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

Ultrasound-guided bupivacaine injection into the CAD muscle reliably induces fully reversible laryngeal hemiplegia in horses.

This study describes a novel ultrasound-guided technique for inducing transient laryngeal hemiplegia (LH) in horses by injecting bupivacaine directly into the cricoarytenoid dorsalis (CAD) muscle, offering an alternative to recurrent laryngeal nerve block methods. The study was conducted in two phases: a cadaveric validation phase using methylene blue injections in five larynges confirmed accurate needle placement via ultrasonography, and an in vivo phase involving nine Thoroughbreds with normal laryngeal function. Using a high-frequency linear ultrasound probe, an 18-gauge spinal needle was guided into the left CAD, and 4.5 mL of 0.5% bupivacaine was injected aseptically. LH of grade IV severity was achieved within five minutes of injection in all horses, persisted throughout maximal treadmill exercise (grade D on dynamic endoscopy), and lasted 70–90 minutes post-exercise. Critically, all horses demonstrated normal CAD echogenicity and complete return of laryngeal function at both rest and exercise one week post-injection, confirming full reversibility without residual muscle damage. This technique provides a reliable, reproducible, and minimally invasive method to create a standardized LH model for clinical research purposes, such as evaluating novel treatments or diagnostic tools. The ultrasound guidance ensures precise intramuscular delivery, overcoming the technical challenges associated with perineural injection. This approach has significant implications for equine respiratory research, allowing controlled induction of LH that is sustainable under physiologically relevant conditions of maximal exercise.

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