Brachycephalic Obstructive Airway Syndrome (BOAS)
“Brachycephalic” means “short-headed”
The French bulldog, bulldog, pug, pekingese, shih tzu, Japanese chin, boxer and Boston terrier are all examples of brachycephalic breeds. The most distinctive feature of these breeds is their short muzzle. Brachycephalic dogs have been bred for centuries to possess a normal-sized lower jaw, and a disproportionately shorter upper jaw.
In recent decades, breeding selection for extreme brachycephalic features has resulted in dogs that are predisposed to upper airway tract obstruction and subsequent respiratory distress, among several other health issues. Although not all brachycephalic dogs suffer clinical signs, the incidence and severity of the respiratory disorders has increased. The respiratory disease related to brachycephalic confirmation is called “brachycephalic obstructive airway syndrome (BOAS).
Pathophysiology of BOAS
Shortening of the skull and upper airway tract lesions
In brachycephalic dogs, while the length of the skeletal muzzle is reduced, there is often no corresponding decrease in the size of the soft tissue contained in the skull. This results in a constricted effect within the nasal cavity and partial obstruction of the pharynx and larynx.
Upper airway tract obstruction & poor thermoregulation
Excessive selection for brachycephaly has deformed the upper airway tract and resulted in obstructions. Affected brachycephalic dogs must increase their inspiratory effort to overcome the resistance generated by the upper airway obstruction and thus achieve sufficient ventilation. This increased inspiratory effort generates a high negative pressure that sucks the soft tissues into the lumen of the airway passage. Eventually, these tissues become hyperplastic and the laryngeal cartilages collapse, further exacerbating the airway obstruction. If this cycle is left uninterrupted, the dog may develop pulmonary oedema, reduced arterial oxygen content, hypertension and right-sided heart failure.
In addition to the respiratory function, BOAS also affects the crucial thermoregulatory role of the canine airway. This is evident in the severe heat susceptibility seen in affected brachycephalic dogs. Dogs do not sweat. Instead, they lower their body temperature by panting. When dogs with elongated skull shapes (like Labradors) pant, they are able to pass air quickly over the throat and nose. The dog cools down as saliva and moisture evaporate when air passes over the tongue and mucous membranes in the oral and nasal cavities. However, in the obstructed brachycephalic dog, the airway tract becomes inflamed and swollen during the panting process. This results in greater obstruction and further over-heating.
Another factor contributing to airway obstruction is obesity. Excessive weight results in fat tissue surrounding and narrowing the airway. Therefore, a weight loss program should be implemented to improve respiratory function in overweight brachycephalic dogs.
- Respiratory noise
- Stenosis of the nostrils
- Gastrointestinal signs
- Obstructive sleep apnea / sleep-disordered breathing
- Heat intolerance
- Cyanosis and collapse
Veterinary assessments for BOAS...
Dogs with normal upper airway tract breathe quietly. Respiratory noises such as snoring and snorting are indicators of airway obstruction. BOAS-affected dogs may present with different types of noise depending on the location of the obstruction: pharynx, larynx, and/or nasal cavity. Some BOAS-affected dogs may only have respiratory noises when they are excited, playing, exercising, eating/drinking or under stress. A thorough veterinary examination is recommended if the respiratory noise is marked.
This type of noise, termed 'stertor' is caused by the elongated and thickened soft palate. The caudal tip of a normal length soft palate should barely touch the epiglottis, so that when the dog pants with an open mouth, the airway is open. However, the soft palate in affected dogs is too long and extends into the opening of the airway (larynx). When the dog pants, extra effort is required to move the soft palate out of the larynx in order to allow air to pass. When the dog breathes through its nose, the increased negative pressure within the upper airway tract during inspiration can trigger vibration of the soft palate and redundant pharyngeal soft tissues - BOAS-affected dogs can be 'awake snorers'.
This type of noise is particularly common in affected pugs. It is called stridor and it is a high-pitched noise, similar to wheezing and different from low-pitched noises like snoring or snorting. Usually this type of noise indicates a narrowed or collapsed larynx. Laryngeal collapse is considered a secondary lesion that may appear as a consequence of leaving primary lesions (e.g., elongated soft palate and narrow nostrils) untreated.
