Brachycephalic Obstructive Airway Syndrome (BOAS) in Dogs and Cats
Brachycephalic obstructive airway syndrome is the term given to the effects that the shortened head of these animals has on the passage of air through the upper airways. The signs can vary in severity from mild snoring or snorting noises to severe breathing problems.Animals suffering from clinically significant BOAS can struggle to breathe during exercise and may suffer collapse due to lack of air. The reliance of dogs on panting to cool themselves also makes animals suffering from BOAS very susceptible to overheating and developing potentially very serious breathing difficulties in hot conditions.
Brachycephalic obstructive airway syndrome (BOAS) is caused by a mismatch between a shortened skull and the soft tissues that have not accompanied this shortening, causing the airway to be crowded and consequently partially obstructed. Breeds commonly affected by this condition include French Bulldogs, English Bulldogs, Pugs, Boston Terriers. The mean age of presentation is usually between 2 and 4 years old, although younger animals are commonly affected. The aim of surgery is to decrease airfl ow resistance by alleviating upper airway obstruction and is still the mainstay of long-term treatment. Prognosis is in general good, with 90% of dogs improving after surgery and a reported mortality rate of only around 4%. Most treated dogs, although improved symptomatically, must never be considered normal dogs and there is an increasing role for the veterinary profession in educating the public about this condition.
- Cause: congenital shortening of the bones of the skull when the soft tissues within the upper airway do not reduce in size proportionally; maldevelopment of the airway cartilages.
- Signs: excessive panting, dyspnea, stertorous and stridorous breathing, respiratory effort, sleep-disordered breathing, exercise and heat intolerance, regurgitation/vomiting, cyanosis and collapse.
- Diagnosis: breed, clinical signs, respiratory function tests, diagnostic imaging, endoscopic examination.
- Treatment: medical treatment and surgical widening of the airways where possible.
- Prognosis: good in mild-moderate cases with surgery.
Pathogenesis
Etiology
- Syndrome comprising one or more of the following components:
-
- Narrowing/stenosis of the external nares, obstruction of nasal vestibule by pronounced ventral alae, aberrant and hypertrophied nasal turbinates with increased mucosal contact point.
- Nasopharyngeal narrowing and collapse.
- Narrow pharyngeal dimensions, pharyngeal collapse, elongated soft palate Soft palate: elongated, macroglossia, and inflamed and extruded tonsils.
- Narrow laryngeal dimensions or laryngeal (cricoid) hypoplasia, everted larnygeal saccules Larynx: miscellaneous conditions Larynx: disease overview, collapse of laryngeal cartilages, redundant laryngeal soft tissues.
- Hypoplastic trachea Trachea: hypoplasia, especially in the bulldog and bronchial collapse (especially in the Pug).
- Skull base malformation (eg medialization of the pterygoid processes).
Predisposing factors
General
- Obesity Obesity.
- Hot weather.
- Exercise.
- Excitement.
- Stress.
- Concurrent cardiac or pulmonary diseases.
- Concurrent or secondary gastro-intestinal diseases with gastro-esophageal reflux.
Pathophysiology
- Airflow through airways is impeded due to abnormal anatomy → noisy breathing and inability to take on board sufficient oxygen to meet increased demands imposed by exercise.
- Restricted airflow → increased inspiratory effort → increased negative pressure within the upper airways leads to a high intrathoracic negative pressure which in turn leads to eversion of laryngeal saccules and airway collapse as well as a sliding hiatal hernia Hiatal hernia of the stomach.
- Increased respiratory effort may → upper airway edema → further obstruction to airflow. Regurgitation Regurgitation leads to acid reflux and increase in pharyngeal and laryngeal inflammation and thus a vicious cycle is set in motion.
- Impeded airflow prevents adequate heat loss through panting so animals rapidly become hyperthermic Hyperthermia in hot weather, following exercise or during stress.
- If the respiratory vicious cycle is left untreated, the dog may develop pulmonary edema, reduced arterial oxygen content, hypertension, and right-sided heart failure.
What breeds are affected?
Brachycephalic breeds include those breeds of dog and cat that have an obvious, characteristic short or squashed nose appearance. Any breed of dog or cat with a brachycephalic head conformation can be affected, e.g.:
Dogs:
- English Bulldog
- French Bulldog
- Dogue de Bordeaux
- Boxer
- Pug
- Cavalier King Charles Spaniel
- Lhasa Apso
- Shih Tsu
- Pekingese
- Other brachycephalic breeds
Cats:
- Persian
- Himalayan
Why do these animals have problems?
There are four main areas of concern in these animals:
- Narrow nares (nostrils)
- Overlong palate
- Secondary effects such as collapse of the larynx (voice box)
- Tracheal hypoplasia (the trachea or windpipe is too small for the size of animal – almost exclusively encountered in the English Bulldog breed)
Whilst dogs are commonly presented to vets with clinically significant problems resulting from BOAS, cats are rarely presented for treatment. This may partly be a reflection of the more sedentary lifestyle of some cats compared to dogs; most cats that are presented only require treatment for narrowed nares (nostrils).
Narrowed nares (nostrils) can dramatically increase the resistance to air flow into the nose of brachycephalic breeds. This, combined with the tendency for the cartilage supports of the nares to collapse during inspiration, requires a much larger effort than normal in order to breathe.
An overlong soft palate can partially obstruct air flow into the trachea (windpipe) and also cause turbulent airflow in the area of the larynx (voicebox). Both of these effects further increase the effort required for an affected animal to breathe in, and can cause significant inflammation of the upper airway.
