Long-term follow-up in patients with Pendred syndrome: vestibular, auditory and other phenotypes

Comparison of vestibular function in hereditary hearing loss patients with GJB2, CDH23, and SLC26A4 variants

To investigate the association between hereditary hearing loss and vestibular function, we compared vestibular function and symptoms among patients with GJB2, SLC26A4, and CDH23 variants. Thirty-nine patients with sensory neural hearing loss (11 males and 28 females) with biallelic pathogenic variants in either GJB2, SLC26A4, or CDH23 were included in this study (13 GJB2, 15 SLC26A4, and 11 CDH23). The patients were examined using caloric testing and cervical and ocular vestibular-evoked myogenic potentials (cVEMP and oVEMP). We also compared vestibular function and symptoms between patients with these gene variants and 78 normal-hearing ears without vestibular symptoms as controls. The frequency of semicircular canal hypofunction in caloric testing was higher in patients with SLC26A4 variants (47%) than in those with GJB2 (0%) and CDH23 variants (27%). According to the cVEMP results, 69% of patients with GJB2 variants had saccular hypofunction, a significantly higher proportion than in those carrying other variants (SLC26A4, 20%; CDH23, 18%). In oVEMP, which reflects utricular function, no difference was observed in the frequency of hypofunction among the three genes (GJB2, 15%; SLC26A4, 40%; and CDH23, 36%). Hence, discernable trends indicate vestibular dysfunction associated with each gene.

Epidemiologic association and shared genetic architecture between cataract and hearing difficulties among middle-aged and older adults

Age-related cataract and hearing difficulties are major sensory disorders that often co-exist in the global-wide elderly and have a tangible influence on the quality of life. However, the epidemiologic association between cataract and hearing difficulties remains unexplored, while little is known about whether the two share their genetic etiology. We first investigated the clinical association between cataract and hearing difficulties using the UK Biobank covering 502,543 individuals. Both unmatched analysis (adjusted for confounders) and a matched analysis (one control matched for each patient with cataract according to confounding factors) were undertaken and confirmed that cataract was associated with hearing difficulties (OR, 2.12; 95% CI, 1.98-2.27; OR, 2.03; 95% CI, 1.86-2.23, respectively). Furthermore, we explored and quantified the shared genetic architecture of these two complex sensory disorders at the common variant level using the bivariate causal mixture model (MiXeR) and conditional/conjunctional false discovery rate method based on the largest available genome-wide association studies of cataract (N = 585,243) and hearing difficulties (N = 323,978). Despite detecting only a negligible genetic correlation, we observe polygenic overlap between cataract and hearing difficulties and identify 6 shared loci with mixed directions of effects. Follow-up analysis of the shared loci implicates candidate genes QKI, STK17A, TYR, NSF, and TCF4 likely contribute to the pathophysiology of cataracts and hearing difficulties. In conclusion, this study demonstrates the presence of epidemiologic association between cataract and hearing difficulties and provides new insights into the shared genetic architecture of these two disorders at the common variant level.

Syndromic Hearing Loss in Children

Pattern recognition of specific temporal bone radiological phenotypes, in association with abnormalities in other organ systems, is critical in the diagnosis and management of syndromic causes of hearing loss. Several recent publications have demonstrated the presence of specific radiological appearances, allowing precise genetic and/or syndromic diagnosis, in the right clinical context. This review article aims to provide an extensive but practical guide to the radiologist dealing with syndromic causes of hearing loss.

Predictive Modeling and Risk Stratification of Patients With Enlarged Vestibular Aqueduct

OBJECTIVES/HYPOTHESIS

To investigate patient-specific characteristics that independently predict for progressive hearing loss in patients with enlarged vestibular aqueduct (EVA). Utilize multivariable predictive models to identify subgroups of patients with significantly different progression risks.

STUDY DESIGN

Retrospective analysis of patients evaluated at an academic tertiary care center. Cohort included 74 ears of patients with a diagnosis of EVA as defined by the Cincinnati criteria.

METHODS

Hearing trajectories were characterized, and a Kaplan-Meier estimator was utilized to determine progressive phenotype probabilities across the first 10 years after diagnosis. Cox proportional hazard regression was used to identify patient characteristics that independently altered this probability. Stratified risk groups were delineated from generated nomogram scores.

RESULTS

Male gender was associated with a 4.53 hazard ratio for progressive hearing loss (95% confidence interval [CI], 2.53 to 12.59). Each millimeter increase in operculum size was independently associated with an 80.40% increase in expected hazard (95% CI, 40.18 to 120.62). Each dB increase in air pure tone average at time of diagnosis decreased expected hazard by 1.59% (95% CI, -3.02 to -0.17). The presence of incomplete partition type II was associated with a 2.44 hazard ratio (95% CI, 1.04 to 5.72). Risk groups stratified by median nomogram score evidenced the discriminative ability of our model with the progression probability in the high-risk group being six times higher at 1 year, nearly five times greater at 3 years, and three times greater at 9 years.

