Upper airway diseases/obstruction are relatively common causes of respiratory distress in dogs and cats. However, because
lung parenchymal diseases are more frequently observed, upper airway problems may be overlooked. In order to fully appreciate
upper airway disease, it is essential to be familiar with the structure, function, and common abnormalities.
The upper airway begins with the mouth, nose and pharynx. The principal role of these structures is the conduction of air.
Additionally, they also serve to filter larger particle debris from the inspired air. Respiratory problems related to the
pharynx generally reflect obstruction to airflow and are characterized by inspiratory distress. Common causes of pharyngeal
airflow obstruction include brachycephalic airway syndrome (excessive soft tissues of the pharynx and elongated soft palate),
abscesses (from penetrating objects like sticks), mucoceles and neoplasia (lymphoma or metastatic diseases). In young cats,
nasopharyngeal polyps may also cause airway obstruction. Diagnosis is based upon visual examination ± biopsy. Therapy for
pharyngeal diseases usually involve weight loss (for excessive tissues) or surgical intervention (elongated soft palate /
nasopharygneal polyps/mucoceles etc).
Laryngeal diseases are fairly common in small animals. The function of the larynx is to permit the flow of air into the lungs
and also to guard the airway against aspiration of foods or liquids. The larynx is responsible for a large portion of the
airflow resistance in the upper airway. Additionally, the larynx is responsible for vocalization. The neuromuscular control
of the larynx is through the recurrent laryngeal nerve for the abductors (dorsal cricoarytenoideus is the most important)
and the cranial laryngeal nerve (for the abductors). Clinical signs of laryngeal disease include noisy breathing, stridorous
breathing, increased inspiratory effort, coughing and voice change. Signs may develop over time or may appear to develop
acutely. Clinical signs are usually pronounced with exercise and may correspond with the first hot and humid days of summer.
Auscultation over the larynx will reveal loud sounds, which may be referred into the thorax. The most common laryngeal problem
in dogs is laryngeal paralysis. Laryngeal paralysis may be congenital (eg. Bouviers, Siberian Huskies, Rottweilers, Dalmatians)
but is much more frequently acquired in large breed dogs. Commonly observed breeds of dogs include the Labrador retriever
and setters although any breed may be affected. The cause is generally not identified. (Idiopathic) Hypothyroidism was historically
thought to be associated with the development of laryngeal paralysis but this is no longer considered true. Rarely, afflicted
dogs will have other signs of a polyneuropathy (megaesophagus, generalized muscle weakness). In contrast to horses, clinical
signs in dogs are usually associated with bilateral paralysis. In brachycephalic breeds (eg. Bulldog) laryngeal collapse
may also occur. Other laryngeal diseases observed in dogs include webbing (after debarking surgery) neoplasia (squamous cell
carcinoma, lymphoma etc) or abscess/granuloma (infectious). Everted laryngeal saccules (lateral ventricles) may develop secondarily
to upper airway obstructions (and resultant translaryngeal pressure changes). Over time, these saccules may become fibrotic
and contribute to permanent airway obstruction.
In cats, laryngeal diseases are much less common. Clinical signs in cats are similar to those observed in dogs. Rarely, cats with
significant volume pleural effusions may appear to have upper airway obstruction. Laryngeal paralysis in the cat is frequently associated with neoplasia or may develop after neck surgeries (thyroidectomies). In cats, unilateral
paralysis seems more likely to cause clinical signs. Cats may also develop laryngeal tumors (SCC, lymphoma) or granulomas.