An In SilicoMethod for Modeling the Nasal Cycle in 3D: Implications for Assessing Human Nasal Form and Function
Authors: Tran Baonhu , Ward Lyndee A. , Thai Elizabeth , Maddux Scott D. 
Published: May 2022

Anthropologists have long posited that geographic‐mediated associations between human nasal morphology and climate evince climatic adaptation. These arguments overwhelmingly focus on the prominent role of the nose in respiratory air‐conditioning, as intranasal heat and moisture exchange in different climates is governed anatomically via the amount of nasal mucosa surface area relative to the volume of air passing through each nasal passage. Yet, the inability to quantitatively account for the nasal cycle (a physiological process in which the left and right nasal passages reciprocally alternate in their mucosal congestion levels) has limited investigation into the adaptive influence of nasal soft‐tissues. Accordingly, the goals of this study were to 1) develop protocols for accurately modeling the three‐dimensional (3D) anatomy of nasal airways with in silico controlled variation in mucosal congestion, and 2) test the hypothesis that mucosal surface area‐to‐volume ratios (SA/V) remain constant throughout the nasal cycle. A computed tomography (CT) scan of one male human head was selected for use in the development of protocols for controlling congestion levels via digital expansion/contraction of the nasal mucosa in Amira‐Avizo. These protocols were then used to generate a fully decongested (left/right = 0/0%) nasal airway model for use as an anatomical baseline comparator. Models were then generated for two different phases of the nasal cycle: asymmetrical (left/right = 90/10%) and mid‐cycle (left/right = 50/50%). Nasal passage surface areas and volumes were collected for each model to permit comparisons of SA/V ratios across different mucosal congestion levels. Following theoretical expectations, the decongested model exhibited a substantially lower SA/V (0.57) than the mid‐cycle (0.72) and asymmetrical (0.74) models. Unilateral analyses also met anatomical expectations, with the highly congested left nasal passage of the asymmetrical model demonstrating a higher SA/V (1.06) compared to the same left passage in the mid‐cycle (0.84) and decongested (0.60) models. Cumulatively, these results suggest that the developed 3D digital methods permit reliable in silico modeling of nasal soft‐tissues, allowing future studies to control for mucosal congestion while investigating the role of nasal morphology on respiratory airflow (using computational fluid dynamics analysis, etc.). Moreover, the similar overall SA/V ratios of the two nasal cycle models appear consistent with the hypothesis that, despite morphological asymmetry, the nose’s overall SA/V and air‐conditioning capacity likely remains relatively stable throughout the nasal cycle. Thus, our study suggests that, rather than morphological variability within the nasal cycle, it is the distinction between the semi‐congested nasal cycle versus complete mucosal decongestion (as seen during strenuous exercise) that may confound functional interpretations of ecogeographic variation in nasal morphology. Consequently, further research is needed to determine how these distinct physiological states impact nasal functional morphology in different climates.