Experimental model for investigating trans-mucosal gas exchanges in the middle ear of the rat

On the functional compartmentalization of the normal middle ear. Morpho-histological modelling parameters of its mucosa

BACKGROUND

Middle ear physiology includes both sound pressure transmission and homeostasis of its static air pressure. Pressure gradients are continuously created by gas exchange over the middle ear mucosa as well as by ambient pressure variations. Gas exchange models require actual values for regional mucosa thickness, blood vessel density, and diffusion distance. Such quantitative data have been scarce and limited to few histological samples from the tympanic cavity (TC) and the antrum. However, a detailed regional description of the morphological differences of the TC and mastoid air cell system (MACS) mucosa has not been available. The aim of the present study was to provide such parameters.

METHODS

The study included sets of three histological H&E-slides from 15 archived healthy temporal bones. We performed a comparison of the mucosa morphology among the following regions: (1) anterior TC; (2) inferior TC; (3) posterior TC; (4) superior TC; (5) MACS antrum; (6) superior MACS; (7) central MACS; (8) inferior MACS.

RESULTS

Regions (1)-(3), situated below the inter-attico-tympanic diaphragm, had the largest proportion of high respiratory epithelium, cilia and loose lamina propria within the mucosa, as well as the thickest mucosa and the largest diffusion distance. Regions (6)-(8), situated above the diaphragm, had the thinnest mucosa, the shortest distance to the blood vessels, together with the largest proportion of flat epithelium and very few cilia. Regions (4)-(5), still supradiaphragmatic, had intermediary values for these parameters, but generally closer to regions (6)-(8). The blood vessel density and the proportion of active mucosa were not significantly different among the regions.

CONCLUSION

Mucosa of regions (1), (2) and (3) represented a predominantly clearance-specific morphology, whereas in regions (4)-(8) it seemed adapted to gas exchange. However, the lack of statistically significant differences in blood vessel density and proportion of active mucosa indicated that all regions could be involved in gas exchange with the highest adaptation in the superior MACS. This pattern divides the middle ear functionally along the inter-attico-tympanic diaphragm rather than the anatomical division into TC and MACS.

The Role of Tympanic Membrane Retractions in Cholesteatoma Pathogenesis

Objective

To analyze the contralateral ear (CLE) of patients with cholesteatoma and to correlate the cholesteatoma growth pattern in the affected ear with the findings in the CLE.

Methods

Videotoscopy of both ears in 432 patients with cholesteatomas classified as posterior epitympanic (PEC), posterior mesotympanic (PMC), two routes, or undetermined. Tympanic membrane (TM) retractions were classified by location and severity and TM perforations according to signs of previous TM retraction.

Results

TM retraction was the most prevalent alteration in the CLE (42.6%). Cholesteatoma was observed in 17.4%. In patients with PEC, the retraction in the CLE was more frequent in the PF (66.7%) than in the PT (1.4%), and in those with two-route cholesteatoma, the retraction in the CLE most frequently involved both the PT and PF (65.6%; p < 0.0001).

Conclusion

Our results confirm the essential role of TM retraction at least in the earlier phases of cholesteatoma pathogenesis.

The Middle Ear Cleft Status in a "Natural" Cohort With Eustachian Tube Dysfunction

BACKGROUND

The Eustachian tube (ET) has a major role in the middle ear (ME) pressure homeostasis. ET dysfunction is the accepted paradigm for pressure-related ME disorders. We studied the ME status in patients with severely diminished ET opening abilities, and anticipated to find ME disorders in most of them.

PATIENTS AND METHODS

ME status was evaluated in unconscious adults, who were hospitalized in a rehabilitation center with severe brain damage, requiring tracheotomy and gastrostomy. These patients were unable to swallow, produce valsalva, yawn, and needed oral suctioning. Examination included fiberoptic nasopharyngoscopy, gag reflex and soft palate assessments, otoscopy, and tympanometry.

RESULTS

Nineteen patients (38 ears) were evaluated: 14 men and 5 women, aged 18 to 93 years (average 59). Duration of gastrostomy and tracheotomy were between 3 months and 18 years. All the patients lacked gag reflex, palatal movements, or supraglottic sensation. Eighteen ears (47%) had otitis media with effusion (OME) (versus ∼3% in the general population, p = 0.00001), none had significant tympanic membrane atelectasis, but 20 (53%) ears were normal. Twenty-two ears (59%) had tympanometry types B/C and 16 (41%) had type A. Cerumen impaction incidence (26 ears, 68%) was significantly higher than in normal adults (10%), mentally retarded (36%), and nursing homes residents (57%).

