What is the medication and dosage used for palmar digital versus abaxial sesamoid blocks in horses?

Palmar Digital versus Abaxial Sesamoid Blocks in Horses

For palmar digital nerve blocks in horses, mepivacaine 2% (30 mg) is the preferred local anesthetic, while for abaxial sesamoid blocks, mepivacaine 2% or ketamine 3% can be used depending on the desired duration of action.

Palmar Digital Nerve (PDN) Block

Medications and Dosages

• First choice: Mepivacaine 2%

  • Dosage: 2-4 mL per site 1
  • Duration: Approximately 186 minutes (3 hours) 2
  • Advantages: More reliable onset and longer duration than lidocaine 3

• Alternative options:

  • Mepivacaine 2% + Dexmedetomidine

    • Dosage: 30 mg mepivacaine + 250 μg dexmedetomidine 2
    • Duration: Approximately 371 minutes (6 hours) 2
    • Advantage: Doubles the duration of analgesia compared to mepivacaine alone

  • Lidocaine 1% + Epinephrine (1:200,000)

    • Advantage: Improved efficacy and prolonged duration compared to lidocaine alone 4
    • Note: Standard lidocaine 2% is not recommended as it may not reliably resolve lameness despite causing skin desensitization 3

Administration Technique

• Inject at the level of the proximal sesamoid bones, targeting the medial and lateral palmar digital nerves
• Use aseptic preparation of the injection site to prevent inadvertent intrasynovial infection 1
• Total volume: 2-4 mL per site (total 4-8 mL for bilateral block) 1

Abaxial Sesamoid Block

Medications and Dosages

• First choice: Mepivacaine 2%

  • Dosage: 2-4 mL per site
  • Duration: Similar to PDN block (approximately 3 hours)

• Alternative option: Ketamine

  • Dosage: 3% solution 5
  • Duration: Short-acting (15 minutes maximum) 5
  • Onset: 2 minutes 5
  • Advantage: Rapid onset when quick, short-duration analgesia is needed

Administration Technique

• Inject at the base of the proximal sesamoid bones where the palmar nerves course abaxial to the sesamoid bones
• Target both medial and lateral aspects

Key Differences Between Block Types

1. Anatomical target:

   • Palmar digital block: Targets only the palmar digital nerves distal to the fetlock
   • Abaxial sesamoid block: Affects both palmar and palmar metacarpal nerves at the level of the fetlock

2. Area of desensitization:

   • Palmar digital block: Desensitizes structures distal to the injection site with minimal proximal diffusion (mean 4.3 cm proximal diffusion) 1
   • Abaxial sesamoid block: Desensitizes a larger area including the fetlock region

3. Clinical applications:

   • Palmar digital block: More specific for distal limb pathology
   • Abaxial sesamoid block: Used when a more proximal block is needed to include the fetlock region

Important Considerations

• Tissue diffusion studies show that anesthetic agents from palmar nerve blocks have limited proximal diffusion (mean total diffusion of 7.1 cm) 1
• Inadvertent intrasynovial injection may occur in approximately 40% of cases, so aseptic technique is essential 1
• Skin desensitization occurs sooner than lameness resolution with mepivacaine, so allow adequate time for full effect 3
• Adding dexmedetomidine to mepivacaine can significantly extend the duration of analgesia without causing systemic sedation 2

By understanding the differences in medications, dosages, and anatomical targets between these two nerve block techniques, veterinarians can select the most appropriate approach based on the suspected location of pathology and required duration of analgesia.