Keywords
nasal airflow simulation - nasal breathing - statistical shape model - diANA - nasal
obstruction - rhinorespiratory homeostasis
Impaired nasal breathing is a frequent problem doctors have to deal with. Estimations
suggest that up to 30% of the population in Europe and North America suffer from a
recurrent or permanent nasal airway obstruction.[1]
[2]
There are three fundamental etiologic groups, and they can be interrelated or coexisting:
Group 1: Deformation of the nasal framework with consecutive changes of the flow domain
of the nasal cavity. The resulting flow field anomalies can also alter the mucous
membranes or erectile tissue with retroaction on the flow domain.[3]
Group 2: Chronic inflammatory diseases or substance-related reactions of the mucous
membranes.
Group 3: Disturbed reception and/or perception of the nasal airflow.
In groups 2 and 3, the first option of treatment is to eliminate the primary causes
or rather the underlying disease. In contrast, group 1 patients can be helped in the
first instance by surgical means restoring the nasal framework and the nasal cavity's
flow domain. For this, objective information about the nasal airflow is essential.
Currently, rhinomanometric methods developed by Vogt and Mlynski are considered the
benchmark in this regard.[4]
[5] In fact, they are only able to provide the value of the total nasal resistance of
each side, which is sufficient for documentation of clear findings. However, these
methods are lacking in details of the airflow within the nasal cavity. Hence, they
are not eligible for identification of a flow-compromising site or structure. Furthermore,
the value of the total nasal resistance is not one-to-one attributed to only a particular
morphology of the nasal cavity. This might be a reason for arbitrariness and polypragmasia
in functional rhinosurgery. It should also be pointed out that a low nasal resistance
does not automatically ensure a physiologic nasal airflow and inner milieu of the
nasal cavity. Not least, the relatively high rate (approximately one fourth or 27%)
of unsatisfactory results of functional surgery on the nasal framework indicates the
need to improve the preoperative diagnostic measures.[5]
[6]
In terms of this necessity, we introduce our methodology of comprehensive assessment
of nasal breathing. It is based on airflow simulation (computational fluid dynamics
[CFD]) in conjunction with a statistical shape model (SSM) of the healthy nose and
an enhanced physiologic approach to nasal breathing—the concept of rhinorespiratory
homeostasis (CRRH).[7]
Fundamentals and Prerequisites
Fundamentals and Prerequisites
Digital Analysis of Nasal Airflow (diANA) merges three innovative methods or scientific
approaches described briefly below. It needs the skills of engineers (fluid mechanics),
computer scientists (image processing), and medical doctors (otorhinolaryngology)
to implement and execute this service.
Computational Fluid Dynamics
Computational fluid dynamics is an advanced technology that is established in the
industry as well as increasingly used in medicine. In particular, it allows for simulation
of the fluid flow within complex geometries by numerically solving equations describing
conservation of energy, mass, and momentum. CFD requires prior reconstruction of the
flow domain and its discretization with finite volumes, generating a grid. Then, calculation
in consideration of the boundary conditions and subsequent visualization can be performed.[8]
[9]
[10]
Typical medical applications are exploration of the blood flow in the heart, aorta,
or brain vessels.[11]
[12] Investigations of the intranasal airflow using CFD is the subject of research for
many years.[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22] In contrast to rhinomanometry, CFD is able to provide user-defined parameters of
the intranasal airstream in a high spatial and temporal resolution. Required is a
Digital Imaging and Communications of Medicine (DICOM) dataset derived from a computed
tomography (CT scan) or optionally from a cone beam CT of the paranasal sinuses. Its
resolution should be according to the recommendations for navigated surgery, and the
nasal cavity has to be completely captured from the tip up to the choanae. Then, sufficient
reconstruction of the nose's three-dimensional (3D) geometry is possible after segmentation.
