In vivo observation with magnetic resonance imaging of middle ear effusion in response to experimental underpressures

A formal description of middle ear pressure-regulation

INTRODUCTION

Middle ear (ME) pressure-regulation (MEPR) is a homeostatic mechanism that maintains the ME-environment pressure-gradient (MEEPG) within a range optimized for "normal" hearing.

OBJECTIVE

Describe MEPR using equations applicable to passive, inter-compartmental gas-exchange and determine if the predictions of that description include the increasing ME pressure observed under certain conditions and interpreted by some as evidencing gas-production by the ME mucosa.

METHODS

MEPR was modeled as the combined effect of passive gas-exchanges between the ME and: perilymph via the round window membrane, the ambient environment via the tympanic membrane, and the local blood via the ME mucosa and of gas flow between the ME and nasopharynx during Eustachian tube openings. The first 3 of these exchanges are described at the species level using the Fick's diffusion equation and the last as a bulk gas transfer governed by Poiseuille's equation. The model structure is a time-iteration of the equation: P^ME_{g(t=(i+1)Δt)} = ∑^s(P^ME_s(t=iΔt)+(1/(β^ME_sV^ME)∑^P(Қ^P_s(P^C_s(t=(iΔt)-P^ME_s(t=(iΔt))). There, P^ME_g(t=iΔt) and P^ME_s(t=iΔt) are the ME total and species-pressures at the indexed times, P^C_s(t=iΔt) is the species-pressure for each exchange-compartment, β^ME_sV^ME is the product of the ME species-capacitance and volume, Қ^P_s is the pathway species-conductance, and ∑^S and ∑^P are operators for summing the expression over all species or exchange pathways.

RESULTS

When calibrated to known values, the model predicts the empirically measured ME species-pressures and the observed time-trajectories for total ME pressure and the MEEPG under a wide variety of physiologic, pathologic and non-physiologic conditions.

CONCLUSIONS

Passive inter-compartmental gas exchange is sole and sufficient to describe MEPR.

Tympanometry Accurately Measures Middle Ear Underpressures in Monkeys

Tympanometry is useful for evaluating middle ear (ME) status, but its accuracy in estimating true ME pressure has been questioned. We evaluated the accuracy of tympanometry in 6 monkeys. Direct application and measurement of ME pressure were achieved with a probe introduced into the mastoid antrum, and tympanometry was done over a large range of applied ME pressures. For all ears, tympanometric pressure was a linear function of applied pressure. At large overpressures, the tympanometric pressure was approximately 40 mm H2O greater than the applied pressure, but there was little error in the measurement for applied underpressures. The measurement error was proportional to the ME pressure multiplied by the ratio of the extant volume displacement of the tympanic membrane to ME volume. These results show that in monkeys, tympanometry provides an accurate, relatively unbiased estimate of ME underpressure and suggest that the measurement error for tympanometry can be predicted for MEs of other species.

Oxymetazoline Applied Topically to the Nasal Mucosa Decreases Trans-Mucosal Nitrous Oxide Exchange for the Middle Ear

OBJECTIVE

Determine if the middle ear (ME) trans-mucosal nitrous oxide (N2O) gas exchange rate can be pharmacologically modulated by the nasal application of a vasoconstrictor.

METHODS

In a randomized, double-blind, crossover study, 20 adults received a nasal spray challenge containing either oxymetazoline or saline (placebo). At each session, subjects were fitted with a non-rebreathing mask and breathed room air for 20 minutes, 50% N2O:50% O2 for 20 minutes, and 100% O2 for 10 minutes. Throughout, heart rate, blood pressure (BP), and blood O2 saturation were monitored, and bilateral ME pressure was recorded by tympanometry every minute. The primary outcome measure was the slope of the ME pressure-time function for the experimental period, a direct measure of the transMEM N2O exchange constant. The effects of treatment, session, and period on the measured vital signs and of treatment, session, disease history, and ear on the ME pressure-time slopes were evaluated for statistical significance using repeated measures ANOVAs.

RESULTS

The analysis documented a significant effect of period on O2 saturation (N2O > room air, P = .03) and of treatment on blood pressure (oxymetazoline > placebo, P < .02) and the ME pressure-time slope (placebo > oxymetazoline, P = .05).