Laryngeal collapse can be temporary and dynamic. During inspiration, the cartilaginous structures are drawn into the tracheal opening. When this phenomenon has happened for an extended period of time, the cartilaginous structures lose rigidity and laryngeal collapse may become permanent.
This type of noise indicates nasal obstruction, usually caused by stenotic nares as well as abnormal growth of nasal turbinates (bony or cartilage scrolls in the nose covered by mucosal membranes). In some dogs, a deviated nasal septum may worsen the situation. The narrowed nasal cavity results in an increase in negative pressure within the airway lumen, which causes soft tissue vibration and noise. This type of noise may be accompanied by nasal flaring, where muscles around the nose contract during nasal breathing. You may also hear a simultaneous, low-pitched and/or high pitched noise.
or it can be a mixed type obstruction, with pharyngeal and nasal noise:
Reverse sneezing is a common event in brachycephalic dogs, the actual causes of the episode are unknown but it is likely to be related to the elongated soft palate that irritates the throat. Episodes of reverse sneezing usually last from a few seconds to one minute. Usually as soon as it passes, the dog breathes normally again. Reverse sneezing rarely needs treatment. Sometimes, after upper airway surgery, reverse sneezing will stop or decrease in frequency. However, for dogs that have turbinectomy surgery, the frequency of episodes might increase until the tissue debris has been cleared out.
Stenotic nares are excessively narrow and often collapse inward during inspiration, making it difficult for the dog to breathe through the nose properly. Stenosis has been reported not only in the exterior nostrils, but also in the inner part of the nasal wing (alar folds). As a result, respiratory effort and open-mouth breathing are commonly seen in brachycephalic dogs. Stenotic nares are considered a risk factor for BOAS, particularly in French bulldogs. French bulldogs with moderate-severe stenosis of nostrils are about 20 times more likely to develop BOAS (reference). Corrective surgery to widen the nostrils is recommended.
The degrees of nostril stenosis in brachycephalic breeds are defined as follows:
Open nostrils - wide opening.
Mild stenosis - Slight narrowing of the nostrils. When the dog is exercising, the nostril wings move dorso-laterally to open on inspiration.
Moderate stenosis - The dorsal part of the nostril wings touch the nasal septum and the nares are only open at the bottom of the nostrils. When the dog is exercising, the nostril wings are not able to move dorso-laterally and there may be nasal flaring (i.e. muscle contraction around the nose trying to enlarge the nostrils).
Severe stenosis - Nostrils are almost closed. The dog may switch to oral breathing from nasal breathing with very gentle exercise or stress.
Eating difficulties are commonly seen in BOAS-affected dogs. The opening of the oesophagus is located dorsal to the airway opening and behind the soft palate. In BOAS-affected dogs, the excessive pharyngeal folds and the elongated soft palate may impede the swallowing function.
(illustration of the location of esophagus and larynx)
Regurgitation is commonly seen in BOAS-affected dogs. It can be caused by esophageal diverticula (esophageal pouches, a congenital condition) and/or hiatal hernia (stomach partially slides into the chest, it may be a congenital or secondary trait). In BOAS-affected dogs, the chronic increase in thoracic airway pressure draws the stomach into the chest, causing gastroesophageal reflux.
(CT: esophageal diverticula) (CT: hiatal hernia) (Video: eating difficulties, by Kate Price)
The canine nasal cavity plays a central role in the dog's ability to regulate its temperature. In order to facilitate heat exchange, a dog's nose is filled with an organized network of thin bone structures that are lined with a highly vascularized mucous membrane. These structures form ducts through which air passes, carrying the released heat away from the body when the dog pants. However, this process is hindered in BOAS-affected dogs that have an obstructed nasal cavity. As a result, BOAS-affected dogs cannot exchange heat as easily as healthy dogs when their body temperature rises during exercise. Because they are not able to cool down, they can suffer from heat stroke, which can lead to organ dysfunction. In addition to an increased body temperature (above 39 C), other symptoms to look for include: excessive panting, excessive drooling, dehydration and rapid heart rate.