Our specific CFD workflow including the used modeling and boundary conditions is in
accordance with those of a prior study.[13]
Statistical Shape Model of the Nasal Cavity
The geometry of the nasal cavity exhibits an enormous heterogeneity, which causes
corresponding variety of the respective airstream.[23] Besides that, there are naturally occurring characteristic morphological structures
such as the septum, isthmus nasi, and turbinates. The latter form the three nasal
meatus on the lateral nasal wall. These topological conditions complicate the comparison
of individual noses and, hence, the related airstream. It makes it challenging to
distinguish between physiologic and pathologic airflow. We address this problem using
a SSM of the nasal cavity. The model was generated from a population of 25 healthy
noses (50 nasal cavities, right and left side) of subjects who did not report impaired
nasal breathing. The patients were selected by one and the same examiner (ear, nose,
and throat [ENT] specialist). The examiner also excluded the existence of inconsistent
findings. The individual geometries were reconstructed from segmented CT scans of
the paranasal sinuses available through the routine ENT practice run by one of the
authors. Subsequently, a specific mathematical algorithm, described by Lamecker and
colleagues in 2009 and 2016, was applied to obtain an appropriate SSM of the healthy
nasal cavity.[24]
[25] The respective averaged geometry shows all regular anatomical landmarks. In addition,
the calculated airflow exhibits all characteristic features, which are described in
the literature as well as observed in our own simulations.[14]
[15]
[17]
[19]
[20]
[21]
[23] We presume that the mean geometry of the SSM represents normal morphology of the
nasal cavity determining physiologic nasal airflow. Consequently, it can serve as
the reference used in diANA intending to delineate impaired from nonimpaired nasal
breathing and to reveal the related noticeable findings in case of problems ([Figs. 1] and [2]).
Fig. 1 Mean geometry of the statistical shape model derived from healthy nasal cavities
viewed from diverse directions, exhibiting the entirety of the common anatomical landmarks.
Fig. 2 Calculated steady airflow (12 L/min according to 6 L/min respiratory minute volume
at rest) in the mean geometry of the statistical shape model derived from healthy
nasal cavities for inspiration and expiration, displayed from left to the right side:
pressure field, velocity field, and distribution of wall shear stress in the left
nasal cavity.
Concept of Rhinorespiratory Homeostasis
Concept of rhinorespiratory homeostasis is an alternative approach to nasal breathing.
It focuses on the parietal effect of the inspiratory and expiratory intranasal airstream
referring to wall shear stress, whereas the conventional view mainly considers the
inhaled air.[3]
[6] The wall shear stress parameter describes the amount of shear force on the nasal
wall through the flowing air. It depends on the near wall flow velocity and correlates
with the mass and heat transfer occurring between the wall and air. Several biological
and physical reactions in/on epithelial and endothelial tissues are attributed to
wall shear stress.[26]
[27]
[28]
[29]
[30] In this context, CRHH addresses the link of the epithelium lining liquid fluid and
nasal breathing.[3] One implication drawn is that the existence of a normal intranasal flow field in
conjunction with adequate secretion is necessary as well as sufficient for maintaining
a physiologic inner milieu (prerequisite for nasal functions and airstream perception),
provided there are no adverse factors such as inflammatory or toxic influences and
destructed epithelium. It suggests that consideration of temperature and humidity
might be dispensable contemplating the link between the interaction of the flow field
and inner milieu of the nose.
Methodology
diANA starts with patient selection and acquisition of detailed information about
medical history, clinical findings, and rhinomanometry, when available. It can be
provided either through personal examination, via teleconsultation, or exchange with
the attending doctor.
The DICOM data set of the CT scan has to be anonymized prior to delivery for segmentation
and subsequent reconstruction of the 3D geometry, which the workflow of CFD initiates.
The calculation of the following flow field parameters is sufficient for diANA: flow
velocity, wall shear stress, pressure field, pressure drop, and the total nasal resistance
of each nasal cavity. We do not take into account temperature and humidity.
The nose is regarded as parallel-connected conduits. Generally, we consider laminar
steady airflow of 12 L/min according to breathing at absolute rest with a respiratory
minute volume of 6 L/min.
The obtained patient-specific nasal geometry and the related flow field parameter
values are compared with those of the SSM or the derived mean geometry, to identify
the differences ([Fig. 3]).
Fig. 3 IN: Individual nasal cavity with respective airflow. SSM: Statistical shape model
of the nasal cavity/derived mean geometry with respective airflow.
The juxtaposition can be executed on three levels. Usually, visual comparison of the
airstream patterns, including the calculated total resistances as well as the nasal
cavity's morphologies with consideration of clinical information, is sufficient to
make a valuable judgment. This complies exclusively with qualitative assessment, which
is level one. In certain cases, we map the patient's geometry and flow field onto
the mean geometry of the SSM and its flow field, respectively. This allows for semiquantitative
calculation of deviations in a point-to-point mode, which is level two. However, there
are no predictions possible whether or not the detected differences are statistically
significant. Only mathematical transformation of the patient's data to those of the
SSM can gain fully quantitative information about aberrations, which is level three.
Finally, after compiling all information and data, elaboration of an expert opinion
including instructive visualizations is executed, using the synergy of advanced technology
and rhinologic expertise. This might support the attending doctor's decisions.
