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Brachycephalic Upper Airway Obstruction Syndrome, BAOS, BAS, BUAOS

Canine Skull Skeleton: French Bulldog with brachycephalic features
Canine Skull Skeleton: French Bulldog with brachycephalic features

Brachycephalic Upper Airway Obstruction Syndrome (BUAOS) refers to an inherited respiratory disorder, a physical abnormality from birth that causes shortening of the bones of the skull when the soft tissues within the upper airways do not reduce in size proportionally plus the maldevelopment of the airway cartilages.

What are the clinical signs of Brachycephalic Upper Airway Obstruction Syndrome (BUAOS)?

The main clinical signs of Brachycephalic Upper Airway Obstruction Syndrome (BUAOS) manifest early in a dog's life, around 2 - 4 years of age, and always progress over time if not treated, and they include:-

  1. Excessive panting, dyspnea.

  2. Stertorous and stridorous breathing.

  3. Respiratory effort.

  4. Exercise and heat intolerance.

  5. Regurgitation and vomiting.

  6. Sleep-disordered breathing.

  7. Cyanosis and sudden collapse, particularly in hot weather.

Almost total upper airway obstruction can lead to non-cardiogenic pulmonary oedema, aspiration pneumonia, and hyperthermia, leading to rapid death without treatment.

Which Breed of dogs is predisposed to Brachycephalic upper airway obstruction syndrome (BUAOS)?


The following dog breeds are highly predisposed:-

  1. Boston terrier

  2. Cavalier King Charles spaniel

  3. Chihuahua

  4. Dogue de Bordeaux

  5. English (British) bulldog

  6. French bulldog

  7. Japanese Chin

  8. Lhasa apso

  9. Pekingese

  10. Pomeranian

  11. Pug

  12. Shar-pei (Chinese shar-pei)

  13. Shih Tzu

  14. Staffordshire bull terrier

How does brachycephalic upper airway obstruction syndrome develop?


The syndrome comprises one or more of the following components:

  1. Narrowing/stenosis of the external nares, obstruction of nasal vestibule by pronounced ventral alae, aberrant and hypertrophied nasal turbinates with increased mucosal contact point.

  2. Nasopharyngeal narrowing and collapse.

  3. Narrow pharyngeal dimensions, pharyngeal collapse, elongated soft palate and inflamed and extruded tonsils.

  4. Narrow laryngeal dimensions, collapse of laryngeal cartilages, and redundant laryngeal soft tissues.

  5. Hypoplastic trachea, especially in the bulldog and bronchial collapse (especially in the Pug).

  6. Skull base malformation (e.g. medialization of the pterygoid processes).

What are the predisposing factors of brachycephalic upper airway obstruction syndrome?

  1. Obesity.

  2. Hot weather.

  3. Exercise.

  4. Excitement.

  5. Stress.

  6. Concurrent cardiac (heart) or pulmonary (respiratory) diseases.

  7. Concurrent or secondary gastrointestinal diseases with gastro-oesophagal reflux.

What is the mechanism (Pathophysiology) of this syndrome?


Airflow through airways is impeded due to abnormal anatomy causing noisy breathing and the inability to take in sufficient oxygen to meet increased demands imposed by exercise. The restricted airflow leads to the increased inspiratory effort that causes increased negative pressure within the upper airways, leading to a high intrathoracic negative pressure which in turn leads to eversion of laryngeal tissues and airway collapse plus a sliding hiatal hernia of the stomach. The increased respiratory effort leads to upper airway oedema that further obstructs airflow. Regurgitation leads to acid reflux and increases pharyngeal and laryngeal inflammation thus, a vicious cycle is set in motion. Impeded airflow prevents adequate heat loss through panting, so animals rapidly become hyperthermic in hot weather, following exercise or during stress. If the respiratory vicious cycle is left untreated, the dog may develop pulmonary oedema, reduced arterial oxygen content, hypertension, and right-sided heart failure.

How is brachycephalic upper airway obstruction syndrome diagnosed?



  1. Exercise intolerance.

  2. Noisy breathing (snorting noise, ‘clicking sound’ when panting).

  3. Loud snoring and disturbed sleeping. The dog may elevate the head when sleeping and/or hold a toy in the mouth to keep the mouth open while sleeping.

  4. Collapse.

  5. Signs may be exacerbated by exercise, excitement or hot weather.

  6. Gagging/retching cough.

  7. Regurgitation during exercise and/or excitement, or after eating/drinking

Clinical signs

  1. Stenotic nares with restricted nasal flaring.

  2. Stertorous or stridorous respiration increases laryngeal noise during laryngeal auscultation and inspiratory effort. Many of these signs only present after exercise. A short (3 minutes) trotting test (a.k.a. respiratory functional grading) is recommended.

