Role of Surgical Procedures on Pressure-Less Acoustic Immittance Measurements

Middle ear diagnostics encompasses a variety of technologies aimed at assessing pressure conditions in the middle ear and measuring middle ear immittance. Standard tympanometry requires a standardized pressure change in the ear canal [1-3]. A frequency is typically selected for this purpose, with 226 Hz used for adults and 1000 Hz for neonates and infants [4]. The introduction of wideband immittance measurement addressed some limitations of single-frequency tympanometry. Wideband tympanometry, also known as wideband acoustic immittance (WAI), tests a frequency range from 226 Hz to 8000 Hz [1, 5]. However, both conventional tympanometry and WAI face two persistent challenges: they often require pressurization and a sealed outer ear canal. To overcome these limitations, a pressure-less acoustic immittance technology called PLAI™ was introduced [6-9]. PLAI provides several parameters with clear pathophysiological correlates. Preliminary examinations of healthy middle ears reveal that tympanometry does not yield normal findings in all test subjects, whereas PLAI consistently provides reliable values, including resonance frequency admittance, peak admittance, quality factor derived from bandwidth, acoustic ear volume, and bandwidth (BW) [10, 11]. Conversely, patients with various middle ear diseases, such as otosclerosis [12], otitis media with effusion [13], and adhesive processes [14], exhibit altered resonance frequencies and peak values. These investigations showed that certain measurement parameters, particularly resonance frequency and peak admittance, vary among different pathologies. It was therefore concluded that the measurement results are influenced by factors such as the condition of the eardrum, the ventilation of the tympanic cavity, the aeration of the mastoid, the function of the Eustachian tube (ET), and the integrity of the ossicular chain. Ultimately, the measurement results reflect all changes occurring in the ear.

Case studies will now be employed to investigate whether treatment effectiveness can be assessed despite the various pathologies of the ear. In the first case of an adhesive process with pronounced myringosclerosis, we will examine whether PLAI measurements in a patient with an open ET indicate normalization of the measurement results following the insertion of a tympanostomy tube and whether artificial closure of the tympanostomy tube confirms this change.

The second case will explore the influence of mastoid volume on the PLAI measurement results, as well as the effects of ear tube insertion and its closure.

The method for PLAI measurements, implemented in the Medwave® device, has been described in detail elsewhere [15]. However, a summary is provided here.

PLAI measures acoustic admittance across a frequency range of 100–3000 Hz without inducing pressure changes in the outer ear canal. By stimulating the ear with a multifrequency acoustic signal, the PLAI device estimates the energy transmitted versus reflected by the middle ear [6, 7, 10, 11]. The resonance frequency and bandwidth of the resonant systems reflect the mass and stiffness of the tympanic membrane and the ossicular chain complex, as well as damping factors in the middle ear and the effective geometry and volume of the external auditory canal [15].

Importantly, the resonance frequency represented is not that of the middle ear itself, but rather that of the admittance, which changes due to alterations in the eardrum and/or the middle ear.

PLAI measurements

The hand-held probe of the Medwave device is inserted into the outer ear canal without manipulating the outer ear canal pressure, meaning that sealing the ear canal is unnecessary.

A multifrequency stimulus is delivered in the range of approximately 100–3000 Hz. Particle velocity is measured at two closely spaced points in the outer ear canal. A complex admittance is derived from the ratio of sound pressure to particle velocity as a function of frequency [6, 7]. The resulting admittance curve displays a resonance peak that is highly correlated with middle ear properties [16] and, consequently, with middle ear pathology [12, 13].

The hand-held probe connects to a touchscreen. Medwave has established age-specific reference values for PLAI measurements in healthy ears [16]. Core parameters are extracted from the frequency-domain curve rather than from a pressure-admittance loop. Current clinical implementations typically extract a compact set of scalar parameters from the admittance magnitude. For clinical application, the resonance frequency and peak of admittance are routinely delivered. For research purposes, additional factors can be derived from the measurements, including acoustic ear volume, band admittance height, minimum and maximum boundary frequency, frequency bandwidth, and quality factor from bandwidth. Acquisition time is a few seconds per ear.

