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

Disclosures None Special Thanks to Danielle Dobbs for illustrations

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

Anatomy

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

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

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

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

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

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) http://www.slideshare.net/schwartzcm/ch-10-senses-part-ii

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 http://www.slideshare.net/schwartzcm/ch-10-senses-part-ii

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 http://galleryhip.com/transmission-of-sound-through-the-ear.html

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

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

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

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

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.

Semicircular Canals

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

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

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

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

Positive Tullio phenomenon (noise induced vertigo) Coronal

Positive Tullio phenomenon (noise induced vertigo) Sagittal

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

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

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

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

Other Causes of the ‘Third Window’

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

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

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.

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

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

Bilateral cavitary otosclerosis Axial sections

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

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.

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