CONCLUSION

The exchange rate across the ME mucosa of inert gases can be decreased by topical treatment of the nasal mucosa with oxymetazoline.

Oral pseudoephedrine decreases the rate of transmucosal nitrous oxide exchange for the middle ear

OBJECTIVES/HYPOTHESIS

Determine if oral treatment with a vasoconstrictor decreases the blood to middle ear exchange rate of the perfusion-limited gas, nitrous oxide (N2O).

STUDY DESIGN

Randomized, double-blind, crossover study.

METHODS

Ten adult subjects with and 10 without past middle ear disease completed paired experimental sessions, identical except for oral treatment with either pseudoephedrine hydrochloride or lactose placebo. At each session, subjects were fitted with a nonrebreathing mask and breathed room air for 20 minutes (acclimation period), 50% N2O:50% O2 for 20 minutes (experimental period), and 100% O2 for 10 minutes (recovery period). Throughout, heart rate, blood pressure, and O2 saturation were monitored, and bilateral middle ear pressures were recorded by tympanometry every minute. The primary outcome was the slope of the middle ear pressure-time function for the experimental period, which estimates the volume N2O exchange rate. Using repeated measures analysis of variance, the effects of group (disease history), treatment (active vs. placebo), and period (1 vs. 2) on the recorded vital signs, and of group, treatment, and ear (left/right) on the middle ear pressure-time slope were evaluated for statistical significance.

RESULTS

Statistically significant effects of period on O2 saturation (period 2 > period 1) and of treatment on heart rate (active > placebo) were documented. Only treatment was statistically significant for the middle ear pressure-time slope, with a shallower slope characterizing the active treatment session.

CONCLUSIONS

The volume exchange rate across the middle ear mucosa of perfusion-limited gases can be modulated pharmacologically. Theoretically, similar drugs can be used to reduce the requisite eustachian tube opening efficiency for adequate middle ear pressure regulation.

LEVEL OF EVIDENCE

1b.

Eustachian tube-tensor veli palatini muscle-cranial base relationships in children and adults: an osteological study

OBJECTIVE

The vector relationships between the Eustachian tube, Tensor veli palatini muscle and cranial base constrain the efficiency of middle ear pressure-regulation and are required parameters for computational modeling of Eustachian tube function. Here, those relationships were reconstructed from skulls and compared between children and adults.

METHOD

Reconstructions were made using modifications of previously described techniques for 18 child skulls aged 3-4 years and 20 adult skulls (10 females, 10 males; >18 years). Measured and calculated variables were compared between groups using a Student's t-test.

RESULTS

Consistent with previous reports, certain variables for adult skulls exhibited sexual dimorphism. Between children and adults, significant differences were documented for measures of cranial base length and width; hard palate width; nasopharyngeal height, width and depth; Eustachian tube length; the maximum and minimum Tensor veli palatini muscle lengths; the angles of deviation of the Tensor veli palatini muscle from the Eustachian tube, and the surface area of the Tensor veli palatini muscle. There were no between-group differences in the angle of Eustachian tube decent from the cranial base, Eustachian tube deviation from the parasagittal plane or the lateral component of the Tensor veli palatine muscle-Eustachian tube angle.

CONCLUSIONS

The differences between children and adults that could account for the observed poorer Eustachian tube function in children include their shorter Eustachian tube, lesser Tensor veli palatine muscle-Eustachian tube vectors, and the lesser Tensor veli palatine muscle surface area. Other observed differences are attributable to growth and development of the craniofacial complex.

Mastoiditis on MRI: Fact or Artifact?

Fluid signal in the mastoid can be an incidental finding on T2-weighted magnetic resonance imaging and often is interpreted as mastoiditis by radiologists. This study examines 28 consecutive cases of such erroneously diagnosed “mastoiditis” and documents the presence or absence of otologic symptoms and clinical signs. We found a very low prevalence of otologic symptoms or pathology and no cases of mastoiditis in these patients, and we determined that magnetic resonance imaging is not an effective screening modality for mastoiditis. We also reviewed the literature on current diagnostic criteria for mastoiditis and propose alternative terminology to replace the use of excessively alarming terms such as “mastoiditis” unless they apply to that specific clinical diagnosis.