CONCLUSIONS

A dysfunctional ET predisposed ME disorders. Yet, ∼50% of the ears were normal, in contrast to the current paradigm. This implies that ME pressure homeostasis is maintained by factors that can compensate for ET dysfunction. Treating cerumen impaction and OME may be beneficial for rehabilitation.

Role of intracranial cavities in avian directional hearing

Whereas it is clear from anatomical studies that all birds have complex interaural canals connecting their middle ears, the effect of interaural coupling on directional hearing has been disputed. A reason for conflicting results in earlier studies may have been that the function of the tympanic ear and hence of the interaural coupling is sensitive to variations in the intracranial air pressure. In awake birds, the middle ears and connected cavities are vented actively through the pharyngotympanic tube. This venting reflex seems to be suppressed in anesthetized birds, leading to increasingly lower pressure in the interaural cavities, stiffening the eardrums, and displacing them medially. This causes the sensitivity, as well as the interaural coupling, to drop. Conversely, when the middle ears are properly vented, robust directional eardrum responses, most likely caused by internal coupling, have been reported. The anatomical basis of this coupling is the 'interaural canal,' which turns out to be a highly complex canal and cavity system, which we describe for the zebra finch. Surprisingly, given the complexity of the interaural canals, simple models of pipe-coupled middle ears fit the eardrum directionality data quite well, but future models taking the complex anatomy into consideration should be developed.

Transmucosal gas-loss rates in middle ears initially filled with O2 or CO2

This study investigates the role of different gases in clearance of gas in the middle ear cavity (ME) by its mucosal blood flow. A rat model was used to measure gas volume changes in the ME cavity at constant pressure without ventilation. We disturbed the normal gas composition of the ME by filling it with O₂ or CO₂, measured the consequent changes in gas volume over time and compared these results with previously obtained ones for air and N₂. The first 5 min of the primary transient phase (phase I) for O₂ or CO₂ was characterized by a volume loss decrease of -0.49 ± 0.34 μL and -46.28 ± 8.49 μL, respectively, with volume loss increase for air and N₂ differing greatly, at +0.17 ± 0.17 and +2.31 ± 0.81, respectively. The CO₂ value of -46.28 μL showed that a volume of gas equivalent to that of the ME cleft volume was eliminated within the first 5 min. In the second phase (phase II), all gases showed a linear decrease in volume, which presumably represents a steady-state gas loss rate. However, the gas loss rate of -0.307 ± 0.170 μL min^-1 for O₂-filled MEs was significantly higher than the mean of -0.124 μL min^-1 for all other gases. We used a previously established mathematical model to calculate the effective ME mucosal blood flow rate under steady-state (phase II) conditions. The blood flow results for O₂-filled MEs differed greatly from those of the other gases (89.0 ± 49.28 vs. 26.5 μL min^-1, on average), which suggest that the model used to calculate blood flow should be modified if used with O₂-filled MEs. Further work should involve a comparison of our method with different methods to verify ME blood flow rate.

Pressure changes in the human middle ear without opening the eustachian tube

CONCLUSION

By means of a direct, though non-invasive experiment on healthy humans we could demonstrate that middle ear (ME) pressure decreases when the eustachian tube (ET) does not open. Thus with a very simple method the basic theory of continuous gas loss from the ME into the circulation and the replenishment of the loss through the ET could be validated.

OBJECTIVES

To record changes in ME pressure over a period of time in normal human ears, while the ET is kept closed.

SUBJECTS AND METHODS

On-line tympanometry was carried out in three subjects, who refrained from swallowing for 20-120 min.

RESULTS

During the time when the ET was kept closed by refraining from swallowing, tympanometric monotonous pressure decrease was recorded. Once the tested subject could not refrain from swallowing any longer and swallowed again, ME pressure equalized immediately.

