Auditory brainstem response audiometry in congenitally hypothyroid children under early replacement therapy

Dysfunction of the Auditory System in Children With Hypothyroidism: A Systematic Review and Meta-Analysis

OBJECTIVE

To examine the prevalence of hearing impairment in children with hypothyroidism, and to characterize clinical and subclinical hearing loss by examining cochlear function, auditory brainstem pathways, and integration of the auditory system as a whole.

DESIGN

An electronic search was conducted using PubMed, Scopus, and Cochrane Library databases. This systematic review was performed in accordance with the PRISMA guidelines. Original observational studies that utilized audiological tests for auditory system evaluations in hypothyroidism were included. A total of 2004 studies were found in the search, with 23 studies meeting the inclusion criteria.

RESULTS

The pooled prevalence of hearing loss was 16.1% [95% confidence interval 10.7, 22.4] for children with congenital hypothyroidism. Hearing thresholds at pure-tone averages (0.5-2 kHz) were 1.6 dB [95% confidence interval 1.7, 4.8] higher for children with hypothyroidism compared to age-matched controls. Cochlear dysfunction was detected at middle frequencies (1-3 kHz) by otoacoustic emission testing, indicating abnormalities of hair cell function or cochlear integration. Retrocochlear involvement was detected on auditory brainstem response (ABR), with prolonged Wave I indicating a peripheral conduction abnormality localized to the middle or inner ear and eighth cranial nerve.

CONCLUSIONS

Children with hypothyroidism have a higher prevalence of hearing loss than children without hypothyroidism. For children with congenital hypothyroidism, evidence of subclinical abnormalities at the level of the cochlea and eighth cranial nerve are present despite early initiation of levothyroxine therapy. Dysfunction of the auditory system might begin with predominance of peripheral conduction abnormalities early in development.

Congenital hypothyroidism as a risk factor for hearing and parents' knowledge about its impact on hearing

Aim

To evaluate the hearing of children with congenital hypothyroidism (CH) and to analyze the knowledge that parents’ have on the possible auditory impacts of the disease.

Methods

A total of 263 parents/guardians were interviewed about aspects of CH and hearing. Audiological evaluation was performed on 80 participants, divided into two groups: with CH (n= 50) and without CH (n=30). Clinical and laboratory CH data were obtained from medical records, pure tone auditory thresholds and acoustic reflexes were analyzed. The auditory data was compared between groups. Student’s t-test and Chi-square were used for statistical analysis at a significance level of 5% (p ≤0.05).

Results

The majority (78%), of the parents were unaware that CH when not treated early is a potential risk to hearing. There was no correlation between socioeconomic class and level of information about CH and hearing (p>0,05; p=0.026). There was a statistically significant difference between the auditory tone thresholds of the groups and between the levels of intensity necessary for the triggering of the acoustic reflex. The group with CH presented the worst results (p≤0.05) and absence of acoustic reflex in a normal tympanometric condition.

Conclusions

Children with CH are more likely to develop damage to the auditory system involving retrocochlear structures when compared to healthy children, and that the disease may have been a risk factor for functional deficits without deteriorating hearing sensitivity. The possible impacts of CH on hearing, when not treated early, should be more publicized among the parents/guardians of this population.

Impact of maternal iron deficiency on the auditory functions in the young and adult guinea pig

OBJECTIVES

The aim of this study was to evaluate the hearing function in the guinea pig offspring at post-natal day (PNd) 24 and PNd84 born from dams suffering from iron deficiency during pregnancy and lactation by using the auditory brainstem response (ABR).

METHOD

Female guinea pigs (n = 24 per dietary group) were fed an iron sufficient (IS) diet (114 mg/kg) or an iron deficient (ID) diet (11.7 mg/kg) during the gestation and lactation periods. Pups in both groups were weaned at PNd9 and given the IS diet. The hematocrit level was measured at every trimester of pregnancy and at the day of sacrifice in dams and at PNd24 and PNd84 in pups. The animal body weight was measured on every second day until the day of sacrifice. The ABR was used in pups to measure the hearing threshold using a broad range of stimulus intensities and latency at 100 and 80 dB in response to 2, 4, 8, 16, and 32 kHz tone pips at PNd24 and 84.

