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Journal of Public Health Advance Access originally published online on June 28, 2005
Journal of Public Health 2005 27(3):298-302; doi:10.1093/pubmed/fdi037
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© The Author 2005, Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved.

Observed and expected prevalence of permanent childhood hearing impairment in Oldham



Julie Mytton
Julie Mytton, Specialist Registrar in Public Health, University of the West of England, Bristol, Glenside Campus, Rm 2E16, Blackberry Hill, Stapleton, Bristol BS16 1DD


Ian Mackenzie
Ian Mackenzie, Consultant Audiological Physician, Oldham Primary Care Trust, Cannon Street Clinic, Cannon Street, Oldham OL9 6EP

Address correspondence to Dr Julie Mytton. E-mail: julie.mytton{at}uwe.ac.uk

Background A perceived high prevalence of permanent childhood hearing impairment in Oldham, particularly in the Asian community, caused concern during the local implementation of the Newborn Hearing Screening Programme.

Methods A retrospective cohort study of cases with dates of birth between 1 January 1986 and 31 May 2003 was undertaken to describe local epidemiology and establish the observed prevalence rate. Expected prevalence was determined by application of published national rates to the susceptible Oldham population.

Results The study identified 132 children in Oldham meeting the case definition. The prevalence of permanent childhood hearing impairment in the non-Asian community (1.34/1000 live births) was equal to published national rates (1.33/1000 live births), but that in the Asian community (4.64/1000 live births) indicated a relative risk of 3.5. Differences in prevalence between observed and expected rates was greater than would have occurred by chance (p<0.001).

Conclusion The clinical suspicion of a raised local prevalence of permanent childhood hearing impairment in Oldham was confirmed. The importance of using locally derived data when implementing national policy is emphasized.

Keywords: permanent childhood hearing impairment, prevalence, hearing screening


   Introduction
TOP
 Introduction
Method
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
The early identification of children with hearing loss allows the introduction of interventions to enable early acquisition of communication skills. Interventions within the first year of life result in a larger vocabulary, and better expressive and receptive language skills than interventions commencing later.1–3 In 1998 the National Screening Committee recommended a national newborn hearing screening programme (NHSP) based on a review of the role of neonatal hearing screening to identify congenital hearing impairment.1 This programme was designed to replace the infant distraction test of hearing, which had been determined to have poor sensitivity1 and risked missed children being diagnosed late. Responsibility for coordinating implementation of the new programme was given to the Medical Research Council’s Institute for Hearing Research. Oldham, Bury and Rochdale Primary Care Trusts (PCTs) became one of 17 second-wave implementation sites for newborn hearing screening in 2002.

1 Oldham is a metropolitan borough of Manchester, England. The population of 217273 has been relatively stable over the last decade with respect to size but not composition.4 Since 1991 the ethnic minority population has increased from 8.7 per cent to 13.9 per cent, with the main ethnic minority groups being Pakistani (6.3 per cent) and Bangladeshi (4.5 per cent) residents. The 2001 Census indicates that the proportion of the population in Oldham that is Asian (i.e. Indian, Pakistani or Bangladeshi) is almost three times that for England (11.9 per cent versus 4.6 per cent). Of all local authorities in England, Oldham ranks 21st with respect to the percentage of all Asian ethnic groups in the authority population. The Borough Council’s population projections5 predict that by 2011 the Pakistani population will reach 10.0 per cent and the Bangladeshi population 7.0 per cent of the total Oldham population. The age structure of the Oldham population is similar overall to that of England and Wales, although the Asian ethnic groups are younger, with larger proportions of children and women of childbearing age.

There was strong local clinical suspicion that the number of hearing impaired children in Oldham exceeded national estimates of prevalence of permanent childhood hearing impairment (PCHI). The risk was considered to be particularly high in Asian families, although there were no data to support these impressions. In the light of these concerns and the local introduction of the newborn hearing screening programme, a review of the local epidemiology of childhood hearing impairment was undertaken, to clarify whether the clinical impression of increased prevalence was true.