The mastoid as a functional rate-limiter of middle ear pressure change

INTRODUCTION

The physiological function of the mastoid air cell system (MACS) with respect to middle ear (ME) pressure-regulation remains controversial because predictive mathematical models and experimental data to formulate and test hypotheses are lacking.

OBJECTIVE

A mathematical description of MACS volume effects on the rate of ME pressure change is presented; the agreement between published data and model prediction is examined for consistency with the hypothesis that the MACS acts as a functional rate-limiter of ME pressure change, and an explanation for the relationship between MACS volume and otitis media is discussed.

METHODS

The mathematical description shows that the value of a single, free parameter, termed the "MACS buffering efficiency" (M) determines if MACS volume affects the rate of ME pressure change caused by diffusive gas exchange. The MACS serves no rate-limiting function for M=0, acts as a gas sink for M>1 and acts as a gas reserve (rate-limiter) for M<1.

RESULTS

Fitting the model equation to published adult human data yielded an estimate for M of 0.2. This implies that larger MACS volumes are associated with lesser rates of change in ME pressure caused by diffusive gas exchange and lesser required frequencies of effective Eustachian tube openings to maintain near ambient ME pressures.

CONCLUSION

If well-controlled studies confirm M<1 for children and adults, larger MACS volumes will increase the time required to develop sufficient ME underpressures to cause otitis media by hydrops ex vacuo during transient or prolonged periods of Eustachian tube dysfunction.

Accuracy of CO2 conductance predicted using a morphometric model of the middle ear mucosa

CONCLUSION

These results hold promise that morphometric analysis can be used to generate transMEM (middle ear mucosa) gas conductance estimates for MEM geometries representative of the shift from healthy to pathologic states (e.g. increased MEM thickness and capillary density).

OBJECTIVES

Novel strategies to treat otitis media with effusion require a better understanding of how MEM geometry affects gas transport. Earlier studies developed techniques to empirically measure transMEM gas conductance and to estimate conductance using morphometric models of MEM geometry. We used chinchillas to determine the correspondence between experimentally measured transMEM CO2 conductance and that predicted by morphometric study of the MEM.

MATERIALS AND METHODS

TransMEM CO2 conductance was measured unilaterally in 10 chinchillas; the animals were killed and the ME was removed and processed for morphometric analyses of MEM geometry.

RESULTS

The average measured and estimated transMEM CO2 conductances were 4.87+/-2.30 x 10-10 and 1.75+/-0.29 x 10-10 mol/s/mmHg, respectively. The magnitude and direction of the estimate error were similar for all ears, suggesting a fixed, negative bias to the estimate. A theoretically consistent source for this bias was identified as the representation of the true diffusional length within a 3-D geometry using a 2-D modeling platform. Best estimate correction for this effect based on available data significantly reduced the estimate bias.

Nasal prostaglandin challenge increases N2O exchange from blood to middle ear

Nasal inflammation (NI) resulting from allergy or virus infection is causally associated with otitis media. Impaired Eustachian tube (ET) function consequent to NI may mediate this relationship. Moreover, the functional demand placed upon the ET for gas supply may be increased by NI, a hypothesis tested here. A total of five experiments were done at a minimum 2-week interval on four monkeys. For each experiment, the monkey was anesthetized and acclimated for 60 min. Then, the monkey was breathed with a 40% N2O gas mixture for 60 min followed by air breathing for 100 min. Fifteen minutes into air breathing, the nose was challenged with one of the five substances: saline, PGD2, capsaicin, histamine or bradykinin. Throughout, middle ear (ME) pressures were recorded at 5-min intervals. All pressure-time curves had a similar form consisting of a curvilinear decrease during acclimation, a linear increase during N2O breathing and a linear decrease during air breathing. The rates of pressure increase were ear-specific and not different across experiments. Compared to saline, the rate of pressure decrease was greater for challenge with all the substances and significantly greater after PGD2 challenge. Like N2, N2O is an inert gas whose transmucosal exchange is perfusion-limited for the ME. The measured pressure change is linearly related to the volume gas exchange between ME and blood. Therefore, nasal challenge with PGD2 and perhaps the other substances increases transmucosal inert gas exchange. ME to blood N2 transfer rate determines the physiological demand placed upon the ET for gas supply. That demand is increased by nasal exposure to certain inflammatory mediators.

