© Borgis - New Medicine 3/1999, s. 56-58
Elżbieta Reroń1, Halina Dziatkowiak2, Paweł Stręk1, Małgorzata Kumorowicz-Kopiec2, Joanna Kazanecka2, Ryszard Ratajczak2
The influence of congenital hypothyroidism on the hearing organ in children
1 Jagiellonian University, Faculty of Medicine, E.N.T. Department, Cracow Jagiellonian University, Polish-American Children´s Hospital
Head: Prof. Eugeniusz Olszewski, M.D.
2 Department of Endocrinology, Cracow
Head: Prof. Hanna Dziatkowiak, M.D.
Summary
The thyroid hormones are the fundamental factor which controls metabolic processes, and determines physical and neuropsychological development from the moment of conception through out the whole life of an individual. Moreover, they have a significant influence on the development of the central nervous system and also, as experiments on animals have shown, on the maturation of the structures of the internal ear. Therefore, an insufficiency of the thyroid hormones can increase the risk of hearing loss in children.
The studies included 75 children aged between 3 and 16 years treated in the Endocrinological Out-Patient Clinic of the Polish-American Children´s University Hospital, in Cracow.
Each child underwent hearing assessment with the use of subjective tests, such as pure tone and verbal audiometry (if possible) and objective tests, such as registration of acoustic brain stem responses (ABR), evoked otoacustic emissions (EOAE) and tympanometry.
The studies showed the correct hearing threshold in 119 ears (79.4%) but in the remaining 31 ears (20.6%) a slight conductive and sensory-nervous hearing loss.
The results of audiometric examinations of children with hearing impairment were correlated with the intensity of hearing loss.
INTRODUCTION
The relationship between hearing loss and hormonal secretion by the thyroid was observed a long time ago, not only in patients with Pendred syndrome (5, 6, 7, 13) or endemic cretinism (9), but also in adults and children with congenital or acquired hypothyroidism (2). Numerous experiments on animals (chickens, rodents and monkeys) have shown that thyroid hormones play an essential role in maturation of the structures of the internal ear (14, 15, 16, 18, 19).
Studies by Meyerhof (16) on guinea pigs and by Uziel et al. (18) on rats, in which hypothyroidism was triggered with radioactive iodine131 or propylthiouracyl within a few dozen days after the birth, showed profound deformities and significant immaturity of the sensory and supportive cells in the organ of Corti - the lesions responsible for hearing loss.
Congenital hypothyroidism increases the risk of hearing loss in children. Its incidence in population a child is 1:3500 to 1:4000. Depending on the functional condition of the mother and the degree of thyroid insufficiency in the foetus, a newborn with congenital hypothyroidism can present a range of clinical manifestations from symptomatic to asymptomatic form.
AIM OF STUDY
A comprehensive, retrospective audiological evaluation of the hearing organ in children with congenital hypothyroidism (CH) was carried out to:
1. Evaluate the hearing acuity in children,
2. Localise the impairment site in cases of hearing loss.
MATERIAL AND METHODS
The studies included 75 children aged from 3 to 16 years (mean age 7.4 years) treated in the Endocrinological Out-patient Clinic of the Polish-American Children´s University Hospital. The control group comprised 30 otologically healthy children of the same age. Audiometric tests were performed in the Audiology Laboratory of the ENT Department, Jagiellonian University Hospital in Cracow.
The comprehensive evaluation of the hearing organ in each of the children was carried out using subjective audiometric tests, such as pure tone audiometry and objective tests, such as (1) registration of acoustic brain stem responses (ABR) (if necessary), (2) registration of evoked otoacoustic emission (EOAE) and (3) tympanometric tests. In pure tone audiometry, hearing acuity was assessed using pure tone average (PTA).
According to the accepted classification (1), hearing loss is determined by a hearing threshold higher than 20 dB for one or both ears. Hearing loss is assessed as slight when the threshold value ranges from 21 to 40 dB and moderate when the threshold value is between 41 and 55 dB.
RESULTS
Analysis of the average values of the hearing threshold using PTA showed that in 119 ears (79.45) hearing was within the norm, and a slight hearing impairment of 2140dB was observed in 31 ears (20.6%) (Tab. 1).
Table 1. Hearing threshold in various types of tympanometry.
| | PTA | Tympanometry |
| A | B | C |
| Hearing within norm | 119 | 79.40% | 101
67.40% | - | 18
12.00% |
| Hearing loss of 21-40 dB | 31 | 20.60% | 16
10.60% | 12
8.00% | 3
2.00% |
Assessing the functional condition of the middle ear with tympanometry, in a group of 119 ears with a normal hearing threshold, in 101 ears (84.8%) type A tympanograms were obtained, whereas in the remaining 18 ears (15.2%) we obtained type C tympanograms. Of 31 ears with hearing impairment, A type tympanograms were obtained in 16 ears (51.6%), type B in 12 ears (38.7%), and type C in 3 ears (9.7%).
