Ultrasound-guided injection of bupivacaine into the cricoarytenoid dorsalis muscle as a novel technique for transiently inducing laryngeal hemiplegia in horses.

Research Overview

• This study developed and tested a new ultrasound-guided technique to temporarily induce laryngeal hemiplegia (LH) in horses by injecting bupivacaine directly into the cricoarytenoid dorsalis (CAD) muscle.
• The technique was shown to reliably cause transient LH, which lasted through maximal exercise and fully reversed within one week.

Background and Objective

• Laryngeal hemiplegia (LH) in horses involves paralysis of the muscles controlling the larynx, often due to recurrent laryngeal nerve dysfunction, leading to airway obstruction and impaired breathing, especially during exercise.
• In clinical research, LH is induced transiently by suppressing nerve function, but existing methods are challenging and may not be reliable or practical.
• The objective was to develop a novel, practical method to transiently induce LH by directly injecting a local anesthetic, bupivacaine, into the CAD muscle using ultrasound guidance.

Methodology

• Subjects: Five cadaver larynges and nine live Thoroughbred horses with normal laryngeal function were used.
• Part 1 – Cadaver Study:
  • Used a high-frequency linear ultrasound probe (6.5-13.0 MHz) to guide injections of methylene blue dye into the CAD muscle of cadaver larynges.
  • This was to confirm the accuracy of ultrasound-guided injection targeting the CAD muscle.
• Part 2 – Live Horses Injection Procedure:
  • The left laryngeal region was aseptically prepared.
  • Ultrasound was used to identify the cricoid, thyroid, and arytenoid cartilages as landmarks.
  • The cricoarytenoid lateralis muscle was visualized first in a dorsal horizontal view, then the arytenoid cartilage and CAD muscle (positioned superficially to the cricoarytenoid joint) were identified.
  • An 18-gauge, 3.5-inch spinal needle was guided into the CAD muscle under ultrasound visualization.
  • Bupivacaine (0.5% solution, 4.5 mL) was injected into the CAD muscle to induce local muscle anesthesia.
• Part 3 – Evaluation of Effects:
  • Laryngeal function was assessed at rest and during maximal treadmill exercise using overground endoscopy.
  • The efficiency of inducing LH (graded severity), duration of hemiplegia, and recovery were monitored.
  • Follow-up laryngeal examinations were conducted one week post-injection to assess reversibility.

Results

• Cadaver Study:
  • Methylene blue dye was successfully deposited in the CAD muscle in all cadaver larynges, confirming that ultrasound provides accurate targeting for injection.
• Live Horse Study:
  • LH was observed within 5 minutes post-injection, reaching grade IV severity at rest (severe dysfunction) and grade D during exercise (consistent with impaired but sustained dysfunction under exercise conditions).
  • LH persisted for 70 to 90 minutes after exercise, demonstrating sustained muscle paralysis during and shortly after intense activity.
  • One week later, horses showed normal CAD muscle ultrasound appearance and fully normal laryngeal function both at rest and during exercise, indicating complete reversibility of the induced paralysis.

Conclusions and Significance

• Injecting bupivacaine into the CAD muscle under ultrasound guidance is a practical, effective, and minimally invasive technique for transiently inducing LH in horses.
• This method provides reliable induction of muscle paralysis that is sustained during maximal exercise and fully reversible within a week.
• The technique could be valuable for research purposes to study LH and potentially for clinical diagnostic or therapeutic applications requiring temporary laryngeal muscle blockade.