Ever Heard of Third Window Hearing Loss? eEdE#: eEdE-155 Control #: 2096 Ever Heard of Third Window Hearing Loss? Sarah J. Moum, MD, MSc1; Alexander W. Korutz, MD1; Achilles G. Karagianis, DO1; Courtney C.J. Voelker, MD, PhD2; Alexander J. Nemeth, MD1 Departments of Radiology1 and Otolaryngology2 Northwestern University Feinberg School of Medicine

Disclosures The authors of this exhibit have no relevant financial or nonfinancial relationships to disclose.

Purpose Unexplained cases of conductive hearing loss have been described in as many as 1 in 3000 patients who have an intact tympanic membrane and no obvious middle ear pathology.1 In the past patients would undergo middle ear explorations and stapedectomy procedures without significant improvement in hearing. Increasing evidence demonstrates that these cases of hearing loss are caused by unrecognized pathologic third windows of the inner ear.2-6

Approach/Methods The mechanism of normal hearing mechanics will be reviewed in addition to the pathophysiology of third windows. A description and classification system for the various pathologic third windows will also be discussed. Data was compiled from a review of the literature. Imaging was obtained from our institutional experience, with the exception of the X-linked deafness case which is courtesy of Massachusetts Eye and Ear Infirmary.

Findings/Discussion: Normal hearing mechanics What is a third window? Pathophysiology of third window hearing loss Classification of pathologic third windows Discrete lesions Diffuse lesions

Normal Hearing Mechanics7 A sound wave causes vibration of the tympanic membrane and transfers energy to the ossicles of the middle ear. As the ossicles move, sound waves are transmitted into the inner ear via the oval window. Within the inner ear the transmitted energy causes displacement of cochlear fluid and the development of a pressure gradient between the scala vestibuli and scala tympani. This gradient causes motion of the basilar membrane, activation of hair cells, and ultimately the perception of sound.

Semicircular Canals Vestibular Aqueduct Fluid Wave Induced by Sound Ossicles Vestibule EAC Scala Vestibuli Basilar Membrane Tympanic Membrane Oval Window Scala Tympani Fluid Wave Cochlear Aqueduct Round Window Cochlea

What is a Third Window? The first and second windows are the oval and round windows.7 The usual compliant membranes of the labyrinthine structures that allow sound transmission to be converted into fluid motion. The fluid motion is then converted into neural stimulation and produces sensations of sound and motion in the brain. A third window refers to an additional compliant area in the labyrinthine structures that modifies fluid motion and may affect neural stimulation in detrimental ways.2,7

Pathophysiology of Third Window Hearing Loss A pathologic third window disrupts the normal transmission of sound. Lesions involving the scala vestibuli side of the cochlea shunt acoustic energy away from the cochlear partition and decrease sensitivity to air-conducted sound.2 Affected patients present clinically with conductive and in some cases mixed hearing loss.2-6

Classification of Pathologic Third Windows Third window lesions have been observed in a variety of disorders. They are classified as discrete or diffuse:2 Discrete lesions involve structures on the vestibular side of the cochlea. They are further classified by location. The vestibule The semicircular canals The scala vestibuli Diffuse lesions demonstrate widespread abnormality throughout the temporal bone. During hearing the diseased bone dissipates sound energy away from the cochlea.

Vestibular Aqueduct: Large Vestibular Aqueduct Syndrome Semicircular Canals Ossicles Fluid Wave Induced by Sound Vestibule EAC Scala Vestibuli Basilar Membrane Tympanic Membrane Oval Window Scala Tympani Fluid Wave Cochlear Aqueduct Round Window Cochlea

Discrete Lesions: The Vestibule Large vestibular aqueduct syndrome. 24-year-old female with hearing loss. Axial CT image demonstrates an enlarged right vestibular aqueduct (arrow) measuring 2.6 mm in maximal diameter.

Semicircular Canals: Superior Canal Dehiscence Lateral Canal Dehiscence Posterior Canal Dehiscence Vestibular Aqueduct Fluid Wave Induced by Sound Ossicles Vestibule EAC Scala Vestibuli Basilar Membrane Tympanic Membrane Oval Window Scala Tympani Fluid Wave Cochlear Aqueduct Round Window Cochlea

Discrete Lesions: The Semicircular Canals Superior semicircular canal dehiscence. Stenvers reformatted CT image demonstrates dehiscence (arrow) of the right superior semicircular canal.

Discrete Lesions: The Semicircular Canals Posterior semicircular canal dehiscence. 29-year-old male with tinnitus and hearing loss. A. Axial and B. coronal CT images demonstrate dehiscence (arrows) of the right posterior semicircular canal.

Discrete Lesions: The Semicircular Canals Lateral semicircular canal dehiscence. Axial CT image demonstrates soft tissue attenuation within the right middle ear cavity with associated dehiscence (arrow) of the right lateral semicircular canal.

Scala Vestibuli: X-linked Deafness Vestibular Aqueduct Semicircular Canals Scala Vestibuli: X-linked Deafness Fluid Wave Induced by Sound Ossicles Vestibule EAC Scala Vestibuli Basilar Membrane Tympanic Membrane Oval Window Scala Tympani Fluid Wave Cochlear Aqueduct Round Window Cochlea

Discrete Lesions: The Scala Vestibuli X-linked deafness. A. and B. Axial CT images demonstrate bilateral dysplatic cochlea with absent modioli (arrowheads). C. and D. Coronal CT images demonstrate bilateral wide internal auditory canals (arrows) communicating with the cochlea.8

Diffuse Lesions Paget disease of the temporal bone. Axial CT image through the level of the left inner ear demonstrates diffuse thickening and coarsening of the left temporal bone. Similar findings were identified in the contralateral temporal bone.

Summary/Conclusion Pathologic third windows of the inner ear can account for otherwise unexplained cases of hearing loss. These lesions produce conductive and in some cases mixed hearing loss by shunting acoustic energy away from the cochlea. This phenomenon has been associated with discrete lesions involving the semicircular canals, the vestibule, and the scala vestibuli.2-6 Diffuse lesions involving the temporal bone, including Paget disease of the temporal bone, have also been described.2

Summary/Conclusion By recognizing imaging findings associated with third window hearing loss and alerting referring clinicians to the possibility of this diagnosis, the radiologist will play a pivotal role in patient care. Moreover, heightened awareness of these disorders will help to ensure that patients receive appropriate treatment and avoid the cost and potential morbidity of unnecessary procedures.

Test Your Knowledge Patients with conductive hearing loss resulting from superior semicircular canal dehiscence will benefit from stapedectomy surgery. (click on the correct answer) True False

Test Your Knowledge Which of the following is true regarding third window hearing loss? (click on the correct answer) Third window lesions are often associated with the scala tympani side of the cochlea. Superior semicircular canal dehiscence is the most common cause of third window hearing loss. Patients clinically present with mixed hearing loss. Pathologic third windows are discrete lesions that shunt acoustic energy away from the cochlea.

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