Studies in otitis media: Children's Hospital of Pittsburgh-University of Pittsburgh progress report--2004

OBJECTIVES/HYPOTHESIS

The present Progress Report has summarized the key otitis media clinical trials and laboratory studies conducted since 1969 by investigators at the Children's Hospital of Pittsburgh-University of Pittsburgh (Pittsburgh, PA).

STUDY DESIGN

Review.

METHODS

Included in the discussion are the following: 1) studies of the epidemiology and risk factors; 2) anatomy and pathology of the eustachian tube-middle ear from human temporal bone histopathological specimens; 3) physiology and pathophysiology of the eustachian tube-middle ear in humans and animal models; 4) pathogenesis; 5) otitis media in special populations (e.g., patients with cleft palate, Native Americans, patients with Down syndrome); 6) microbiology; 7) diagnosis; 8) outcomes of randomized clinical trials that evaluated efficacy of nonsurgical and surgical methods of treatment and prevention; 9) studies of certain complications and sequelae (e.g., effect of middle-ear effusion on hearing, early child development, and the vestibular system; chronic suppurative otitis media). Also included are relevant summary tables and 256 references.

Barotrauma during air travel: predictions of a mathematical model

Middle ear barotrauma during flight is a painful disorder experienced by passengers who cannot properly regulate their middle ear pressure in response to the changing cabin pressures during ascent and descent. Previous reports emphasized the important role of poor eustachian tube function in disease pathogenesis but paid little attention to other moderating factors. Here we describe a mathematical model of middle ear pressure regulation and simulate the pressure response to the changes in cabin pressure experienced over typical flights. The results document buffering mechanisms that decrease the requisite efficiency of active, muscle-assisted eustachian tube opening for disease-free flight. These include the relative difference between destination and departure elevations and the ratio of maximum tympanic membrane volume displacement to middle ear volume, where greater absolute values require lesser efficiencies for disease-free flight. Also, the specific type of functional deficit is important since ears with a completely obstructed eustachian tube can be less susceptible to barotrauma than those with a eustachian tube that passively opens but fails to dilate in response to muscle activity. These buffering systems can explain why some children and adults with poor eustachian tube function do not experience middle ear barotrauma.

Prevention of otitis media caused by viral upper respiratory tract infection: vaccines, antivirals, and other approaches

Otitis media (OM) imposes significant morbidity on the pediatric age group and a large financial burden on the general population. Because standard medical treatments are not highly efficacious in resolving the accompanying middle ear (ME) inflammation, a goal of current research is OM prevention. Past studies show that new episodes of OM are usually a complication of viral upper respiratory infection (vURI), and therefore, a rational approach to achieving that goal is to develop intervention strategies that target vURI-associated OM. However, past experiences with antibiotics show that, in the absence of well-defined treatment protocols that maximize expected efficacy, the adoption of prophylactic or active treatments for OM can have negative consequences for the patient and for the general population. In this review, we discuss the hypothesized mechanisms by which a vURI is translated into an acute OM episode and describe different strategies for aborting that process. Limitations to deployment of each strategy are outlined.

The effect of immobilizing the gerbil's pars flaccida on the middle-ear's response to static pressure

The pars flaccida of the tympanic membrane has a small role in regulating middle-ear static pressure (Acta Physiol. Scand. 118 (1983) 337; Hear. Res. 118 (1998) 35) and can also modify the response of the middle ear to low-frequency sound pressures by shunting ear-canal volume velocity around the pars tensa (Hear. Res. 13 (1984) 83; Hear. Res. 106 (1997) 39; Diversity in Auditory Mechanics (1997) 129; Audiol. Neuro-Otol. 4 (1999) 129). It has been hypothesized that these two functions can interact to reduce the effect of middle-ear static pressure on sound transmission through the middle ear (Hear. Res. 153 (2001) 146). This paper tests this hypothesis by measuring the effect of static pressure on the sensitivity of the p. tensa and the coupled malleus to sound, before and after immobilizing the p. flaccida. The results are consistent with a limited role of the p. flaccida in influencing the effect of static pressure on the p. tensa's acoustic response. However, this effect is only observed at low frequencies and over the +/-1 cm H(2)0 range of middle-ear static pressures. The results also suggest that large negative middle-ear pressures can induce a change in the mode of tympanic membrane motion regardless of the state of the p. flaccida.

