Conductive hearing loss caused by third-window lesions of the inner ear

Abstract

Background: Various authors have described conductive hearing loss (CHL), defined as an air-bone gap on audiometry, in patients without obvious middle ear pathologic findings. Recent investigations have suggested that many of these cases are due to disorders of the inner ear, resulting in pathologic third windows.

Objective: To provide an overview of lesions of the inner ear resulting in a CHL due to a third-window mechanism. The mechanism of the CHL is explained along with a classification scheme for these disorders. We also discuss methods for diagnosis of these disorders.

Data sources: The data were compiled from a review of the literature and recent published research on middle and inner ear mechanics from our laboratory.

Conclusion: A number of disparate disorders affecting the labyrinth can produce CHL by acting as a pathologic third window in the inner ear. The common denominator is that these conditions result in a mobile window on the scala vestibuli side of the cochlear partition. The CHL results by the dual mechanism of worsening of air conduction thresholds and improvement of bone conduction thresholds. Such lesions may be anatomically discrete or diffuse. Anatomically discrete lesions may be classified by location: semicircular canals (superior, lateral, or posterior canal dehiscence), bony vestibule (large vestibular aqueduct syndrome, other inner ear malformations), or the cochlea (carotid-cochlear dehiscence, X-linked deafness with stapes gusher, etc.). An example of an anatomically diffuse lesion is Paget disease, which may behave as a distributed or diffuse third window. Third-window lesions should be considered in the differential diagnosis of CHL in patients with an intact tympanic membrane and an aerated, otherwise healthy, middle ear. Clues to suspect such a lesion include a low-frequency air-bone gap with supranormal thresholds for bone conduction, and presence of acoustic reflexes, vestibular evoked myogenic responses, or otoacoustic emission responses despite the CHL. Imaging studies can help confirm the diagnosis.

FIG. 1

Schematic representations of mechanism of air-bone gap in third-window lesions. A, Normal ear, air conduction. Air-conducted sound stimuli enter the vestibule through motion of the stapes. There is a pressure difference between the scala vestibuli and the scala tympani, resulting in motion of the cochlear partition. The volume velocities of the oval and round windows are equal in magnitude but opposite in phase. B, Third-window lesions, air conduction. It is hypothesized that a third window (in one of the canals, the vestibule or the scala vestibuli) allows a portion of the acoustic energy entering the vestibule through motion of the stapes to be shunted away from the cochlea. The shunting occurs primarily at low frequencies, resulting in a hearing loss by air conduction. C, Normal ear, bone conduction. Compression of inner ear fluid by bone-conducted sound results in a hearing percept because of an inequality in the impedance between the scala vestibuli side and the scala tympani side of the cochlear partition. This inequality is primarily due to a difference in the impedance between the oval and windows. As a result, there is a pressure difference across the cochlear partition, resulting in motion of the basilar membrane that leads to perception of bone-conducted sound. D, Third-window lesions, bone conduction. A third window increases the difference between the impedance on the scala vestibuli side and the scala tympani side of the cochlear partition by lowering the impedance on the vestibuli side, thereby improving the cochlear response to bone conduction. In patients with healthy cochleae as in SCD, supranormal bone conduction thresholds may be evident. In other patients with an accompanying true sensorineural hearing as in DFN-3, LVAS, etc., the improved bone conduction due to the third-window mechanism may not result in supranormal thresholds. LVAS indicates large vestibular aqueduct syndrome.