CONCLUSIONS

EVA patient characteristics can be used to predict hearing loss probability with a high degree of accuracy (C-index of 0.79). This can help clinicians make more proactive management decisions by identifying patients at high risk for hearing loss.

LEVEL OF EVIDENCE

4 Laryngoscope, 2021.

Vestibular Function in Pendred Syndrome: Intact High Frequency VOR and Saccular Hypersensitivity

OBJECTIVE

Although Pendred syndrome involves anatomical abnormalities in the vestibular system and patient-perceived dizziness, the literature on vestibular function is scarce. The objective was to evaluate objective vestibular function in patients with PS using the video head impulse test (VHIT) and the cervical vestibular evoked myogenic potential (cVEMP) test, assessing the high frequency vestibulo-ocular reflex (VOR) and the saccular function, respectively.

METHODS

Twenty-six subjects (52 ears) with Pendred Syndrome and severe-to-profound hearing loss (cochlear implant candidacy) were studied retrospectively. Main outcome measures were VHIT VOR gain, refixation saccades, cVEMP stimulus thresholds and amplitudes.

RESULTS

In total, 4 of 52 ears (8%) had pathological VHIT outcomes (2 ears with low VHIT gain and 2 ears with saccades). The mean VHIT gain value was 0.96 (range 0.63-2.02). No patients had complete VOR loss. CVEMP responses were present in 76% of tested ears and absent in 24%. The mean cVEMP amplitude value was 192 μV. Absence of cVEMP response was associated with the presence of VHIT saccades (p = 0.038) and advanced age (rs = -0.34; p < 0.05). cVEMP amplitude was correlated with VHIT gain (rs = 0.46; p < 0.001).

CONCLUSION

Despite dysmorphic vestibular anatomy and severe loss of hearing, the VHIT VOR function is normal in patients with Pendred Syndrome. However, saccular function appears to be abnormally sensitive, as documented by low cVEMP thresholds and high amplitudes, which agrees with a "third window" effect secondary to the large vestibular aqueduct.

Systematic Review on Vestibular Symptoms in Patients With Enlarged Vestibular Aqueducts

OBJECTIVE

Enlarged vestibular aqueduct (EVA) is a congenital condition that can lead to various outcomes in pediatric patients including hearing loss and vestibular dysfunction. Our goal was to critically appraise the literature on the proportion of patients with EVA who report vestibular dysfunction, determine relevant risk factors for the development of these symptoms, and describe vestibular tests and interventions used to improve outcomes.

METHODS

A systematic review was performed in accordance with the PRISMA guidelines. We queried the EMBASE, Ovid Medline, and Cochrane Library databases for relevant literature. Studies were included if they had n > 10, reported vestibular symptoms or vestibular function testing in patients with EVA, and were published in English. Nonhuman studies, systematic reviews, and review articles were excluded.

RESULTS

Of 808 identified studies, 20 met inclusion criteria. Subjective vestibular symptoms included dizziness, episodic vertigo, and imbalance. Seventeen studies reported subjective vestibular symptoms, ranging from 2% to 71% of patients between studies. Seventeen studies performed some form of vestibular function test, including physical exam maneuvers (Dix-Hallpike), caloric testing, electronystagmography, and vestibular evoked myogenic potentials. Of those who had vestibular function testing, 7% to 92% had an abnormal result. Two studies identified head trauma as a risk factor. One study successfully treated patients with BPPV using the Epley maneuver, but other vestibular symptoms were not targeted with treatment.

CONCLUSION

The degree to which vestibular symptoms impact patients with EVA varies significantly. Performing vestibular function testing may help identify asymptomatic patients with vestibular dysfunction. Future studies should target improving treatment of vestibular symptoms in EVA patients. Laryngoscope, 2021.

Rare Disorders of the Vestibular Labyrinth: of Zebras, Chameleons and Wolves in Sheep's Clothing

The differential diagnosis of vertigo syndromes is a challenging issue, as many - and in particular - rare disorders of the vestibular labyrinth can hide behind the very common symptoms of "vertigo" and "dizziness". The following article presents an overview of those rare disorders of the balance organ that are of special interest for the otorhinolaryngologist dealing with vertigo disorders. For a better orientation, these disorders are categorized as acute (AVS), episodic (EVS) and chronic vestibular syndromes (CVS) according to their clinical presentation. The main focus lies on EVS sorted by their duration and the presence/absence of triggering factors (seconds, no triggers: vestibular paroxysmia, Tumarkin attacks; seconds, sound and pressure induced: "third window" syndromes; seconds to minutes, positional: rare variants and differential diagnoses of benign paroxysmal positional vertigo; hours to days, spontaneous: intralabyrinthine schwannomas, endolymphatic sac tumors, autoimmune disorders of the inner ear). Furthermore, rare causes of AVS (inferior vestibular neuritis, otolith organ specific dysfunction, vascular labyrinthine disorders, acute bilateral vestibulopathy) and CVS (chronic bilateral vestibulopathy) are covered. In each case, special emphasis is laid on the decisive diagnostic test for the identification of the rare disease and "red flags" for potentially dangerous disorders (e. g. labyrinthine infarction/hemorrhage). Thus, this chapter may serve as a clinical companion for the otorhinolaryngologist aiding in the efficient diagnosis and treatment of rare disorders of the vestibular labyrinth.