Mean hearing threshold values (Tab. 2) in children with normal hearing did not exceed 10 dB irrespective of the tympanogram type, whereas in children with impaired hearing the threshold values were highest in type A tympanometry (28.12dB) and the lowest in children with type C tympanometry (25.0dB).
In the group of ears with increased hearing threshold, in 16 ears sensory-nervous hearing impairment was observed, located in the cochlea, which was confirmed by the decreased threshold of the stapedius reflex and the lack of evoked otoemission. In the remaing 15 ears, conductive hearing loss was observed, which in 12 ears was caused by otitis media with effusion and in 3 ears by dysfunction of the Eustachian tubes. All children with conductive hearing loss were treated pharmacologically, which resulted in the return of the threshold values to the norm.
Table 2. Mean hearing threshold in various types of tympanometry.
| | PTA | Tympanometry |
| A | B | C |
| Hearing within norm | 119 | 9.37
N = 101 | - | 9.99
N = 18 |
| Hearing loss of 21-40 dB | 31 | 28.12
N = 16 | 27.70
N = 12 | 25.00
N = 3 |
Table 3. EOAE in various types of tympanograms taking hearing threshold into consideration.
| | PTA | EOAE |
| Tympanometry type A | Tympanometry type B | Tympanometry type C |
| + | - | + | - | + | - |
| Hearing within norm | 119 | 101 | - | - | - | 16 | 2 |
| Hearing loss of 21-40 dB | 31 | - | 16 | - | 12 | 2 | 1 |
Table 4. Mean EOAE in various types of tympanograms taking hearing threshold into consideration.
| | EOAE |
| Tympanogram A | Tympanogram B | Tympanogram C |
| % of reproduction | amplitude | % of reproduction | amplitude | % of reproduction | amplitude |
Hearing within norm
N = 119 | 82.06 | 14.47 | - | - | 79.62 | 11.21 |
Hearing loss of 21-40 dB
N = 31 | - | - | - | - | 67.00 | 8.95 |
Control group
N = 60 | 94.80 | 22.80 | - | - | - | - |
Click-evoked otoacoustic emission (Click EOAE) (Tab.3) was noted in 117 ears (98.3%) of the children with normal hearing, a lack being observed bilaterally in one child (1.6%) despite normal threshold values and type C tympanograms. In the group of 31 ears with hearing impairment, otoemission was registered in only 2 ears in one child with a type C tympanogram.
The comparison of mean values of otoemission (Tab.4), that is, the percentage of its reproducibility and size of the amplitude in children with normal hearing and type A tympanogram with otoemission registered in the control group, reveals a considerable decrease in reproducibility and amplitude size in children with congenital hypothyroidism despite slight differences in the hearing threshold between the control group and children with CH, amounting to 5.6dB and 9.37dB, respectively.
DISCUSSION
Impaired function of the nervous system and hearing loss are the components of the clinical manifestation of hypothyroidism. In the material studied, impaired hearing was observed in 20.6% of cases. Hearing loss, except in one child, was bilateral, symmetric and insignificant. Moderate or severe hearing loss was not observed in any of the children. Both conductive and sensory-nervous hearing impairment located in the cochlea occurred with the same frequency.
The incidence of hearing loss in children with congenital hypothyroidism in our studies is in accordance with that observed by Rovet et al. (17), Brucker-Davis et al. (4) and Francis et al. (10, 11) who noted the hearing loss in 18 to 20% of children with CH.
A survey of the literature shows that the rate of hearing loss observed in our studies is significantly different from that reported in the 1970s and 1980s, that is, before the introduction of screening examinations for congenital hypothyroidism in newborns in Cracow in 1985, where this type of examination was initiated. As a country-wide programme, screening was introduced only just in 1994. The incidence of slight, moderate and severe hearing loss, requiring correction with a hearing aid, in the 70s and 80s amounted to 43-45% (3, 12), 50% (8) and 80-85% (2), respectively in children with CH, as observed by Vantt Hoff (20).
CONCLUSIONS
1. A slight hearing loss still accompanies hypothyroidism despite early treatment, however the intensity is considerably lower than that observed before the introduction of newborn screening.
2. Hearing loss found in 20.6% of children with CH is always bilateral, symmetric and of a slight degree.
3. Hearing loss can be both conductive and sensory-nervous with cochlear localisation.
4. Retrospective studies revealed conductive hearing loss, which after pharmacological therapy usually returns to normal.
5. Children with sensory-nervous hearing loss must undergo regular check-up examinations so that in cases of exacerbation of hearing loss, immediate correction with hearing aids can be started.
