Contribution of the N-acetyltransferase 2 polymorphism NAT2*6A to age-related hearing impairment

BACKGROUND

Age-related hearing impairment (ARHI) is the most common sensory impairment in older people, affecting 50% of those aged 80 years. The proportion of older people is increasing in the general population, and as a consequence, the number of people affected with ARHI is growing. ARHI is a complex disorder, with both environmental and genetic factors contributing to the disease. The first studies to elucidate these genetic factors were recently performed, resulting in the identification of the first two susceptibility genes for ARHI, NAT2 and KCNQ4.

METHODS

In the present study, the association between ARHI and polymorphisms in genes that contribute to the defence against reactive oxygen species, including GSTT1, GSTM1 and NAT2, was tested. Samples originated from seven different countries and were combined into two test population samples, the general European population and the Finnish population. Two distinct phenotypes for ARHI were studied, Z(low) and Z(high), representing hearing in the low and high frequencies, respectively. Statistical analysis was performed for single polymorphisms (GSTM1, GSTT1, NAT2*5A, NAT2*6A, and NAT2*7A), haplotypes, and gene-environment and gene-gene interactions.

RESULTS

We found an association between ARHI and GSTT1 and GSTM1 in the Finnish population sample, and with NAT2*6A in the general European population sample. The latter finding replicates previously published data.

CONCLUSION

As replication is considered the ultimate proof of true associations in the study of complex disorders, this study provides further support for the involvement of NAT2*6A in ARHI.

Familial aggregation of tinnitus: a European multicentre study

INTRODUCTION AND AIM

Tinnitus is a common condition affecting approximately 20% of the older population. There is increasing evidence that changes in the central auditory system following cochlear malfunctioning are responsible for tinnitus. To date, few investigators have studied the influence of genetic factors on tinnitus. The present report investigates the presence of a familial effect in tinnitus subjects.

METHODS

In a European multicentre study, 198 families were recruited in seven European countries. Each family had at least 3 siblings. Subjects were screened for causes of hearing loss other than presbyacusis by clinical examination and a questionnaire. The presence of tinnitus was evaluated with the question "Nowadays, do you ever get noises in your head or ear (tinnitus) which usually last longer than five minutes". Familial aggregation was tested using three methods: a mixed model approach, calculating familial correlations, and estimating the risk of a subject having tinnitus if the disorder is present in another family member.

RESULTS

All methods demonstrated a significant familial effect for tinnitus. The effect persisted after correction for the effect of other risk factors such as hearing loss, gender and age. The size of the familial effect is smaller than that for age-related hearing impairment, with a familial correlation of 0.15.

CONCLUSION

The presence of a familial effect for tinnitus opens the door to specific studies that can determine whether this effect is due to a shared familial environment or the involvement of genetic factors. Subsequent association studies may result in the identification of the factors responsible. In addition, more emphasis should be placed on the effect of role models in the treatment of tinnitus.

Working memory and novel word learning in children with hearing impairment and children with specific language impairment

BACKGROUND

Working memory is considered to influence a range of linguistic skills, i.e. vocabulary acquisition, sentence comprehension and reading. Several studies have pointed to limitations of working memory in children with specific language impairment. Few studies, however, have explored the role of working memory for language deficits in children with hearing impairment.

AIMS

The first aim was to compare children with mild-to-moderate bilateral sensorineural hearing impairment, children with a preschool diagnosis of specific language impairment and children with normal language development, aged 9-12 years, for language and working memory. The special focus was on the role of working memory in learning new words for primary school age children.

METHODS & PROCEDURES

The assessment of working memory included tests of phonological short-term memory and complex working memory. Novel word learning was assessed according to the methods of. In addition, a range of language tests was used to assess language comprehension, output phonology and reading.

OUTCOMES & RESULTS

Children with hearing impairment performed significantly better than children with a preschool diagnosis of specific language impairment on tasks assessing novel word learning, complex working memory, sentence comprehension and reading accuracy. No significant correlation was found between phonological short-term memory and novel word learning in any group. The best predictor of novel word learning in children with specific language impairment and in children with hearing impairment was complex working memory. Furthermore, there was a close relationship between complex working memory and language in children with a preschool diagnosis of specific language impairment but not in children with hearing impairment.

