How to Ultrasound the Equine Larynx

Abnormalities of the upper airway (UA) are frequently implicated as a cause of poor performance, exercise intolerance, or abnormal respiratory noise in the equine. Common abnormalities involving the function of the arytenoid cartilages include recurrent laryngeal neuropathy, arytenoid chondritis, and laryngeal dysplasia. Dorsal displacement of the soft palate (DDSP) and pharyngeal collapse affect the pharyngeal region. Congenital abnormalities and dynamic larynx collapse associated with poll flexion can affect multiple regions of the larynx and pharynx. Resting UA endoscopy is widely available and is often the first diagnostic tool used in evaluation of UA abnormalities. In many cases, resting endoscopy is sufficient for diagnosis. However, dynamic evaluation, either using a treadmill or a portable over ground videoendoscopy system, is considered to provide a more accurate diagnosis than resting endoscopy. Unfortunately, dynamic UA examination requires specialized equipment that may not be available locally, may be cost prohibitive for some clients, and may not be appropriate for some horses. Additionally, some UA abnormalities cannot be fully assessed using UA endoscopy. Laryngeal ultrasonography, first described by Chalmers et al. in 2006, is a relatively new addition to the diagnostic repertoire for UA disorders. It has proven to be useful clinically in the diagnosis of many conditions of the UA, especially disorders resulting in abnormal arytenoid movement including recurrent laryngeal neuropathy, arytenoid chondritis, and laryngeal dysplasia, a congenital malformation of the larynx. Laryngeal ultrasonography can be especially helpful in cases where a dynamic examination cannot be performed. An appreciation of the pathologic changes that occur in diseases of the UA is helpful in understanding the ultrasonographic changes that accompany each disease. Recurrent laryngeal neuropathy results in denervation atrophy and subsequent loss of function of the intrinsic laryngeal muscles innervated by the recurrent laryngeal nerve, including the cricoarytenoideus dorsalis (CADM), cricoarytenoideus lateralis (CALM), transversus arytenoideus, vocalis, and ventricularis muscles. Clinically, this condition manifests itself as decreased or absent arytenoid abduction and affects the left arytenoid more commonly than the right arytenoid. Denervated muscle has a characteristic ultrasonographic appearance; the muscle becomes hyperechoic to normal muscle and has a more homogeneous appearance with a loss of the normal striated pattern. These changes have been observed ultrasonographically in the CALM and CADM of horses with treadmill UA endoscopy-confirmed recurrent laryngeal neuropathy. The precise etiology of arytenoid chondritis remains unclear but likely results from local trauma