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.

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.