The abnormally dilated internal auditory canal: a non-specific finding or a distinctive pathologic entity

Audiovestibular Quantification in Rare Third Window Disorders in Children

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.

The Normal Adult Human Internal Auditory Canal: A Volumetric Multidetector Computed Tomography Study

OBJECTIVE

The purpose of this study was to demonstrate that volumetric analysis of multidetector computed tomography (CT) images can be used to calculate the volume of the adult human internal auditory canal (IAC) reproducibly, and to describe the range of normal IAC volumes in the adult population with subgroup analysis of sex, age, and laterality.

BACKGROUND

Previous studies of the IAC have typically used measurements in two dimensions or by using casts of cadavers to measure IAC volumes. This study is the first to report the normal ranges of IAC volumes measured by CT.

METHODS

Two hundred eighty-one CT scans were assessed. Of the CT scans that met the inclusion criteria, a software package was used to manually contour the IACs in each subject to calculate the volumes in cubic millimeters. Subgroup analysis of laterality, sex, and age was evaluated. Interobserver agreement was calculated for the first 59 patients (118 canals).

RESULTS

Two hundred fifty-nine scans (518 canals) met the inclusion criteria. The volumes ranged from 74 to 502 mm, with no statistically significant difference between left and right (p value = 0.69). In males, the range of volumes measured 74 to 502 mm while in females it ranged from 78 to 416 mm. Males had larger IAC volumes than females (Wilcoxon rank-sum test: S = 14,845.0, p value = 0.01 on the right, and S = 14,646, p value = 0.004 on the left). No correlation was found with age (Spearman: -0.10, p value = 0.09 on the right and -0.04, p value = 0.50 on the left). Excellent interobserver agreement was found.

CONCLUSION

IAC volumes of normal adult subjects, measured by CT, were larger in males and not significantly different with respect to age or laterality.

The internal acoustic canal--another review area in paediatric sensorineural hearing loss

Morphological abnormalities of the internal acoustic canal (IAC), albeit rare, are sometimes associated with hearing loss in children. We present an illustration of the spectrum of IAC abnormalities together with a brief review of the embryology and anatomy of the IAC and the techniques used when imaging the petrous temporal bone. This review focuses on morphological abnormalities of the IAC together with their clinical implications and impact on clinical management.

Hearing loss and enlarged internal auditory canal in children

INTRODUCTION

Among the temporal bone abnormalities that can be found in the etiological study of paediatric sensorineural hearing loss (SNHL) by imaging techniques, those related to the internal auditory canal (IAC) are the least frequent. The most prevalent of these abnormalities that is associated with SNHL is stenotic IAC due to its association with cochlear nerve deficiencies. Less frequent and less concomitant with SNHL is the finding of an enlarged IAC (>8mm).

METHODS

Retrospective and descriptive review of clinical associations, imaging, audiological patterns and treatment of 9 children with hearing loss and enlarged IAC in the period 1999 to 2012.

RESULTS

Two groups of patients are described. The first, without association with vestibulocochlear dysplasias, consisted of: 2 patients with SNHL without other temporal bone or systemic abnormalities, one with bilateral mixed HL from chromosome 18q deletion, one with a genetic X-linked DFN3 hearing loss, one with unilateral hearing loss in neurofibromatosis type 2 with bilateral acoustic neuroma, and one with unilateral hearing loss with cochlear nerve deficiency. The second group, with association with vestibulocochlear dysplasias, was comprised of: one patient with moderate bilateral mixed hearing loss in branchio-oto-renal syndrome, one with profound unilateral SNHL with recurrent meningitis, and another with profound bilateral SNHL with congenital hypothyroidism.

CONCLUSIONS

The presence of an enlarged IAC in children can be found in different clinical and audiological settings with relevancies that can range from life-threatening situations, such as recurrent meningitis, to isolated hearing loss with no other associations.