CONCLUSIONS

Complex working memory seems to play a significant role in vocabulary acquisition in primary school age children. The interpretation is that the results support theories suggesting a weakened influence of phonological short-term memory on novel word learning in school age children.

Linguistic and motor abilities of low-birthweight children as assessed by parents and teachers at 8 years of age

Linguistic and motor abilities among low-birthweight 8-y-old children in the northern Finland Birth Cohort for 1985-1986 (n = 9322) were studied using parental and teacher evaluations. The parents of 8370 (90%) and teachers of 8525 (92%) children returned a mailed questionnaire concerning the children's speech, language, learning and motor abilities. Low-birthweight (LBW, < 2500 g) children (n = 279) appeared to have experienced more difficulties than normal-birthweight (NBW, > or = 2500 g) children (n = 8091). The parents evaluated the LBW boys to be the poorest in linguistic and motor skills compared with the other boys or any of the groups of girls. They are therefore presumably at risk of having problems at school, which was confirmed by the teachers' reports. There was also a clear relationship between speech/linguistic and motor disabilities.

CONCLUSION

Multivariate logistic regression analyses showed that the lower birthweight and some sociodemographic factors, for example the mother's age being between 20 and 24 y, having more than four children in the family, a reconstructed family, as well as hearing impairment and male gender were the most important determinants of poor speech and language abilities at 8 y of age, with and without adjustment for neonatal risk factors (asphyxia, convulsions, respiratory distress syndrome, bronchopulmonary dysplasia, patent ductus arteriosus). Smallness for gestational age was also a risk factor for poor speech and language skills. Preterm birth was associated with poor skills only after adjustment for the neonatal risk factor. The parental and teacher evaluations were concordant concerning the LBW children's outcome.

Can the impact of hearing impairments be modelled?

The effects of two simulated hearing impairments (presbyacusis) on speech perception were investigated in two groups of normally hearing subjects (30 and 48 subjects, respectively). Eight patients with presbyacusis with similar hearing impairment to those in the latter simulation served as the clinical material. Word recognition scores were measured in quiet and in cocktail party noise (S/N + 18 dB and + 12 dB). The recognition scores deteriorated in the less favourable listening conditions. Somewhat surprisingly, the hearing impaired subjects showed better word recognition scores in quiet and especially in moderate noise. Subjective complaints on speech perception difficulties in noise on a visual analogue scale behaved quite logically; the hearing impaired subjects' subjective experiences were no milder than those of the normally hearing during the simulation. The better results in word recognition shown by the hearing aid candidates may be explained by habituation to the hearing impairment.

Hearing and occupation

Not only does the environment play a role as a source of risk factors for a hearing impairment, but a hearing impairment itself can adversely affect interaction with family members, workmates and friends, thus reducing social well-being. The number of work-related hearing impairments has been decreasing for last five years, but noise-induced sensorineural hearing loss is still the second most common work-related disease in Finland. The financial burden related to occupational hearing impairments includes costs of compensation, salaries of screening personnel, equipment, maintenance costs, costs resulting from loss of work for the employer and referrals to specialist clinics etc, which until now have not been calculated in Finland. Numerous questions still remain to be answered regarding the association of age, socioacousis, occupation and leisure activities with the development of sensorineural hearing impairment. Can hearing impairment acquired in childhood or in early adolescence predict the development of occupational hearing loss? What is the interactive role of such factors as ageing, chemicals, diet, environmental noise, genetic susceptibility and the individual's other diseases in the development of noise-induced hearing impairment?

Effect of hearing impairment on educational outcomes and employment up to the age of 25 years in northern Finland

