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INTRODUCTION

Diagnostic investigation in otolaryngology is usually complex, requiring sophisticated devices and advanced technology such as computed tomography, magnetic resonance imaging, ultrasound, fiber-optic endoscopy, etc.. The most recent innovations include methods based on the physical principle of acoustic reflection, which have gained importance for being extremely useful in clinical and experimental settings. Typical examples for the assessment of upper airways would be rhinometry and acoustic pharyngometry.

Rhinometry and acoustic pharyngometry assess the geometry of the oropharyngeal cavity1,2,3 using a reflected acoustic signal (filtered click) emitted from a device and sent into the oropharynx. This principle is based on the relation:

Z = rC/A

Where: Z is impedance (resistance), r is the density of gas traversed by the acoustic wave, C is the wave velocity, and A is the diameter of the tube. If we consider r a constant and C equal to 0, we may deduce that the impedance is inversely proportional to the cross-section of the tube:

A = 1/Z

The amplitude and frequency of reflected waves depend on the airway area, while the time taken for the reflected wave to return is a function of distance. Therefore the relation "pressure/time" may be changed into "cross-sectional area/distance".

Consequently, reductions in the anatomical space - in particular the diameter - will produce changes in the intensity of the reflected wave and in the time taken for the reflected wave to return from a given anatomical structure to the microphone.4

Fredberg and col.5 were the first to hypothesize the possibility of using acoustic reflection in the mouth to study the airwave network geometry and to define reproducible, accurate and variable parameters for acoustic pharyngometry.

Further studies compared acoustic pharyngometry and other traditional diagnostic methods such as cephalometrics, computed tomography, and magnetic resonance imaging6,7 as applied to certain airway obstructive diseases, especially the obstructive sleep apnea syndrome (OSAS). These studies found no statistically significant difference between these methods. As such, acoustic pharyngometry was considered a valid, non-invasive, and easily reproducible tool to study the permeability of upper airways.

The aim of this study was to assess pharyngometric parameter variations in a group of patients with variable levels of sleep-disordered breathing (SDB) of exclusively obstructive origin. Clinical, anthropometric, and instrumental data were carefully studied to define whether acoustic pharyngometry could become a reliable diagnostic tool for SDB (snoring, OSAS) by measuring variations in the morphology and volume of the mouth, pharynx, and larynx.

SERIES AND METHOD

110 patients (58 men and 52 women) aged between 18 and 70 years (mean age 44 years) were examined. 70 of these patients (64%) composed the sample group that included SDB patients diagnosed from their clinical history and confirmed by polysomnography. Patients were divided into two groups according to polysomnographic results, depending on the severity of the respiratory disorder:8 snoring only and OSAS of various degrees subdivided into three stages according to the Respiratory Disturbance Index (RDI), and the degree of daytime drowsiness calculated according to Epworth's scale (ESS). (Tab. I)

The study was approved by the director of the hospital unit according to current guidelines in Italy; the study was noninvasive and carried no direct or indirect risk for the patients.

The control group involved 40 patients (36%) selected according to inclusion criteria. (Tab. II)

All of the patients first underwent rhinoscopy with morphological assessments of the nasal septum and the lower turbinates. This study of the mouth aimed to investigate the existing relations between the tongue and the oral cavity according to the modified Mallanpati scale, to identify the degree of palatine tonsillar hypertrophy according to Brodsky's classification, to check the existence of eventual size increases of the soft palate, to measure the length and thickness of the uvula, and finally to assess orthodontic structures (dental malocclusion, prognathism, progenism, micrognathia, retrognathia, etc.).

Flexible fibronasopharyngolaryngoscopy (VISION SCIENCES - ENT/2000 - 3.4mm) was also done to uncover other possible obstructive diseases such as nasal polyposis, rhinopharyngeal tumors, stenosis or atresia of the choanae, hypertrophy of the lingual tonsil, morphological and functional changes of the hypopharynx and the glottis, etc.. Müller's maneuver was done in all patients during this exam to identify the degree of hypopharyngeal wall collapse.

