Biomechanics of Third Window Syndrome

Abstract

Third window syndrome describes a set of vestibular and auditory symptoms that arise when a pathological third mobile window is present in the bony labyrinth of the inner ear. The pathological mobile window (or windows) adds to the oval and round windows, disrupting normal auditory and vestibular function by altering biomechanics of the inner ear. The most commonly occurring third window syndrome arises from superior semicircular canal dehiscence (SSCD), where a section of bone overlying the superior semicircular canal is absent or thinned (near-dehiscence). The presentation of SSCD syndrome is well characterized by clinical audiological and vestibular tests. In this review, we describe how the third compliant window introduced by a SSCD alters the biomechanics of the inner ear and thereby leads to vestibular and auditory symptoms. Understanding the biomechanical origins of SSCD further provides insight into other third window syndromes and the potential of restoring function or reducing symptoms through surgical repair.

Keywords: air-bone gap; biomechanics; canal dehiscence; dizziness; superior semicircular canal dehiscence; third window; vertigo; vestibular.

Figure 1

Audiogram from a patient with SSCD before and after superior canal plugging surgery. (A) Preoperative audiogram has a low-frequency air-bone gap of up to 25 dB. (B) Postoperative audiogram shows resolution of the air-bone gap, with a high-frequency sensorineural loss at 8 kHz. Patient experienced resolution of vestibular symptoms after surgery (41).

Figure 2

Lumped parameter network model of the inner and middle ear with and without SSCD. (A) Air-conduction model where the drive is sound pressure from the ear canal, P_TM. (B) Bone-conduction model where the drive is effective sound pressure of the vibratory bone-conducted stimulus, P_BC. (C) The peak in the bone-conduction thresholds is due to a parallel resonance between the compliance of the middle ear load and the inertance of the fluid in the canal limbs. A smaller dehiscence would shift both curves left to lower frequencies. (D) Predicted velocity of vestibular lymph fluids in an SSCD with air-conducted sound. Republished from (49), with permission. The Creative Commons license does not apply to this content. Use of the material in any format is prohibited without written permission from the publisher, Wolters Kluwer Health, Inc. Please contact permissions@lww.com for further information.

Figure 3

Eye positions recorded from a patient with SSCD. (A) Sound-evoked eye movements with 2 kHz tone at 110 dB presented to the left dehiscent ear. Slow phase components are directed upward and clockwise with respect to the patient's point of view, consistent with excitation of the left superior semicircular canal. (B) Pressure-evoked eye movements with glottic Valsalva. Slow phase components are principally downward and counterclockwise consistent with inhibition of the left superior semicircular canal. Release causes reversal of the evoked eye movements. Republished from (22), with permission.

Figure 4

VEMPs from dehiscent (left) and patent (right) ear. (A) cVEMP shows increased amplitudes in the dehiscent ear. (B) oVEMP shows increased amplitudes as well as abnormal response at super high frequency 4,000 Hz (double arrow). Republished from (70), with permission.

Figure 5

ECoG in a dehiscent ear before and after canal surgery. Preoperative ECoG response shows an elevated SP/AP ratio (>0.4) that normalizes after surgical canal plugging. Republished from (75), with permission. The Creative Commons license does not apply to this content. Use of the material in any format is prohibited without written permission from the publisher, Wolters Kluwer Health, Inc. Please contact permissions@lww.com for further information.

Figure 6

Vestibular afferent neuron responses evoked by fluid vibration and pumping. (A) Sustained changes in firing rate in a superior canal afferent neuron after a dehiscence is made in chinchilla superior semicircular canal. Sound evokes a decrease (125 Hz) or increase in afferent firing rate (250, 500, 750, 1,000 Hz). Rise time follows the slow mechanical time constant of the canal. Republished from (83), with permission. (B) Phase-locked responses in a superior canal afferent neuron after dehiscence is made in guinea pig superior canal. Sound and bone-conducted vibration at auditory frequencies evoke phase-locking in this irregularly discharging calyx-bearing unit. Republished from (24), with permission. The Creative Commons license does not apply to this content. Use of the material in any format is prohibited without written permission from the publisher, Wolters Kluwer Health, Inc. Please contact permissions@lww.com for further information.

Figure 7

Computational model of a human semicircular canal. (A,B) Auditory frequency stimulation at 419 Hz (A) and 790 Hz (B) evokes slowly developing endolymph pressure distribution (yellow: high; red: zero; and black: low) and a pressure gradient across the cupula (C). Waves travel along the membranous labyrinth away from the site of the dehiscence (transmembrane pressure: black solid line relative to gray dotted line) causing vibration of hair bundles at the stimulus frequency and pumping of endolymph (q) in either direction, ampullofugal for 419 Hz (A) and ampullopetal for 790 Hz (B). (i,ii) Cupula displacement where black is the mechanical cupula volume displacement responsible for sustained afferent responses, blue is the cycle-by-cycle cupula vibration responsible for phase-locked afferent responses at 419 Hz (i) and 790 Hz (ii). Based on (34).