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During exercise and sleep, BOAS-affected dogs have great difficulty breathing because of the soft tissue lesions associated with the syndrome. As a result, they may not be able to meet their oxygen requirements. When their blood is inadequately oxygenated, their skin presents a bluish discoloration, which is an obvious sign of cyanosis that can be easily recognized in the dog's tongue and gums. If the dog's oxygen levels are not stabilized immediately, he/she may collapse or become unconscious.
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BOAS clinical signs may not be present at rest in some moderately affected dogs. Therefore, functional grading based on a 3-minute trotting exercise tolerance test is suggested. Instructions on how to perform function grading can be found here. Grade 0 and I are considered clinically unaffected; Grade II and III are considered clinically BOAS-affected and they require management and/or treatment.
Test Result reference:
Grade 0 – BOAS free; annual health check is suggested if the dog is under 2 years old.
Grade I – clinically unaffected but with mild respiratory signs, annual health check is suggested if the dog is under 3 years old.
Grade II – BOAS affected with moderate respiratory signs. The dog has a clinically relevant disease and requires medical attention, including weight loss and/or surgical intervention.
Grade III –BOAS affected with severe respiratory signs. The dog should have a thorough veterinary examination with surgical intervention.
(video to be uploaded)
Breathing patterns of a bulldog before and after exercise:
Whole-body barometric plethysmography (WBBP) is a non-invasive respiratory function test that can provide objective information on the dog's airflow. We can use this information to calculate a BOAS Index (0-100%), which can be used to indicate the relative severity of the disease. The procedure takes about 20-30 mins. WBBP is the only non-invasive clinical tool used so far to measure respiratory function objectively. Sedation and anaesthetics are not required. For more information about WBBP...
Head/neck and thoracic radiography or CT scans are part of the diagnostic workup for BOAS. Thoracic radiography/CT scans can assist in the diagnosis of tracheal/bronchial hypoplasia/collapse, esophageal diverticula, hiatal hernia, and aspiration pneumonia. Head/neck CT scans can provide guidance for the planning of surgical techniques such as folded flap palatoplasty, alar fold resection, and turbinectomy.
Soft palate: Elongated soft palate refers to a condition where the soft palate is longer than normal, so that it overlaps with the tip of the epiglottis and extends into the larynx, interfering with respiration. Sometimes the soft palate can be thickened as well. A thickened soft palate can further narrow the pharyngeal and nasopharyngeal area (the airway pathway between the nasal cavity and the oral cavity). CT images are helpful in identifying the thickening of the soft palate and planning the appropriate surgical technique.
Nasal cavity: Canine noses contain a complex network of turbinates—branches of soft tissue that play a crucial role in warming and moistening the incoming air. Brachycephalic dogs often have hypertrophic turbinates that are relatively thicker and less branched. These turbinates are squashed inside the reduced nasal cavity and they can block the air flow in the nose and the nasopharynx. As a result, the respiratory and thermoregulatory function of the nose is greatly reduced. CT scans also allow identification of septal deviation. We can also observe second stenosis of the nostrils, where the inner part of the nostrils collapses (this is not visible from the outside).
Trachea: Hypoplastic trachea is also seen in some brachycephalic breeds, particularly in English bulldogs. The trachea may lose its structural support, making breathing more difficult. It may initiate the development of a chronic cough and respiratory resistance.
Laryngoscopy and endoscopy
Pharyngeal, laryngeal, and nasal cavity lesions can be assessed via laryngoscopy/endoscopy under sedation or general anesthesia.
Pharynx: soft palate and tonsils
Pharyngeal lesions include elongated and thickened soft palate, enlarged and extruded tonsils, as well as redundant soft tissues and proportionally enlarged tongue.
