Brachycephalic obstructive airway syndrome: a growing problem

Anybody reading the popular and celebrity press cannot help but notice the increasing popularity of the brachycephalic breeds, particularly the bulldog breeds and the pug. The pug is now one of the top 10 breeds in the list of Kennel Club annual registrations with total registrations having climbed from 1675 in 2004 to 8071 in 2013. French bulldog registrations have similarly risen from 350 to 6990 in the same period. Although these breeds are prone to various problems one of the most significant is brachycephalic obstructive airway syndrome (BOAS). This syndrome results in a number of signs including snoring, panting, overheating, exercise intolerance, prolonged recovery from exercise, cyanosis, retching, gastrointestinal signs and disturbed sleep patterns. For many affected dogs this condition can have a significant effect on quality of life and can also be life threatening. The importance and potential severity of this syndrome are illustrated by the number of dogs affected with this condition in the paper by Trinterud et al. (2014), concerning the use of silicone tracheal stoma for temporary tracheostomy in this issue of JSAP. The brachycephalic breeds have been bred with a shortened skull and muzzle. This conformation is associated with a number of anatomical abnormalities that cause increased resistance to inspiratory airflow. The classically described primary abnormalities are stenotic nares and overlong soft palate. This reported focus is probably simply because they can be surgically improved. However, the whole anatomy of the airway in these breeds predisposes to increased airway resistance. A number of other abnormalities are reported including hypoplastic trachea, main stem bronchus collapse, aberrant conchane, nasopharyngeal turbinates, enlarged tonsils, macroglossia, excessive periepiglottic folds, epiglottic cyst and laryngeal granulomas. The role of these various abnormalities in the severity of signs in an individual is unknown. As a result of these abnormalities there is resistance to inspiratory airflow which results in increased inspiratory effort and the generation of supraphysiological negative airway pressures. This in turn causes oedema and stretching of the soft tissues of the airway and collapse of the cartilages of the larynx. There are three stages of laryngeal collapse described in dogs. Stage I is eversion of the laryngeal ventricles, which we identify in the majority of dogs presenting for surgery. In stage II, the cuneiform processes collapse medially and in stage III the corniculate processes collapse. These secondary changes to the airway further reduce the glottic diameter and increase the degree of airway obstruction. The timing of onset of laryngeal collapse in the disease process is variable between individuals and breeds, but has been reported in dogs as young as 4 · 5 months (Pink et al. 2006). The significance of the development of laryngeal collapse is not fully understood. Although it is considered to worsen prognosis, this is not consistently seen and some dogs with laryngeal collapse will have good outcomes from surgery. There is a spectrum from a "normal" brachycephalic dog to a dog clinically affected with BOAS syndrome. Evaluating the severity of airway obstruction and deciding which dogs will benefit from surgery can sometimes be a difficult decision. Currently this is generally subjective based on severity of signs. Current research using whole body barometric plethysmography may be one way to better define the severity of the obstruction. There is an argument that any dog with identified anatomical abnormalities, whatever signs are present, should be corrected to reduce the risk of later problems due to progressive secondary changes. However, the potential benefits need to be balanced against the risks of surgery. The natural progression of brachycephalic dogs with and without BOAS that do not undergo surgery has not been studied. Current standard surgical options for the treatment of BOAS combine staphylectomy, stenotic nares correction and excision of everted laryngeal ventricles. Across several studies, results are reasonably consistent with approximately 90% of dogs having improved clinical signs (Poncet et al. 2006, Torrez & Hunt 2006, Rieks et al. 2007, Fasanella et al. 2010). However, only approximately half of these cases have significant improvement or an excellent outcome. Across various studies, approximately 10% of dogs have no improvement following surgery. These results have led to the investigation of novel techniques such as folded palatoplasty and laser excision of nasal turbinates in an attempt to further improve outcome, but these have yet to be reported in large numbers or compared directly with the current standard surgical techniques. Surgical management of laryngeal collapse using arytenoid lateralisation has been reported with success in a limited number of cases. It appears likely that the reason for limited improvement following surgery is the presence of a number of anatomical abnormalities relating to conformation that cannot be surgically corrected and the presence of irreversible secondary changes. Identifying patients that will have limited improvement or complications from standard surgery is currently frustrating for surgeons. Factors such as age and presence of laryngeal collapse have been suggested as risk factors for poor outcome following surgery but are not proven consistently (Torrez & Hunt 2006). Interestingly, a large proportion of the dogs in the paper by Trinterud et al. (2014) requiring tracheostomy were ones with laryngeal collapse. Intuitively one would assume that dogs with more severe conformational abnormalities would be at higher risk, but again this has not been proven and requires further research. Significantly thicker soft palates based on computed tomography (CT) evaluation have been associated with more severe signs (Grand & Bureau 2011), but this has not been assessed in relation to postoperative outcome. As palate thickness is not addressed by standard soft palate surgery this could also be a risk factor for poorer outcome following surgery. Surgery carries some risk with a proportion of dogs having significant postoperative airway compromise resulting in the need for temporary tracheostomy. Postsurgical complication rates of 12% have been reported with mortality rates of up to 3% (Poncet et al. 2006, Riecks et al. 2007). Although life saving in many patients, trachesotomy tubes require intensive nursing and even with this, significant complications and mortality occur. In the study by Nicholson & Baines (2012) there was an 86% complication rate and 19% mortality rate. Again the brachycephalic breeds were over-represented in patients suffering complications. The novel tracheal stoma described by Trinterud et al. (2014) demonstrates that it can be successfully used in brachycepahlic breeds and offers the potential to reduce complication rates, but requires further evaluation in a larger number of cases. The Cavalier King Charles spaniel (CKCS) is often considered a brachycephalic breed and can present with typical brachycephalic airway signs such as respiratory noise, snoring, stertor and exercise intolerance. They are represented in reviews of dogs undergoing corrective airway surgery for BOAS and dogs requiring tracheostomies (Torrez & Hunt 2006). However these dogs often do not demonstrate the typical abnormalities of the brachycephalic breeds. They often do not have an overlong palate as judged by standard criteria or stenotic nares or everted laryngeal ventricles. Some present with isolated laryngeal collapse and we have seen some dogs with concurrent laryngeal paralysis. As a breed they often have a particularly thick soft palate and small nasopharynx which may be the main factors driving the signs in this breed. Other breeds that can present with BOAS-like symptoms but often without the typical abnormalities include the bull terrier and Mastiff breeds. Like the CKCS, these breeds often do not have a surgically correctable problem. BOAS is a complex problem which currently we do not fully understand. We need to better understand the anatomical and breed variations in these cases and their relationship to severity of signs and postsurgical outcome. Current surgical options offer a way to alleviate signs and can provide significant improvement. However, there is still a frustrating subset of cases with poor outcomes that we struggle to identify pre-operatively. Surgery should be performed on affected dogs to improve quality of life and as a welfare issue. However, ultimately, the only way to truly improve this condition is by education of the public and breeders regarding the problems of these breeds and with breeding aimed at improving conformation. Terry Emmerson graduated from Cambridge University in 1994. Following this, he completed an internship and a 3-year residency in small animal surgery at the Royal Veterinary College, University of London. He became a Diplomate of the European College of Veterinary Surgeons in 1999 and an RCVS-recognised Specialist in Small Animal Surgery in 2006. After completing his residency he entered private practice and co-founded North Downs Specialist Referrals where he is Head of Surgery. He is interested in all aspects of soft tissue surgery but has particular interest in airway and oncological surgery.