Analyses of the Relation between BPPV and Thyroid Diseases: A Nested Case-Control Study

BACKGROUND

This study investigated relationship between multiple thyroid disorders and benign paroxysmal positional vertigo (BPPV), adjusting for levothyroxine medication.

METHODS

The Korean National Health Insurance Service-Health Screening Cohort data from 2002 to 2015 were used. A total of 19,071 patients with BPPV were matched with 76,284 participants of a control group in a ratio of 1:4 for age, sex, income, and region of residence. The previous histories of thyroid disorders such as goiter, hypothyroidism, thyroiditis, hyperthyroidism, and autoimmune thyroiditis were investigated in both the BPPV and control groups. The odds ratios (ORs) for BPPV in thyroid diseases were calculated using conditional logistic regression analyses.

RESULTS

The histories of goiter (5.5% vs. 4.1%), hypothyroidism (4.7% vs. 3.7%), thyroiditis (2.1% vs. 1.6%), and hyperthyroidism (3.1% vs. 2.5%) were higher in the BPPV group than in the control group (all p < 0.001). Goiter, hypothyroidism, thyroiditis, and hyperthyroidism were associated with BPPV (adjusted OR = 1.28 (95% CI = 1.17-1.39) for goiter, 1.23 (95% CI = 1.10-1.37) for hypothyroidism, 1.13 (95% CI = 1.02-1.26) for hyperthyroidism, each p < 0.05).

CONCLUSIONS

BPPV was associated with thyroid disorders such as goiter, hypothyroidism, thyroiditis, and hyperthyroidism.

Could ionic regulation disorders explain the overlap between meniere's disease and migraine?

Ménière's disease (MD) is an inner ear disorder characterized by a burden of symptoms and comorbidities, including migraine. In both disorders, ionic dysregulation may play a role as a predisposing factor. In recent years. aquaporins have been widely investigated, but the results are far from conclusive. We recently studied the genetics of ionic transporters and the hormone endogenous ouabain as predisposing factors for development of MD. In particular, we found two genetic polymorphisms associated with MD: 1) rs3746951, a missense variant (Gly180Ser) in the salt-inducible kinase-1 (SIK1) gene encoding a Na+, K+ ATPase; 2) rs487119, an intronic variant of gene SLC8A1 coding for a Na+, Ca++ exchanger (NCX-1). Ionic concentration in the brain also plays a role in the pathophysiology of migraine. In this brief review we summarize what has been published on MD and migraine.

[Enlarged vestibular aqueduct syndrome-dehiscence syndromes-honeycomb mastoid : Pathophysiology and evidence for clinical differentiation]

BACKGROUND

Differential diagnosis of dizziness with hearing loss requires standardized procedures for detection and classification of rare congenital and acquired malformations of the petrous part of the temporal bone.

OBJECTIVE

The aim of this study was to present the physiology and pathophysiology of endolymphatic and perilymphatic pressure regulation, diagnostic guidelines, and aspects of prognosis and treatment.

MATERIALS AND METHODS

Relevant publications and guidelines were evaluated and own cases are reported.

RESULTS

Enlarged vestibular aqueduct (EVA) is the most frequently observed malformation of the inner ear, which leads to increased internal hydrostatic pressure and cochleovestibular dysfunction. Non-syndromic and syndromic forms, e.g., Pendred syndrome, are known. Other pressure-relevant malformations are semicircular canal dehiscence syndrome (SCDS) and enlargement of the cochlear aqueduct. There are currently no treatment options for EVAS and enlarged cochlear aqueduct. Pendred syndrome generally requires treatment with cochlea implants (CI) in early childhood. Dizziness and autophony in patients with SCDS syndrome can be effectively treated by semicircular canal occlusion and coverage.

CONCLUSION

Complaints in non-syndromic EVA and SCD syndrome are mainly caused by exposure of the inner ear to provoked and spontaneous pressure increases. Deafness and vestibular dysfunction in syndromic EVA (i.e., Pendred syndrome, branchiootorenal syndrome) are caused by malformation of the cochlea, genetic maldevelopment of the hair cells, and pressure effects.