[Effects of nitrous oxide on middle ear pressure]

INTRODUCTION

Recent literature data suggest that permanent or reversible hearing loss may occur after general anesthesia. The etiology varies, while hearing loss following middle ear surgery is attributed to exposure to nitrous oxide (N,0). The objective of our study was to measure, using tympanometry, the middle air pressure change caused by nitrous oxide during general anesthesia and to establish its emetogenic effects during the postoperative period.

MATERIAL AND METHODS

This academic (non-commercial) prospective study included two groups of patients (a total of 58), with ASA status I, II and III. The study group (n 30) consisted of patients undergoing unilateral ear surgery. In this group, the intratympanic pressure was measured in the unoperated (healthy) ear before and during the surgery. The control group (n 28) patients underwent nose, throat or neck surgical interventions. This group underwent measurement of bilateral intratympanic pressure in healthy ears, before and during the surgery. Both groups were operated under general balanced anesthesia. Pain, nausea and antiemetics were monitored during the first 24 postoperative hours. Statistical analysis was performed using the Mann-Whitney-Wilcoxon test.

RESULTS

This perioperative study confirmed the following: highly significant (p < 0.001) increase in intratympanic pressure in nonoperated ears in the study group and significant (p < 0.05) in controls. However, there was no statistical significance (p > 0.05) between groups. Pain was more freguent in controls, and nausea in the study group, but without significant difference (p > 0.05).

CONCLUSIONS

Postoperative audiometry findings showed no conductive or sensorineural hearing loss after interventions. Nitrous oxide can be used in general balanced anesthesia with discontinuation 15 to 45 minutes before insertion of the tvmpanic membrane and completion of middle ear surgery.

High resolution gas volume change sensor

Changes of gas quantity in a system can be measured either by measuring pressure changes or by measuring volume changes. As sensitive pressure sensors are readily available, pressure change is the commonly used technique. In many physiologic systems, however, buildup of pressure influences the gas exchange mechanisms, thus changing the gas quantity change rate. If one wants to study the gas flow in or out of a biological gas pocket, measurements need to be done at constant pressure. In this article we present a highly sensitive sensor for quantitative measurements of gas volume change at constant pressure. The sensor is based on optical detection of the movement of a droplet of fluid enclosed in a capillary. The device is easy to use and delivers gas volume data at a rate of more than 15 measurements/s and a resolution better than 0.06 microl. At the onset of a gas quantity change the sensor shows a small pressure artifact of less than 15 Pa, and at constant change rates the pressure artifact is smaller than 10 Pa or 0.01% of ambient pressure.

Middle ear gas loss in inflammatory conditions: The role of mucosa thickness and blood flow

Several middle ear (ME) pathologies are associated with ME gas deficit. These include in particular the chronic otitis media diseases that are associated with inflammation (hence, increased ME mucosal blood flow) and/or reduced Eustachian tube (ET) function. The present study models the trans-mucosal gas exchange in normal and inflamed middle ears of rats. The model evaluates the role of the gas diffusion path in the ME mucosa using mucosa thickness as its index and the role of the mucosal blood flow rate on ME gas economy in order to compare between normal and inflamed MEs. An experimental method employing ME gas volume changes at constant pressure due to trans-mucosal gas exchange, and blood gas values from the literature, was used in anaesthetized rats to corroborate the model. Mucosa thickness was measured as an index of the gas diffusion path between the ME space and the ME circulation. ME inner surface area was estimated from its measured gas volume. Inflammation was inflicted by applying lipopolysaccharide (LPS) into one ear. The contralateral ear served as control. ME gas volume decreased significantly faster with time (p=0.02) in inflamed ears (-0.107 microL min(-1) +/- 0.034 S.D., n=10) versus control ears (-0.067 microL min(-1) +/- 0.036 S.D., n=10). Mucosa thickness was significantly thicker in inflamed ears (48.4 microm +/- 11.0 S.D.) versus controls (20.5 microm +/- 10.1 S.D.). The mathematical model, the experimental results, and the blood gas values were used to estimate the relative effective mucosal blood flow rate. The model predicts that in spite of almost doubling mucosa thickness in LPS treated ears, the increased gas loss in inflamed ears may be explained by increased mucosal blood flow rate. We suggest that the ability to estimate ME blood flow as obtained by applying the model to the measurements, is relevant to medical management of inflamed ME.

