine: Provide systemic analgesia, which can interfere with the examination results.
Technical Procedures and Safety:
Aseptic Technique: Alcohol is sufficient for distal nerve blocks, but intra-articular blocks require a 3–5 minute antiseptic scrub.
Needle Handling: Always detach the needle from the syringe before insertion to prevent breakage; direct needles distally in the distal limb to avoid proximal anesthetic migration.
Evaluation Timing: Gait should be re-evaluated within 15 minutes for distal blocks and 15–30 minutes for proximal blocks to ensure accuracy.
Safety Precautions:
Do not use stocks for distal limb blocks.
Bandage the distal limb when performing proximal pelvic limb blocks, as horses frequently stumble due to altered proprioception.
Forelimb Blocking Sequence:
1. Palmar Digital Nerve (PDN) Block: Anesthetizes the entire sole, foot, and the coffin joint.
2. Abaxial Sesamoid Block: Localizes pain to the pastern and fetlock regions.
3. Low Palmar (Low 4-Point) Block: Pinpoints the fetlock as the primary source of pain.
4. High Palmar (High 4-Point) Block: Desensitizes the proximal metacarpus and distal carpus.
5. Proximal Blocks: Includes intra-articular anesthesia of the carpus, elbow, or shoulder, or a simultaneous median and ulnar nerve block.
Pelvic (Hind) Limb Blocking Sequence:
Distal Blocks: Similar protocols to the forelimb (PDN and Low 4-Point).
Low 6-Point Block: Includes the dorsal metatarsal nerves to provide more comprehensive distal anesthesia.
Deep Branch of the Lateral Plantar Nerve Block: A specific diagnostic tool for proximal suspensory ligament pain.
Tibial and Peroneal Nerve Blocks: Typically performed together to determine if pain originates in the hock region.
Intra-Articular (Joint) Anesthesia:
Mechanism: Identifies a specific joint as the site of pain; resistant injection often indicates incorrect needle placement.
Standard Volumes:
Tarsometatarsal: 2 mL.
Coffin (DIP) Joint: 5 mL.
Fetlock Joint: 8 mL.
Stifle (Medial Femorotibial): Up to 30 mL.
Chondrotoxicity: Clinicians should consider that local anesthetics can be toxic to joint cartilage and synoviocytes.