Two-Dimensional Geometric Morphometric Analysis of Nasopharyngeal Shape

Abstract Body : The nasopharynx is an important anatomical structure of the upper respiratory tract, which functions in respiration, vocalization, and deglutition. Understanding variations in nasopharyngeal anatomy is essential for otorhinolaryngologists, as alterations in its shape have severe clinical implications. For instance, a reduction in nasopharyngeal transverse diameter can affect upper respiratory airway patency increasing the risk of developing sleep disorders. Humans have taller and shorter airways than our closest relatives, which already results in lower velocity, higher pressure air entering the airway. However, there is limited research on variation in the structure of the nasopharynx and its influence on physiological processes. This study aims to assess shape variation of the nasopharynx and its relationship to nasopharyngeal size to better understand adaptive changes that may be occurring in this region. We used ImageJ to collect two-dimensional coordinate data on 10 to 15 transverse CT slices from 69 Rhesus macaques from the California National Primate Research Center. Data was collected starting from the most cranial boundary of the nasopharynx, as visible on transverse CT slices, to its most inferior boundary as demarcated by the beginning of the oropharynx. Landmarks were placed on clearly identifiable hard and soft tissues demarcating the maximum anteroposterior and transverse margins of the nasopharynx. We analyzed the trajectory of shape change along the nasopharyngeal tract using geometric morphometrics methods on MorphoJ and assessed the relationship of size to shape using linear regression. Homologous landmarks were standardized with Procrustes superimposition to a common orientation. Principal Components Analysis was used to assess shape variation for the dataset. The nasopharynx of Rhesus macaques tends to increase in size caudally, changing from a narrow diamond shape to a more triangular shape with the area of greatest transverse breadth located more dorsally. Individuals vary in the location of the shape change of the nasopharynx, and this variation may translate to altered respiratory functions. Shape is significantly associated with size in the nasopharynx but only in the upper regions of the tract (p < 0.001 upper, R2 = 0.215 versus p = 0.198-0.579 lower, R2 = 0.112). By recognizing the relationship of size and shape in the upper respiratory region, clinicians can be on the look-out for potential pathologies arising from morphological variations in the cranial aspects of the nasopharynx. In humans, individuals with obstructive sleep apnea have smaller pharyngeal size than those without. In Apert, Crouzon, and Pfeiffer syndromes, reduced nasopharyngeal volume affects airway patency and presents as stridor and obstructive sleep apnea. Early recognition can lead to better prognosis and therapeutic options in these patient populations.