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The Function of the Extrinsic and Intrinsic Musculature in Stabilizing the Upper Airways
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Introduction:
Dynamic upper respiratory obstructive diseases are common in equine athletes and many of these disorders are attributed to dysfunction of skeletal muscles that control airway caliber. As veterinarians, we are familiar with most of the laryngeal obstructive diseases that involve collapse of the arytenoid cartilages and/or vocal folds across the rima glottis, impairing ventilation and causing poor performance. Nasopharyngeal obstructive diseases are as career limiting for performance horses as laryngeal obstruction, but the etiologies and effective treatments of these syndromes are much less clear.
Nasopharyngeal dynamic collapse occurs in horses during strenuous exercise, unlike obstructive sleep apnea in people. The diseases are similar in that collapse of the pharynx just rostral to the epiglottis, particularly the caudal portion of the soft palate, is implicated in both syndromes. Dorsal displacement of the soft palate (DDSP) is the most common manifestation of nasopharyngeal dynamic collapse in horses. Other forms of nasopharyngeal collapse include restriction in both the dorsoventral and axial-abaxial planes. These upper airway obstructive diseases likely have multiple etiologies. Treatments range from anti-inflammatory therapy to surgical intervention that modifies the pharyngeal airway by extirpation of soft tissue or modification of the underlying craniofacial skeleton.
This presentation will focus on intrinsic and extrinsic muscles that control the dilation and stability of the nasopharynx. We will present information that suggests that anomalies of soft palate musculature and its innervation may occur but are not an essential requirement for nasopharynx instability.
Anatomy:
The nasopharynx is a musculomembranous channel originating just caudal to the nasal septum and extending to the larynx. The nasopharynx must withstand large changes in intraluminal pressure, from 30-60 cm H2O during inspiration and expiration, without cartilage or bony support. In addition, the nasopharynx is exposed to turbulent airflow at peak velocities of 90 to100 L/s that causes vibration of the soft tissues within the nasopharynx that tend to lift the soft palate during expiration. The stability of the nasopharynx during these huge changes in intraluminal pressures and airflow is achieved by contraction of skeletal muscles. These muscles include extrinsic pharyngeal muscles, such as the muscles that alter the shape and position of the tongue and muscles that control the position of the hyoid apparatus, and intrinsic pharyngeal muscles, which include a constrictor group of muscles located in the dorsal pharynx and a group of muscles that regulate the position of the soft palate. Local sensory reflexes play an important role in signaling contraction of the extrinsic and intrinsic pharyngeal musculature during changes in pressures and airflow. [...]
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