   Method
TOP
 Introduction
 Method
Results
 Discussion
 Conclusions
 Acknowledgements
 References
 
Definition of permanent childhood hearing impairment
PCHI is classified either by the severity or the causation of the hearing loss. Severity of hearing loss is established using a variety of techniques. In neonates and the very young, objective tests (otoacoustic emissions and the auditory brainstem response) establish hearing levels. In older infants and preschool children, behavioural tests (distraction testing and visual reinforcement audiometry) are used. Prior to school entry, true hearing thresholds can be established in decibels hearing level (dB HL) using pure tone audiometry. The average of hearing thresholds across a range of frequencies in the better hearing ear determines the severity of the hearing loss. The severity is usually categorised as ‘mild’, ‘moderate’, ‘severe’ or ‘profound’. Different authors allocate different ranges of dB HL to each category, and determine thresholds across different ranges of frequencies, limiting the comparability and generalizability of published reports.

The cause of PCHI is classified into Sensorineural (failure of the sensory or neural mechanism of hearing), Conductive (failure of transmission of sound to the cochlea), Mixed or Non-organic. Within the first three categories a distinction is made between congenital (prenatal) and acquired (postnatal) causes. Sensorineural hearing loss can also occur as a consequence of perinatal exposures. Investigation to establish the cause of a child’s hearing impairment may be limited by poor adherence to local protocols, lack of local facilities or parents declining investigation, such that frequently no cause is identified. Failing to take into account children labelled as having acquired disease, risks missing cases of congenital impairment incorrectly classified as acquired.

A decision was therefore taken to identify the total prevalence of PCHI in Oldham, and to distinguish between congenital and acquired cases where possible. In the absence of either an identifiable exogenous cause of the impairment or a confirmed late onset, a case was considered congenital. This definition by exclusion was published by Fortnum6 and its use enabled comparison of local findings with published rates. For the same reason, Fortnum’s classification of severity of hearing impairment6 was chosen, i.e. mild ≤39 dB HL, moderate=40–69 dB HL, severe=70–94 dB HL and profound ≥95 dB HL. A case of permanent childhood hearing impairment was therefore defined as a child with bilateral hearing impairment of ≥40 dB HL in the better hearing ear, as determined by audiological assessment of hearing thresholds across the frequencies 0.5, 1, 2 and 4kHz, and resident in Oldham at the time of the study.

Estimation of expected cases
A literature review was conducted to identify reports of the prevalence of PCHI and congenital hearing impairment in the UK. Fourteen reports were identified6–19 including three reporting prevalence in Asian communities.14,17,18 The two largest studies were conducted by Fortnum and colleagues.6,12,19 The prevalence estimates from Fortnum’s Trent Regional Study6 were used by the Institute of Hearing Research to calculate the number of neonates that could potentially be detected in an area about to commence newborn hearing screening. Therefore this rate was applied to the number of children in Oldham susceptible to hearing impairment to calculate the expected numbers of cases.

Identification of observed cases
The database of the local paediatric audiology clinic was chosen as the primary source for identification of the observed number of cases of PCHI. The Child Health database at the PCT was used as a secondary source for missing data, and the records of the Preschool Teacher for the Hearing Impaired (PTHI) at the local Education department were used as a validation source. An audit of the clinical records of a retrospective cohort of children born between 1 January 1986 and 31 May 2003 was undertaken to establish the number of observed cases. A data extraction sheet was designed and piloted. Data were collected anonymously from all available records meeting the case definition. An assessment of population movements into and out of Oldham was made using published reports.5 SPSS software was used for analysis.

Due to poor recording of ethnicity in the clinical records of cases, an ethnic group code was allocated by the manager of the Ethnic Care Service, Pennine Acute Hospitals NHS Trust, based on the surname and postcode of each case. The manager was unaware of the nature of the study for which codes were being allocated. Cases not allocated a minority ethnic group code were considered White British by default. Previous validation20 of this method of ethnic group allocation indicated an error rate of 0.84 per cent, suggesting that up to two cases may have been inappropriately assigned ethnic groups in this study.

Use of the term ‘prevalence’
A prevalence rate is ‘the number of persons who have a disease at a particular time (or during a particular period) divided by the population at risk of having the disease at this point in time or midway through the period’.21 As the diagnosis of PCHI is a relatively rare event, the numerator is usually the number of cases accumulating over several years. This can create difficulty in defining an accurate denominator. The majority of published reports of prevalence of hearing impairment use the cumulative number of live births as a proxy measure for the at-risk population at the period midpoint. For comparability, the cumulative number of live births in the Oldham PCT area was therefore used as the denominator.

