The rigid bony and cartilaginous structures of the upper airway are minimally affected by changes in airway dynamics. However, less rigid soft tissue and muscle structures, such as the nares, nasopharynx, and larynx, rely on neuromuscular activity to maintain stability during respiration. Any functional weakness or structural defect in these structures may result in inability to resist airway pressure gradients and airflows. This becomes increasingly important during exercise, when gradients and flows are high. Any abnormality of the upper airway that decreases the diameter or function of the airway can therefore limit respiratory function, especially in the athletic horse.

Diagnosis

Accurate history and thorough physical examination are essential and should always be performed when horses are evaluated for upper respiratory tract dysfunction. Common presenting complaints include abnormal respiratory noise, exercise intolerance, and poor performance. In the racehorse, abrupt decline in performance, gradual deterioration at the end of the race, or other ill-defined forms of poor performance are also common complaints. In sport horses, the most common historical finding is that the horse makes abnormal respiratory noise. Horses competing in disciplines that require enforced poll flexion or tension may make abnormal noises while exercising in this position. Physical examination is centered on the head and neck, and should include external manual palpation of the larynx for detection of arytenoid deformities or prominence of the muscular process that develops secondary to atrophy of the cricoarytenoideus dorsalis muscle in horses with RLN. Thickening of the ventral throatlatch region or the lateral aspect of the larynx may be detected in horses that have undergone previous airway surgery.

Complete laryngeal paralysis and structural upper airway abnormalities are easily identifiable during resting endoscopic examination. However, many obstructions are dynamic, and resting observations are notoriously unreliable in predicting exercising upper airway function. Exercising endoscopic evaluation of the upper airway is required to correctly diagnose the obstruction and is indicated in horses that do not have obvious abnormalities at rest, those with questionable laryngeal or pharyngeal function, and those that have any history of making an abnormal respiratory noise during exercise.

The alar fold is a thick fold of skin extending rostrad from the ventral nasal concha. The false nostril is the space dorsal to the alar fold. During exercise, the alar fold is tensed and the space is obliterated. Inappropriate nostril dilation or redundant tissue results in alar fold collapse during exercise. The condition was originally described in Standardbred racehorses. Affected horses are normal at rest but make a loud objectionable fluttering expiratory noise during exercise. The diagnosis is made by placing a suture through the alar folds and securing them in a dorsal position. Exercising noise is greatly diminished or eliminated after suture placement. Treatment includes surgically resecting the folds or securing them during exercise. Resolution of noise and return to racing is the outcome in most surgically corrected horses.