Clinical Investigation and Mechanism of Air-Bone Gaps in Large Vestibular Aqueduct Syndrome

Abstract

Objectives-Patients with large vestibular aqueduct syndrome (LVAS) often demonstrate an airbone gap at the low frequencies on audiometric testing. The mechanism causing such a gap has not been well elucidated. We investigated middle ear sound transmission in patients with LVAS, and present a hypothesis to explain the air-bone gap.Methods-Observations were made on 8 ears from 5 individuals with LVAS. The diagnosis of LVAS was made by computed tomography in all cases. Investigations included standard audiometry and measurements of umbo velocity by laser Doppler vibrometry (LDV) in all cases, as well as tympanometry, acoustic reflex testing, vestibular evoked myogenic potential (VEMP) testing, distortion product otoacoustic emission (DPOAE) testing, and middle ear exploration in some ears.Results-One ear with LVAS had anacusis. The other 7 ears demonstrated air-bone gaps at the low frequencies, with mean gaps of 51 dB at 250 Hz, 31 dB at 500 Hz, and 12 dB at 1,000 Hz. In these 7 ears with air-bone gaps, LDV showed the umbo velocity to be normal or high normal in all 7; tympanometry was normal in all 6 ears tested; acoustic reflexes were present in 3 of the 4 ears tested; VEMP responses were present in all 3 ears tested; DPOAEs were present in 1 of the 2 ears tested, and exploratory tympanotomy in 1 case showed a normal middle ear. The above data suggest that an air-bone gap in LVAS is not due to disease in the middle ear. The data are consistent with the hypothesis that a large vestibular aqueduct introduces a third mobile window into the inner ear, which can produce an air-bone gap by 1) shunting air-conducted sound away from the cochlea, thus elevating air conduction thresholds, and 2) increasing the difference in impedance between the scala vestibuli side and the scala tympani side of the cochlear partition during bone conduction testing, thus improving thresholds for bone-conducted sound.Conclusions-We conclude that LVAS can present with an air-bone gap that can mimic middle ear disease. Diagnostic testing using acoustic reflexes, VEMPs, DPOAEs, and LDV can help to identify a non-middle ear source for such a gap, thereby avoiding negative middle ear exploration. A large vestibular aqueduct may act as a third mobile window in the inner ear, resulting in an airbone gap at low frequencies.

A possible third window for bone conducted hearing: Cochlear aqueduct vs. vestibular aqueduct

“…It should be noted that the pressure decrease due to the VA was observed at low frequencies (below about 1 kHz). This is consistent with the previous studies [4,5]. Those studies showed that the VA or the semicircular canal dehiscence can be a TW and that the sound energy is shunted through the TW at low frequencies in AC.…”

“…Therefore, a third window (TW), which is another fluid pathway in the cochlea except the OW and RW, was suggested as a possibile factor to explain the asymmetry of the volume velocity ratio. However, in spite of many studies for potential TWs [1,4,5], it is still unclear which opening in the real cochlea acts as the TW causing the asymmetry of the ratio. In order to further investigate the effect of the TWs on the asymmetry of the volume velocity ratio, we used a 3-D finite-element (FE) human-ear model consisting of the middle ear and the cochlea.…”

Conductive Hearing Loss Caused by Third-Window Lesions of the Inner Ear

“…These patients often present with a mixed hearing loss. The lack of pathologic findings in the middle ear in these patients has been shown by a number of studies using tests such as tympanometry, acoustic reflexes, laser Doppler vibrometry, air-conducted VEMP and OAE testing, and middle ear explorations (30). The enlarged vestibular aqueduct probably provides a pathologically large communication between the bony vestibule and the cranial cavity, resulting in an air-bone gap, similar to that observed in SCD.…”

“…For example, patients with SCD may exhibit sound-and/or pressure-induced vertigo or demonstrate low thresholds for air-conducted vestibular evoked myogenic potential (VEMP) responses (10,11,29). Similarly, patients with an enlarged vestibular aqueduct may have low thresholds on air-conducted VEMP testing (30). It is not known why some individuals with third-window lesions have solely auditory or vestibular manifestations, whereas others have both.…”

“…Various authors (reviewed by Merchant et al [30]) have reported a low-frequency air-bone gap in patients with an enlarged vestibular aqueduct. These patients often present with a mixed hearing loss.…”