Birth registration data provided the cumulative number of live births. However, information relating to the ethnicity of live births was not available via this source. The Royal Oldham Hospital (ROH) had collected information on the ethnic group of its maternities from 1991. By using this as a proxy measure for the ethnic group of the child, the ethnic diversity of the live births in Oldham could be estimated. The vast majority of Oldham resident mothers gave birth in the ROH maternity unit; deliveries at home or in the nearest alternative unit during 2002 totalled only 138.


   Results
TOP
 Introduction
 Method
 Results
Discussion
 Conclusions
 Acknowledgements
 References
 
Expected cases
The Office of National Statistics (ONS) birth data for Oldham PCT between 1986 and 2002 indicated the average number of live births to be approximately 3200 per year. Therefore, using Fortnum’s definitions of congenital hearing impairment,6 about four cases of PCHI could be expected to be born each year in the area, most of which would be potentially diagnosable through the NHSP (Table 1).


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  		Table 1 Expected cases of PCHI and congenital hearing impairment in Oldham

 

Observed cases
A total of 132 cases of PCHI ≥40dB HL born between 1 January 1986 and 31 May 2003 were identified from audiology clinic records (85 male and 47 female). Of these, 100 cases were congenital, including one due to rubella and two due to cytomegalovirus infections, and 32 cases were acquired or progressive, of which five were secondary to meningitis.

Of these cases 52.3 per cent came from White ethnic groups, 46.2 per cent from Asian ethnic groups, and 1.6 per cent from other ethnic groups. The proportion of children with hearing impairment from Asian ethnic groups is high compared with the proportion of the population from which they arise as identified through the 1991 and 2001 Census (Table 2).


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  		Table 2 Number and proportion of cases of PCHI from different ethnic groups (January 1986 to May 2003) compared with total population proportions for Oldham, Census 1991 and 2001

 

The severity of PCHI by ethnic group is summarized in Fig. 1. In each category Asian ethnic groups contributed a large proportion of all cases, especially severe and profound. This was predominantly due to cases from the Pakistani community.


Figure 1
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  		Figure 1 Cases of PCHI by severity of hearing loss and ethnic group.

 

One hundred and thirty cases of PCHI were identified during the 17 completed years studied (January 1986 to December 2002 inclusive), an average of 7.65 cases per year. Of these, 98 could be considered congenital in origin, averaging 5.76 congenital cases per year. There were 54448 live births in Oldham during this period. The prevalence of PCHI was therefore 2.39/1000 live births (95 per cent CI 1.98–2.80), and for congenital cases, 1.80 cases/1000 live births (95 per cent CI 1.44–2.16).

Maternal ethnicity data from the Royal Oldham Hospital were available for the period 1991 to 2002, during which time an average of 3172 babies were delivered each year. By using this as a proxy measure for the ethnic group of the child, the prevalence of observed cases of PCHI by ethnic group was calculated (Table 3), indicating a relative risk of hearing impairment in the Asian population of 3.5 times that in the non-Asian population. Due to the relatively small numbers involved it was not felt appropriate to subdivide the Asian and non-Asian ethnic groups further.


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  		Table 3 Comparison of observed prevalence rates of PCHI in Asian and non-Asian ethnic groups, 1991–2002

 

Chi-squared analysis indicates that there is a statistically significant difference in the prevalence of PCHI in the Asian community in Oldham compared with the non-Asian community ({chi}2=36.45, p<0.001). Having been used nationally to predict numbers of affected children potentially diagnosable through the NHSP, the prevalence rate determined by Fortnum in Trent region (1.33/1000 live births) was chosen as a proxy for the UK prevalence. Chi-squared analysis indicated that there is a statistically significant difference in the prevalence of PCHI in Oldham compared with Trent Region ({chi}2 =16.33, p<0.001).