Discussion
Added Value
In contrast to rhinomanometry, the nose is no longer considered a black box but as
a system of two parallel conduits. In addition, to our knowledge, diANA is the first
and currently only tool that employs a SSM of the healthy nose as a reference for
the evaluation of nasal breathing. This enables morphometric aberrations and consecutive
flow field anomalies to localize and quantify within the nasal cavity. Furthermore,
the wall shear stress parameter provides a measure of the mass and heat transfer as
well as mechanical forces. Both values stimulate receptors in the epithelium that
is required for the perception of the nasal airstream and regulating processes.
The isthmus nasi is the essential bulk flow formation structure.[31]
[32]
[33]
[34] However, it is impossible to gain all flow-relevant parameters of this structure
through a clinical or even CT scan examination. Only 3D reconstruction of the nasal
cavity's geometry in conjunction with airstream simulation used in diANA allows for
millimeter accurate determination of its position, shape, and extent in all three
dimensions. At the same time, it is possible to assess how airstream is influenced
in context with the adjacent flow domain.
Factors such as size of the nose, body constitution, gender, ethnicity, and the respiratory
minute volume affect the total nasal resistance but have little or no influence on
the airstream pattern. This also applies to physiological phenomena such as nasal
cycle, compensatory turbinate hyperplasia, or a moderate diffuse swelling of the entire
inner lining. Airstream simulations performed by us and other research groups account
for this assertion.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22] Consequently, the results of diANA are relatively independent of the listed influences
due to the topological comparison modus.
Limitations
Besides deformations of the nasal framework, the geometry of the flow domain can also
be significantly modified through manifest diseases of the mucous membranes that is
impractical or impossible to determine. Therefore, their presence should be excluded.
However, in case of minor affections, for example, due to mucous formations, thin
adhesions/synechiae, small polyps, or circumscribed perforations, reconstruction of
a consistent geometry succeeds within certain limits through interpolation.
During strong inhalation, one can observe an inward movement of the alar wings due
to the resulting transmural force. This is a phenomenon of fluid–structure interaction
(FSI) and might serve as protection of the nasal or olfactory epithelium.[17]
[35] The respective functional region in the nasal cavity works according to a Starling
resistor limiting the inflow depending on its strength.[36] Commonly, however, with rising physical stress a mouth bypass is initiated, and
no onset of the so-called nasal valve mechanism occurs. Nevertheless, diANA intends
to consider nasal breathing only at absolute rest enabling predominant or even exclusive
respiration through the nose. Overall, effects of FSI appear to us to be negligible.
In case of incongruity of airflow perception and diANA results, determining to what
extent problems with reception or the subsequent signal processing are involved, is
challenging. Using diANA, one has to be aware that questions only regarding to the
nose's morphology or the respective airflow can be addressed.
Case
A 28-year-old female patient, operated on the nasal framework elsewhere, complained
about persisting impaired nasal breathing on the right side. A minor ipsilateral septum
deviation was the only clinical correlate. The CT scan of the paranasal sinuses did
not show a relevant chronic rhinosinusitis. Furthermore, rhinomanometry revealed resistance
values on both sides in the normal range. There was uncertainty about the functional
relevance of the septum deviation. Therefore, we applied diANA using the existing
CT scan data. Qualitative comparison of the morphology and the pressure drop with
the reference showed a substantial isthmus stenosis on the right side ([Fig. 4]). In contrast to the measured resistances, the calculated values were in accordance
with the patient's complains. We speculate that the rhinomanometer mask lateralized
the cheek's soft tissue and, hence, generated false values. We proposed an extracorporeal
septum correction, introduced by Gubisch and colleagues, to the patient.[37]
[38]
Fig. 4 Significant changes of the isthmus' morphology compared with the statistical shape
model (SSM) reference on the right side, associated with increased pressure drop.
This example illustrates our opinion that conventional assessment of the isthmus nasi
is not sufficiently reliable.
Outlook
Only recently we implemented diANA after 2 years of testing, but there is a constant
need for enhancement. For example, we are continuously increasing the population of
healthy nasal cavities to improve the quality and reliability of the SSM. Furthermore,
complete automation of various processes, for example, segmentation, will help reduce
the costs, diminish the investigator bias, and eventually support establishing the
method.
Currently, we are working on the project making it possible to display obstructing
regions or structures within the nasal cavity in terms of augmented reality during
endoscopy or navigated surgery.
Conclusion
diANA is a new tool for the evaluation of the intranasal airstream. We have developed
a technique that allows, in contrast to established methods, for a more comprehensive
understanding of compromised nasal breathing and flow-related changes of the inner
milieu. Using diANA intends to distinguish between normal and abnormal nasal breathing
as well as to detect the responsible findings in case of impairment. This has the
potential to avoid unnecessary surgery on the one hand, and on the other to improve
target-orientation and precision of functionally indicated operations on the nasal
framework.
