Characteristics of sensorineural hearing loss in children with inner ear anomalies

Abstract

Purpose

To determine whether hearing loss in children with inner ear anomalies has some distinctive characteristics when compared to children with hearing loss but without inner ear anomalies.

Methods

Temporal bone computed tomography scans of 69 patients with sensorineural hearing loss were examined for inner ear abnormalities of which 17 were identified. The medical histories of these patients were reviewed for the characteristics of their hearing loss, including initial presentation, natural history, and nature of loss, as well as the family history of hearing loss and risk factors for hearing loss. These were compared to age-matched controls with hearing loss but without inner ear anomalies.

Results

Seventeen patients had inner ear anomalies. Records of 14 of these patients were compared to patients without inner ear anomalies. Regarding age of onset, 71.4% of patients with anomalies had onset of their hearing loss at less than 2 years old vs 78.6% without anomalies. Regarding unilateral vs bilateral, 42.9% of patients with anomalies were unilateral vs 28.6% of patients without anomalies. For patients with anomalies, 85.7% were stable and 14.3% were progressive; without anomalies, 71.4% were stable, 21.4% were progressive, and 7.1% were fluctuating. Regarding family history, only 14.3% of patients without anomalies had a positive family history vs 56% of patients with anomalies.

Conclusions

Children with inner ear anomalies and sensorineural hearing loss have an increased incidence of unilateral hearing loss and stable hearing loss as compared to controls with sensorineural hearing loss without inner ear anomalies. In addition, children with inner ear anomalies and sensorineural hearing loss are less likely to have a family history of hearing loss.

Introduction

Congenital sensorineural hearing loss (SNHL) is a relatively common diagnosis in children with incidence rates ranging from 1:1000 to 1:2000 [1], [2], [3] . Prompt diagnosis and treatment of congenital SNHL are essential as significant developmental delays in speech and language can ensue. Unfortunately, because of the variety of etiologies and presentations of SNHL in pediatric populations, diagnosis and initiation of proper treatment can be difficult. One common attribute that appears in at least a proportion of children with SNHL is an inner ear anomaly (IEA).

It is generally accepted that approximately 50% of congenital hearing loss is attributed to genetics, 25% to environmental factors such as perinatal infection, ototoxic drug use, or trauma, and 25% is idiopathic [2], [3]. Congenital hearing loss due to genetic factors is often due to single-gene mutations that can lead to manifestations that are either syndromic or nonsyndromic, and are inherited in either an autosomal dominant, autosomal recessive, X-linked, or mitochondrial fashion [3]. Syndromic hearing impairment describes clinical states in which hearing loss is just one finding in a constellation of pathologies affecting multiple organ systems. Dozens of such syndromes exist with examples including Usher's syndrome in which progressive SNHL and retinitis pigmentosa are seen and Pendred's syndrome in which hearing loss is combined with euthyroid goiter [4], [5]. Nonsyndromic hearing impairment describes isolated hearing impairment without other symptoms. Diagnosis can be quite difficult in suspected cases of nonsyndromic hearing loss because of extreme genetic heterogeneity. Indeed, at least 80 gene loci have been identified, and the list is ever growing [2].

Although the root causes of congenital SNHL, whether genetic or environmental, continue to be elucidated, the underlying mechanism producing SNHL is not completely understood. However, radiological studies of the temporal bone have demonstrated the presence of an IEA in 20% to 30% of patients with SNHL [6], [7], [8]. In several forms of syndromic and nonsyndromic genetic hearing loss, inner ear abnormalities have been identified. For instance, labyrinthine dysplasia has been associated with Usher's syndrome [4]. In addition, when a POU transcription gene, part of a set of genes implicated in an X-linked form of nonsyndromic hearing loss, was inactivated in mouse models, the gene loss resulted in the abnormal development of the bony labyrinth and ossicles of the middle ear [3]. Congenital inner ear abnormalities are not only a genetic phenomenon, but also can occur sporadically as well.

The embryology of the inner ear is complex. The structure of the inner ear is composed of a membranous labyrinth surrounded by a bony labyrinth within the petrous temporal bone. Beginning at early third week of development, slight thickening of the ectoderm occurs on both sides of the still-open neural canal. By 18 days, the thickening becomes apparent and is termed the auditory or otic placode. The otic placode invaginates at the 22nd day and continues to deepen to form the auditory pit. By the 25th day, the auditory pit has deepened and pinched off its exposure to the external environment becoming an otocyst or auditory vesicle [9]. During the fourth to fifth weeks of development, the auditory vesicle enlarges and elongates dorsoventrally, and a diverticulum forms on the medial side of the auditory vesicle that later becomes the endolymphatic sac and duct. The major compartment of the auditory vesicle is termed the utriculo-saccular chamber. From the dorsal portion of this chamber, 3 disc-like outpouchings appear, and by 9 weeks of development form the semicircular canals. From the ventral part of the chamber, a single outpouching pushes medially as the cochlear duct, and by 12 weeks the defined cochlea is formed. The central portion of the chamber becomes the membranous vestibule with the utricle developing dorsally and the saccule ventrally. At 8 weeks of development, the mesoderm that invests the auditory vesicle begins conversion into a cartilaginous capsule. Within this cartilage, the scala tympani and scala vestibuli extend along each side of the cochlear duct. Ossification of the cartilaginous capsule begins and is completed between the 16th and 23rd weeks [10].

Malformations of the inner ear result from abnormal development of the membranous and/or osseous labyrinth. These anomalies include, but are not limited to, complete labyrinthine aplasia (Michel deformity), cochlear aplasia, cochlear hypoplasia, incomplete cochlear partition (Mondini deformity), common cavity, the Scheibe deformity, the Alexander deformity, enlarged cochlear aqueduct, and an enlarged vestibular aqueduct (EVA). Mondini's deformity is attributed to arrest of cochlear maturation at the sixth or seventh week of gestation and results in a small, flat cochlea with only 1.5 turns and an incomplete, intrascalar septa [8], [11]. The Scheibe deformity, also known as cochleosaccular dysplasia, results in a poorly differentiated saccule and cochlear duct [11]. The Alexander deformity is characterized by a cochlear base deformity. The Michel deformity represents a complete failure of inner ear development [8], [11]. Lastly, the most commonly observed IEA, EVA, is defined as a vestibular aqueduct with an anteroposterior diameter greater than 1.5 mm [12], [13]. Enlarged vestibular aqueduct can present independently or in combination with other deformities.

Identification of patients with SNHL secondary to an IEA is critical for proper treatment and prevention of further loss. Although temporal bone computed tomography (CT) scanning is the diagnostic tool of choice for determining an IEA, other, less costly, means of screening for these patients would be of great benefit. The goal of this study is characterize the population of patients with SNHL and an IEA with regard to the presentation, nature, and natural history of this patient population as well as any associated genetic mutations, family history, or risk factors. If certain clinical characteristics could be identified that differentiate patients with SNHL due to an IEA vs those with SNHL without an IEA, these could be used as screening criteria for selecting patients for temporal bone CT scans.