CHAPTER 56 Disorders of the Trachea
The equine trachea is a rigid yet flexible cartilage-supported tube approximately 75 to 80 cm long in adult Thoroughbred-type horses and extends from the larynx to the carina, where it divides into the two mainstem bronchi at the level of the fifth to sixth ribs. Primary tracheal disorders are relatively uncommon in horses. In horses with pulmonary disorders, however, the tracheal mucosa may be inflamed, manifested as redness of the mucosa and blunting of the carina; and the lumen of the rostral, intrathoracic portion of the trachea may contain excessive volumes of abnormal (usually mucopurulent to purulent) respiratory secretions. Consequently endoscopic examination of the trachea is an essential part of any in-depth pulmonary examination in horses.
Lateral tracheal flattening, also known as scabbard trachea, occurs to some degree in the first few tracheal rings of many Thoroughbred-type horses, but rarely results in clinical airflow obstruction. This feature can be recognized by palpation of the ventral aspect of the V-shaped tracheal rings that protrude through the overlying muscles in the rostral or cervical portion of the trachea. Endoscopic evaluation enables confirmation of this slight lateral flattening of the upper tracheal lumen, but invariably reveals the airway to be adequate.
Dorsoventral tracheal collapse is a common disorder of small ponies, especially Shetland Ponies and Miniature horses, but the condition can also develop in larger pony breeds and has a high prevalence in donkeys. In many small ponies and donkeys, the disorder remains undiagnosed because the limited degree of tracheal collapse that develops does not limit the usual low workload of these animals.
Dorsoventral tracheal collapse is most commonly caused by a cartilage ring deformity, with the normal round- or oval-shaped tracheal ring replaced by a dorsoventrally flattened tracheal ring in which a greatly reduced dorsoventral distance is further decreased by excessive dorsoventral movement of an elongated and slack dorsal tracheal ligament. In addition to dorsoventrally flattened tracheal rings, especially in the lower portion of the cervical trachea, many affected ponies and donkeys also have other types of tracheal ring deformities at other sites that also restrict tracheal lumen size. Other animals can have relatively normally shaped tracheal rings but have separation of the dorsal ligament from the usual attachments to the dorsal aspect of the tracheal rings. Tracheal collapse most commonly involves the distal portion of the cervical segment of the trachea and rostral portion of the intrathoracic segment of the trachea, but it occasionally involves the remaining segments of trachea and even the mainstem bronchi. Some affected ponies may also have some degree of tracheal rotation, with the dorsal aspect of the trachea facing dorsolaterally.
A similar disorder in miniature dogs is associated with hypocellularity and decreased chondroitin sulfate content of the tracheal cartilage, suggesting that an inherited defect in chondrogenesis is the underlying cause, but such studies are lacking in horses. Most affected ponies develop clinical signs of tracheal collapse in middle age, indicating that a degenerative process of the tracheal cartilage or connective tissue, including separation of the dorsal ligament from the cartilage, also plays a role in the pathogenesis of the disorder, in addition to the apparent developmental cartilage deformity.
The usual clinical signs in affected ponies and donkeys are respiratory distress and stridor, which often occur for short periods during hot or humid weather and regress in the autumn and winter. The finding of stridor, which indicates upper respiratory tract obstruction, can help to differentiate this disorder from summer pasture–associated obstructive pulmonary disease, in which respiratory distress without stridor is the main presenting sign. Severe respiratory distress may develop even at rest in animals with tracheal collapse because the increased intrathoracic pressure changes caused by the partial tracheal obstruction causes further dynamic tracheal collapse in a self-perpetuating cycle. In severe cases, exercise-induced pulmonary hemorrhage may occur, presumably because of the increasingly subatmospheric intra-alveolar pressures that are a consequence of airway obstruction.
Palpation of a dorsoventrally collapsed distal portion of the cervical trachea may reveal a distinct, even sharp, edge on the lateral aspect of a trachea at the usual site of the rounded, lateral aspect of a normal trachea. Additionally, a flattened, deformed trachea does not roll away under lateral digital pressure, as will a normal rounded trachea. However, palpation of the lateral edges of the flattened trachea can be difficult because the deeper, distal portion of the cervical segment of the trachea is most commonly affected, and some small ponies have thick subcutaneous tissues overlying this area.