  3. Cyanosis (the tongue turns blue due to reduced oxygen in circulation).

  4. Hyperthermia following exercise or stress due to the inability to cool body temperature by panting.

Laboratory investigation


Blood gas analysis -reduced arterial blood oxygen saturation in severe cases.

Whole-body barometric plethysmography (WBBP):

Whole-body barometric plethysmography (WBBP) is a non-invasive method that allows safe and repeated quantitative measurements of respiratory cycles on unsedated dogs.

WBBP flow trace shows a fixed-type obstruction, a dynamic-type obstruction, or both.

A BOAS index is used to calculate the respiratory patterns giving a severity score from 0 (BOAS free) to 100% (severe BOAS).

Endoscopic evaluation:

Oral examination and pharyngoscopy:

  • Pharyngeal narrowing.

  • Length of the soft palate.


  • Laryngeal dimensions and degree of collapse of the larynx

  • Degree of oedema of the laryngeal mucosa.

  • Laryngeal granuloma.

Tracheobronchoscopy (Tracheoscopy):

  • Tracheal collapse

  • Bronchial collapse.

Rhinoscopy and nasopharyngoscopy:

  • Narrowing of the nasal vestibule (ventral alae).

  • Increased mucosal contact point.

  • Nasopharyngeal turbinate protrusion.

  • Nasopharyngeal cyst.

Diagnostic imaging

Thoracic and head radiography:

  • Hypoplastic trachea.

  • Aspiration pneumonia.

  • Severe hiatal hernia.

Head and neck x-ray of a dog
Head and neck x-ray of a dog

Computed tomography of the thorax, neck and head

  • Lower airway and the heart structure.

  • Nasopharyngeal dimension.

  • Swollen lateral nasal glands.

  • Soft palate length and thickness.

  • The thickness of the tongue.

  • Skull base abnormalities.

  • Hypoplastic and collapsed trachea and bronchi.

  • Stenotic nares and collapsed nasal vestibules.

  • Hiatal hernia.

  • Oesophagal diverticula/ redundant oesophagal mucosa.

How is brachycephalic upper airway obstruction syndrome treated?


a.) In case of an emergency:

  1. The animal is sedated using either Acepromazine maleate (0.02-0.05 mg/kg) IV, IM or SQ, or Diazepam (0.2 mg/kg) IV may be combined with oxymorphone (0.5 mg/kg) or Butorphanol tartrate (0.3 mg/kg) IV, IM or SC.

  2. A glucocorticoid (Prednisolone 0.5 mg/kg BID or dexamethasone 1-2 mg/kg IV) is administered to reduce laryngeal oedema.

  3. Temporary tracheostomy may be required in severely cyanotic (blue) patients. A tracheostomy is an opening created at the front of the neck so that a tube can be inserted into the windpipe (trachea) to help an animal breathe. If necessary, the tube can be connected to an oxygen supply, and a breathing machine called a ventilator.

  4. Endotracheal intubation may be considered during the crisis as an option.

  5. Supplemental oxygen may be needed.

b) Removal exacerbating factors:

  1. Cage rest to reduce stress and excitement.

  2. Trazodone administration to reduce anxiety.

  3. Cool patient if hyperthermic with fans, alcohol baths or cold water sprays.

Sitting French Bulldog dog wearing blue cooling vest harness to lower body temperature on a hot day.
Sitting French Bulldog dog wearing blue cooling vest harness to lower body temperature on a hot day.

c) Standard treatment:

  1. Weight management (ideal body condition score 4-5/9).

  2. Surgical correction of anatomical abnormalities:

    1. Correction of stenotic nares enlargement and the enlarged and collapsed alar folds at nasal vestibules.

    2. Correction of the oversized soft palate.

    3. Correction of laryngeal collapse.

    4. Removal of protruded tonsils.

    5. Removal of the obstructive nasal turbinates (laser-assisted turbinectomy Laser-assisted turbinectomy (LATE)).

  3. Medical management of gastrointestinal signs.

How is brachycephalic airway obstruction syndrome prevented?


Breeding should only be done with dogs that have been selected based on exercise tolerance tests (e.g. respiratory functional grading) and that have had airway evaluation by veterinary healthcare professionals to encourage group eradication.



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  • Cambridge BOAS Research Group:

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