   Discussion
TOP
 Introduction
 Method
 Results
 Discussion
Conclusions
 Acknowledgements
 References
 
The observed period prevalence of PCHI and congenital hearing impairment were higher in Oldham than nationally, and higher in the Asian community than the non-Asian community. The prevalence of PCHI found in the Oldham population of Asian residents is similar to other UK published rates.17

The main risk factors for development of PCHI are admission for ≥48h to a neonatal intensive care unit, a family history of hearing impairment arising in childhood and cranio-facial abnormality. Additional risk factors are infective (e.g. meningitis, rubella, toxoplasmosis), metabolic (e.g. hypoxia, hyperbilirubinaemia) and cultural (practice of consanguineous marriage). One reason why Asian ethnic groups are thought to have higher prevalence rates is the practice of consanguineous marriage.22 Over 30 genes have been mapped for autosomal recessive, non-syndromic hereditary deafness.23 Consanguinity increases the likelihood that two parents may each give a child the same recessive gene inherited from a common ancestor, increasing the chance of expression of inherited hearing impairment.24 The custom of consanguineous marriage is practiced by some sectors of the Oldham Asian community, in particular within the Pakistani community.

The results illustrate the need to consider local factors when implementing a national policy. Had a more sophisticated model for estimating the number of infants potentially diagnosed through the NHSP been available, a more accurate estimate of set-up infrastructure and impact on existing services would have been possible by the Institute for Hearing Research. Such a model could utilize published rates of prevalence in different ethic groups in the UK where these exist.

Methodological issues
Annual numbers of cases are relatively small and therefore likely to vary in frequency by chance. Cases are probably underestimated, as they do not include children that cope functionally in the community despite a hearing loss (and do not, therefore, get referred). Including only cases with an Oldham postcode in the numerator risked excluding cases living in Oldham but attending clinics elsewhere. Enquiries at the nearest alternative clinic identified no further cases.

The disadvantage of using a cumulative number of live births as a denominator is that it fails to account for population movements into and out of the area. Migration is dependent on economic circumstances, legislation governing immigration control, and whether or not British ethnic minority youth continue to seek marriage partners in their country of family origin. Incorporating the migration statistics from the 2001 Census and estimates from the in-country migration that can be monitored through the change of patient registration documented at the National Health Service Central Register in Southport, an estimated 303 Pakistani and Bangladeshi people will migrate into Oldham each year between 1996 and 2011.5 The numbers migrating out are harder to estimate, but as the overall population of Oldham is projected to remain constant or slightly fall, the migration in and out are likely to be similar. The ethnic group of the people migrating out is more likely to be White. Migration into and out of Oldham is unlikely to have a marked impact on the estimated prevalence of PCHI.

Classification of congenital and acquired hearing impairment
Fortnum’s definition of congenital hearing impairment as one of default, i.e. hearing impairment should be considered congenital unless there is evidence of a late onset, progressive or exogenous cause6 was used in this study for comparability. This may risk overestimating the number of congenital cases, e.g. children with Down’s syndrome are classified as congenital hearing impairment even though they are not born deaf, and are therefore not likely to be detected by the NHSP.


   Conclusions
TOP
 Introduction
 Method
 Results
 Discussion
 Conclusions
Acknowledgements
 References
 
This epidemiological study has shown that there are, on average, 7.65 cases of PCHI in Oldham per year (2.39/1000 live births), when national rates would suggest 4.27 cases per year. It has indicated that the prevalence of PCHI in the non-Asian community in Oldham (1.34/1000 live births) is equivalent to published national rates (1.33/1000 live births), but that the relative risk of PCHI in the Asian community (4.64/1000 live births) is 3.5 times that in the non-Asian community, and that these differences are statistically significant, thereby confirming the clinical suspicions of the audiologists. This study has illustrated the importance of using locally derived and locally relevant data when implementing a national policy.


   Acknowledgements
TOP
 Introduction
 Method
 Results
 Discussion
 Conclusions
 Acknowledgements
References
 
We acknowledge the support of the staff at the Cannon Street Clinic, Oldham Primary Care Trust who provided access to the data, in particular to Mr Frank Renzulli, Mrs Barbara Wilson and Mrs Jean Brookes. In addition we thank Mr Julian Bonnebaight and Mrs Andrea Biggs from Pennine Acute Trust for the provision of maternity and ethnicity information, respectively, and Mrs Chris Alvi from the Education Department in Oldham. Dr Jane Rossini provided supervision during the data collection process and Dr Selena Gray provided advice and reviewed drafts of this paper for which we are grateful.


   References
TOP
 Introduction
 Method
 Results
 Discussion
 Conclusions
 Acknowledgements
 References
 

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