RESULTS AND DISCUSSION

No significant difference between dietary groups was measured in hearing threshold and absolute latencies in pups at PNd24 and PNd84. Although the ID offspring (n = 16) did not differ in brainstem transmission times (BTTs) at 80 dB compare to the IS siblings (n = 25) at PNd24, they showed significant delayed inter-peak latency (IPL) I-IV at 100 dB suggesting a delayed BTT. At PNd84, the latency of all peaks including IPL I-IV at 80 and 100 dB significantly decreased and was also similar in pups from both dietary groups suggesting a better brain maturation. This is the first study investigating the long-term impact of maternal iron deficiency on the auditory functions in the guinea pig offspring during early development to adulthood.

Mechanisms involved in hearing disorders of thyroid ontogeny: a literature review

Endocochlear, retrocochlear and/or central origin hearing damage may be related to the absence of appropriate levels of thyroid hormone during morphogenesis and/or auditory system development. Hearing disorders related to the thyroid are not well studied, despite speculation on the pathophysiological mechanisms. The objective of this review was to characterize the main pathophysiological mechanisms of congenital hypothyroidism and to evaluate the relationship with central and peripheral hearing disorders. We conducted a literature review using the databases MedLine, LILACS, Cochrane Library, SciELO, Institute for Scientific Information (ISI), Embase, and Science Direct between July and September on 2016. We identified the studies that address hearing disorder mechanisms on the congenital hypothyroidism. Congenital hypothyroidism may have clinical and subclinical manifestations that affect the auditory system and may be a potential risk factor for hearing impairment. Hearing impairment can severely impact quality-of-life, which emphasizes the importance of monitoring and evaluating hearing during the clinical routine of these patients.

Neurophysiologic evaluation of infants with congenital hypothyroidism before and after treatment

We performed a systematic neurophysiological evaluation of newborns-infants newly diagnosed with congenital hypothyroidism and started on replacement therapy, in order to document the maturation of visual, auditory and somesthetic pathways and to evaluate the influence of treatment. Twenty-one patients (9 boys, 12 girls) were studied. They underwent neurophysiological evaluation consisting of visual, auditory, and somatosensory evoked potentials at diagnosis, as well as 6 and 12 months after initiation of treatment. At the time of diagnosis, 47.61 % of the patients had abnormal evoked potentials, with visual evoked potentials being most commonly abnormal. Twelve months after the onset of treatment, abnormal evoked potentials were detected in 33.3 % of the patients. In newly diagnosed infants with congenital hypothyroidism there is a high relevance of abnormal evoked potentials (47.61 %) at the time of diagnosis, declining with time and not correlating with the severity of the disease at diagnosis, the time of diagnosis or the initial dose of thyroxine.

Congenital hypothyroidism: a clinical update of long-term outcome in young adults

Congenital hypothyroidism (CH) is the most common congenital endocrine disorder. The early treatment of CH patients has successfully improved the prognosis and management of this disorder. Optimal treatment and management throughout the patient's life, beginning in the neonatal period, are required to ensure long-term health. Affected patients should be offered assessments of associated medical conditions and provided with accurate information about their condition throughout their lives, but particularly during the transition from pediatric to adult services. This review provides a summary of current knowledge about the long-term outcomes of these patients and appropriate management into early adulthood. We carried out a systematic search of the Medline database to identify relevant articles. Despite major improvements in prognosis, the impact of CH is clearly not uniform, and management should take into account a broader range of relevant indicators, including CH severity, associated comorbid conditions and the adequacy of treatment during childhood and adulthood. The early diagnosis and management of associated medical conditions, and better educational strategies to improve compliance with treatment, should improve the long-term prognosis. Further studies are required to explore changes with aging.

Mild iron deficiency anaemia during pregnancy and lactation in guinea pigs alters amplitudes and auditory nerve velocity, but not brainstem transmission times in the offspring's auditory brainstem response

OBJECTIVES

It is well known that postnatal/early childhood iron deficiency (ID) anaemia (IDA) adversely affects infants' cognitive development and neurophysiology. However, the effects of IDA during gestation and lactation on the offspring are largely unknown. To address this health issue, the impact of mild IDA during gestation and lactation on the offsprings' neural maturation was studied in the guinea pig, using auditory brainstem responses (ABRs) latencies and amplitudes.

METHODS

Female guinea pigs (n = 10/group) were fed an iron sufficient (ISD) or deficient diet (IDD) (144 and 11.7 mg iron/kg) during the gestation and lactation periods. From postnatal day (PNd) 9 onward, the ISD was given to both groups of weaned offspring. The offsprings' ABRs were collected on PNd24 using a broad range of stimulus intensities in response to 2, 4, 8, 16, and 32 kHz tone pips.

