Download Classification of Pathologic Third Windows

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
Ossicles
Fluid Wave
Induced by
Sound
Vestibule
EAC
Scala Vestibuli
Basilar
Membrane
Oval
Window
Scala Tympani
Cochlear
Aqueduct
Cochlea
Fluid
Wave
Round
Window
Tympanic
Membrane
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
Oval
Window
Scala Tympani
Cochlear
Aqueduct
Cochlea
Fluid
Wave
Round
Window
Tympanic
Membrane
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:
Vestibular
Aqueduct
Superior Canal Dehiscence
Lateral Canal Dehiscence
Posterior Canal Dehiscence
Fluid Wave
Induced by
Sound
Ossicles
Vestibule
EAC
Scala Vestibuli
Basilar
Membrane
Oval
Window
Scala Tympani
Cochlear
Aqueduct
Cochlea
Fluid
Wave
Round
Window
Tympanic
Membrane
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.
Vestibular
Aqueduct
Semicircular
Canals
Scala Vestibuli:
X-linked Deafness
Ossicles
Fluid Wave
Induced by
Sound
Vestibule
EAC
Scala Vestibuli
Basilar
Membrane
Oval
Window
Scala Tympani
Cochlear
Aqueduct
Cochlea
Fluid
Wave
Round
Window
Tympanic
Membrane
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)
o True
o False
Test Your Knowledge
• Which of the following is true regarding third window
hearing loss? (click on the correct answer)
o Third window lesions are often associated with the
scala tympani side of the cochlea.
o Superior semicircular canal dehiscence is the most
common cause of third window hearing loss.
o Patients clinically present with mixed hearing loss.
o Pathologic third windows are discrete lesions that
shunt acoustic energy away from the cochlea.
References
1.
2.
3.
4.
5.
6.
7.
8.
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;270:3057-62
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