The association between hearing impairment in adolescence and school performance and the outcome of education was studied among 25-year-old subjects followed since pregnancy in the Northern Finland birth cohort. The series, 395 subjects with abnormal hearing and 977 randomly selected controls, was based on a questionnaire on hearing and school achievement sent to 11780 members of the cohort alive at the age of 14 years, and on audiometric screening test requested from health centers. Hearing loss was defined as 'clinically significant' if the pure tone average (PTA; mean of the thresholds at 0.5, 1 and 2 kHz) exceeded 25 dB in the better ear; a threshold of > or = 30 dB at 4 kHz and a PTA of < or = 25 dB as '4 kHz loss'; and as 'slightly abnormal' if any of the thresholds exceeded 20 dB at any frequency and the case did not belong to the above two categories. The more severe the hearing impairment, the poorer was the child's performance at elementary school. Those with normal hearing and those with a slightly abnormal or 4 kHz loss were equally often accepted for intermediate education (88%), while those with a clinically significant loss had the lowest acceptance figures (64%). When adjusting for neurological and social confounders, excluding mental disability, the risk of not qualifying from intermediate or higher education at all was twice as high among those with a clinically significant loss as among the controls (OR 2.1, 95% CI 1.13-3.8), and was still elevated after adjustment had been made for all the relevant perinatal, neurological and social factors (OR 1.9, 95% CI 1.02-3.6). 14% of those with a clinically significant hearing loss, 9% of the subjects with a 4 kHz loss and 7% of those with normal hearing were unemployed at the age of 25 years. Hearing impairment appears to have effects on both the outcome of education and employment status.

Prevalence figures for mild hearing impairments cannot be based on clinical data

The target population of the present study consisted of a 1-year (July 1985-June 1986) birth cohort from northern Finland. The prevalence of even slight hearing impairments (any threshold from 0.5 to 4 kHz > or = 25 db) at the age of 7 years was investigated among those 8,713 children still living in the area. The subjects for clinical audiometry were obtained in two ways. First, the standard clinical practice brought about 541 children, either with non-confirming results from their child welfare clinic screenings, suspected by their parents or already diagnosed as hearing impaired at a hospital. Secondly, in addition to this group, a random sample of 1,009 children, out of the 8,172 children not suspected, were also invited for audiometry. Of the clinical material, 101 children out of the 438 investigated were found to have impaired hearing according to the above criteria, and another 27 children out of the 789 investigated were obtained from the random sample. The estimated over-all prevalence of hearing impairments, even with the slight ones included, turned out to be 3.9% (95% confidence interval, CI, 2.7-5.7). Only 32% of the hearing impairments could be obtained according to the standard clinical policy! In conclusion, one cannot rely on clinical data when calculating prevalence figures for mild hearing impairments.

Hearing in 55 to 75 year old people in northern Finland--a comparison of two classifications of hearing impairment

A total of 5,400 persons from different age groups was randomly selected from the population register for the study, and of the 1,620 55 to 75 year old persons invited, 1,233 attended the check-ups (76.1%). Air conduction pure tone thresholds were measured at the frequencies of 0.25, 0.5, 1, 2, 3, 4, 6 and 8 kHz. The findings were interpreted according to both the EU and WHO classifications. According to the WHO classification, 1,046 subjects (85.0%) had normal hearing and 124 persons (10.1%) had a mild hearing impairment, 44 (3.6%) a moderate, 13 (1.1%) a severe and 3 (0.2%) had a profound hearing impairment. According to the EU classification, 758 subjects (61.6%) were found to have normal hearing, a mild impairment was observed in 363 persons (29.5%), a moderate hearing impairment in 96 (7.8%) and a severe impairment in 13 (1.1%) persons. None had a profound hearing impairment. The difference between these classifications is remarkable as is evident in even this small study. A thorough discussion on the role of different gradings for hearing is needed.

Otitis media and long-term follow-up of hearing

The present material is based on a birth cohort of 2,512 children followed up from the antenatal period. Data on middle ear infections were collected until the children were 7 years. At the age of 7, audiometry was performed on 298 of them. For reasons other than middle ear infection, 32 children were excluded and the final material comprised 266 children. Only those children with clear differences in their history for otitis media were analysed. Although the pure tone averages (0.5-3 kHz) showed no major changes, different mean air-conduction (AC) thresholds were associated with different histories of otitis media. The 35 healthy children (with no otitis media episodes until 2 years of age) showed the best mean AC thresholds. The 51 children with recurrent acute otitis media (RAOM; > or = 4 episodes of acute otitis media) showed worse thresholds at the high frequencies and those 13 who had had secretory otitis media (SOM) at all frequencies. The children with a history of otitis media, either RAOM or SOM, more often had hearing thresholds exceeding 20 dB. Our results might indicate inner ear involvement in these long-term follow-up hearing thresholds.