Research explores the effect of pterygoid bone medialization on BOAS severity and surgical outcome in brachycephalic dogs
7 February 2023
University of Cambridge vet student Sze Lynn Yuen reports on her BSAVA PetSavers-funded research project entitled Effect of pterygoid bone medialization on nasopharyngeal narrowing and surgical outcome in dogs with brachycephalic obstructive airway syndrome.
Brachycephalic obstructive airway syndrome (BOAS) is a multi-lesional disease arising from the artificial selection of the ‘flat-faced’ feature consisting of a shortened snout. During breeding selection of flat-faced breeds, the skull becomes shortened but the upper airway soft tissues do not reduce in size proportionally and thus lead to airway obstruction. The most common clinical signs of BOAS include loud breathing noises, exercise and heat intolerance, disturbed breathing during sleep, and gastrointestinal disorders such as regurgitation and vomiting. The average lifespan of brachycephalic breeds is also reduced when compared to that of nonbrachycephalic breeds of a similar body size. BOAS-related issues have been compounded by the increased popularity of flat-faced breeds. The fact that 60% of owners are unaware of the clinical signs of BOAS suggests that the true prevalence of BOAS could be much higher.
Although many anatomical lesions are described in the literature as being associated with BOAS (including soft palate thickness, tracheal diameter and aberrant nasal turbinate conformations), the effect of bony malformation on nasopharyngeal narrowing has not been reported. Through clinical observation and experience, it was suspected that BOAS-affected dogs that persistently did not respond to surgery often had a narrower nasopharynx, and many cases were attributed to their medialized pterygoid bones. This study was therefore the first known to investigate the effect of pterygoid bone medialization on BOAS severity and surgical outcome. Other than the soft tissue structures involved in causing respiratory signs, the focus of the project investigated whether nasopharyngeal narrowing was associated with malformed pterygoid bones and could be a potential risk factor for poor surgical outcomes. The three main brachycephalic breeds investigated in this study were French Bulldogs (FB), Pugs and Bulldogs (BD), which are currently the most popular brachycephalic breeds.
Of the methods developed to quantify BOAS severity, the study used the BOAS index for each patient which is considered a more objective numerical scale than the respiratory functional grading system. A BOAS index of 0% indicates that the dog is BOAS-free, whereas 100% indicates severe BOAS. Based on this, the threshold for a poor post-operative outcome was determined to be ≥50% of the BOAS index scale on post-operative assessment.
The study analyzed computed tomography (CT) images of both BOAS-affected dogs and nonbrachycephalic control dogs (Figure). The results showed that brachycephalic dogs had a more medialized pterygoid bone conformation than control dogs. When comparing the relationship between pterygoid medialization and the cross-sectional area of the nasopharynx (CCSA), all brachycephalic breeds showed that more medialized pterygoid bones corresponded to a smaller CCSA. However, the most important finding revealed that FBs with a poor surgical outcome had a more medialized pterygoid conformation compared to those with normal pterygoids. Overall, this showed that pterygoid bone medialization is a feature that is more apparent in brachycephalic dogs compared to non brachycephalic controls. Pterygoid bone medialization was also shown to be related to nasopharyngeal narrowing and pterygoid bone malformation to limit surgical effectiveness in BOAS-affected FBs.
Given the multitude of lesions contributing to BOAS, there are limitations to what surgical intervention can do to alleviate respiratory and other BOAS-related signs in affected dogs (FBs in particular). There is currently no effective surgical option to correct the pterygoid bone conformation in affected dogs. The findings of this study therefore provide significant clinical benefit in guiding clinicians to advise owners about the surgical prognosis in BOAS-affected dogs. For clinicians working in general practice, simple palpation of the pterygoid bone during surgery can also provide an indication of how medialized the pterygoid bones are. Additionally, quantifying pterygoid medialization and overall conformation can be used as a tool to aid breeders in deciding which dogs are most appropriate for breeding.
Experiencing the BSAVA PetSavers Student Research Project
I first came across the BSAVA PetSavers Student Research Project through my role as the PetSavers representative at Cambridge. This was my first experience completing a veterinary research project and it gave me an insight into what veterinary clinical research was like. I was keen on completing a project with the BOAS group due to my long-standing interest in BOAS from when I observed clinical practice prior to attending university, and this experience was nothing short of amazing.
I thoroughly enjoyed the project and felt that my knowledge about BOAS was further heightened. I was able to fully immerse myself with the members of the group, such as having organic discussions with my supervisors as well as participating in other routine tasks being carried out at the lab. I understood the reasons for selecting different surgical procedures for each patient by being exposed to the BOAS assessment protocol and observed the use of the whole-body barometric plethysmography chamber and laser-assisted turbinectomy procedure. The diversity of the cases presented at BOAS clinics and the multitude of lesions characteristic of BOAS patients highlighted how personalized medicine played an important role as there was not one definitive answer to every problem or individual. The same idea can therefore be echoed in the case of conducting clinical research; there are many ways to solve a problem, and there is a never-ending quest to finding the solution to many questions and problems.
Other than the skills directly related to BOAS, I also honed my ability to interpret various imaging modalities (especially that of CT images) and simple coding, which I hope to apply when I begin practicing as a vet. Overall, I found the entire process of applying for the grant simple and straightforward with PetSavers being very responsive and providing support throughout the process. I am extremely grateful to PetSavers for funding the project and to my mentors for their excellent guidance.