1. 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

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

3. 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

4. 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.

5. 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

6. 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.

7. 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

8. 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

9. 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

11. 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.

12. 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

13. 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.

14. 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

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

16. 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.

17. 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.

18. 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

19. 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

20. 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.

21. 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

22. 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.

23. 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

24. 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.

25. References
Schuknecht HF. Otologic mystery. Am J Otolaryngol 1987;8:182–3
Chien WW, Carey JP, Minor LB. Canal Dehiscence. Curr Opin Neurol 2011;24(1):25-31
Choi BY, An YH, Park JH, et al. Audiological and surgical evidence for the presence of a third window effect for the conductive hearing loss in DFNX2 deafness irrespective of types of mutations. Eur Arch Otorhinolaryngol 2013;270:
Merchant SN, Rosowski JJ. Conductive hearing loss caused by third-window lesions of the inner ear. Otol Neurotol 2008;29:282–289
Minor LB, Solomon D, Zinreich JS, et al. Sound- and/or pressure-induced vertigo due to bone dehiscence of the superior semicircular canal. Arch Otolaryngol Head Neck Surg 1998;124:249-58
Kim HH, Wilson DF. A third mobile window at the cochlear apex. Otolaryngol Head Neck Surg 2006;135:965–6
Huttenbrink KB. The mechanics and function of the middle ear. Part 1: The ossicular chain and middle ear muscles. Laryngorhinootologie 1992;71(11):545-51
Karagianis A. Head and Neck Imaging Variants 1st ed. New York: McGraw-Hill; 2016