Merck Manual

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Professional Version

Laryngeal Dysplasia in Horses

(Fourth Branchial Arch Defect, Rostral Displacement of the Palatopharyngeal Arch)


Kara M. Lascola

, DVM, MS, DACVIM, Auburn University College of Veterinary Medicine

Reviewed/Revised Jul 2023

Equine laryngeal dysplasia is a congenital syndrome recognized in horses of various breeds. Also referred to as 4th branchial arch disease (4-BAD), this syndrome involves developmental malformation of laryngeal structures formed from the 4th and, occasionally 6th, branchial arches. Aplasia or hypoplasia of various laryngeal structures may occur unilaterally or bilaterally but defects are most often right-sided. Numerous developmental abnormalities involving the cricoid and thyroid cartilages and pharyngeal muscles have been identified.

Developmental anomalies most often include:

  • Malformations of thyroid and cricoid cartilages

  • Absent crico-thyroid and crico-arytenoid articulations

  • Hypoplasia or aplasia of the cricothyroideus or cricopharyngeus muscles

  • Malformation of upper esophageal sphincter muscles

Clinical signs of laryngeal dysplasia can first occur in foals or adult horses, with the severity of clinical signs dependent upon the extent of the defect, specific structures affected, and the use of the horse. The most common clinical sign is respiratory noise during exercise, although dyspnea, exercise intolerance, dysphagia, and mild eructation or colic (secondary to aerophagia) are reported. In some horses, laryngeal dysplasia is diagnosed as an incidental finding.

Diagnosis can often be made via laryngeal palpation and endoscopy (rest or dynamic). Dynamic endoscopy is essential for assessing laryngeal function during exercise and for prognostic and treatment decisions. Laryngeal ultrasound or radiography are useful in confirming the diagnosis. CT or MRI are the most accurate diagnostic tool.

Diagnostic findings can include:

  • Palpation of the larynx consistent with an absent cricothyroid articulation—palpable space between the cricoid and thyroid cartilages; absence of one or both wings of the thyroid cartilage

  • Endoscopic examination at rest or dynamic—reduced right-sided arytenoid abduction (most common), vocal fold collapse, rostral displacement of the palatopharyngeal arch, and aryepiglottic fold collapse

  • Radiography—dilation of the cricopharynx with a continuous column of air from the pharynx to the cervical esophagus

  • Laryngeal ultrasonography—absent cricothyroid articulation, dorsal extension of thyroid cartilage

Numerous surgical treatment approaches have been proposed and include ventriculocordectomy, arytenoidectomy, laryngoplasty, and laser thermoplasty. Choice of treatment and outcome depend on the nature and severity of structural abnormalities and goals for athletic performance. In general, affected horses are unlikely to become effective athletes.

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