Assessment of the Clinical Benefit of Imaging in Children With Unilateral Sensorineural Hearing Loss: A Systematic Review and Meta-analysis

IMPORTANCE

Imaging used to determine the cause of unilateral sensorineural hearing loss (USNHL) in children is often justified by the high likelihood of detecting abnormalities, which implies that these abnormalities are associated with hearing loss and that imaging has a positive contribution to patient outcome or well-being by providing information on the prognosis, hereditary factors, or cause of hearing loss.

OBJECTIVES

To evaluate the diagnostic yield of computed tomography (CT) and magnetic resonance imaging (MRI) in children with isolated unexplained USNHL and investigate the clinical relevance of these findings.

EVIDENCE REVIEW

Cochrane Library, Embase, PubMed, and Web of Science databases were searched for articles published from 1978 to 2017 on studies of children with USNHL who underwent CT and/or MRI of the temporal bone. Two authors (F.G.R. and E.N.B.P.) independently extracted information on population characteristics, imaging modality, and the prevalence of abnormalities and assessed the studies for risk of bias. Eligibility criteria included studies with 20 or more patients with USNHL who had CT and/or MRI scans, a population younger than 18 years, and those published in English.

MAIN OUTCOMES AND MEASURES

The pooled prevalence with 95% CI of inner ear abnormalities grouped according to finding and imaging modality.

FINDINGS

Of 1562 studies, 18 were included with a total of 1504 participants included in the analysis. Fifteen studies were consecutive case studies and 3 were retrospective cohort studies. The pooled diagnostic yield for pathophysiologic relevant findings in patients with unexplained USNHL was 37% for CT (95% CI, 25%-48%) and 35% for MRI (95% CI, 22%-49%). Cochleovestibular abnormalities were found with a pooled frequency of 19% for CT (95% CI, 14%-25%) and 16% for MRI (95% CI, 7%-25%). Cochlear nerve deficiency and associated cochlear aperture stenosis had a pooled frequency of 16% for MRI (95% CI, 3%-29%) and 44% for CT (95% CI, 36%-53%), respectively. Enlarged vestibular aqueduct (EVA) was detected with a pooled frequency of 7% for CT and 12% for MRI in children with USNHL.

CONCLUSIONS AND RELEVANCE

Imaging provided insight into the cause of hearing loss in a pooled frequency of about 35% to 37% in children with isolated unexplained USNHL. However, none of these findings had therapeutic consequences, and imaging provided information on prognosis and hereditary factors only in a small proportion of children, namely those with EVA. Thus, there is currently no convincing evidence supporting a strong recommendation for imaging in children who present with USNHL. The advantages of imaging should be carefully balanced against the drawbacks during shared decision making.

[The clinical definition and etiology of Pendred syndrome (a review of the literature and clinical observations)]

The aim of this work was a clinical study of the patients with mutations in the SLC26A4 gene and clinical diagnosis of the Pendred syndrome. The Pendred syndrome is a hereditary autosomal recessive disorder characterized by combined pathology of the inner ear and the thyroid gland. CT of the temporal bones demonstrates the Mondini-type structural anomaly in the inner ear and enlarged vestibular aqueduct. Examination of the thyroid gland reveals hypothyroidism and euthyroid goiter. A total of 20 unrelated children at the age from 2 to 16 years presenting with the hearing loss of different severity were available for the examination. High-resolution CT of the temporal bones demonstrated abnormal development of the inner ear including the Mondini-type structural anomaly and enlarged vestibular aqueduct. Five children with congenital hypothyroidism suffered from bilateral sensorineural impairment of hearing. The routine methods of audiological and molecular genetic examination were used throughout the study.

RESULTS

As a result of molecular genetic studies, four out of the 20 patients were found to carry six recessive mutations of the SLC26A4 gene in the compound heterozygous and one such gene in the homozygous state which confirmed the hereditary nature of the disease. The children suffered the hearing loss of varying severity diagnosed at different age. The thyroid hypofunction in one child was identified when it was 2 years of age, and in two children at the age of 8 and 9 years.

CONCLUSION

The first step in the diagnosis of the Pendred syndrome among children with congenital hearing loss was a CT scan of the temporal bones that showed incomplete separation of the curls of the cochlea and enlarged vestibular aqueduct. It is necessary to continue to study epidemiology, clinical and molecular genetics of the Pendred syndrome in the Russian population.

Progressive Hearing Loss and Head Trauma in Enlarged Vestibular Aqueduct

Objective

Enlarged vestibular aqueduct is the most common radiographically identified cause of congenital sensorineural hearing loss and is frequently progressive. Imaging is often ordered during the workup of children with congenital sensorineural hearing loss in part to identify enlarged vestibular aqueduct given concern for progression with head trauma. However, this association has not been systematically evaluated. We aimed to determine the rate of progression and association with head trauma in individuals with enlarged vestibular aqueduct.

Data Sources

Systematic review of primary studies identified through PubMed, Embase, Cochrane, and Web of Science.

