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.