KY Chan, F Lee, CB Chow, CC Shek, R Mak

Abstract

From a cohort of 6,127 live births in Princess Margaret Hospital, 309 (5%) were identified as being at risk of hearing impairment. The at risk infants were screened at postconceptual age of 37 weeks or before age of 3 months when they were in stable condition by measuring the auditory brainstem response (ABR). Thirty-five (11%) of 309 at risk infants failed the ABR test. Twenty-six infants were further evaluated by ABR examination, distraction test and impedence test. Twelve were confirmed to have moderate to severe hearing loss. Their risk factors were as follows - 4 babies with Down syndrome, 2 with severe ear malformation, 1 preterm baby, 1 baby with severe asphyxia, 3 with severe neonatal jaundice with serum bilirubin of >= 340 μmol/L and 1 with elevated netromycin level. The overall rate of hearing impariment of at risk neonates was 3.8% and the incidence of moderate to severe deafness for the cohort was 1.9 per 1000 live births. Auditory training was provided to all severely hearing impaired infants before one year of age. Auditory brainstem response remains to be the best currently available method for early detection of hearing loss in at risk neonates.

Introduction

The national incidence of deafness is reported at 1-2 per 1000 live births.¹ The prevalence of moderate to severe hearing impairment among high risk neonates has been estimated between 2-12%.² The early identification and habilitation of infants with severe hearing loss is important for development of language in affected children. In Hong Kong, there is territory wide hearing screening programme for all infants aged more than 4 months in the Maternal and Child Health Centre (MCHC) by distraction tests and the number of infants confirmed to have hearing defect below age of 2 years is very low either because the method is not very sensitive or there is sub-optimal population coverage. Moreover, at risk neonates may have multiple medical problems that delay their visits to MCHC for hearing screening test.

Measurement of the Auditory Brainstem response (ABR) is considered to be the most sensitive method of assessing the auditory acuity of neonates.³ A hearing screening programme for infants at risk of deafness was introduced in the Neonatal Unit of Princess Margaret Hospital on a neonatal cohort born between January 1993 to June 1994. We report here the results of our screening.

Subjects and Methods

From January 1993 to June 1994, all newborns delivered at Princess Margaret Hospital formed the cohort for infant hearing screening programme. Infants at risk for congenital or early onset deafness will be recruited for screening for hearing. The American Joint Committee on infant hearing at risk registry was used , with some modifications (Table I).

Infants who had one or more risk factors were enrolled to the programme. Medical and nursing staff were taught to identify at risk infants using the risk criteria, Infants not admitted into special care baby unit were identified during the newborn screening examination by medical staff. Each baby was assigned one major risk factor. The Medelac 92A ST10 was used for testing the auditory brainstem response (ABR). Where possible neonates were tested at a post-conceptional age of 37 weeks or before 3 months of age and when they were in a stable condition. Pass criterion for ABR screening was a response from each ear at intensity levels 40dBnHL or less, and normal ABR wave I, V latency.

Comprehensive audiologic evaluation were performed at Child Assessment Centre for infants who failed the screening. The evaluation included ABR, behavioural testing and impedence test. Infants with abnormal hearing response will be assessed by ENT surgeon followed by auditory training. The definitions of hearing loss are as follows: mild, 26-40 dB hearing loss; moderate, 41-70 dB hearing loss; severe, 71-90 dB hearing loss; and profound, > 90 dB hearing loss and high frequency hearing loss means hearing loss at 4000 Hz.

All parents of at risk infants were given education pamphlets (Table II) on early awareness of hearing abnormality. At risk infants with normal screening results will be seen in MCHC for behavioral hearing test. (Table III)

Table II

Table III Protocol for Infant Hearing Screening Programme

Results

From a cohort of 6,127 live births, 309 were identified as being at risk of hearing impairment. Thirty five (11%) of 309 at risk infants had failed ABR test. Further audiological assessment was performed for them by either ABR examination, impedence test or distraction tests. Among 35 babies, 20 were confirmed to have mild to severe hearing loss; 7 babies defaulted follow up tests despite repeated calls to the families; 2 died of sepsis and further tests could not be done; 6 babies showed normal results by ABR and/or distraction tests. The overall false positive rate was 23% and the drop out rate was 20%. (Fig.1)

After comprehensive audiological evaluation, 20 out of the 309 at risk infants confirmed to have mild to severe hearing loss. Their associated risk factors are listed in Table IV. Of the 20 cases of hearing loss 5 were mild, 9 moderate, 3 severe and 3 high frequency hearing loss (Table V). Altogether, there were 12 cases having moderate to severe hearing loss giving a moderate to severe deafness incidence of 1.9 per 1,000 live births.

For the four cases of neonatal meningitis, all were bacterial culture negative and 2 were due to enterovirus infections. Of the 121 cases of neonatal jaundice, 80 had a serum bilirubin level of 340 - 400 μmol/L and 2 had unilateral moderate hearing loss. Twenty-four cases had exchange transfusion performed and 2 had bilateral hearing loss.

Discussion

Significant bilateral sensorineural hearing loss occurs between 1.5 to 3 per thousand livebirths.⁴ Accurate data is not available in Hong Kong. In 1991, 103 out of 128792 children were screened to have hearing impairment at the MCHC giving an incidence of 0.8 per 1,000 live-birth.⁵ The figure is probably under estimated as some high risk cases may have been followed up in hospitals did not attend MCHC. Our figure of 1.9 per 1000 live birth having hearing impairment may be high as the sample is biased being heavily represented with high risk pregnancies. The incidence of deafness in Hong Kong is probably 1-2 per 1,000 live births.