Role of nitrogen in transmucosal gas exchange rate in the rat middle ear

This study investigates the role of nitrogen (N2) in transmucosal gas exchange of the middle ear (ME). We used an experimental rat model to measure gas volume variations in the ME cavity at constant pressure. We disturbed the steady-state gas composition with either air or N2 to measure resulting changes in volume at ambient pressure. Changes in gas volume over time could be characterized by three phases: a primary transient increase with time (phase I), followed by a linear decrease (phase II), and then a gradual decrease (phase III). The mean slope of phase II was −0.128 μl/min (SD 0.023) in the air group ( n = 10) and −0.105 μl/min (SD 0.032) in the N2 group ( n = 10), but the difference was not significant ( P = 0.13), which suggests that the rate of gas loss can be attributed mainly to the same steady-state partial pressure gradient of N2 reached in this phase. Furthermore, a mathematical model was developed analyzing the transmucosal N2 exchange in phase II. The model takes gas diffusion into account, predicting that, in the absence of change in mucosal blood flow rate, gas volume in the ME should show a linear decrease with time after steady-state conditions and gas composition are established. In accordance with the experimental results, the mathematical model also suggested that transmucosal gas absorption of the rat ME during steady-state conditions is governed mainly by diffusive N2 exchange between the ME gas and its mucosal blood circulation.

High-resolution measurements of middle ear gas volume changes in the rabbit enables estimation of its mucosal CO(2) conductance

Transmucosal CO(2) exchange in the middle ear (ME) of the New Zealand White rabbit (Oryctolagus cuniculus) was studied using an accurate novel detecting and recording system for measuring gas volume changes at constant pressure, based on a principle that was previously used by Kania et al. (Acta Otolaryngol 124:408-410, 2004). After the ME cavity was washed with ambient air, the initial diffusion rate of CO(2) (V(.-)(i)CO(2)) from the blood perfusing the ME mucosa was calculated from gas volume change measurements. In nine cases, the (V(.-)(i)CO(2))calculated after normalization due to shifts in baseline was 314+/-112 microL x h(-1) (mean +/- SD). In two cases where normalization was not needed, (V(.-)(i)CO(2)) was 409 microL x h(-1) (276 and 543 microL x h(-1)). Normalization of (V(.-)(i)CO(2)) data was also made in five additional cases where secretion of fluids from the lining of the ear canal was observed. In these cases (V(.-)(i)CO(2)) was 245 +/- 142 microL x h(-1). No differences were found between results obtained in the three groups. Thus, an overall mean value of (V(.-)(i)CO(2)) of 305 +/- 131 microL x h(-1) (n = 16) was calculated. An effective coefficient of conductance of CO(2) (G(CO(2))) between the mucosal circulation and the ME gas cavity of the New Zealand White rabbit was estimated to be approximately 0.05 microL (h x Pa)(-1) and compared to the G(CO(2)) estimated for humans in a different study.

In vivo demonstration of the absorptive function of the middle ear epithelium

The present study investigated in vivo fluid and ion transport across the middle ear epithelium. The tympanic membrane of rats was punctured under general anesthesia. A capillary tube was fitted to the external auditory canal and the bulla filled with various solutions. Middle ear (ME) fluid volume variations were then measured at constant pressure. When saline was used, a linear decrease of fluid volume was apparent. Replacement of sodium with a non-permeable cation (N-methyl-D-glucamin) reduced the absorption rate from 0.065+/-0.008 to 0.019+/-0.003 microl/min (P<0.05, n=6). Similarly, amiloride (10(-3)M), a sodium channel antagonist, reduced the absorption rate to 0.027+/-0.006 microl/min (P<0.05, n=6). Net absorption was abolished when chloride was substituted with gluconate: -0.008+/-0.004 microl/min (P<0.02, n=6), which might have been related (i) to the role of chloride as a diffusible anion through the paracellular pathway, or (ii) to the secretion of chloride through apical channels. However in this condition, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, a chloride channel blocker, did not affect the rate of fluid exchange -0.008+/-0.007 microl/min (P=0.75, n=6). This model provides the first in vivo evidence for the absorptive function of the ME. Fluid introduced into the ME cavity disappears due to active transport through the mucosa. This process is sodium-dependent and can be hindered by high concentration of amiloride. The rate of absorption is high enough to allow total clearance of fluid from the cavity of the middle ear within 13 h. This process might play a role in the maintaining a fluid-free and gas-filled middle ear cavity.

