Air- and Bone-Conducted Sources of Feedback With an Active Middle Ear Implant.

OBJECTIVES: Active middle ear implants (AMEI) have been used to treat hearing loss in patients for whom conventional hearing aids are unsuccessful for varied biologic or personal reasons. Several studies have discussed feedback as a potential complication of AMEI usage, though the feedback pathway is not well understood. While reverse propagation of an acoustic signal through the ossicular chain and tympanic membrane constitutes an air-conducted source of feedback, the implanted nature of the device microphone near the mastoid cortex suggests that bone conduction pathways may potentially be another significant factor. This study examines the relative contributions of potential sources of feedback during stimulation with an AMEI. DESIGN: Four fresh-frozen, hemi-sectioned, human cadaver specimens were prepared with a mastoid antrostomy and atticotomy to visualize the posterior incus body. A Carina active middle ear implant actuator (Cochlear Ltd., Boulder, CO) was coupled to the incus by two means: (1) a stereotactic arm mounted independently of the specimen and (2) a fixation bracket anchored directly to the mastoid cortical bone. The actuator was driven with pure-tone frequencies in 1/4 octave steps from 500 to 6000 Hz. Acoustic sound intensity in the ear canal was measured with a probe tube microphone (Bruel & Kjær, Nærum, Denmark). Bone-conducted vibration was quantified with a single-axis laser Doppler vibrometer (Polytec Inc., Irvine, CA) from both a piece of reflective tape placed on the skin overlying the mastoid and a bone-anchored titanium screw and pedestal (Cochlear Ltd., Centennial, CO) implanted in the cortical mastoid bone. RESULTS: Microphone measurements revealed ear-canal pressures of 60-89 dB SPL, peaking in the frequency range below 2 kHz. Peak LDV measurements were greatest on the mastoid bone (0.32-0.79 mm/s with mounting bracket and 0.21-0.36 mm/s with the stereotactic suspension); peak measurements on the skin ranged from 0.05 to 0.15 mm/s with the bracket and 0.03 to 0.13 mm/s with stereotactic suspension. CONCLUSION: AMEI produce both air- and bone-conducted signals of adequate strength to be detected by the implanted device microphone, potentially resulting in reamplification. Understanding the relative contribution of these sources may play an important role in the development of targeted mitigation algorithms, as well as surgical techniques emphasizing acoustic isolation.

Radiological Evaluation of Middle Cranial Fossa Dura in Patients with Unilateral Cholesteatoma.

Cholesteatoma is a common disease of the middle ear cleft associated with several complications, Surgery is the mainstay for the management of such a pathological condition, however, complications may occur including intracranial ones due to violation of middle cranial fossa dura. Anatomical variations have been reported and High-resolution CT scan is the major tool for detecting such variations. In this study, we aim to determine if there is a difference between tegmen height between both sides of patients with unilateral cholesteatoma. 21 patients were recruited for such study all underwent HRCT and the tegmen height was determined in two coronal planes, the first between the central point of the Henle spine and the tegmen, the other between the plane of both lateral canals and the roof of the middle ear. The tegmen height measurements showed that the affected side had significantly lower tegmen height Henle spine (6.30 ± 1.54 vs. 9.03 ± 1.34 (mm); p < 0.001) relative to the healthy side. Also, coronal plane measurement has shown statistical significance for tegmen height (2.98 ± 0.79 vs. 4.15 ± 0.76 (mm); p = 0.01) in comparison to the healthy side. The percentage of difference between both sides as regards tegmen height of Henle's spine and coronal measurement was 30.32% and 28.2%, respectively. A significant difference was found in tegmen height between healthy and diseased sides in patients with unilateral cholesteatoma.

Intracochlear Pressures in Simulated Otitis Media With Effusion: A Temporal Bone Study.

HYPOTHESIS: Simulated otitis media with effusion reduces intracochlear pressures comparable to umbo velocity. BACKGROUND: Otitis media with effusion is a common cause of temporary hearing loss, particularly in children, producing deficits of 30 to 40 dB. Previous studies measured the effects of simulated effusion on ossicular mechanics; however, no studies have measured cochlear stimulation directly. Here, we compare pressures in the scala vestibuli and tympani to umbo velocity, before and after induction of simulated effusion in cadaveric human specimens. METHODS: Eight cadaveric, hemi-cephalic human heads were prepared with complete mastoidectomies. Intracochlear pressures were measured with fiber optic pressure probes, and umbo velocity measured via laser Doppler vibrometry (LDV). Stimuli were pure tones (0.1-14 kHz) presented in the ear canal via a custom speculum sealed with a glass cover slip. Effusion was simulated by filling the mastoid cavity and middle ear space with water. RESULTS: Acoustic stimulation with middle ear effusion resulted in decreased umbo velocity up to ∼26 dB, whereas differential pressure (PDiff) at the base of the cochlea decreased by only ∼16 dB. CONCLUSION: Simulating effusion leads to a frequency-dependent reduction in intracochlear sound pressure levels consistent with audiological presentation and prior reports. Results reveal that intracochlear pressure measurements (PSV and PST) decrease less than expected, and less than the decrease in PDiff. The observed decrease in umbo velocity is greater than in the differential intracochlear pressures, suggesting that umbo velocity overestimates the induced conductive hearing loss. These results suggest that an alternate sound conduction pathway transmits sound to the inner ear during effusion.

