Causes of Brachycephalic Airway Syndrome
Brachcephalic airway syndrome (BAS) is caused by several abnormal physical attributes of affected dogs. First, brachycephalic breeds have structural deformities in their skulls and nasal passageways, caused by generations of selective breeding (by people) to create the exaggerated domed heads, broad foreheads, flat faces and almost nonexistent muzzles characteristic of these breeds. The narrowed nostril openings (stenotic nares) that are so typical of dogs with BAS prevent the normal flow of air into the upper respiratory tract when the dog breathes in (on inspiration). Dogs with BAS also have an elongated soft palate, which is the fleshy structure that separates the back of the mouth from the end of the nasal passages. This redundant soft tissue contributes to increased upper airway resistance and breathing difficulty (dypsnea), because it acts as a physical barrier to the free passage of air. The combination of these anatomical abnormalities leads to inflammation and swelling of the lining of the nasal passages, pharynx (throat), larynx (”voice box”) and trachea (“windpipe”), which further exacerbates the dyspnea already associated with this syndrome.
There is no realistic way to prevent a dog from developing brachycephalic airway syndrome. Affected dogs are born with the anatomical abnormalities that cause the respiratory difficulties associated with this condition. Episodes of respiratory distress may be able to be avoided or at least minimized by reducing the stress in the dog’s environment, limiting its exercise or exertion, avoiding high temperature enclosed environments and preventing obesity.
Brachycephalic airway syndrome frequently leads to secondary changes to upper respiratory tract tissues because of the increased airway resistance caused by the condition. These commonly include laryngeal collapse and laryngeal saccule eversion. The larynx is the muscular and cartilaginous structure situated at the top end of the trachea, behind the root of the tongue. It is made up of 9 separate cartilages that are held together by ligaments and muscles. The larynx is lined with mucous membrane tissue and contains the dog’s vocal cords. Another name for the larynx is the “voice box.” The laryngeal saccules are small paired structures lining the laryngeal ventricle, which is simply an outpocketing of the larynx. When the laryngeal saccules evert, or if the larynx itself collapses (several of the cartilages fold inward), the passage of air from the mouth through the larynx and trachea to the lungs is further obstructed, making the dog’s breathing much more labored, especially when breathing in. In medical lingo, this is called “increased inspiratory effort.”
Hot, humid weather can cause rapid deterioration and severe respiratory distress (difficulty breathing; dyspnea). Confinement in an area with high ambient temperature, such as in a car with poor ventilation, and exercise or exertion, also exacerbate the symptoms of this syndrome.