Audiovestibular Quantification in Rare Third Window Disorders in Children

Abstract

Third window disorders are structural abnormalities in the bony otic capsule that establish a connection between the middle/inner ear or the inner ear/cranial cavity. Investigated extensively in adults, they have hardly been studied in children. This study is a retrospective study of children (aged 5-17 years) diagnosed with rare third window disorders (third window disorders reported rarely or not reported in children) in a tertiary pediatric vestibular unit in the United Kingdom. It aimed to investigate audiovestibular function in these children. Final diagnosis was achieved by high resolution CT scan of the temporal bones. Of 920 children attending for audiovestibular assessment over a 42 month period, rare third windows were observed in 8 (<1%). These included posterior semicircular canal dehiscence (n = 3, 0.3%), posterior semicircular canal thinning (n = 2, 0.2%), X linked gusher (n = 2, 0.2%), and a combination of dilated internal auditory meatus/irregular cochlear partition/deficient facial nerve canal (n = 1, 0.1%). The majority of them (87.5%) demonstrated a mixed/conductive hearing loss with an air-bone gap in the presence of normal tympanometry (100%). Transient otoacoustic emissions were absent with a simultaneous cochlear pathology in 50% of the cohort. Features of disequilibrium were observed in 75% and about a third showed deranged vestibular function tests. Video head impulse test abnormalities were detected in 50% localizing to the side of the lesion. Cervical vestibular evoked myogenic potential test abnormalities were observed in all children in the cohort undergoing the test where low thresholds and high amplitudes classically found in third window disorders localized to the side of the defects in 28.5%. In the series, 71.4% also demonstrated absent responses/amplitude asymmetry, some of which did not localize to the ipsilesional side. Two children presented with typical third window symptoms. This study observes 2 new rare pediatric third window phenotypes and the presence of a cochlear hearing loss in these disorders. It emphasizes that these disorders should be considered as an etiology of hearing loss/disequilibrium in children. It also suggests that pediatric third window disorders may not present with classical third window features and are variable in their presentations/audiovestibular functions.

Keywords: HRCT; X linked gusher; audiovestibular; cVEMP; children; semicircular canal dehiscence; third window; vHIT.

Figure 1

Case number 2—Unilateral right posterior semicircular dehiscence. (A), Pure tone audiometry showing mild conductive low frequency hearing loss on the right. (B), Clinically significant cVEMP amplitude asymmetry with the right weaker than the left. This child perceived significant balance issues but his vestibular tests including the vHIT was normal. (C)—CT scan images of the right petrous temporal bone in the coronal plane (A) demonstrates a high riding jugular bulb. The axial image (B) the sagittal oblique reconstruction parallel to the posterior semi-circular canal (C) demonstrate dehiscence of the posterior semi-circular canal at its junction with the jugular bulb.

Figure 2

Case number 5—Bilateral posterior semicircular canal thinning. (A), Pure tone audiometry showing right mixed hearing loss. (B), cVEMP showing absent response on the right and normal amplitude on the left; in this child vHIT was normal and there were no symptoms of balance problems. (C)—CT scan images of the right (A) and left (B) petrous temporal bone in the axial plane demonstrates apparent dehiscence of the posterior semi-circular canal (white arrows). Sagittal oblique reconstruction of the right (C) and left (D) petrous temporal bone parallel to the plane of the posterior semi-circular canal demonstrates thinning of the overlying bone (dotted arrows) measuring 0.5 mm in thickness on both sides.

Figure 3

Case number 6, X linked gusher. (A), Pure tone audiometry showing significant bilateral mixed hearing loss. (B), Catch up saccades on both sides on the vHIT (arrow). (C), Absent cVEMP response on the side of the greater hearing loss and normal amplitude and threshold on the other side. (D), Axial CT scan (A) and T2 DRIVE MRI image (B) of the petrous temporal bone demonstrating bilateral bulbous dilatation involving the fundus of the internal auditory canal (arrow) and bilateral incomplete separation of the basal turn of the cochlea (arrow head) from the fundus of the internal acoustic canal (dotted arrow) classical of X linked gusher disorder.

Figure 4

Case number 8, Multiple abnormalities. (A), Pure tone audiometry showing left mixed hearing loss (A) with a normal right side. (B), vHIT showing left sided high frequency multiple canal dysfunction with catch up saccades (arrow) with a normal right side. (C), No response on the cVEMP on the left with a normal right side. (D), CT scan of the left petrous temporal bone in the axial plane (B) and coronal plane (D) demonstrate irregular widening of the internal auditory canal (white arrow) the labyrinthine segment of the left facial nerve canal has an ill-defined bony wall (dotted white arrow). The right petrous temporal bone demonstrates a normal sized internal auditory canal (C black arrow) on the axial and coronal planes and the bony wall of the right facial nerve canal appears normal (A dotted black arrow).