PLAI-based measurements after Valsalva and Toynbee maneuvers

A baseline PLAI measurement is recorded. The patient/test subject is then instructed to hold their nose closed and drink water 2 or 3 times while maintaining this position, or to swallow (Toynbee maneuver) or press (Valsalva maneuver).

The measurement time remains a few seconds per ear, and the difference in the resonance frequency of the admittance is recorded.

PLAI-based stapedial reflex measurement

A baseline PLAI measurement is recorded at moderate intensity. A stapedial reflex-exciting stimulus, either ipsilateral or contralateral, is delivered [16]. A second PLAI measurement is recorded during or after the reflex, resulting in a frequency shift toward higher frequencies, changes in peak values, and alterations in bandwidth and band admittance height.

Ethical considerations

The intraoperative PLAI measurements were conducted in strict adherence to the revised version of the Helsinki Declaration. Both patients were informed about the objectives of the measurements and provided their written consent for the measurements and publication of the data.

Case 1

In a case involving a calcified and stiff tympanic membrane with meso- and epitympanic retraction and partial mesotympanic adhesion, a tympanogram type B was detected (Figure 1). An ear canal volume of 1.2 mL was recorded. PLAI measurements indicated a frequency of admittance before pneumatic otoscopy of 495.3 Hz and a peak of admittance of 2.13 × 10-2 mmho (after pneumatic otoscopy: 506 Hz and 1.83 × 10-2 mmho). Pure tone audiometry revealed a slight conductive hearing loss. Given the increased resonance frequency of admittance, the elevated peak of admittance, and the negative Valsalva maneuver, an adhesive process was suspected.

The partial adhesive process was confirmed through pneumatic otoscopy.

To enhance ET function before tympanoplasty, ET dilatation was performed [14]. Three weeks post-procedure, ET function was re-evaluated. The Valsalva maneuver showed no clear bulging of the calcified tympanic membrane, resulting in an ambiguous maneuver outcome. Figure 2 and 3 illustrate the PLAI measurements before and after treatment.

PLAI measurements indicate that after the insertion of a ventilation tube, there was a greater decrease in resonance frequency, peak measures, quality factor, and frequency bandwidth values compared to paracentesis, while the ear canal volume increased. Artificial blockage of the ventilation tube resulted in a renewed increase in resonance frequency and a decrease in Vea, Q, and BW measures (Figure 3, Table 1).

Case 2

In a case of Ménière’s disease with a normal tympanic membrane and normal ET function, saccus decompression and insertion of a ventilation tube for further intratympanic medication were indicated. PLAI measurements showed a frequency of admittance of 419.9 Hz and a peak of admittance of 1.96 × 10-2 mmho, resulting in a normal finding on the dashboard (Figure 4). Over several weeks, pure tone audiometry revealed slight lowfrequency inner ear hearing loss, with occasional normal hearing. The Valsalva maneuver was positive.

Serial PLAI measurements were performed during surgery before and after mastoidectomy, after artificial blockage of the antrum, after paracentesis, and after insertion of a ventilation tube (Figures 5). To assess the effect of the continuity of the grommet, the opening of the grommet was sealed with Gelaspone.

Mastoidectomy with an open ET, an intact tympanic membrane, and an air-filled tympanic cavity results in a decrease in several parameters, primarily resonance frequency, peak admittance, quality factor derived from bandwidth, and bandwidth, while the acoustic ear volume increases (Figure 6, Table 2). Only the resonance frequency of admittance returns to its initial value after the artificial closure of the drilled-out mastoid cavity, while the other parameters remain decreased. This case of a well-aerated middle ear with an open ET also demonstrates that the resonance frequency of admittance decreases following the insertion of a ventilation tube. A blockage of the ventilation tube opening with blood (not shown) or Gelaspone restores the resonance frequency of admittance in the demonstrated normal ear to higher values, similar to those observed before any manipulation of the tympanic membrane.