Finally patients underwent computerized active anterior rhinomanometry (AAR) (RHINO-SISTEM-MENFIS-BIOMEDICA SRL-ITALY), both in baseline conditions and after using topical nasal decongesting agents, to quantify the degree of nasal obstruction.

The body mass index (BMI) was calculated; values over 30 were defined as altered; the height and neck circumference were also measured (normal neck circumference was defined as values below 42 cm).

The acoustic pharyngometer (Eccovision Acoustic Rhinometer - HOOD Laboratories, Boston, MA (USA) - Fig. 1) was used to assess the area and volume of the mouth, the pharynx, and the larynx.

Figure 1 - Pharyngometer (device)

Pharyngometry

Procedure/Interpretation

Patients are kept awake and seated (the back should be vertical) during the exam. The sound wave generator tube, which should always be horizontal, is placed close to the mouth.

The tube has a disposable plastic mouthpiece over which the patient closes his/her mouth, avoiding sound wave loss. The patient is guided not to move, to maintain his/her head still, and to slowly breathe through their mouth. It is important to avoid nasal breathing, as the opening of the velopharyngeal space would increase the calculated volume. The nostrils should be compressed externally throughout the exam. During the exam the patient pronounces the phoneme "oooh" without issuing sound, so that the tongue is kept relaxed on the floor of the mouth to assure complete closure of the velopharyngeal space and to avoid measurements of the nasal volume. (Fig. 2)

Figure 2 - The exam

The sound wave emitted by the generator (loudspeaker) placed at the base of the pharyngometer tube travels along the tube and the airways of the patient, generating reflected waves. These waves are recorded by a microphone and have an amplitude and frequency that depend on the area of the airway; the time taken for the reflected waves to return is a function of the distance traveled. (Fig. 3)

Figure 3 - Outline of acoustic pharyngometry

As mentioned before, we use the relation "pressure/time", converted by an analyzer into the relation "cross-sectional area/distance" measured as cm2/cm, from which a chart - the pharyngogram - is obtained.

This chart (Fig. 4 A-B) is shown on a cartesian system, where the x-axis is the airway distance (calculated in cm) and the y-axis is the airway area (calculated in cm2). It may be subdivided into three regions:

a) "Oral region" from the incisors to the soft palate;

b) "Pharyngeal region" from the soft palate to the hypopharynx;

c) "Laryngeal region".

Figure 4 A - Pharyngogram: morphology

Figure 4 B - Pharyngogram: corresponding anatomical sites

On the chart (Fig. 4) the distance zero is the endpiece of the tube placed over the incisors. The first wave (A) originates from this point, indicating the beginning of the mouth; its peak (B) is found at an approximate distance of 2 cm, and its area is roughly 6.5cm2. The curve, therefore, begins with a deflection reaching a minimal area value of roughly 1.4 cm2 at a distance of 8cm (C), which anatomically is the anterior margin of the soft palate (oropharyngeal junction).

The curve on the chart begins to move up reaching its peak at about 13cm on the x-axis; its area is 3cm2, which is the hypopharynx. This wave segment has two secondary peaks, which are usually not visible; the first one is the posterior oropharynx (D), and the second one is the hypopharynx (E).

The F point on the chart is placed at approximately 16cm from the incisors, at its area is roughly 1cm2. This is the glottic region, always seen as the narrowest area of the curve. This deflection is followed by an area of subglottic expansion (G).

Each segment of the pharyngogram may be numerically analyzed in greater detail (area, volume, distance from incisors) by changing the position of the segment limits under analysis on the x-axis. Care must be taken when analyzing data obtained from a significant post-stenotic area, which might limit the return of the reflected wave, generating shadow zones.

The following exam parameters were assessed in this study: (Fig. 5 A-B)

- Wave I amplitude (identifies changes in the volume of the tongue);

- Extension of the O-F segment (assesses the surfaces that make contact in the oropharynx);

- Amplitude of the O-F segment (indicates decreased area in the oropharynx);

- Area of the hypopharynx (considered as that area below the oropharynx).

Figure 5 A - Main pharyngometric parameters