Fourth branchial arch defect in a Welsh section A pony mare

Fourth branchial arch defect is a term used to describe a syndrome of congenital defects affecting the extrinsic structures of the larynx, due to a failure of development of structures that derive from the fourth branchial arch. This Case Report describes a 6-year-old Welsh section A mare presented for investigation of a chronic cough that was exacerbated on exercise. A fourth branchial arch defect was identified on resting upper airway endoscopy, with dorsal pharyngeal collapse evident on dynamic respiratory endoscopy, although full abduction of both the left and right arytenoid could be achieved and maintained at exercise.