33rd Annual Scientific Meeting proceedings

The lesion sites that are linked to BOAS include stenotic nares, a hyperplastic soft palate, laryngeal collapse, hypoplastic trachea and bronchial collapse, macroglossia and excessive turbinate crowding in the nasal cavity. 

The early literature describing BOAS^1,2,3 was working from radiographs and direct observation of the upper airway lesions and thus could not assess the nasal component. Oechtering⁴ published on the CT imaging of the brachycephalic nose in 2007. He found abnormal conchal formation in these dogs, both rostrally positioned ventral turbinates and caudal aberrant turbinates, originating from the middle or ventral nasal conchae (seen in 43% of these dogs). The turbinates, in addition to being abnormally positioned, had a lower level of branching compared with a normocephalic dog. These findings were mirrored on rhinoscopic examination where rostrally positioned overcrowded turbinates resulted in nasal airflow obstruction⁵. Oechtering published treating hypertrophic and malformed nasal turbinates in the extreme brachycephalic breeds using laser turbinectomy in the same year⁶.

These lesion sites are breed specific and it is unusual to see an English bulldog with nasal obstruction whereas most French bulldogs and many pugs in the UK have some degree of nasal obstruction.  

Assessment
We assess the dogs that present with brachycephalic obstructive airway syndrome using a combination of the following:

• History
• Clinical signs at rest and during exercise- functional respiratory grading
• Whole body barometric plethysmography
• Oral examination under sedation/ GA
• Head/ thoracic CT or radiographs
• Examination of the nasopharynx with an endoscope

With the history, for nasal obstruction we would expect noisy breathing (often less so on exercise) and loud snoring. Other indicators of nasal obstruction are sleep disorders, heat intolerance and regurgitation (particularly in the French bulldog). On clinical examination we are looking for nasal/ nasopharyngeal stertor and stenotic nostrils.  The respiratory functional grading (RFG) scheme is a clinical evaluation of the upper airway that includes an exercise test to increase breathing demand⁶. We exercise the dogs at a moderate trot (4 mph) for 3 minutes and note refusals, airway noise, effort, and extreme respiratory signs pre and post exercise.  The grades are:

• 0 – BOAS not present
• 1 – mild BOAS (respiratory noise only audible with a stethoscope)
• 2 – moderate BOAS (noise constant and heard without a stethoscope)
• 3 – severe (severe stertor, dyspnoea)

Many dogs with marked nasal obstruction have moderate stertor which is most noticeable at rest. These dogs will often mouth breath within 30 seconds of starting exercise and will be reluctant to mouth breath after exercise. It is not unusual for body temperature to raise 2-3^o over the 3-minute period. In addition to the RFG Scheme, we use whole body barometric plethysmography (WBBP). Parameters generated from WBBP include breathing frequency; total expiratory and inspiratory volume, time and peak flow; lags between breath cycles (if any) and minute respiration volume. The traces pre and post-surgery are noticeably different, with the dynamic component (peak initial expiratory flow) much reduced⁷. In dogs with a fixed obstruction, such as the nasal cavity, the respiratory flow traces are often very flattened, with a restricted flow rate.

In dogs that are surgical candidates, we usually assess the dogs pre surgery with CT and rhinoscopy. On CT the soft tissue: air proportion at the level of the caudal nasopharyngeal meatus is a good indicator of whether the dog will require a LATE in the future (> 55% predicts LATE surgery in French bulldogs, > 64% in pugs in our cohort¹⁴), however we combine this with the clinical signs and only operate if the STNM is high and the dog is RFG grade II/III. On rhinoscopy a rough and ready assessment of likely turbinectomy requirement is whether it is possible to scope through to the nasopharynx with a 1.9 sheathed rhinoscope. After imaging, we perform the multilevel surgery (alavestibuloplasty, tonsillectomy, modified folding flap palatoplasty and laryngeal surgery as required). Dogs are re-evaluated 8-12 weeks post-surgery and those that have a RFG score 2/3 with clinical signs of nasal obstruction, regurgitation and sleep disorders are offered surgery to improve the nasal airflow – usually laser assisted turbinectomy (LATE). These dogs will usually have an improved BOAS index (80/90s usually improve to around 65-70%) but flattened traces. If the BOAS index is < 50 % we rarely require a LATE.

Very occasionally we see dogs that have minimal pharyngeal or laryngeal airway nose and clinical signs very localisable to the nasal cavity. In these dogs we will offer LATE and alavestibuloplasty as the first surgery.

Surgical Results
Conventional surgical results are encouraging with most papers reporting a good outcome for most patients with about a 10% risk of post-operative complications, including death^1,8,9,10,11 .Torrez in 2006 was a little more guarded with regard to prognosis, with just over 50% much improved after surgery, 32% having some improvement and 10% of patients showing no improvement¹². The authors concluded that very few patients would have resolution of clinical signs post-surgery.

In our objective assessment of improvement post-surgery, about 40% of dogs improve to a clinically unaffected status⁷.  The remainder of dogs remain clinically affected though usually improved. LATE surgery for dogs that have little response to the first surgery results in a further 80% being classified as clinically unaffected¹³.

Professor Oechtering at Liepzig offers LATE as part of the initial multilevel BOAS surgery to good effect¹⁴. Whilst there is good anatomical support for this approach in French bulldogs and pugs we found in our clinic there was an increase in surgical morbidity with this approach. LATE has a steep learning curve and we found that operating on nose and pharyngeal/ laryngeal areas simultaneously resulted in an increased sedation rate post-operatively and increase in use of tracheostomy tubes. I prefer to reduce hospitalisation duration and post-operative interventions in the BOAS patients. The LATE surgeries are usually done as day cases and morbidity is low.

References
Available on request.