Unfortunately, the extra effort required to breathe in (inspiration) eventually results in secondary problems, since the airway is not adapted to cope with the turbulence and increased negative pressures which develop.
These secondary effects include:
- Gradual collapse of the larynx (voice-box) termed laryngeal collapse. Veterinary surgeons grade the severity of collapse from 1 to 3, with 3 being the most severe. Unfortunately, the collapse is likely to be progressive, although the rate of progression may be able to be slowed with appropriate treatment.
- Tonsillar eversion and hypertrophy. The tonsils usually lie in shallow recesses towards the back of the mouth. The increased negative pressures can cause them to enlarge and protrude into the back of the mouth, further narrowing the airway.
- Pharyngeal muscle hypertrophy. The pharynx is the term used to describe the area of the back of the mouth and entrance to the larynx. These muscles can become enlarged (hypertrophied) due to the altered pressures in the upper airway.
- Acid reflux from the stomach into the oesophagus (food pipe). This can result in inflammation of the oesophagus and sometimes in ulceration of the stomach. Severe cases may suffer from a hiatal hernia, when part of the stomach can become displaced into the chest cavity during breathing.
- Heart failure – this is less common than the other secondary effects of BOAS.
How is BOAS diagnosed?
When carrying out a physical examination, your primary care vet will assess your dog or cat’s anatomy (some heads are shorter than others) and your pet’s behaviour in certain circumstances.
Most brachycephalic animals will experience a degree of upper airway obstruction, usually evident as snorting or snoring noises. The key is for you and your veterinary surgeon to decide between you whether your pet is experiencing a significant level of signs or showing an anatomical conformation that may predispose him/her to problems. The discussion will involve how your pet copes with exercise or warm conditions and also any likely susceptibility to secondary deterioration within the airway.
Unfortunately, only the nares (nostrils) can be properly assessed without the requirement for a general anaesthetic. Initially therefore, the diagnosis is based on the veterinary surgeon’s overall assessment but will require further investigation to confirm the diagnosis.
How is BOAS investigated by the veterinary surgeon?
There are a number of tests that may be required, depending on the findings of the initial physical examination. These can include blood tests, cardiac (heart) assessment and chest X-rays. However, full airway assessment will require an anaesthetic.
General anaesthesia in brachycephalic dogs can be associated with increased risk, particularly during the induction and recovery phases.
Many primary care veterinary surgeons would choose to refer these cases to an experienced surgeon to carry out an assessment of the airway under general anaesthesia, with the advantage that surgical treatment can be carried out under the same anaesthetic and therefore reduce the risks during the recovery phase.
The surgeon may use an endoscope (small camera) to examine the oesophagus for signs of gastric reflux.
What treatment options are available?
Stenotic Nares (narrowed nostrils)
Removal of a section of the cartilage of the front of the nose or a section of skin next to the nose can improve the airflow through the nares.
Overlong Soft Palate
The excess length of the soft palate can be reduced by surgery in order to reduce the interference with air flow into the larynx (voice box).
Laryngeal Collapse
Early laryngeal collapse can sometimes be improved by removal of some of the tissue that is slowing airflow. The surgeon will assess the larynx (voice box) and decide whether additional surgery in this area is appropriate or likely to be beneficial. Sometimes, this additional procedure may need to be carried out at a later date.
Severe (Grade 3) collapse is unlikely to benefit greatly from a minor procedure and may require a permanent tracheostomy (breathing hole in the neck), or can sometimes be temporarily improved with a tieback procedure (see laryngeal paralysis information sheet).
Everted tonsils
Some cases may benefit from tonsillectomy (removal of the tonsils). The decision on whether this is appropriate is made at the time of assessment and surgery.
Acid reflux
Treatment for gastric reflux usually involves regular medication, rather like one might take for heartburn.
Hiatal Hernia
In general, conservative management is initially employed in cases suffering from displacement of a section of the stomach into the chest, although some cases do require surgical treatment.
Weight management
The lifestyle of many of these patients can predisposed them to obesity. Weight loss can dramatically improve the airflow through the pharynx and neck and MUST form part of the treatment protocol in overweight patients.
Treatment and prevention of brachycephalic airway syndrome in dogs and cats
- Brachycephalic Obstructive Airway Syndrome Surgery: Surgical intervention to address the physical abnormalities is the hallmark of BOAS treatment. Surgical options include widening of the nostrils, shortening of the soft palate, and the removal of the laryngeal saccules (if everted). It is important to note that surgery is not considered curative in these breeds. Surgery is instead aimed at reducing the stress on the airways as much as possible and therefore increasing the animal’s quality of life. It is recommended for corrective airway surgery to be performed in brachycephalic animals early on in their lives to reduce the chance of secondary effects of BOAS such as airway collapse and eversion of laryngeal saccules.
- Exercise and temperature management:Aim to exercise and walk animals during the coolest times of the day (e.g. dusk and dawn), be sure not to over exercise your brachycephalic pet and always ensure your pet has access to fresh water.
- Taking care in choosing your pet if buying from a breeder:Be sure to meet your puppy or kitten’s parents as well as the breeder in person to discuss BOAS.
- Visit the vet early for assessment:This involves a physical examination, but your vet may also recommend an airway examination under sedation or anaesthesia and/or radiographs depending on clinical signs. In more severe cases this may also include referral to a veterinary specialist for assessment and advanced imaging.
- DR ROHIT RAJ, CANINE CONSULTANT,KOLKATA
- REFERENCE-ON REQUEST
https://kb.rspca.org.au/knowledge-base/what-is-brachycephalic-obstructive-airway-syndrome-boas/