RESULTS

Although the IDA siblings (n = 8) did not differ in brainstem transmission times (BTTs) compared to the IS siblings (n = 8), they showed significant delayed peak I latency at 100 and 80 dB, respectively. Additionally, significantly higher ABR wave amplitudes were observed in the IDA female offspring between 35 and 50 dB (4 kHz), a phenomenon suggestive of a neural hyperactivity (hyperacusis).

DISCUSSION

In support to our previous findings, the present results indicate that a mild IDA during gestation and lactation can have detrimental effects on early development of the offsprings' hearing and nervous systems, particularly on neural synchrony and auditory nerve conduction velocity, but not on BTT.

Factors associated with hearing impairment in patients with congenital hypothyroidism treated since the neonatal period: a national population-based study

CONTEXT

Untreated hypothyroidism is known to impair hearing, but little is known about the long-term hearing of patients treated for congenital hypothyroidism (CH) since the neonatal period.

OBJECTIVE

The purpose of this study was to assess hearing and its determinants in a population-based registry of young adult patients with CH.

DESIGN, SETTING, AND PARTICIPANTS

Self-declared hearing loss was evaluated in 1202 of the 1748 eligible patients with CH who completed a questionnaire on health status at a median age of 23.4 years. Audiograms were obtained for one third of the patients declaring hearing loss (37 of 107).

MAIN OUTCOME MEASURES

Self-declared hearing loss and audiogram characteristics for patients reporting hearing impairment were measured.

RESULTS

These patients had a risk of self-declared hearing loss more than 3 times higher than that for the reference population (relative risk [RR] = 3.7; 95% confidence interval [CI], 2.9-4.7). Hearing impairment was diagnosed at a median age of 7.0 (25th-75th percentiles, 3.4-19.0) years, and 17% of affected patients required hearing support in early adulthood. Hearing loss was associated with the type of CH (patients with athyreosis and gland in situ were more frequently affected than those with an ectopic gland [RR = 2.61; 95% CI, 1.77-3.88]), with disease severity, as assessed by bone maturation delay at the time of diagnosis, with at least one knee epiphyseal ossification center absent in the most severe form (RR = 2.29; 95% CI, 1.39-3.79), and with other associated chronic diseases (RR = 3.64; 95% CI, 2.35-5.62). A trend for association with serum free T4 concentration at diagnosis was also observed (RR = 1.47; 95% CI, 0.96-2.23). Hearing loss was mostly bilateral (90%), mild to moderate (96%), of the sensorineural type (76%), and concerned high or very high frequencies.

CONCLUSION

Despite major improvements in prognosis, hearing loss remains a significant problem, particularly in patients with severe CH. Parents and primary care providers should be aware of this risk, because early diagnosis and intervention could improve the long-term prognosis in these patients.

Iodine deficiency, thyroid function and hearing deficit: a review

Iodine deficiency affects an estimated 241 million school-aged children in the world. Little is known about iodine deficiency in relation to auditory function, except for the fact that deaf–mutism is one of the features of cretinism. In the present review, we documented the scientific knowledge on the role of iodine and hypothyroidism in the auditory system. We found that ear development and hearing function depend on thyroid hormones. Multiple pathways are involved in this, including both inner ear morphology as well as neurological processes. Conductive as well as sensorineural hearing loss is found in studies with animals that were rendered hypothyroidic. In humans, auditory impairment is reported frequently in relation to hypothyroidism, ranging from mild disturbances to severe handicap. Auditory impairment has been related more explicitly to congenital hypothyroidism than to acquired hypothyroidism. The critical period for thyroid function-related hearing maturation is the first and second trimesters of pregnancy. Although only a limited number of studies have directly investigated the relationship between iodine deficiency and auditory function, most studies point toward an association. However, evidence from good randomised controlled trials is lacking. Inclusion of auditory outcomes in iodine supplementation studies is therefore to be recommended, especially for trials in pregnancy. Hearing deficit is an invisible abnormality, but has major consequences for educational and social skills if not detected. In view of this, auditory impairment should be mapped in iodine-deficient areas in order to realistically estimate the magnitude of the problem.

