Left recurrent laryngeal hemiplegia is characterized by paresis or paralysis of the left arytenoid cartilage and vocal fold. It manifests clinically as exercise intolerance and inspiratory respiratory noise (“roaring”) during exercise. Right-sided hemiplegia and bilateral (paraplegia) arytenoid dysfunction are uncommon.
Progressive loss of the large myelinated fibers in the distal portion of the recurrent laryngeal nerves results in neurogenic atrophy of the intrinsic laryngeal musculature, the most crucial of which is the cricoarytenoideus dorsalis muscle. Axonal dystrophy of the left recurrent nerve occurs more commonly than the right, perhaps due to its extended length around the base of the heart. Left laryngeal hemiplegia is likely heritable. Less common causes include direct trauma to the recurrent laryngeal nerve, accidental perivascular injection of irritating substances, and plant (eg, Cicer arietinum [chick peas] and Lathyrus spp) and chemical intoxications. Lead toxicity should be suspected in horses with bilateral laryngeal paralysis. The peroneal nerve (similar length to the left recurrent laryngeal) may be affected with toxic insults, and axonal dystrophy of the peroneal nerve may manifest as stringhalt (see Stringhalt). Although all breeds are affected, prevalence is higher in males and long-necked/larger breeds. The prevalence in young Thoroughbreds presented for sale is estimated to be ~3%–5%.
Loss of neuromuscular control of the abductor muscle results in collapse of the arytenoid cartilage and vocal fold, which reduces the glottal cross-sectional area. The resistance to airflow necessitates greater respiratory effort. Because of the pliable nature of the glottis, the exaggerated subatmospheric pressure in the airway results in further collapse of the arytenoid cartilage and exacerbation of the impedance to airflow. Upon inspiration during strenuous exercise, the affected side is drawn across the midline (by negative pressure in the airway) until it abuts the abducted normal arytenoid, effectively occluding the airway (dynamic collapse). The characteristic inspiratory whistle results from resonance within the open ventricle on the affected side. The harsher stridor, or roar, is produced by vortex shedding from the edges of the arytenoid cartilage and vocal fold.
The principal clinical signs are inspiratory noise during exercise and exercise intolerance. Affected horses are asymptomatic at rest but may have an unusual whinny. Diagnosis is confirmed by endoscopic observation of reduced or absent mobility of the arytenoid cartilage and vocal fold. With laryngeal hemiplegia, the arytenoid cartilage and vocal fold are located in a median position within the laryngeal lumen and are immobile. Asynchronous movements of the laryngeal cartilages occur commonly, with variable clinical relevance. Horses with laryngeal asynchrony, exercise intolerance, and respiratory noise during exercise should have their laryngeal function evaluated endoscopically during treadmill exercise to confirm laryngeal dysfunction.
Differential diagnoses include other pharyngeal conditions that produce upper airway obstruction and exercise intolerance. Most of these conditions are easily differentiated from laryngeal hemiplegia during endoscopic examination. Although arytenoid chondritis may be confused with laryngeal hemiplegia, misdiagnosis can be avoided by observation of the shape and size of the arytenoid cartilages. In arytenoid chondritis, the arytenoids thicken transversely and lose their characteristic “bean” shape. Abduction and adduction are usually limited. The axial (medial) surface of the arytenoid cartilage may be distorted with granulation tissue protruding through the mucosa, and a contact (kissing) lesion may be present on the contralateral arytenoid cartilage. Arytenoid chondritis should always be considered if motility of the right arytenoid is reduced. Radiographic examination of the pharynx may reveal mineralization within the arytenoid cartilages in cases of chondritis.
Prosthetic laryngoplasty can stabilize the affected side of the larynx during inspiration and prevent dynamic collapse of the airway during exercise. Laryngeal ventriculectomy performed via laryngotomy, or ventriculocordectomy performed via transendoscopic laser, improves airflow and reduces the “roaring” sound during exercise. Prosthetic laryngoplasty is commonly done in racing horses and is the only technique that satisfactorily reduces the impedance to inspiratory flow. Postoperative complications include chronic cough, chronic aspiration of feed, implant failure, and implant infection. Athletic performance will improve after surgery; however, horses are more likely to experience inflammatory airway disease and exercise-induced pulmonary hemorrhage, have fewer race starts, and are unlikely to develop their predicted performance potential.