Fig. Outcome of screening 6,127 neonates for deafness

It has been demonstrated that all children with significant hearing loss would benefit from amplification with hearing aid or other devices.⁶ Early intervention, preferably before 6 months improves outcome and acquisition of communication in these hearing-impaired children.⁷ The National Institute of Health, United States recommended that infants in neonatal intensive unit be screened for hearing loss before discharge from the unit and that all infants be screened in the first months of life.⁸ The Joint Committee on Infant Hearing also endorsed the goal of universal detection of infants with hearing loss as early as possible and all infants with hearing loss should be identified before 3 months of age and receive intervention by 6 months of age.⁹

There are definite benefits in early diagnosis of deafness. There are several approaches to hearing screening with different sensitivity and cost. (Table VJ)^10-11 Recently, a National Institutes of Health Consensus Statement recommend screening for hearing impairment of all infants within the first 3 months of life, and preferably before discharge from the hospital newborn nursery.⁸ This was also endorsed by the Joint Committee on Infant Hearing in the 1994 Position Statement.⁹ The statement also recommended maintaining a role for the high risk factors for the early detection of hearing impairment in children.¹² However, objections to this universal screening were raised on grounds of practicability, effectiveness, cost and harm-benefit ratio.¹³ The UK Third Joint Working Party on Child Health Surveillance does not recommend universal neonatal nor 'intermediate' (preschool hearing screen after the age of 7 months) screening at this stage. It recommends the setting up of a well-organized audiology service with a district coordinator, a systemic approach to increasing parental awareness about hearing loss; school entry sweep test of hearing and selective or high risk screening. The group has also identified many hazards and problems of the distraction test and set out prerequsite conditions for high quality cost-effective programme.¹¹

In Hong Kong, hearing screening, which intends to cover all infants, is performed at MCHC using behavioural distraction test at the age of 6 to 9 months. This is followed by a Combined Screening Programme, which covers all Primary 1 student in ordinary schools, identifies children with moderate or severe hearing problems. Of the 103 cases of deafness diagnosed by the MCHC screening programme in 1991, only 34 were diagnosed before 1 year of age and the mean age of diagnosis is 2 to 3 years.⁸ Problems of the programme include high repeat and low coverage rate which is quite similar to problems encountered elsewhere: (1) traditional behavioural audiometry in a 6-month-old infant requires skilled personnel and is time-consuming; (2) unlike newborn testing, the evaluation of older infants requires reasonable access to a testing facility; (3) testing is most difficult in developmentally delayed infants who are at high risk; and (4) some hearing impairments would not be treated until after 1 year of life because of a lack of lead time to implement intervention.⁸ The cost-effectiveness of the programme has never been evaluated but from the above figure it seems to be not very effective in the early detection of hearing problems in infancy.

In our high risk neonatal hearing screening programme, majority of the cases were diagnosed within 6 months and all had intervention started by one year of age. It has been estimated that screening of at risk groups should be possible to detect between 50% and 70% of severely deaf neonates.¹⁴ We were not able to estimate the percentage of deafness detected by our at risk neonatal screening programme but it should be more than 50%. Nor do we have figures on false negative rate as we do not have follow up data on infants who passed ABR. With the high default rate of 20%, it will be very difficult to assess the effectiveness of our programme. However, the overall rate for moderate to severe hearing impairment in our cohort of 3.8% is very comparable to study performed elsewhere.¹⁵ The preliminary data does suggest the introduction of a high risk neonatal hearing screening programme in all hospitals in Hong Kong. More detailed study is required to define the cost-effectiveness of such programme.(Table VI)

The results of the study confirmed ABR with a lower stimulus intensity of 40dBnHL can identify deafness. ABR assesses aspects of the functioning of the auditory system, but not the conscious hearing. The high false positive rate (23%) can be explained by high incidence of transient conductive hearing loss due to middle ear disorder. This is in accordance with the experience of others. In the NICU setting, for every child with significant hearing impairment detected, approximately 6 babies are referred for follow up. In the well-baby nursery, where the prevalence of hearing impairment is far lower, for every child with significant hearing impairment, more than 100 babies are referred. This high referral rate may cause undue parental anxiety.⁸ Thus all infants who fail ABR must be followed up with a detailed battery of auditory tests preferably at about 6 months if possible. The mean age at diagnosis for those identified by the neonatal screening was significantly lower than for those identified by the infant distraction test. Since ABR screening and follow up are expensive and require trained personnel, this method has been applied principally to newborns who are at high risk for hearing impairment (those in the NICU or HRC).

One major difficulty we encountered is the tracking of defaulters who are mainly illegal immigrants from China. The tracking system for this group of children should be improved. With the handing over of the sovereignty of Hong Kong back to China, the problem hopefully will be settled in the near future.

The hearing screening programme for at risk neonates is of significant value and should be recommended in all regional hospitals. There is great need to strengthen the follow up of these at risk infants to minimize default. However, this programme can only identify congenital and early onset deafness. Neonates with the following risk factors including family history of deafness, congenital infection, meningitis, persistent pulmonary hypertension and chromosomal defect may have delayed hearing loss with normal initial ABR response. Audiological reevaluation should be conducted every 6-9 months up to age of 2 years. However, the traditional hearing screening programme, sensitive parental and professional observation should be continued and enforced. Early screening will be of little value if follow up services, such as confirmatory and rehabilitation services are inadequate or poorly coordinated. For efficient and effective follow up service, a central registry with regional coordinator should be established.

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