Auditory brainstem evoked potentials in early-treated congenital hypothyroidism

To evaluate the effect of early treatment of congenital hypothyroidism on central nervous system development, auditory brainstem evoked potentials were determined in 32 patients with hyperthyrotropinemia diagnosed during neonatal screening. The patients included 27 with congenital hypothyroidism and 5 with transient hypothyroidism. Abnormal auditory brainstem evoked potential tracings were found in 8 patients (congenital hypothyroidism in 7 and transient hypothyroidism in 1). Four of these patients had increased peripheral conduction time (wave I prolongation), and the other 4 had increased central conduction time (wave III or V prolongation). The patients with abnormal auditory brainstem evoked potentials did not show increased initial manifestations, yet 6 of them had lower initial thyroxine levels. Specific auditory brainstem evoked potential abnormalities were found in 25% of early-treated patients with congenital hypothyroidism. The possible causal relationship between deviant auditory brainstem evoked potential patterns and later neurodevelopment demands further clarification. This study suggests the usefulness of auditory brainstem evoked potential assessment to provide information about electrophysiologic deviation of the auditory pathway in patients with early-treated congenital hypothyroidism.

The recovery potential of central conduction disorder in hypothyroid rats

In an aim to detect the dysfunction of central nervous system among rats with varied durations of hypothyroidism and to elucidate the recovery potential after thyroxine replacement, a series of BAEP were conducted and compared with age-matched controls. BAEP was performed in five groups of the hypothyroid animals 1, 3, 5, 7, and 9 months after thyroidectomy respectively. Following initial electrophysiological assessment, thyroxine replacement was administered to each group of hypothyroid rats, and BAEP was performed at two month intervals, up to two successive normal studies or six months after the initiation of therapy, whichever came first. Before thyroxine treatment, prolonged I-V interpeak latency was the most consistent abnormal finding in all groups of hypothyroid rats, and longer hypothyroid state correlated well with more severe central conduction disorder. Hearing impairment was also noted among those with long duration of hypothyroidism. After thyroxine replacement, the central conduction dysfunction usually returned to normal if the hypothyroid state was not more than 5 months in duration. However, when hypothyroid state persisted over 7 months or more, there would be an incomplete recovery for central conduction disorder. The present study brings out the concept of 'therapeutic window' in reversing the central nervous dysfunction caused by hypothyroidism in adult rats.

Relation of basic and clinical research on foetal and neonatal thyroid pathology to neonatal thyroid screening

In this short review the importance is emphasized of basic and clinical research in regard to the interpretation of screening results. This review covers foetal thyroid physiology during the first 48 h of life, the importance of thyroid hormones for normal brain development, the definition of a critical period which might explain intellectual abnormalities in 10-15% of detected hypothyroid children and the importance of autoimmunity in the aetiology and diagnosis of transient congenital hypothyroidism.

A longitudinal study of central and peripheral nerve conduction in hypothyroid rats

In order to detect the dysfunction of central and peripheral nervous systems among rats with varied duration of hypothyroidism and to elucidate the pattern of recovery after thyroxine replacement, a series of BAEP and PNCS (Peripheral Nerve Conduction Study) were conducted and compared with age-matched controls. BAEP and PNCS were performed in three groups of hypothyroid animals 1, 3 and 5 months after thyroidectomy, respectively. Following initial electrophysiological assessment, thyroxine replacement was administered to each group of hypothyroid rats, and BAEP and PNCS were performed at two month intervals, up to two successive normal studies, or six months after the initiation of therapy, whichever came first. For BAEP, prolonged I-V interpeak latency was the most consistent abnormal finding in all groups of hypothyroid rats, and longer hypothyroid states correlated well with more severe central conduction disorder. Nevertheless, these abnormalities usually returned to normal after thyroxine replacement if the duration of hypothyroidism was less than 5 months. Regarding PNCS, all groups of thyroidectomized rats showed normal conduction before and after thyroxine therapy. The present study indicates that, in rats: (1) the peripheral nervous system seems to be more resistant to hypothyroidism than the central nervous system, or (2) the pathogenesis of central and peripheral nerve dysfunction in hypothyroid rats may occur through different mechanisms.

Development of vestibular and auditory function: effects of hypothyroidism and thyroxine replacement therapy on nystagmus and auditory evoked potentials in the pigmented rat

The functional development of semicircular canals and some brainstem structures of the auditory system was followed in parallel with time in control and propylthiouracyl-induced hypothyroid pigmented rats by respective recording of postrotatory nystagmus response and auditory evoked brainstem potentials, with the aim of discovering the timing of permanent alterations of these responses in congenital hypothyroidism. A group of hypothyroid rats which under went thyroxine-replacement therapy from postnatal day 12 onward was also included in our studies to corroborate the involvement of thyroid hormones in these effects. Postrotatory nystagmus and auditory evoked responses were absent in congenital hypothyroid rats. In the thyroxine-replaced group postrotatory nystagmus values showed no differences from the control group from postnatal day 28 onward. Auditory evoked potentials in thyroxine-replaced animals could not be elicited at 30 dB, but by increasing the intensity of stimulus to 70 dB, values of latencies of the four waves composing the response were indistinguishable from controls from postnatal day 39 and thereafter. These results show that hypothyroidism affects both semicircular canal and auditory function, the latter more severely than the former, but that these effects can be prevented when thyroxine replacement treatment is started in early stages of postnatal development.

