Mechanisms responsible for postnatal middle ear amniotic fluid clearance

How Is the Cochlea Activated in Response to Soft Tissue Auditory Stimulation in the Occluded Ear?

Soft tissue conduction is an additional mode of auditory stimulation which can be initiated either by applying an external vibrator to skin sites not overlying skull bone such as the neck (so it is not bone conduction) or by intrinsic body vibrations resulting, for example, from the heartbeat and vocalization. The soft tissue vibrations thereby induced are conducted by the soft tissues to all parts of the body, including the walls of the external auditory canal. In order for soft tissue conduction to elicit hearing, the soft tissue vibrations which are induced must penetrate into the cochlea in order to excite the inner ear hair cells and auditory nerve fibers. This final stage can be achieved either by an osseous bone conduction mechanism, or, more likely, by the occlusion effect: the vibrations of the walls of the occluded canal induce air pressures in the canal which drive the tympanic membrane and middle ear ossicles and activate the inner ear, acting by means of a more air conduction-like mechanism. In fact, when the clinician applies his stethoscope to the body surface of his patient in order to detect heart sounds or pulmonary air flow, he is detecting soft tissue vibrations.

Effect of delivery mode on neonate auditory brainstem responses to air- and bone-conducted stimuli

OBJECTIVES

The object of this study was to examine if caesarean section delivered neonates have different middle ear function relative to neonates with vaginal delivery.

METHODS

Auditory brainstem responses (ABRs) were examined in caesarean section delivered (n = 23) and vaginally delivered (n = 29) neonates. ABRs were also evoked with air- and bone-conducted stimuli (i.e., clicks and CE-Chirps) and presented at a screening intensity level (i.e., 30 dB nHL). Wave V latencies and amplitudes were examined as a function of mode of delivery and stimuli.

RESULTS

Statistically significant longer wave V latencies evoked with air-conducted stimuli were seen in caesarean section delivered neonates (p = .042). There was no statistically significant difference in wave V latencies with bone-conducted stimuli among the two groups of neonates (p = .42). There were no significant differences in wave V amplitude between neonates with caesarean section and vaginal delivery for air-conducted (p = .42) stimuli. Wave V amplitudes were not significantly different as a function of mode of delivery with CE-Chirp stimulus (p = .41). Wave V amplitudes were significantly larger for the caesarean section delivered neonates with the bone-conducted click stimulus (p = .036).

CONCLUSIONS

The ABR wave V latency disparity with air- and bone-conducted stimuli support the notion that differences in middle ear function exist between the two groups of newborns. It was speculated that delayed fluid resorption in the middle ear exists in neonates with caesarean section delivery compared to those with vaginal delivery.

Age and Other Factors Affecting the Outcome of AABR Screening in Neonates

BACKGROUND

Although the utility of universal newborn hearing screening is undisputed, testing protocols vary. In particular, the impact of the infant's age at the time of automated auditory brainstem response (AABR) screening has not been well studied.

METHODS

We conducted a retrospective review of newborn hearing screening data in 6817 low-risk, term and late-preterm newborns at our large, urban, academic medical center for a 1-year period to analyze the impact of age and other factors on the screening failure rate and referral for diagnostic testing.

RESULTS

AABR screening failure rates decreased with postnatal age over the first 48 hours; 13.3% failed at <24 hours versus 3.8% at ≥48 hours (P < .0001). Infants who were initially tested at ≥36 hours failed repeat testing more often than those who were tested at <36 hours (11.5% vs 18.9%; P = .03). Other factors that were associated with failure included being a boy and of a race other than white. Sensorineural hearing loss (SNHL) was diagnosed in 18.6% of infants who failed their final screening at ≥48 hours compared with 2.8% of those whose final screening occurred earlier (P = .03). SNHL was more likely in infants who failed their first screening bilaterally (21.2%) than unilaterally (4.4%); P = .03).

CONCLUSIONS

Among healthy newborns, delaying AABR screening in the first 48 hours minimized failure rates. SNHL was 6 times as likely in infants who failed their final screening at ≥48 hours compared with those who were screened at <48 hours of age. In our study, we offer guidance for nursery directors and audiologists who determine hearing screening protocols and counsel families about results.

Newborn hearing screening failure and maternal factors during pregnancy

OBJECTIVE

Temporary conductive hearing loss due to amniotic fluid accumulation in the middle ear cavity may lead to failure (false positive) in newborn hearing screening tests. The aim of this study was to identify whether amniotic fluid index has association with failure of the initial newborn otoacoustic emission (OAE) screening test.

METHODS

A cohort study in a tertiary hospital center (Royal Victoria Hospital, Montréal) was constructed from 70 newborns that failed the OAE test, but passed a subsequent auditory brainstem response (ABR) test, and 75 randomly selected newborns that passed initial otoacoustic emission testing. Maternal (including the amniotic fluid index in the third trimester) and newborn clinical data were extracted from medical records. Statistical association models were built to determine variables that influenced hearing screen passage or failure.