Middle ear volume and pressure effects on tympanometric middle ear pressure determination: model experiments with special reference to secretory otitis media

OBJECTIVE

Middle ear pressure (P(m)) measured by tympanometry has revealed high negative values in patients with secretory otitis media (SOM) in contrast to direct measurement. This may be explained by errors in tympanometry caused by volume displacement of the tympanic membrane (TM) affecting the volume of the middle ear (V(m)) and the P(m) according to Boyle's Law. Such errors are susceptible to the size of V(m).

METHODS

A realistic middle ear model based on previous clinical studies of normal pressure-volume relations of the middle ear system (MES) was constructed. In this model non-linear behaviour and hysteresis of the MES was imitated and P(m) as well as V(m) could be controlled.

RESULTS

Tympanometrically estimated P(m) decreased on average 38 daPa, when V(m) was changed from 21 to 1 cm(3). The decrease was most pronounced, when V(m) became smaller than 5 cm(3). Moreover, tympanometry showed a linear numerical overestimation of P(m) by a factor 2.31 compared with model P(m).

CONCLUSION

A curve fit was derived describing the tympanometric P(m) as a function of V(m). This demonstrated that tympanometric P(m) approached -infinity daPa, when middle ear volume approached 0 cm(3), which indicates that negative tympanometric recordings and B curves can be found in ears with normal P(m) entirely due to very small V(m)'s. This explains the discrepancy between direct and tympanometric measurements of P(m) in SOM, since the effusion replaces the air filled expandable volume resulting in a very small 'functional' V(m). Numerical overestimation of P(m) by tympanometry was explained by hysteresis, which reflected the viscoelastic properties of the MES. These results question the significance of negative P(m)'s as a pathogenetic factor in SOM.

Mathematical model explaining the sources of error in certain estimates of the gas exchange constants for the middle ear

Mover-Lev and colleagues reported a carbon dioxide-oxygen time-constant ratio of 3.9 for transmucosal gas exchange in guinea pigs under conditions of a large positive oxygen pressure gradient and a negative carbon dioxide gradient. That ratio is much less than the value of 19 reported previously for monkeys and used in predictive models of middle ear pressure regulation. In this report, the mathematics that underlie models of transmucosal gas exchange are developed and the conditions that allow accurate estimation of time constants are defined. The results demonstrate that the experimental and analytic methods used by Mover-Lev et al do not control for certain confounding effects or concurrently measure all required system parameters. Under the most realistic conditions, their ratio of 3.9 represents a significant underestimation of a true value on the order of 10. Also, their expectation of nonvarying, transmucosal time constant ratios for pairings that include reactive gases is simplistic and true only for identical experimental contexts.

The effects of changing middle ear pressure and gas partial pressure on mucosal blood flow and vascular permeability in the chinchilla

OBJECTIVE

To determine if middle ear (ME) gas composition and/or total pressure regulates local mucosal blood flow (MBF) and vascular permeability. The hypotheses tested are: (1) relatively high local CO2 tensions and/or low O2 tensions increase the ME MBF and vascular permeability; and (2) sub-atmospheric total ME pressure provokes similar effects.

METHODS

The responses of ME MBF and vascular permeability parameters were measured during 60 min exposures of chinchilla MEs to one of two test gas mixtures (16.3% O2, 5.1% CO2, balance N2, or 5.3% O2, 15.6% CO2, balance N2) applied at different levels of underpressure (ref. ambient). In the first set of experiments (n = 19), mucosal perfusion parameters were recorded using a Laser Doppler Flowmeter for 60 min before and 60 min after exposure to the experimental conditions. In the second set of experiments (n = 19 chinchillas, 38 ears), the MEs were exposed to the gas mixtures and then maintained for 60 min at ambient pressure or at negative pressures of -200, -400, -600 mmH2O. Fifty minutes into the experiment, the animals were injected intravenously with 60 mg/kg of horseradish peroxidase (HRP). The animals were killed and existing effusion was aspirated, its volume recorded and then analyzed for total protein. From surface preparations of the ME mucosa, vascular leakage sites were measured as percent total surface area using an image analysis program with the threshold window set to discriminate HRP stain.

