The extrinsic structures of the larynx, such as the wing of the thyroid cartilage, cricothyroid muscle, and upper esophageal sphincter, develop from the fourth branchial arches. Aplasia or hypoplasia of one or more of these structures may occur unilaterally or bilaterally. Right-sided defects are more common than bilateral or left-sided defects. The severity of clinical manifestation ranges broadly and is based on the degree of the defect. The most common clinical sign is respiratory noise, although mild dysphagia, eructation, and cough have been reported. Palpation of the larynx reveals absence of one or both wings of the thyroid cartilage, resulting in failure of the cricothyroid articulation and a palpable space between the cricoid and thyroid cartilages. Radiographic evidence of a fourth branchial arch defect includes dilation of the cricopharynx with a continuous column of air from the pharynx to the cervical esophagus. Rostral displacement of the palatopharyngeal arch may or may not be detected during endoscopic examination. Endoscopic examination during treadmill exercise may reveal dynamic collapse of the vocal folds. Affected horses are unlikely to become effective athletes. Partial arytenoidectomy may improve airway dynamics sufficiently for pleasure riding.
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