Intracochlear pressure measurements during acoustic shock wave exposure.

INTRODUCTION: Injuries to the peripheral auditory system are among the most common results of high intensity impulsive acoustic exposure. Prior studies of high intensity sound transmission by the ossicular chain have relied upon measurements in animal models, measurements at more moderate sound levels (i.e. < 130 dB SPL), and/or measured responses to steady-state noise. Here, we directly measure intracochlear pressure in human cadaveric temporal bones, with fiber optic pressure sensors placed in scala vestibuli (SV) and tympani (ST), during exposure to shock waves with peak positive pressures between ∼7 and 83 kPa. METHODS: Eight full-cephalic human cadaver heads were exposed, face-on, to acoustic shock waves in a 45 cm diameter shock tube. Specimens were exposed to impulses with nominal peak overpressures of 7, 28, 55, & 83 kPa (171, 183, 189, & 192 dB pSPL), measured in the free field adjacent to the forehead. Specimens were prepared bilaterally by mastoidectomy and extended facial recess to expose the ossicular chain. Ear canal (EAC), middle ear, and intracochlear sound pressure levels were measured with fiber-optic pressure sensors. Surface-mounted sensors measured SPL and skull strain near the opening of each EAC and at the forehead. RESULTS: Measurements on the forehead showed incident peak pressures approximately twice that measured by adjacent free-field and EAC entrance sensors, as expected based on the sensor orientation (normal vs tangential to the shock wave propagation). At 7 kPa, EAC pressure showed gain, calculated from the frequency spectra, consistent with the ear canal resonance, and gain in the intracochlear pressures (normalized to the EAC pressure) were consistent with (though somewhat lower than) previously reported middle ear transfer functions. Responses to higher intensity impulses tended to show lower intracochlear gain relative to EAC, suggesting sound transmission efficiency along the ossicular chain is reduced at high intensities. Tympanic membrane (TM) rupture was observed following nearly every exposure 55 kPa or higher. CONCLUSIONS: Intracochlear pressures reveal lower middle-ear transfer function magnitudes (i.e. reduced gain relative to the ear canal) for high sound pressure levels, thus revealing lower than expected cochlear exposure based on extrapolation from cochlear pressures measured at more moderate sound levels. These results are consistent with lowered transmissivity of the ossicular chain at high intensities, and are consistent with our prior report measuring middle ear transfer functions in human cadaveric temporal bones with high intensity tone pips.

Anatomical Relationship of the Middle Cranial Fossa Dura to Surface Landmarks of the Temporal Bone.

HYPOTHESIS: The suprameatal crest and temporal line provides a reliable landmark to the middle fossa dura. BACKGROUND: Surface anatomy of the temporal bone is used to guide mastoid surgery, but studies investigating these landmarks are limited. The aim of this study was to examine the anatomical relationship of the middle fossa dura to the temporal line. METHODS: Thirty-two fresh hemicephalic temporal bones were prepared by drawing four lines along the mastoid including the suprameatal crest and temporal line (line 2), one line 5 mm superior to line 2 (line 1), and one 5 mm inferior to line 2 (line 3), and at Reid's base line (line 4). Four points were marked along these lines anterior to posterior 3 mm apart. A 1 mm bur was used to drill perpendicular to these points to examine the relationship to the middle fossa dura. RESULTS: The dura was found inferior to line 2 in 6.3% at point 1, 6.3% at point 2, 9.4% at point 3, and 18.8% at point 4. The dura in line 1 was found inferior to point 1 in 52.1%, point 2 in 46.9%, point 3 in 56.3%, and point 4 in 62.5%. Only one specimen (3.1%) had dura lying inferior to line 3. No specimens were inferior line 4 at any point. CONCLUSION: The dura of the middle fossa lies superior the temporal line in >80% of specimens and at least 5 mm superior in nearly half. This indicates the temporal line or a line slightly inferior to this is reliably inferior the middle fossa dura.