Review Methods

Meta-analysis was performed on patient-level data describing enlarged vestibular aqueduct, progressive sensorineural hearing loss, and head trauma extracted from articles identified on systematic review according to PRISMA guidelines.

Results

Twenty-three studies (1115 ears with enlarged vestibular aqueduct) met inclusion criteria. Progressive sensorineural hearing loss was found in 39.6% of ears, with trauma-associated progression in 12%. Limited case-control data show no difference in the incidence of progression between patients with and without head trauma.

Conclusions

Long-term progressive sensorineural hearing loss is common in enlarged vestibular aqueduct, but its association with head trauma is not strongly supported.

Vestibular Dysfunction in Patients with Enlarged Vestibular Aqueduct

OBJECTIVE

Enlarged vestibular aqueduct (EVA) is the most common inner ear malformation. While a strong correlative relationship between EVA and hearing loss is well established, its association with vestibular dysfunction is less well understood. In this study, we examine the effects of EVA on the vestibular system in patients with EVA.

STUDY DESIGN

Prospective, cross-sectional study of a cohort ascertained between 1999 and 2013.

SETTING

National Institutes of Health Clinical Center, a federal biomedical research facility.

SUBJECTS AND METHODS

In total, 106 patients with unilateral or bilateral EVA, defined as a midpoint diameter greater than 1.5 mm, were referred or self-referred to participate in a study of the clinical and molecular aspects of EVA. Clinical history was ascertained with respect to the presence or absence of various vestibular signs and symptoms and history of head trauma. Videonystagmography (VNG), cervical vestibular evoked myogenic potential (cVEMP), and rotational vestibular testing (RVT) were performed to assess the vestibular function.

RESULTS

Of the patients with EVA, 45% had vestibular signs and symptoms, and 44% of tested patients had abnormal VNG test results. An increased number of vestibular signs and symptoms was correlated with the presence of bilateral EVA (P = .008) and a history of head injury (P < .001). Abnormal VNG results also correlated with a history of head injury (P = .018).

CONCLUSION

Vestibular dysfunction is common in patients with EVA. However, not all patients with vestibular signs and symptoms have abnormal vestibular test results. Clinicians should be aware of the high prevalence of vestibular dysfunction in patients with EVA.

Nystagmus in Enlarged Vestibular Aqueduct: A Case Series

Enlarged vestibular aqueduct (EVA) is one of the commonly identified congenital temporal bone abnormalities associated with sensorineural hearing loss. Hearing loss may be unilateral or bilateral, and typically presents at birth or in early childhood. Vestibular symptoms have been reported in up to 50% of affected individuals, and may be delayed in onset until adulthood. The details of nystagmus in patients with EVA have not been previously reported.

The objectives were to describe the clinical history, vestibular test findings and nystagmus seen in a case series of patients with enlarged vestibular aqueduct anomaly.

Chart review, included computed tomography temporal bones, infrared nystagmography with positional and positioning testing, caloric testing, rotary chair and vibration testing.

Clinical history and nystagmus varied among the five patients in this series. All patients were initially presumed to have benign paroxysmal positional vertigo, but repositioning treatments were not effective, prompting referral, further testing and evaluation. In three patients with longstanding vestibular complaints, positional nystagmus was consistently present. One patient had distinct recurrent severe episodes of positional nystagmus. Nystagmus was unidirectional and horizontal. In one case horizontal nystagmus was consistently reproducible with seated head turn to the affected side, and reached 48 d/s. Nystagmus associated with enlarged vestibular aqueduct is often positional, and can be confused with benign paroxysmal positional vertigo. Unexplained vestibular symptoms in patients with unilateral or bilateral sensorineural hearing loss should prompt diagnostic consideration of EVA.

Goiter and hearing impairment: A case of a male patient with Pendred syndrome

Pendred syndrome is a rare genetic disease that causes a disturbance in thyroid hormone synthesis, which results in thyroid dysfunction and the development of goiter and sensorineural deafness. The present report describes the case of a young euthyroid male, who developed a large goiter and hearing impairment, despite no family history of these conditions. A left lobectomy and a subtotal right lobectomy were performed, and the patient was administered permanent hormone replacement therapy with thyroxine. Patients with Pendred syndrome exhibit distinct clinical features and the mechanisms associated with the molecular genetics of this disease have been clarified. Thus, gene detection is considered to be a reliable diagnostic method. Certain patients require surgical intervention in order to relieve the symptoms. Misdiagnosis can be significantly reduced by increasing the understanding of Pendred syndrome.

Mouse models for pendrin-associated loss of cochlear and vestibular function

The human gene SLC26A4 and the mouse ortholog Slc26a4 code for the protein pendrin, which is an anion exchanger expressed in apical membranes of selected epithelia. In the inner ear, pendrin is expressed in the cochlea, the vestibular labyrinth and the endolymphatic sac. Loss-of-function and hypo-functional mutations cause an enlargement of the vestibular aqueduct (EVA) and sensorineural hearing loss. The relatively high prevalence of SLC26A4 mutations provides a strong imperative to develop rational interventions that delay, ameliorate or prevent pendrin-associated loss of cochlear and vestibular function. This review summarizes recent studies in mouse models that have been developed to delineate the role of pendrin in the physiology of hearing and balance and that have brought forward the concept that a temporally and spatially limited therapy may be sufficient to secure a life-time of normal hearing in children bearing mutations of SLC26A4.

