Nasal Structural and Aerodynamic Features That May Benefit Normal Olfactory Sensitivity

Abstract

Nasal airflow that effectively transports ambient odors to the olfactory receptors is important for human olfaction. Yet, the impact of nasal anatomical variations on airflow pattern and olfactory function is not fully understood. In this study, 22 healthy volunteers were recruited and underwent computed tomographic scans for computational simulations of nasal airflow patterns. Unilateral odor detection thresholds (ODT) to l-carvone, phenylethyl alcohol (PEA) and d-limonene were also obtained for all participants. Significant normative variations in both nasal anatomy and aerodynamics were found. The most prominent was the formation of an anterior dorsal airflow vortex in some but not all subjects, with the vortex size being significantly correlated with ODT of l-carvone (r = 0.31, P < 0.05). The formation of the vortex is likely the result of anterior nasal morphology, with the vortex size varying significantly with the nasal index (ratio of the width and height of external nose, r = -0.59, P < 0.001) and nasal vestibule "notch" index (r = 0.76, P < 0.001). The "notch" is a narrowing of the upper nasal vestibule cartilage region. The degree of the notch also significantly correlates with ODT for PEA (r = 0.32, P < 0.05) and l-carvone (r = 0.33, P < 0.05). ODT of d-limonene, a low mucosal soluble odor, does not correlate with any of the anatomical or aerodynamic variables. The current study revealed that nasal anatomy and aerodynamics might have a significant impact on normal olfactory sensitivity, with greater airflow vortex and a narrower vestibule region likely intensifying the airflow vortex toward the olfactory region and resulting in greater olfactory sensitivity to high mucosal soluble odors.

Figure 1.

Facial reconstruction based on CT scan, and measurement of nasal index as the ratio of external nasal width and height.

Figure 2.

CT-based computational model. Side-by-side comparison of the CT scan and CFD model from sagittal and coronal views, respectively. The dashed line indicates the slice cut on the sagittal plane. The perspective view of the 3D model and its dimensions are shown on the right top plot. In a close-up view (right bottom), layers of small and fine elements along the wall can be seen; these capture the rapid near wall changes of air velocity and odorant concentration and are essential for accurate numerical simulations.

Figure 3.

Measurements of vortex index (a) and notch index (b). The vortex index is defined as the ratio of vortex length (D) and nasal cavity length (L). The notch index is defined as the ratio of notch depth (h) and nasal cavity length (L). In plot (b), point A indicates the deepest point of the nasal notch. The line BC is the extension line along the tangential direction of the anterior dorsal curve. The notch depth (h) is defined as the perpendicular distance from point A to line BC.

Figure 4.

Endoscopic view of the nasal valve region of a significant (a), a small (b), and an absence of notch (c). The views are generated by ParaView 5.1.2 (Kitware Inc.) based on CT scans.

Figure 5.

Sagittal view showing external nose (gray transparent) and morphology of the nasal vestibule airway (gold solid) for each phenotype. (a) significant notch (notch index > 5%). (b) small notch (notch index < 5%). (c) no notch (notch index = 0%). The “n” values indicate the number of sides that were categorized into each phenotype. Airflow streamline patterns in the nasal cavity were categorized based on the formation of anterior–superior airflow vortex. Depending on its nasal notch index (=0%; <5%; >5%) and vortex index (=0%; <20%; >20%), unilateral nasal cavities were categorized into nine different types. The “n” values indicate the number of sides of all subjects that were categorized into each type.

Figure 6.

Nasal index distribution for all subjects with various scores of the vortex index (a) and notch index (b). The nasal index for the subjects with significant vortex (vortex index > 20%) was significantly lower than that of the subjects with no vortex (vortex index = 0%) in their nasal airflow. Similarly, the nasal index for the subjects with significant notch (notch index > 5%) was significantly lower than that of the subjects with no notch (notch index = 0%) for their nasal anatomy.

Figure 7.

Scatter diagram of the notch index (a) and vortex index (b) between left and right side of the same subjects. Among 22 tested healthy controls, there was no significant correlation between left and right side of the nose for either the notch index or the vortex index.

Figure 8.

ODT for L-Carvone (a, d), PEA (b, e), and D-Limonene (c, f). Subjects with significant nasal valve notch (notch index > 5%) and more intense anterior airflow vortex (vortex index > 20%) are likely to have better olfactory sensitivity to an odorant with high mucosal solubility (L-Carvone and PEA), but not an odorant with low mucosal solubility (D-Limonene).