Chapter 161 Obstructive Upper Airway Disorders
Obstructive upper airway diseases typically are insidious in onset and result in progressively worsening respiratory stridor and dyspnea. The dimensions of the upper airway play a fundamental role in the efficiency of breathing and progression of disease. In narrowed regions of the upper airway, air velocity is higher and pressure is lower than elsewhere (Bernoulli effect), which tends to narrow susceptible regions of the upper airway still more.
Congenital obstructive upper airway diseases are diagnosed so commonly in brachycephalic dogs that they are referred to collectively as the brachycephalic syndrome. Most other obstructive upper airway diseases are acquired and are diagnosed in middle-aged and older dogs. Obstructive upper airway diseases are diagnosed less frequently in cats.
Laryngeal collapse is recognized most commonly in brachycephalic dogs with chronic obstructive upper respiratory disease (brachycephalic syndrome). Loss of the supporting function of the laryngeal cartilages results from pressure changes within the larynx induced by obstruction rostral to the rima glottidis. Laryngeal collapse is considered a progressive, end-stage disease. However, some dogs have a mild to moderate form of the condition and can do reasonably well with medical management.
This condition is seen primarily in old, large-breed, and giant-breed dogs. Interruption of the innervation to the intrinsic muscles of the larynx, particularly the cricoarytenoideus dorsalis muscle, results in failure of the arytenoid cartilages and vocal folds to abduct during inspiration. The condition is a congenital anomaly in some breeds, such as the Bouvier des Flandres, Siberian husky, and dalmatians. Laryngeal paralysis also has been associated with hypothyroidism (see Chapter 31) and diffuse polyneuropathies in dogs (see Chapter 129). In many cases laryngeal paralysis is idiopathic.
Tracheal foreign bodies rarely cause complete obstruction; large items usually are retained at the carina, whereas small ones often pass into the bronchi, leading to bronchial obstruction and pneumonia.
Segmental tracheal stenosis can be caused by compression from extraluminal masses such as parasitic granulomas (Oslerus [Filaroides] osleri), thyroid carcinoma (see Chapter 31), hilar lymphadenopathy (see Chapter 165), left atrial enlargement (see Chapter 149), mediastinal lymphoma (see Chapter 27), and mediastinal lipoma.
Fluoroscopy allows continual assessment of the tracheal lumen diameter during all phases of the respiratory cycle. In dogs suspected of having tracheal collapse, evaluate the entire trachea and both main stem bronchi during quiet respiration and induced coughing.