Brachycephalic airway syndrome (Proceedings)
Brachycephalic breeds (Shih Tzu, boxer English and French Bulldog, Pekingese, pug and Boston terrier) have a shortened skull compared to the other breeds. Compression of the nasal passage and distortion of the pharyngeal tissue result in an increase in airway resistance. Brachycephalic airway syndrome includes stenotic nares, elongated soft palate, everted laryngeal saccules and laryngeal collapse. There is a high incidence of hypoplastic trachea found in brachycephalic dogs that contributes to airway distress.
Stenotic nares and elongated soft palate are the primary anatomic components of the syndrome while everted laryngeal saccules with laryngeal collapse are thought to be secondary. Excessive negative pressure generated at inspiration because of stenotic nares creates inflammation and stretching of soft tissue and eventually eversion of the laryngeal saccules and laryngeal collapse. Stenotic or obstructed nares affect the mechanics of the lungs and provoke degenerative changes of the nasal mucous membrane. Severe upper airway obstruction can result in pulmonary edema because of a reduction of intrathoracic pressure. The greatest changes are observed in dogs with partial bilateral nasal obstruction and high nasal resistance. Inadequate pulmonary ventilation due to upper airway obstruction can lead to a reduction of arterial oxygen content. The hypoxia is a potent pulmonary vasoconstrictor to divert away blood from poorly ventilated alveoli. Pulmonary vasoconstriction and pulmonary hypertension result in cor pulmonale and right sided heart failure.
Clinical Findings And DiagnosisBrachycephalic breeds are presented for excessive noisy breathing and inspiratory dyspnea. Inspiratory dyspnea is exacerbated by exercise and augmentation of the ambient temperature. Some English Bulldogs have been presented for vomiting not associated with meals. An increase frequency of hiatal hernia seems to be present in English Bulldogs with brachycephalic airway syndrome. The mean age for dogs presented is 3 to 4 years old. Many of these animals have a high potential to decompensate and develop acute respiratory distress. Therefore, they must be handled carefully to prevent stress and acute decompensation. It is important to keep the animal calm and in a cool environment. Supplemental oxygen might be required.
Physical examination of the nares for stenosis should be performed. Breathing pattern should be observed. Brachycephalic dogs are presented with an inspiratory dyspnea that is corrected by open mouth breathing if only the nares are involved in the syndrome. If the soft palate is elongated, the laryngeal saccules and/or the larynx collapse the dyspnea is inspiratory and expiratory. The severity of inspiratory dyspnea depends on the length and congestion of the soft palate and other restrictive or obstructive conditions present. An obstructive breathing pattern, characterized by a slow inspiratory phase followed by a rapid expiratory phase is seen frequently in brachycephalic breed even if the airway diameter is not comprised more than 50%. In nonbrachycephalic breed, a reduction of more than 50% of the airway diameter is required to modify the breathing pattern. The greatest airway noise is usually noticed in the larynx. Auscultation of the lung field is difficult because of enhanced upper airway sounds.
A radiological examination of the larynx shows an elongated soft palate protruding in the rima glottidis. It is also important to evaluate the diameter of the trachea in dogs with brachycephalic syndrome. It is very common to diagnose an hypoplastic trachea, which worsens the prognosis. Comparison of the diameters of the thoracic inlet (TI) and the tracheal lumen (TD) makes the diagnosis of tracheal hypoplasia. In normal Bulldogs the ratio TD/TI is 0.106 English Bulldogs have the highest incidence of hypoplastic trachea within brachyocephalic breeds (55%). Thoracic radiographs allow evaluation of lung fields for signs of pulmonary edema, pneumonia and the heart for signs of right-sided dilation. If there is cardiac enlargement an echocardiography and an electrocardiogram are required to evaluate myocardial function and arrhythmias. Diagnosis of hiatal hernia can also be done with radiographs.
Blood work is usually within normal limits since the animals are young at the time of diagnosis. An augmentation of the pack cell volume might be indicative of a mild to moderate hypoxia
A laryngeal examination is required under light general anesthesia to visualize the soft palate, the laryngeal saccules and the function of the larynx. The soft palate should not extend passed the tip of the epiglottis. Position of the soft palate is influenced by the position of the head, traction on the tongue and presence of an endotracheal tube. Evaluation of the soft palate should be performed without an endotracheal tube in place and with the tongue in a normal position. Everted laryngeal saccules are white shiny dome shape structures located cranial to the vocal cords. Tonsils should also be evaluated as well as the presence of redundant mucosal folds in the pharynx/larynx. A medial tipping of both corniculate processes both and medial flattening of the cuneiform processes of the arytenoid cartilage characterize a laryngeal collapse. The vocal cords are usually not visualized if the larynx is collapse. Usually the corrective surgery is performed during the same anesthesia because recovery from anesthesia with compromise airways could be life threatening.
Stenotic nares are frequently diagnosed in younger, brachycephalic dogs (less than 2 years) with an overlong soft palate and have a favorable prognosis after surgical treatment. In brachycephalic dogs older than 2 years, stenotic nares are associated with additional airway obstruction, and these patients have a guarded prognosis even with treatment. Surgical treatment is therefore recommended as soon as possible to prevent further deterioration of the animal condition and prognosis. English Bulldogs are not responding as well as the other breeds to surgery probably because of the higher incidence of hypoplastic trachea in this breed.