Computed Tomography positioning and nasopharynx allometry in dogs affected by brachycephalic airway syndrome

Background: Brachycephalic dogs are often affected by the Brachycephalic Airway Syndrome (BAS). Literature is mainly concentrated on the size and shape of the soft palate compared to the clinical presentation, but CT features of the nasophaynx have been rarely discussed. Purpose: The aim of the work is to identify a standardized positioning of skull and neck in the CT evaluation of the upper airways and to compare skull measurements with functional and absolute volumetric measurements of the rostral nasopharynx, in brachycephalic dogs with different degrees of BAS. Methods: Twenty-five brachycephalic dogs with BAS, 13 of them with nasal stertor on clinical presentation, were investigated. Each dog, intubated during general anaesthesia, underwent upper airway CT in two different positions: with the skull elevated higher than the neck; with the skull and neck at the same level. Functional volume and absolute volume measurements of the rostral nasopharynx were obtained by a thresholding-based segmentation algorithm. Volume of the mucosa was calculated as the difference between absolute and functional volume. To normalize differences in size, the volumes were statistically correlated (Shapiro-Wilk, Pearson tests) with the Cranial Index (CI), i.e. the ratio between cranial width and cranial length. Findings: Volumes of the nasopharynx appeared reduced when the skull was in an elevated position. The caudal nasopharynx showed an ellipsoid shape due to a reduced dorsoventral expansion, compared to a circular appearance when head and neck were placed at the same level. An inverse relationship between CI and functional volume was observed (P=0,037) in all the patients regardless of their clinical signs, but no relationship between CI and absolute volume or volume of the mucosa were detected. Consequently, the reduction in the volume of air does not seem to be related to a redundant presence of soft tissues in this region, but primarily with a reduction of the space in which they lie. Interestingly, Cranial Index was the highest in Pugs, suggesting that this value may represent a negative predictive value for the functional volume of the nasopharynx. Finally, unexpectedly, all the patients showed a prominent median septum localized at the level of the vomer, associated with an elliptic shape of the region. Conclusions: Due to the mobility of the soft palate, skull positioning seems to play a key role in the evaluation of the throat and positioning with the head and neck at the same level provides consistently higher volume measurements. Nasopharynx allometry suggests that the morphology of this region alone cannot be the only responsible factor for clinical presentation. The median nasal septum and the elliptical shape of rostral nasopharynx are likely to modify the column of air and create a more turbulent airflow, especially in dogs with the highest CI. Future perspectives will be oriented to the dynamic study of airflow in patients with BAS.