Differences in the pathogenicity of the p.H723R mutation of the common deafness-associated SLC26A4 gene in humans and mice

Mutations in the SLC26A4 gene are a common cause of human hereditary hearing impairment worldwide. Previous studies have demonstrated that different SLC26A4 mutations have different pathogenetic mechanisms. By using a genotype-driven approach, we established a knock-in mouse model (i.e., Slc26a4^{tm2Dontuh/tm2Dontuh} mice) homozygous for the common p.H723R mutation in the East Asian population. To verify the pathogenicity of the p.H723R allele in mice, we further generated mice with compound heterozygous mutations (i.e., Slc26a4^{tm1Dontuh/tm2Dontuh}) by intercrossing Slc26a4^+/tm2Dontuh mice with Slc26a4^{tm1Dontuh/tm1Dontuh} mice, which segregated the c.919-2A>G mutation with an abolished Slc26a4 function. Mice were then subjected to audiologic assessments, a battery of vestibular evaluations, inner ear morphological studies, and noise exposure experiments. The results were unexpected; both Slc26a4^{tm2Dontuh/tm2Dontuh} and Slc26a4^{tm1Dontuh/tm2Dontuh} mice showed normal audiovestibular phenotypes and inner ear morphology, and they did not show significantly higher shifts in hearing thresholds after noise exposure than the wild-type mice. The results indicated not only the p.H723R allele was non-pathogenic in mice, but also a single p.H723R allele was sufficient to maintain normal inner ear physiology in heterozygous compound mice. There might be discrepancies in the pathogenicity of specific SLC26A4 mutations in humans and mice; therefore, precautions should be taken when extrapolating the results of animal studies to humans.

The role of pendrin in the development of the murine inner ear

Enlargement of the vestibular aqueduct (EVA) is a common inner ear malformation found in children with sensorineural hearing loss that is frequently associated with loss-of-function or hypo-function mutations of SLC26A4. SLC26A4 codes for pendrin, which is a protein that is expressed in apical membranes of selected epithelia and functions as an anion exchanger. The comparatively high prevalence of EVA provides a strong imperative to develop rational interventions that delay, ameliorate or prevent hearing loss associated with this phenotype. The development of rational interventions requires a fundamental understanding of the role that pendrin plays in the normal development of hearing, as well as a detailed understanding of the pathobiologic mechanisms that, in the absence of fully functional pendrin, lead to an unstable hearing phenotype, with fluctuating or progressive loss of hearing. This review summarizes studies in mouse models that have focused on delineating the role of pendrin in the physiology of the inner ear and the pathobiology that leads to hearing loss.

Integration of human and mouse genetics reveals pendrin function in hearing and deafness

Genomic technology has completely changed the way in which we are able to diagnose human genetic mutations. Genomic techniques such as the polymerase chain reaction, linkage analysis, Sanger sequencing, and most recently, massively parallel sequencing, have allowed researchers and clinicians to identify mutations for patients with Pendred syndrome and DFNB4 non-syndromic hearing loss. While thus far most of the mutations have been in the SLC26A4 gene coding for the pendrin protein, other genetic mutations may contribute to these phenotypes as well. Furthermore, mouse models for deafness have been invaluable to help determine the mechanisms for SLC26A4-associated deafness. Further work in these areas of research will help define genotype-phenotype correlations and develop methods for therapy in the future.

Absence of primary hypothyroidism and goiter in Slc26a4 (-/-) mice fed on a low iodine diet

BACKGROUND

Mutations in the SLC26A4 gene, coding for the anion transporter pendrin, are responsible for Pendred syndrome, characterized by congenital sensorineural deafness and dyshormonogenic goiter. The physiological role of pendrin in the thyroid is still unclear and the lack of a thyroid phenotype in some patients with SLC26A4 mutations and in Slc26a4 (-/-) mice indicate the existence of environmental or individual modifiers able to compensate for pendrin inactivation in the thyroid. Since pendrin can transport iodide in vitro, variations in iodide supply have been claimed to account for the thyroid phenotype associated with pendrin defects.

AIM

The Slc26a4 (-/-) mouse model was used to test the hypothesis that iodide supply may influence the penetrance and expressivity of SLC26A4 mutations.

MATERIALS AND METHODS

Slc26a4 (-/-) and (+/+) mice were fed up to 6 months on a standard or low iodine diet and were evaluated for thyroid structural abnormalities or biochemical hypothyroidism.