This case study illustrates that various artificial alterations in the aeration of the mastoid and modifications to the ventilation tube opening, regardless of the ET’s status, can be monitored using PLAI. Previous studies have shown that artificial changes in middle ear pressure (via the Valsalva and Toynbee maneuvers) in subjects with an intact tympanic membrane result in increased stiffness of the tympanic membrane and ossicular chain, with greater stiffness observed following the Valsalva maneuver [17]. Both maneuvers lead to an increase in the resonance frequency of admittance. A lack of change in frequency after these maneuvers can reliably indicate that the tube is not opening. ET dilatation without the insertion of a ventilation tube results in a decrease in the resonance frequency of admittance in cases of adhesive otitis media [14]. Regardless of the opening of the ET, the opening of a ventilation tube further decreases the resonance frequency of admittance. Artificially closing the ET opening leads to a significant increase in resonance frequency. This raises the question of whether a single PLAI measurement can clarify whether a tympanostomy tube is open, particularly in children. To address this, a study is needed analyze middle ear disease using conventional methods (otoscopy, pneumatic otoscopy, tympanometry, audiometry) before treatment, followed by PLAI measurements after the insertion of a ventilation tube. Serial measurements combined with otoscopy are necessary to check the opening of the ventilation tube. However, it is already evident that a single measurement cannot definitively answer this question. This limitation arises because PLAI measurements are influenced by numerous factors, primarily the pathology of the tympanic membrane [12], the middle ear, and particularly [10, 11, 13, 14], the ossicular chain. Therefore, the results of the PLAI measurements should be interpreted as the outcome of multiple factors. A subsequent study could provide insights into the pathogenesis of otitis media with effusion. The ventilation of the middle ear and mastoid cells plays a crucial role in the pathogenesis of the disease [18, 19]. A CT scan of the temporal bone is performed to visualize ventilation. To minimize the number of CT examinations and reduce radiation exposure for patients, especially children, thereby avoiding potential neoplasms, it is essential to investigate whether PLAI measurements (acoustic ear volume) [20] correlates with the volume of the temporal bone (ear canal, middle ear, mastoid), similar to tympanometry in patients with chronic otitis media (ear canal volume).

According to the consensus document on the diagnosis of Ménière’s disease [21] from various societies, including the Bárány Society, the Japan Society for Equilibrium Research, the European Academy of Otology and Neurotology, the American Academy of Otolaryngology-Head and Neck Surgery, and the Korean Balance Society, the diagnosis of Ménière’s disease is based on several criteria derived from medical history, audiological assessments, and vestibular tests. Typically, patients do not exhibit middle ear involvement, as middle ear function tests, particularly tympanometry, yield normal results. Our patient presented with a preoperative tympanogram type A and a normal PLAI diagnosis. Additionally, the tube function was also normal. Consequently, all measurements were performed on a patient with a healthy middle ear. The change in mastoid volume during a mastoidectomy, with an intact eardrum and normal tube function, led to changes in alterations in PLAI measurement outcomes. This indicates that in individuals with healthy middle ears, PLAI results are influenced by mastoid ventilation. Previous studies have demonstrated that PLAI results are affected by middle ear [13, 14] and ET ventilation [11]. Variations in mastoid aeration may contribute to the discrepancies observed in PLAI and tympanometric measurements in healthy individuals. Research has indicated that healthy subjects with normal eardrums and tube function can exhibit inconsistent tympanometric results and low variability in PLAI measurement results [10]. A correlative analysis of CT morphological data from subjects with healthy middle ears, alongside tympanometric and admittance measurement results, could yield insights into this phenomenon. Further, such a study could enhance our understanding of normal hearing, which has primarily been assessed through air conduction thresholds in tone audiometry. Incorporating additional middle ear function tests, such as PLAI measurements correlated with the volumes of the middle ear, mastoid, and outer ear canal, could provide further valuable information.

In a study involving normal-hearing subjects, objective middle ear function tests—including tympanometry, wideband absorbance, stapedius reflex measurements, and otoacoustic emissions—were conducted alongside pure tone audiometric assessments [5]. The reference values obtained in this study revealed significant interindividual variability in tympanic membrane compliance. It was concluded that evaluating middle ear function could offer supplementary information to confirm normal hearing. However, the reasons for the variability in measured values, such as compliance, resonance frequency, and absorbance curves, in normal-hearing individuals, remain unexplained.

The two cases demonstrate that various artificial alterations in mastoid aeration and modifications to the ventilation tube opening, irrespective of the ET’s status, can be effectively monitored using PLAI. Further research is necessary to elucidate the significance of mastoid aeration’s influence on PLAI measurement results in healthy individuals with normal hearing.

None.

The authors declare that there is no conflict of interest.