RESULTS

Throughout the 120 min in the first set of experiments, the measured MBF parameters decreased in all exposure groups. Comparisons among groups for the absolute magnitude of the change from baseline showed that high local CO2 partial pressures decreased MBF and ME underpressures increased MBF, but the effects did not achieve statistical significance. The results of the second set of experiments demonstrated no effect of gas composition on any of the measured parameters of vascular permeability. All measures of permeability were linearly related to the magnitude of the underpressure.

CONCLUSION

These data support a role for total ME pressure, but not CO2 partial pressure in regulating ME MBF and vascular permeability.

Middle ear inflation for diagnosis and treatment of otitis media with effusion

An adult (18 years), healthy, male subject with persistent bilateral middle ear (ME) underpressure and a history of recurrent otitis media into his teen years was identified. The response of his MEs to air inflation was evaluated and showed an immediate pressure increase after a Valsalva maneuver followed by a rapid pressure drop to approach the pre-inflation levels. That response is consistent with the presence of ME effusion, which was not diagnosed by otoendoscopy or tympanometry, but was visualized bilaterally within the mastoid regions using magnetic resonance imaging (MRI). The patient was treated for 25 days with ME inflation (3/day) and then re-examined. On each treatment day, he recorded his ME pressure using tympanometry before and after one inflation maneuver. The patient's compliance with the treatment protocol was high, and successful gas transfers were documented on most days. Over the course of treatment, pre-inflation ME pressure became more normal bilaterally. When compared to the pre-treatment test, the post-treatment inflation test showed a similar rate of ME pressure decrease, but significantly higher terminal pressures. On follow-up but not during the pre-treatment period, discrete changes in ME pressure attributable to ET openings were noted during test sessions. MRI documented lesser amounts of effusion in the mastoid, but not complete disease resolution. The significance of these observations to the design of a well controlled clinical trail of ME inflation as a treatment for otitis media is discussed.

MRI validation of the accuracy of tympanometric gradient for the diagnosis of OME

Tympanometry provides a rapid, non-invasive and objective assessment of middle ear (ME) status and is widely used for the clinical diagnosis and follow-up of otitis media with effusion (OME). ME pressure, acoustic admittance and tympanometric gradient are the main test parameters used in making assignments to diagnostic classes (i.e. presence or absence of effusion, effusion quantity). Of these, the tympanometric gradient was suggested to be more sensitive to the presence of effusion, but this has not been demonstrated conclusively and no standard definition of that gradient is accepted. In this study, 10 cynomolgus monkeys with experimental OME were used to compare the diagnosis of OME made using three different methods to estimate tympanometric gradient with that provided by simultaneous magnetic resonance imaging (MRI) of the ME. All three methods of tympanometric gradient measurement were highly correlated with the quantity of ME effusion measured by the MRI. Although not significant, the MRI results were better correlated with those for the 'width' method when compared to either the 'difference' or the 'ratio' method of gradient estimation. This study demonstrates the use of MRI as a gold standard for evaluating the accuracy of other methods to diagnose ME effusion.

Repeated inflation does not prevent otitis media with effusion in a monkey model

OBJECTIVE

Investigate the efficacy of repeated middle ear inflation with an inert gas (argon) for preventing the development of middle ear effusion in monkeys with functional eustachian tube obstruction.

STUDY DESIGN

Prospective controlled trial of daily middle ear inflation with five monkeys assigned to the inflation group and four to the control group.

METHODS

The right tensor veli palatini muscle of nine monkeys was paralyzed with botulinum toxin. Tympanometry was done before the procedure and then daily for 21 days. Presence and distribution of effusion were assessed before paralysis and on day 15 using magnetic resonance imaging (MRI). In five right ears inflation was done beginning at the first observation of negative middle ear pressure of < or =200 mm H2O and repeated on all days with pressures < or =-100 mm H2O. Four right ears served as uninflated controls.

