1. Authors: Rickin Shah, MD Ashok Srinivasan, MD
Windows to the Acoustic World: A Review of Pathologies Involving the Oval, Round, and ‘Third’ Windows.
Authors: Rickin Shah, MD
Ashok Srinivasan, MD

2. Disclosures None Special Thanks to Danielle Dobbs for illustrations

3. Objectives
To review physiology of hearing and the importance of the oval and round windows in this process
To discuss pathologies involving the oval and
round windows
To describe the 'third window' phenomenon and present illustrated examples of different causative etiologies

4. Anatomy

5. CT Anatomy – Axial SectionsB
3 – Apical cochlear turn
4 – Middle cochlear turn
5 – Basal cochlear turn
Courtesy of Dr. Hoeffner

6. CT Anatomy – Axial SectionsD
M – malleus
I – incus
V – vestibule
IAC – internal auditory canal
Courtesy of Dr. Hoeffner

7. CT Anatomy – Axial SectionsF
V – vestibule
LSSC – lateral semicircular canal
Mo - Modiolus
Courtesy of Dr. Hoeffner

8. CT Anatomy – Coronal SectionsG H
1 – Facial nerve recess
2 – Sinus tympani
Courtesy of Dr. Hoeffner

9. MR Anatomy I J Axial Section Asterisk - cochlea Coronal Section
‘White arrow heads – Internal auditory canal
Courtesy of Dr. Hoeffner

10. Sound Transmission
Outer ear channels sound (pressure waves) to the tympanic membrane
The pressure wave is comprised of alternating high and low pressures which are converted to mechanical energy at the tympanic membrane (TM).
High pressure causes compression (inward displacement of TM) while low pressure causes rarefaction (outward displacement of TM)

11. Sound Transmission
The ossicular chain is attached to the TM and amplifies and transmits the mechanical energy to the oval window. Due to mechanical advantage, the displacement of the stapes is greater than the malleus which allows ability to hear faint noises
Oval and round windows have comparatively larger surface area than length to minimize impedance of fluid and transmit sound (pressure waves)
Cochlea is filled with perilymph (incompressible fluid)
Each compression of the oval window converts the mechanical energy as bulk motion of perilymph fluid

12. Cochlea is lined by cells called Organ of Corti which contain hair cells.
The movement of fluid travels along the scala vestibula (upper compartment) from the oval window then along the scala tympani to the round window.
Sound Transmission

13. Sound Transmission
Each sound wave bends certain hair cell projections based on their natural frequency
Each bend of the hair cells converts the mechanical energy into electric energy which is sent to the brain
Semicircular canals do not play a role in hearing but used to detect rotational acceleration in their planes

15. Oval Window Pathology Diagnosis: Oval Window Atresia
Absent cleavage plane between lateral
semicircular canal above and cochlear
promontory below.
Abnormal fixation of stapes on ossified web
over the oval window which results in
conductive hearing loss
Associated with anomalous stapes and
malpositioned facial nerve.
CT findings:
Ossified web replaced normal oval window
Inferomedial positioned tympanic CN 7
Pearl:
Must locate CN 7 for surgeon to ensure safe
correction

16. Oval Window Pathology
Diagnosis : Calcified annular ligament and crura of the stapes
Results in abnormal fixation of stapes on oval window
Clinically can present with conductive hearing loss
CT features:
Ossification or calcification at the insertion of the stapes on the oval window

17. Round Window Pathology
Very rare
Can have atresia of the round window

18. Dehiscence
There is a ‘third’ window to the membranous labyrinth that a dissipates a fraction the sound toward the semicircular canals
This sound wave causes deflections of the cupula which stimulates the sensation of movement.

19. Semicircular Canals

21. Dehiscence Sound and/or pressure induced vestibular symptoms
Increased sensitivity of bone conducted sound
Decreased sensitivity of air conducted sound

22. Sound induced vestibular symptoms
Flow of fluid pressure wave through the dehiscence and away from the vestibule and cochlea
There is excitation of the cupula resulting in motion sensation
and decrease sound conduction in cochlea

23. Impact on Bone Conducted Hearing
Normally, there is pressure difference between the scala vestibula and scala tympani which allows for perception of bone conducted sound
A pathologic window on the scala vestibula side increases the pressure difference
This causes larger fluid movement of the vestibula side and improves the cochlear response to bone conducted sound

24. Impact on Air Conducted Hearing
Mechanical pressure wave transmitted onto oval window by stapes
Some perilymph fluid is displaced away from cochlea
This causes decreased response of cochlea to air conducted sound

25. Positive Tullio phenomenon (noise induced vertigo)
Coronal

26. Positive Tullio phenomenon (noise induced vertigo)
Sagittal

27. Superior Semicircular Canal Dehiscence
Absence of bony roof of the SSC of unclear etiology
Thinning of tegmen tympani maybe associated
Tullio phenomenon is a clinical manifestation with sound induced vertigo and/or nystagmus
Should be considered in patients with suspected conductive hearing loss but intact TM and normal middle ears
Pearl: asymptomatic thinning of SSC can occur and usually only seen on one coronal or axial section

28. Treatment Fascia and/or bone chip plugging of dehiscence
Resurfacing of the dehiscence with fascia and bone graft

29. Poschl and Stenver views are helpful in diagnosis to see > 2 mm of dehiscence.
Stenver Poschl

30. Positive Tullio phenomenon (noise induced vertigo)
Stenver’s view

31. Other Causes of the ‘Third Window’

32. Posterior Semicircular Canal (PSCC) Dehiscence
Axial section demonstrating dehiscence of the
posterior semicircular canal from a high riding jugular bulb.

33. Vestibular Aqueduct Dehiscence
Axial section demonstrating dehiscence of the
vestibular aqueduct from a high riding jugular bulb

34. PSCC and Vestibular Aqueduct Dehiscence
Reported incidence is lower than superior semicircular dehiscence
Frequently due to a high riding jugular bulb
Diagnosis must be made in conjunction with clinical exam and tests.

35. Otosclerosis
Lytic spongy bone of unclear etiology which starts just anterior to oval window (fissula ante fenestram)
Can progress along medial wall of the middle ear and may involve the cochlear bony labyrinth
It can cross the stapedial annular ligament and fixate the stapes to the oval window causing conductive hearing loss

36. Cavitary otosclerosis
This can create a connection between the membranous labyrinth and middle ear results in a pathologic third window
Fenestral and cochlear otosclerosis do not cause a ‘third’ window phenomenon

37. Bilateral cavitary otosclerosis
Axial sections

38. Cholesteatoma Comprised of squamous epithelium and keratin
Appears pearly white or pale yellow
CT - Erosion into cochlear promontory
Creates pathologic ‘third’ window
Coronal

39. Summary
Understanding the complex temporal anatomy is key for interpretation.
There are many structural causes for conductive hearing loss and identifying the oval and round window pathologies is important.
There are many causes for the ‘third window’ phenomenon and identifying the various entities is important for clinical management.

40. References
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