Study of the auditory tube by ventilation scintigraphy with technetium-99m

The two essential regulating mechanisms of the middle ear pressure are the trans-mucosal gas exchange in the middle ear and the ventilation function of the eustachian tube (ET). The physiological mechanism of these both functions is not yet clear. The purpose of this study was to evaluate the role of the ET pressure equilibrium function by ventilation scintigraphy with technetium-99m. The rabbit animal model in vivo was used to study the presence and role of the ventilation of the tympanic cavity via auditory tube. The obtained results did not show any ventilation function of the ET despite active opening by muscle movement. In our experience, ventilation scintigraphy with technetium-99m is not a reliable method to study the auditory tube pressure equilibrium function in physiological conditions.

Update on eustachian tube dysfunction and the patulous eustachian tube

PURPOSE OF REVIEW

The purpose of this review is to summarize the recent knowledge on eustachian tube dysfunction and the patulous eustachian tube.

RECENT FINDINGS

A clinically useful test for eustachian tube function is still lacking. Narrowing of the isthmus alone was demonstrated to be an insufficient cause of otitis media. Inflammatory mediators identified within the eustachian tube and middle ear cells were causally linked with otitis media with effusion. Increasing evidence was found that allergic disease and reflux may be two of the most important contributors of tubal inflammation causing otitis media with effusion. The adenoid size and proximity to the torus tubaris may also be important in considering which patients with persistent otitis media with effusion may benefit from adenoidectomy. Computed tomography scan has documented loss of soft tissue within the cartilaginous eustachian tube in patients with patulous eustachian tubes. An endoscopic approach to seal the tubal lumen has been found to be effective in relieving patulous symptoms.

SUMMARY

These studies suggest that allergic rhinitis and gastroesophageal reflux should be investigated in patients with eustachian tube dysfunction. Adenoidectomy should also be considered in patients who have adenoids that obstruct the torus tubaris. Patients with a patulous eustachian tube may benefit from an endoscopic closure. Further research is needed to identify a clinically useful test for eustachian tube dysfunction.

Technical pitfalls in middle ear gas studies: errors introduced by the gas permeability of tubing and additional dead space

CONCLUSIONS

We demonstrated errors introduced by the gas permeability of the tubing and additional dead space. Materials with practically no permeability, such as glass, must be used to overcome the loss or gain of gas through the walls of tubes used for studying ME gas variations. Experiments conducted at a constant volume (variable pressure) require the smallest possible tubing volume in order to obtain good sensitivity and improve the accuracy of the results.

OBJECTIVES

Experimental studies that investigate middle ear (ME) gas exchanges, using either pressure measurements or volume changes, are conducted using various tubing connections between the ME and a measuring device. The aims of this study were to highlight experimental problems due to the errors introduced by (i) the gas permeability of the tubing used and (ii) additional dead space in experiments conducted at constant volume.

MATERIAL AND METHODS

The problem of the gas permeability of the tubing was addressed by comparing three types of tube (silicone, polyethylene, PVC) with a glass tube. Horizontally placed tubes were connected to a syringe filled with pure CO2 via a gas-tight valve. The end of each tube tested was plunged into colored water (5% Coomassie Brilliant Blue R-250). The tube was washed out with CO2 until gas bubbles were seen leaving it. The valve was then closed and the movement of water in the tube was observed. The same experiments were repeated for pure O2. The problem of the error introduced by the additional dead space volume was addressed at a theoretical level using the well-known gas laws. Two conditions were studied: condition A, in which the experiment was conducted at constant pressure and the volume was measured; and condition B, in which the experiment was conducted at constant volume and the pressure was measured. The main outcome measure of each condition was the calculated variation in the final number of moles of gas involved.

RESULTS

No water movement was observed in glass tubes. In contrast, plastic tubes exhibited significant gas permeability effects for both CO2 and O2. The colored solution advanced at a faster rate with CO2 than O2 but differently for each type of tubing. For condition A, gas transfer was independent of the volume of the external tubing and was accurately measured by the displacement of the droplet in the lumen. In contrast, for condition B, the pressure variations were influenced by the volume of the tubing.