RESULTS

Right middle ear pressure decreased in all animals over the course of the study. Pressure returned to near-ambient levels immediately following the argon inflation but was decreased to control levels at the subsequent observation on the following day. MRI at day 15 documented effusion in all right ears with no quantifiable differences in amount or distribution between ears that were and were not inflated with argon.

CONCLUSIONS

Repeated inflation with an inert gas does not prevent middle ear effusion in monkeys with functional eustachian tube obstruction.

Trans-mucosal inert gas exchange constants for the monkey middle ear

OBJECTIVE

The purpose of the study was to estimate the exchange constants for Argon and N2 across the middle ear mucosa of monkeys for use in mathematical models of middle ear pressure regulation.

METHODS

In five anesthetized monkeys, the tympanic membrane was perforated and, with the animal breathing room air, a 13-ml probe attached to an Argon gas source and to a pressure transducer was introduced into the ear canal and sealed. The probe and middle ear volume was washed with Argon and then closed to the gas source. Gas samples were withdrawn from the probe at 15-min intervals for composition analysis using an online mass spectrometer. Probe partial pressures of Argon and N2 were regressed on time and the slopes of those functions were divided by the respective average partial pressure gradient for the interval to estimate the exchange constants.

RESULTS

The average trans-mucosal exchange constants for Argon and N2 were 0.0007 +/- 0.0001 and 0.0003 +/- 0.0001/min, respectively. The average ratio of the two constants was 2.60 +/- 0.36 which is not different from the value of 2.3 predicted for perfusion limited gas exchange.

CONCLUSION

These results confirm a very slow, perfusion limited trans-mucosal exchange of N2 and other inert gases across the middle ear mucosa. They infer that the required frequency of Eustachian tube openings to prevent the development of middle ear effusion is low and on the order of one/day.

A model to explain the rapid pressure decrease after air-inflation of diseased middle ears

OBJECTIVES

Air-inflation in humans and monkeys with significant negative middle ear pressure or with middle ear inflammation was shown to cause greater than ambient middle ear pressure initially, followed by a rapid rate of pressure decrease to approach the preinflation value.

STUDY DESIGN

A mathematical model of middle ear pressure regulation is presented and used to simulate air-inflation of the normal and diseased middle ear.

MATERIALS AND METHODS

The model represents the total volume of the middle ear as consisting of three subcompartments representing the airspace, effusion, and mucosa/blood. Gas exchange among those compartments was assumed to be diffusion limited, and the gas exchange between the mucosa/blood compartment and systemic blood was assumed to be perfusion limited. Disease was modeled as an increase in mucosal blood flow or, alternatively, as an increase in the volumes of the effusion and mucosa/blood compartments.

RESULTS

The predictions of the model agree better with the experimental data when the increased rate of pressure change after middle ear inflation in diseased ears is driven by an increased volume of the effusion compartment as opposed to an increased perfusion rate. The responsible mechanism is a rapid redistribution among subcompartments of the gas volume introduced into the air compartment.

CONCLUSIONS

These results suggest that middle ear inflation with inert gas can be used to diagnose the presence and relative amount of middle ear effusion, and that current protocols for treating otitis media with effusion using inflation need to be modified to optimize their intended effect.

Epidemiology and pathogenesis of chronic suppurative otitis media: implications for prevention and treatment

Despite advances in public health and medical care, chronic suppurative otitis media is still prevalent around the world. It is most common in developing countries and in certain high risk populations in developed nations, as well as among children who have tympanostomy tubes inserted. Since this chronic infection is caused by persistent acute otorrhea, which in turn is usually secondary to acute otitis media, prevention should be directed toward prompt and appropriate treatment of the acute middle-ear infection. Repair of chronic perforations should prevent recurrence, since reinfection is due either to reflux of pathogenic organisms from the nasopharynx into the middle ear, or water contamination from the external canal. Information from epidemiological studies, which show that populations can be categorized into highest, high, low and lowest prevalence, can be helpful in setting national priorities for prevention and treatment.