Brachycephalic obstructive airway syndrome

What it is

Brachycephalic obstructive airway syndrome (BOAS) is a set of anatomical abnormalities that results from selecting for a short muzzle: the bones of the face are shortened, but the soft tissues are not reduced to the same extent and are crowded into a smaller space. The primary components are stenotic nares, an elongated and thickened soft palate, aberrant nasal turbinates that obstruct the nasal cavity and nasopharynx, macroglossia and a hypoplastic trachea. The increased negative pressure needed to breathe through these obstructions causes secondary changes: eversion of the laryngeal saccules, tonsillar eversion and progressive laryngeal collapse. Gastrointestinal signs (regurgitation, vomiting, hiatal hernia) are frequently associated.

Who it affects

Brachycephalic breeds: English bulldog, French bulldog, pug, Boston terrier, Pekingese, shih tzu, boxer, Cavalier King Charles spaniel and others. Signs are often evident from a young age and worsen with time, obesity and heat.

Anatomy involved

• Skull: shortened facial skeleton (maxilla, incisive and nasal bones) with a short nasal cavity; the conformation is the root of the syndrome.
• Nostrils, nasal cavity and nasopharynx: narrow nares and aberrant turbinates.
• Soft palate: extends past the tip of the epiglottis and obstructs the laryngeal entrance.
• Larynx: everted saccules and, in advanced cases, collapse of the cuneiform and corniculate processes of the arytenoid cartilages.
• Trachea: a hypoplastic trachea with a small diameter relative to the thoracic inlet, mainly in English bulldogs.

Clinical signs

Snoring and stertor, inspiratory stridor, open-mouth breathing, exercise and heat intolerance, sleep-disordered breathing and sleeping in a sitting position, gagging and retching. Severe cases show cyanosis, syncope and life-threatening crises with hyperthermia, especially in hot weather or under stress. Aspiration pneumonia and non-cardiogenic pulmonary oedema may complicate them.

Diagnosis

• Physical examination: visible stenotic nares, noisy breathing; functional exercise tests are used in some breeding schemes.
• Examination under anaesthesia (oral and laryngeal): assesses the length and thickness of the soft palate, saccules, tonsils and the degree of laryngeal collapse; it is often done immediately before surgery.
• Cervical and thoracic radiographs: tracheal diameter relative to the thoracic inlet, aspiration pneumonia, hiatal hernia.
• Rhinoscopy and CT: for aberrant turbinates and nasopharyngeal obstruction.

Treatment

Weight control, a harness, and avoiding heat and strenuous exercise in all cases. Surgical correction is best done early, before secondary changes develop: alarplasty (widening the nares), staphylectomy or folded-flap palatoplasty (shortening and thinning the soft palate), and resection of everted saccules; laser-assisted turbinectomy is done in selected cases. Severe laryngeal collapse may require a permanent tracheostomy. Because the problem is conformational, breeding toward a longer muzzle and open nares is the long-term solution.

In the cat

Brachycephalic cats (Persian, Exotic Shorthair, Himalayan and, to a lesser extent, British Shorthair) have a similar syndrome, although severe respiratory crises are less common than in the dog and laryngeal collapse is rare. The most consistent finding is stenotic nares; some cats also have an elongated soft palate and aberrant nasal turbinates, and brachycephaly has been associated with more respiratory noise and signs. Extreme shortening of the face also causes problems beyond the airway: distortion of the nasolacrimal duct with epiphora and tear staining, nasal folds rubbing the cornea, prominent eyes with a greater risk of ulcers and corneal sequestrum, dental crowding and malocclusion, and skull malformations with internal hydrocephalus in the flattest-faced Persians. Cats with very narrow nares benefit from surgical widening; as in the dog, the long-term solution is selecting less extreme faces.

Key studies in the cat

• Schlueter et al. (2009), CT and anatomical study of 31 brachycephalic and 15 mesocephalic cats. The more brachycephalic the head, the more the facial bones and upper canine teeth are rotated dorsally. In severe cases the nasolacrimal duct is forced into a steep, V-shaped course under the canine tooth and seems to drain poorly.
• Farnworth et al. (2016), owner survey of 1000 cats with facial measurements taken from photographs. A shorter muzzle and a higher position of the nose were associated with more respiratory noise during sleep and more breathing difficulty after activity, and that respiratory score was also associated with tear staining. Signs were reported by owners.
• Schmidt et al. (2017), CT of 92 Persian and 10 domestic shorthair cats. Persians with extreme brachycephaly ("peke-face") had shorter nasal bones and malformations of the skull and teeth, and much larger cerebral ventricles than less brachycephalic ("doll-face") Persians (ventricle to brain ratio 0.159 versus 0.015).
• Sieslack et al. (2021), CT of 69 Persian and 10 domestic shorthair cats. Higher grades of brachycephaly were associated with more of the eyeball lying outside the bony orbit, a lower bony nasal aperture and more displacement of the upper premolars, while other measurements of the nasal airway did not correlate. Some Persians overlapped with the domestic shorthairs.
• Gleason et al. (2023), owner questionnaire on 194 brachycephalic and 1003 non-brachycephalic cats. Brachycephalic cats more often snored (odds ratio 6.9), had nasal discharge (8.3), dyspnoea (5.3) and difficulty chewing (5.2), became dyspnoeic sooner during activity and recovered more slowly. Signs were reported by owners.