Focal Point Key Facts

Idiopathic laryngeal hemiplegia (ILH) is the most common cause of upper respiratory unsoundness in horses. Classified as a distal axonopathy, ILH primarily affects the intrinsic laryngeal muscles on the left side. The adductor muscles are more severely affected at an earlier stage than is the primary abductor muscle. Endoscopic evaluation in standing or exercising horses can result in a definitive diagnosis of ILH. Quantitative methods to further evaluate laryngeal function include morphometric evaluation of laryngeal images, airflow mechanics, and tidal breathing flow–volume loop analysis. Four potential treatment options for ILH are currently available: prosthetic laryngoplasty, ventriculectomy/cordectomy, arytenoidectomy, and laryngeal reinnervation. Idiopathic laryngeal hemiplegia (ILH; Figure 1), a common upper airway condition, occurs in approximately 8% of large sport horses. Affected animals have upper airway obstruction and produce a characteristic respiratory noise during exercise. Both the airway obstruction and the respiratory noise can be career limiting; the former can be severe enough to cause exercise intolerance, whereas the latter may eliminate show horses from competition. ILH presents a challenge to veterinarians because upper airway anatomy and physiology are complex, the pathophysiology of ILH is poorly understood, evaluation of upper airway function requires specialized expertise, and several surgical options—each with its own complications—are available. This article reviews the structure and function of the normal equine larynx and summarizes what is known about the pathogenesis of ILH. In addition, upper airway assessment and surgical treatment options are discussed. LARYNGEAL ANATOMY Cartilages The equine larynx consists of three unpaired and two paired cartilages (Figure 2). The thyroid cartilage, the largest of the unpaired cartilages, consists of two lateral laminae that are fused rostroventrally. Each lamina projects a rostral cornu, which articulates with the thyrohyoid bone, and a caudal cornu, which articulates with the cricoid cartilage. A foramen lies ventral to the rostral cornu and contains the cranial laryngeal nerve. The cricothyroid ligament runs from the border of the thyroid to the ventral notch of the cricoid cartilage. CE n Horses with ILH have upper airway obstruction and produce a characteristic respiratory noise during exercise, p. 85. n Horses affected with subclinical and clinical ILH show evidence of neurogenic atrophy of the intrinsic laryngeal muscles, p. 89. n The exact reason for the differential involvement of abductor and adductor muscle atrophy in ILH is unknown, but alterations in axonal transport appear to be the most likely