RESULTS

A 27-fold iodide restriction induced similar modifications in thyroid histology, but no differences in thyroid size, T4 or TSH levels were observed between between Slc26a4 (-/-) and (+/+) mice, either in standard conditions and during iodine restriction.

CONCLUSIONS

Iodide restriction is not able to induce a thyroid phenotype in Slc26a4 (-/-) mice. These experimental data, together with those coming from a review of familial Pendred cases leaving in regions either with low or sufficient iodide supply, support the idea that the expression of thyroid phenotype in Pendred syndrome is more powerfully influenced by individual factors than by dietary iodide.

Establishment of a knock-in mouse model with the SLC26A4 c.919-2A>G mutation and characterization of its pathology

Recessive mutations in the SLC26A4 gene are a common cause of hereditary hearing impairment worldwide. Previous studies have demonstrated that different SLC26A4 mutations may have different pathogenetic mechanisms. In the present study, we established a knock-in mouse model (i.e., Slc26a4^{tm1Dontuh/tm1Dontuh} mice) homozygous for the c.919-2A>G mutation, which is a common mutation in East Asians. Mice were then subjected to audiologic assessment, a battery of vestibular evaluations, and inner ear morphological studies. All Slc26a4^{tm1Dontuh/tm1Dontuh} mice revealed profound hearing loss, whereas 46% mice demonstrated pronounced head tilting and circling behaviors. There was a significant difference in the vestibular performance between wild-type and Slc26a4^{tm1Dontuh/tm1Dontuh} mice, especially those exhibiting circling behavior. Inner ear morphological examination of Slc26a4^{tm1Dontuh/tm1Dontuh} mice revealed an enlarged endolymphatic duct, vestibular aqueduct and sac, atrophy of stria vascularis, deformity of otoconia in the vestibular organs, consistent degeneration of cochlear hair cells, and variable degeneration of vestibular hair cells. Audiologic and inner ear morphological features of Slc26a4^{tm1Dontuh/tm1Dontuh} mice were reminiscent of those observed in humans. These features were also similar to those previously reported in both knock-out Slc26a4^−/− mice and Slc26a4^loop/loop mice with the Slc26a4 p.S408F mutation, albeit the severity of vestibular hair cell degeneration appeared different among the three mouse strains.

SLC26A4 genotype, but not cochlear radiologic structure, is correlated with hearing loss in ears with an enlarged vestibular aqueduct

OBJECTIVES/HYPOTHESIS

Identify correlations among SLC26A4 genotype, cochlear structural anomalies, and hearing loss associated with enlargement of the vestibular aqueduct (EVA).

STUDY DESIGN

Prospective cohort survey, National Institutes of Health, Clinical Center, a federal biomedical research facility.

METHODS

Eighty-three individuals, 11 months to 59 years of age, with EVA in at least one ear were studied. Correlations among pure-tone hearing thresholds, number of mutant SLC26A4 alleles, and the presence of cochlear anomalies detected by computed tomography or magnetic resonance imaging were examined.

RESULTS

Linear mixed-effects model indicated significantly poorer hearing in ears with EVA in individuals with two mutant alleles of SLC26A4 than in those with EVA and a single mutant allele (P = .012) or no mutant alleles (P = .007) in this gene. There was no detectable relationship between degree of hearing loss and the presence of structural cochlear anomalies.

CONCLUSIONS

The number of mutant alleles of SLC26A4, but not the presence of cochlear anomalies, has a significant association with severity of hearing loss in ears with EVA. This information will be useful for prognostic counseling of patients and families with EVA.

Pendred syndrome

Pendred syndrome (PDS) is an autosomal recessive disorder clinically characterized by sensorineural hearing loss and goiter. PDS is mainly caused by mutations in the SLC26A4 gene, although a few cases are due to mutations in the FOXI1 gene. SLC26A4 encodes pendrin, a sodium-independent transporter of iodide/chloride, chloride/formate and bicarbonate, that is expressed in the inner ear, thyroid gland, syncytiotrophoblast cells, endometrium and kidney. FOXI1 encodes a transcription factor necessary for pendrin expression. Patients with PDS show a bilateral and severe-to-profound hearing loss, although some cases present with a slowly progressive and fluctuating course. Temporal bone abnormalities with enlargement of the vestibular aqueduct, alone or with Mondini dysplasia, are common. Goiter appears most frequently in the second decade of life with a range of variations in size, depending on the amount of iodide intake and the effect that the mutation causes in pendrin function in any individual patient. A standard thyroid hormone-replacement regimen should be given to PDS patients with hypothyroidism to re-establish euthyroidism and prevent or decrease goiter growth. Total or partial thyroidectomy is occasionally the treatment of choice. Hearing aids and proper educational programs should also be offered to patients.

