Discussion
To date, there is no consensus on nasopharyngeal depth normative data or a reliable method to estimate it in adults. Our results showed that the average nasopharyngeal depth in adult men is 10.1 cm (SD 0.9) and 9.4 cm in adult women (SD 0.86). Among the external facial distances analyzed, the distance anteriorly from the tragus to a plane perpendicular to the philtrum was the most strongly correlated to the nasopharyngeal depth. This clinical measurement can predict the nasopharyngeal depth quite accurately, with an average error estimation as small as 0.1 cm. Self-reported ethnicity or age, does not seem to interfere with this premise.
Both the New England Journal of Medicine and the Centers for Disease Control have advocated for the performance of NP swab by inserting along the floor of the nose until resistance is met, or to a distance equivalent of the ear canal to the nose; however, they do not further specify the directionality (along the curvature of the face or perpendicular from the ear), nor do they give specifics about what part of the ear or nose to start and end the measurement, the average distance, or evidence for this recommendation. In the performance of a nasopharyngeal swab, premature resistance from irregularities including nasal deviation, polyposis, and inferior turbinate hypertrophy could lead to sub-optimal swab insertion depth. Conversely, overestimation of the nasopharyngeal depth or incorrect directionality can increase the risk of complications ranging from epistaxis to the injury of the skull base in rare cases, especially for inexperienced providers.8,9 Attempts to train healthcare workers on proper NP swab technique have improved procedural accuracy and self-confidence using group training sessions and simulation.12,13 Heterogeneous NP depth insertion among guidelines14 for this procedure have implications extending beyond the test accuracy, and these measurements should help standardize what is regarded as a “nasopharyngeal swab”. However, since the performance of these swabs is based on tactile feedback along the patient’s nasal cavity, estimating individualized NP depth seems necessary as a pretest method to predict the appropriate depth of swab insertion and appropriately label the performed swab (mid-turbinate versus anterior nasal versus nasopharyngeal).
Considering the degree of association between the distance anteriorly from the tragus to a plane perpendicular to the philtrum and ND on our study’s correlation and regression analyses, this perpendicular distance seems to be the most appropriate external facial measurement for predicting NP depth. These results can support updated instructional guidelines on effective nasopharyngeal swab procedures.
Our ND results are in line with a study from Lim et al. (2014) which assessed nasopharyngeal depth for temperature probe positioning in 200 participants from Korea (mean age 52 yrs., SD 12) via nasoendoscopy and reported an average ND of 9.4 cm (SD 0.6), and 10 cm (SD 0.5) in women and men, respectively.10 They also measured the distance from philtrum to tragus along the facial curvature, similar to the ”curved” distance in our study, as a potential predictor of ND. However, while the reported endoscopic depth is similar to our results, their participants’ curved distance was 3 cm longer than ours for both women and men. In their study, measurements were performed in the supine position and beyond the alar-facial groove to the midline philtrum, which could account for the difference. In addition, the authors showed a weak correlation between ND and curved distance. They did suggest a regression model to predict ND using curved distance, though it was not validated. Lastly, the authors stated that the difference from their ”curved distance” to ND was, on average 5 cm, suggesting that prediction of ND could be made by simply subtracting 5 cm from the tragus-to-philtrum distance. Since their measurement parameters differ from our study, this delta cannot be compared to our Δ Curved.
Nasopharyngeal depth in adults was also assessed by Callesen et al. (2021) in 109 adult participants in Denmark (mean age 34 yrs., SD 13) using a nasal swab guided by a video endoscope on the opposite nostril; however, they did not correlate this with any external measure.1 Their results showed an overall ND of 9.4 cm (SD 0.64), lower than our total sample’s ND (i.e., 9.81 cm, SD 0.94). Based on their results, the authors suggested a minimum swab insertion depth of 8 cm in adults. Although we acknowledge that a minimum value could reduce the odds of sub-optimal swab insertion on nasopharyngeal tests in adults, a clinical predictor for individualized ND is more practical. According to our results, predicting NP depth before procedures when appropriate is simple and straightforward.
The CT distance assessed in our study does not seem to be a good clinical predictor since it may result in ND underestimation of 1.2 cm (SD 0.8) and it relies on availability of sinus CT images. However, Lee et al. (2014) assessed the distance from the nares to the closest proximity portion of the nasopharyngeal mucosa through the inferior meatus in sagittal images of 100 patients’ CT scans (females 50 yrs., and males 52yrs.).11 Their results show an average ND of 9.1 cm (95% CI 8.1 to 10.2) and 9.7 cm (95% CI 8.6 to 1.8) for women and men, respectively, which are considerably higher than ours and more realistically approximate our average ND results. The difference is likely accounted for by measurements in the sagittal plane, which differs from the axial view used in our study, which increases variability based on tilt. Due to missing sagittal plane reconstructions in a number of subjects, we elected to use axial images. Accordingly, future CT measurements should be conducted in the sagittal plane for consistency among studies.
We acknowledge that the generalizability of our results is limited to adults belonging to our normally distributed study population. As such, the ratios between external facial measurement and NP depth can be expected to differ in those excluded from study participation, including those with a history of mid-face surgery and craniofacial abnormalities. Additionally, our findings should not be generalized to children whose craniofacial ratios continue to change with sinus development and facial elongation until adulthood. It is noted that although our findings do not demonstrate a significant ethnicity-related variation in NP depth, our sample was predominantly Caucasian. Validation in other populations would help to broaden applicability.
The consistency between our nasopharyngeal depth data and previously published studies from other countries reassures that our results are likely similar throughout adult populations. Our predictive method gains validity from measurements made by otorhinolaryngologists with considerable experience in nasal endoscopy and anatomy, and consistency amongst data collection sites confirms a robust and reproducible methodology. The narrow difference from NP depth to perpendicular measurement allows for simple external estimation by any healthcare provider.