Thyroid hormone receptor beta is essential for development of auditory function

Congenital thyroid disorders are often associated with profound deafness, indicating a requirement for thyroid hormone (T3) and its receptors in the development of hearing. Two T3 receptor genes, Tr alpha and Tr beta are differentially expressed, although in overlapping patterns, during development. Thus, the extent to which they mediate unique or redundant functions is unclear. We demonstrate that Tr beta-deficient (Thrb-/-) mice exhibit a permanent deficit in auditory function across a wide range of frequencies, although they show no other overt neurological defects. The auditory-evoked brainstem response (ABR) in Thrb-/- mice, although greatly diminished, displayed normal waveforms, which suggested that the primary defect resides in the cochlea. Although hypothyroidism causes cochlear malformation, there was no evidence of this in Thrb-/- mice. These findings suggest that Tr beta controls the maturation of auditory function but not morphogenesis of the cochlea. Thrb-/- mice provide a model for the human endocrine disorder of resistance to thyroid hormone (RTH), which is typically associated with dominant mutations in Tr beta. However, deafness is generally absent in RTH, indicating that dominant and recessive mutations in Tr beta have different consequences on the auditory system. Our results identify Tr beta as an essential transcription factor for auditory development and indicate that distinct Tr genes serve certain unique functions.

Long-term sequelae of hearing impairment in congenital hypothyroidism

Hearing loss and its functional consequences were evaluated retrospectively in children with congenital hypothyroidism. From a cohort of 101 children followed longitudinally to evaluate newborn screening, 75 with previous hearing tests were studied. Fifteen (20%) were found to have hearing problems. Of these, nine had unilateral or sensorineural loss mostly at high frequencies, five had a conductive loss, and one had both problems. Hearing impaired children differed from children with normal hearing in age of treatment onset (22 vs 14 days) but not disease severity or duration. A comparison of language and auditory processing skills at ages 3, 5, and 7 years revealed that early speech was delayed in hearing impaired children, whereas deficits persisted in later receptive language and auditory discrimination skills. Comparing hearing impaired children and children with normal hearing with matched control subjects at grade 3 showed that hearing impaired children were poorer readers because of less adequate phonologic processing skills.

Audiological assessment of eleven congenital hypothyroid infants before and after treatment

Eleven congenital hypothyroid newborns were consecutively referred for audiological evaluation before any treatment. Evoked otoacoustic emissions were present bilaterally in 9 newborns. These results suggest the normality outer hair cells and contrast with results obtained in experimental studies. The auditory brainstem responses in 6 newborns showed a prolonged wave I, as previously reported by other authors. At 9 to 12 months of age, all children treated had evoked otoacoustic emissions in at least one ear. In 9 cases, the auditory threshold was normal (equal to or below 20 dB) as determined by behavioural tests.

Somatosensory evoked potentials and auditory brain-stem responses in congenital hypothyroidism. II. A cross-sectional study in childhood. Correlations with hormonal levels and developmental quotients

We report the results of somatosensory evoked potentials (SEPs) and auditory brain-stem responses (ABRs) done in 48 congenital hypothyroid (CH) treated children, early detected and aged 18 months, 3 and 5-9 years of age. We report also the results of SEP and ABR done in 9 3-year-old CH children, before and after a 1 month therapy interruption to reassess the thyroid status. The more frequent abnormalities were increased wave I latencies for 18 month and 3-year-old CH children. In some children, these increases were not associated with signs of otitis media. No significant difference was seen between CH children and controls for SEP latencies and ABR and SEP interpeak latencies (IPLs). Similarly, no significant difference was seen between the two recording sessions in the 9 3-year-old CH children for ABR and SEP. On an individual basis, we observed abnormalities of central conduction time for ABR and SEP in several CH children. Moreover, significant partial correlations were found between ABR and SEP IPLs and thyroxine (T4) serum levels at diagnosis and thyrotropin (TSH) serum levels at the time of recording. There was also a significant partial correlation between N19-P22 IPLs and the practical reasoning scale of the Griffiths test considering the whole group of CH children. These results indicate that SEP might eventually be used to detect CH children at risk of presenting developmental abnormalities.