Clinical characteristics and genotype-phenotype correlation of hearing loss patients with SLC26A4 mutations

CONCLUSIONS

The present study confirmed the clinical characteristics of patients with SLC26A4 mutations: congenital, fluctuating, and progressive hearing loss usually associated with vertigo and/or goiter during long-term follow-up. This clarification should help to facilitate appropriate genetic counseling and proper medical management for patients with these mutations, but there was no particular genotype-phenotype correlation among them, suggesting that other factors may contribute to such variability.

OBJECTIVES

Due to the wide range of phenotypes caused by SLC26A4 mutations, there is controversy with regard to genotype-phenotype correlation. The present study was performed: (1) to determine phenotypic range in patients with biallelic SLC26A4 mutations, and (2) to evaluate whether possible genotype-phenotype correlation exists.

SUBJECTS AND METHODS

Phenotypes in 39 hearing loss patients with SLC26A4 mutations were summarized and genotype-phenotype correlation was analyzed.

RESULTS

Hearing level varied in the individuals from mild to profound severity. Most of the patients had fluctuating and progressive hearing loss that may have been of prelingual onset. Twenty-four (70.6%) patients had episodes of vertigo, and 10 (27.8%) patients had goiter, which had appeared at age 12 or older. In contrast to such phenotypic variabilities, no apparent correlation was found between these phenotypes and their genotypes.

Vestibular symptoms in children with enlarged vestibular aqueduct anomaly

OBJECTIVE

The objective of this study is to describe the vestibular symptoms in pediatric patients with enlarged vestibular aqueduct (EVA) anomaly.

METHODS

Retrospective chart review of pediatric and adult patients with EVA anomaly who were treated at the University of Utah Hospital or Primary Children's Medical Center, between 1995 and 2005. Radiographs were reviewed to confirm the diagnosis. Comparisons were made between adult and pediatric patients.

RESULTS

Thirty-two patients were included in the study, 17 females and 15 males. Twenty-one patients were under the age of 18 and 11 patients were age 18 or older. On initial audiometric evaluation at a tertiary hospital, the pure tone average in the right ear was 75.0 dB and the pure tone average in the left ear was 80.4 dB. The incidence of vestibular symptoms in adult patients was 45.5% and in pediatric patients was 48.0%. Fourteen patients underwent cochlear implantation. Four patients (28.6%) who previously denied vestibular symptoms experienced post-operative vertigo after cochlear implantation.

CONCLUSIONS

About half of the patients with EVA in our series experienced vestibular symptoms. Pediatric patients in our series experienced vertigo and vestibular symptoms with equal frequency when compared to adult patients. Some patients with EVA undergoing cochlear implantation experienced vestibular symptoms in the post-operative period.

The H723R mutation in the PDS/SLC26A4 gene is associated with typical Pendred syndrome in Korean patients

Inherited as an autosomal recessive trait, Pendred syndrome is a disease that shows congenital sensorineural hearing loss and goiter, with a positive finding in the perchlorate discharge test. Pendred syndrome results from various mutations in the PDS/SLC26A4 gene that cause production of an abnormal pendrin protein. More than 90 mutations in the PDS/SLC26A4 gene have been reported throughout the world. A recent study of 26 Korean patients with a relatively high frequency (65%) of a mutated PDS/SLC26A4 gene exhibited nonsyndromic deafness and an enlarged vestibular aqueduct. We report two patients with characteristics of typical Pendred syndrome, a 26-yr-old female and a 61-yr-old male, who were both homozygous for a previously reported missense mutation, H723R (Histidine 723Arginine) in the PDS/SLC26A4 gene.

Lateral Semicircular Canal and Vertigo in Patients With Large Vestibular Aqueduct Syndrome

OBJECTIVE

To evaluate the hypothesis that there are differences in the morphology of the lateral semicircular canal (LSCC) between patients with large vestibular aqueduct syndrome (LVAS) and control subjects and to investigate the clinical implications of these differences.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

Nine patients (two male patients and seven female patients; age range, 8-54 yr) with LVAS (one patient had unilateral LVAS, and eight patients had bilateral LVAS). Five patients had vertigo, and four patients, including the one with unilateral LVAS, did not have vertigo.

MAIN OUTCOME MEASURES

The area of the LSCC was traced on the magnetic resonance imaging console and compared between LVAS patients and 12 control subjects who did not have sensorineural hearing loss. The LSCC fluid-containing area was divided by the sum of the LSCC inner area and the LSCC fluid-containing area for evaluation of the degree of the LSCC dysplasia.

RESULTS

The LSCC fluid-containing ratio was significantly larger in LVAS patients than in control subjects. Moreover, the LSCC fluid-containing ratio was significantly larger in the eight ears with vertigo than in the nine ears without vertigo. There was no relationship between hearing level and the LSCC fluid-containing ratio.

CONCLUSION

Patients with LVAS may have disturbed morphogenesis of both membranous and bony labyrinths. Our results reveal that the morphology of semicircular canals is clinically associated with vertigo.