Larynx: cartilages and saccules
The larynx is a cartilaginous structure controlling the airway opening between the oral cavity and the trachea. The increase in negative pressure within the airway lumen chronically draws the laryngeal soft tissues (saccules) and cartilages into the airway lumen. Eversion of the laryngeal saccules is a condition where the soft tissue located in front of the vocal cords becomes swollen and inflamed. These protruded laryngeal tissues are then pulled into the airway lumen, acquiring the shape of saccules and partially obstructing airflow. In the more severe cases, the cartilage that supports the larynx may also collapse.
Nasal CavityRhinoscopy is particularly helpful for assessing nasal obstruction, and planning for turbinectomy. Lesions include septal deviation, hypertrophic nasal turbinates, and normal growth of the turbinates.
Comparing body weight to the breed average weight is not a reliable way to assess a dog’s ideal weight. Body condition score (BCS) is a more useful scoring system to assess whether an animal is underweight or overweight.
Our research findings show that obesity (i.e. BCS≥7) is significantly associated with BOAS. Fat tissue within the airway tract can further narrow the lumen. For obese dogs with mild to moderate BOAS signs, weight loss is suggested prior to surgical intervention.
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While it is important that you exercise your dog regularly and keep him/her fit, it is also important to remember that brachycephalic breeds, particularly BOAS-affected dogs, may have difficulty regulating their body temperature. Therefore, it is recommended that you avoid exercising your dog excessively or during hot summer days. In general, it is best to keep your dog away from hot environments or situations that may cause too much excitement or stress in him/her. Signs of overheating include: heavy panting, elevated body temperature, glazed eyes, increased pulse, vomiting/diarrhoea, excessive thirst, dark red tongue, excessive drooling, staggering. In severe cases, they may seizure, collapse, become unconsciousness, or even die. If you notice signs of overheating in your dog, you should move him/her to a cool place with good air conditioning. You should keep providing him/her with a small amount of water to drink to avoid dehydration, and cool him/her down by soaking with cool water. Contact your vet for further treatment and evaluation.
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The main BOAS lesions at the pharynx are the oversized soft palate and the everted/hypertrophic tonsils. Several surgical techniques have been reported to shorten and thin the soft palate. The elongated soft palate should be trimmed to the tip of the epiglottis. If the soft palate is thickened, a folded flap palatoplasty may be used.
Everted/hypertrophic tonsils are commonly seen in BOAS-affected dogs, where the tonsils extrude from the crypt and narrow the pharynx. Partial removal of the extruding tonsils increases the pharyngeal space.
Some BOAS affected dogs present with laryngeal collapse. Everted laryngeal saccules may cause additional airway obstruction. Removal of everted laryngeal saccules is a simple and effective procedure to open the ventral part of the larynx.
Laryngoplasty (e.g. trimming of the deformed/collapsed cuneiform processes) may be performed in dogs with Grade II and III of laryngeal collapse. Further laryngeal lateralization (laryngeal “tie-back”) may be required for dogs with Grade III laryngeal collapse.
Wedge resection of the nostrils and Trader’s technique for amputating the nostril wings are both effective surgical techniques to widen the external nostrils. Resection of the interior alar folds opens the inner part of the nares.
Laser-assisted turbinectomy has been used to address the intranasal obstruction, particularly in pugs and French bulldogs. The aberrant and hypertrophic ventral conchae are removed. LATE surgery is indicated for dogs that tend to mouth breathe when at rest, dogs that present with excessive panting during exercise, and dogs that had little or no improvement after conventional surgery (e.g. soft palate shortening and widening of the nostrils). Rhinitis and reverse sneezing may be present after the surgery, but the symptoms usually resolve spontaneously.
Post-operative recovery fromanaesthetic is the major risk related to the surgery. Inflammation and swelling of the airway may occur post-operatively. A temporary tracheostomy tube can be placed if necessary. Aspiration pneumonia caused by regurgitation is also a risk after surgery. Gastro-protectants are given to surgical patients.
The surgical results are variable and depend on the severity of the disease. Dogs with Grade II or III laryngeal collapse usually have a poorer prognosis. BOAS is not curable. Nevertheless, upper airway corrective surgery can provide a better quality of life. Regular post-operative rechecks are suggested. Revision surgery might be required in some severely affected dogs.