RESULTS

The two arms of the cohort had no significant differences in maternal or child clinical indices, including in amniotic fluid index. Calculated as individual odds ratios, maternal tobacco [95% CI of odds ratio: 0.04, 0.59, p = 0.0078], and drug use [95% CI of odds ratio: 0.0065, 0.72, p = 0.058] [borderline significance] were associated with failing the otoacoustic emission testing.

CONCLUSIONS

Amniotic fluid index was not found to be associated with failure of otoacoustic emission screening in newborns. However, our study unveiled an interesting unexpected association of OAE failure with maternal smoking and/or drug use. This finding can help alleviate some of the time, cost and parental anxiety related to failed OAE screening. In selected cases of maternal smoking or drug use we might want to replace or add OAE to the ABR test in newborn hearing screening protocols, that don't perform both tests before discharge.

Newborn hearing screening may predict Eustachian tube dysfunction

INTRODUCTION

There is evidence for temporary hearing loss in neonates immediately after birth because of residual liquid derived from amniotic fluid in the ME cavity. This study examines whether a referred newborn hearing screen (NBHS) with subsequent testing confirming normal hearing can be attributed to persistence of middle ear effusion and predict poor Eustachian tube function manifested as recurrent otitis media or otitis media with effusion in the first year of life. The aims of the present study are to investigate the following: (1) whether infants who fail a neonatal hearing screen and subsequently pass are more likely to experience recurrent otitis media or otitis media with effusion, (2) whether these infants are more likely to obtain tympanostomy tubes.

METHODS

This retrospective cohort study examined newborns who referred their NBHS and were subsequently noted to have normal hearing and a control group comprised of newborns who passed their NBHS. Univariate and multivariate analysis was performed on the data collected as well as generation mean cumulative function plots.

RESULTS

The baseline characteristics of the case and control groups are not statistically significant with regards to gender, number of otitis media (OM), delivery mode, or the need for tubes in the follow up period. Within the refer group, those with bilateral refers were twice as likely to have otitis media than those with a unilateral refer (p=0.012). The logistic regression model for odds of subsequent otitis media was not statistically significant for bilateral or unilateral refer though the logistic regression model for odds of tubes demonstrated a statistically significant increased risk in bilateral refer patients. With time to event analysis, it was seen that bilateral refer patients are more likely to have OM than control and unilateral refer patients.

CONCLUSION

There is no difference in the incidence of subsequent OM between those infants who passed the NBHS versus those who initially referred and then passed subsequent audiology examination. However there was a difference in the number of otitis media between those infants who referred bilaterally versus those who referred unilaterally.

Mechanism and Rate of Middle Ear Fluid Absorption

Several mechanisms have been suggested to explain the clearance of fluids from the middle ear. These include a pumping action through the eustachian tube, mucociliary beating through the tube, outflow of water to the blood due to osmotic gradients and an active Na(+) transport driving water absorption. In order to assess these mechanisms, the middle ear cavity of paralyzed, ventilated (eustachian tube occluded) guinea pigs was filled with fluids varying in osmotic pressure (hypotonic, isotonic, hypertonic) to which a vertical tube was attached. The change in height of fluid in the tube was taken as a measure of changes in middle ear fluid volume. A greater fluid volume reduction was seen with the hypotonic (1/5 saline) solution. A small volume increase was observed with the hypertonic solution. These results provide evidence that in these experimental conditions, water absorption due to osmotic gradients can contribute to middle ear fluid clearance.

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.

Quantitative Experimental Assessment of the Factors Contributing to Hearing Loss in Serous Otitis Media

OBJECTIVE

This study was designed to quantitatively assess the contribution of various factors to the conductive hearing loss in otitis media.

BACKGROUND

In the conductive hearing loss seen in cases of serous otitis media, various volumes of fluid of different viscosities along with subatmospheric (negative) pressure are found in the middle ear.

METHODS

To evaluate the contribution of each of these factors to hearing loss, various volumes of saline, whole blood, or glycerol were applied to the open middle ear cavity of guinea pigs for short periods of time and auditory function was evaluated by recording the threshold of auditory nerve-brainstem evoked responses. In some of the saline experiments, the bulla cavity was also sealed, allowing a subatmospheric (negative) pressure to develop in the cavity as water was osmotically absorbed because of the gradient in colloid osmotic pressure between saline and blood in the vessels lining the middle ear cavity. In other experiments, a thoracic drainage system was connected to the middle ear cavity to induce desired negative middle ear pressures.

RESULTS

The degree of hearing loss increased as larger volumes of fluid were introduced into the middle ear, reaching a maximum of 15 to 16 dB. There was no difference in the degree of hearing loss induced by saline or by fluids with viscosities up to 1,000 times greater than that of water (glycerol). A subatmospheric pressure in the middle ear contributed only a small additional (1-2 dB) threshold elevation.

CONCLUSION

The major factor contributing to hearing loss in serous otitis media is the volume of fluid in the middle ear, irrespective of its viscosity. The contribution of negative middle ear pressure is much smaller.