Horses are obligate nose breathers, so such collapse is a true problem. Inflammation of the nasal sinus, sinusitis are a common problem with many etiologies of upper respiratory tract disease. The pharynx is a relatively large space, compared to other structures in the airway but pharyngitis, if severe enough, can impede airflow. Pharyngeal lymphoid hyperplasia is a condition that is not uncommon. Here lymphoid tissue swells and can create inspiratory noise.
Diagnosis is by endoscopy.
Air is channeled over the hard palate and soft palate towards the opening of the trachea. The soft palate must be rigid enough so as not to displace during inspiration. If the palate is too flexible, or if the tongue pushes hard enough, the palate will displace dorsally, obstructing airflow. Displacement may only occur under extreme air movements such as characterized during racing. Thus diagnosis may require endoscopy on the treadmill.