1 epidemiology of permanent childhood hearing impairment · 1 epidemiology of permanent childhood...

1 Epidemiology of permanent childhood hearing impairment

A. Davis, K. Davis and G. Mencher

INTRODUCTION

Hearing impairment is the most frequent sensory impairment in humans, with signifi cant social and psychological implications. The effect of the impairment will vary from individual to individual due to factors such as severity, age of onset, treatment / management options and the hearing status of their parents. It is likely that the greatest impact of hearing impairment upon a child is on the acquisition of language and development of communication, which in turn can lead to poor literacy skills1,2 and altered long-term employment opportunities.3,4 It is likely that other areas of development will also be affected, for example, mental health,5,6 with one study fi nding 50% of a sample of hearing-impaired 11- to 16-year-olds met diagnostic criteria for a mental illness.7

Despite these diffi culties, it is possible that, given adequate support, their impact may be reduced. For example, language development may be enhanced through the use of language support programmes, and residual hearing may be used effectively through adequate amplifi ca-tion from hearing aids or cochlear implants.4,8 It has long been suspected that earlier diagnosis leads to better adjustment,9 and evidence increasingly shows that a support programme starting in the fi rst few months of life, used in tandem with early identifi cation procedures, is benefi cial for hearing-impaired children and their families (see Davis et al.10 for an overview or Barton et al.11 for an example). Support may be available through educational services, audiology services, social services and mental health services – and this support should be individualised, family-friendly and culturally sensitive.10,12,13

In the light of the impact that permanent childhood hearing impairment (PCHI) can have on children and their families, the importance of epidemiological studies cannot be underesti-mated. Epidemiological studies can provide information concerning the aetiology of hearing impairment and the groups within a population who are most at risk, which can be used to plan primary prevention by modifying relevant risk factors; it can be used to target those most likely to become hearing impaired and help detect them. They can also provide information on the overall prevalence of hearing impairment that can help estimate how many children have PCHI in different areas, helping plan secondary prevention of complications. Demo-graphic and follow-up data can be used to make sure the services on offer are appropriate for users. For meaningful epidemiological studies, hearing impairment needs to be classifi ed. Defi nitions may take into account not only the severity of the hearing impairment, but also the pathology and ontogenesis of the impairment, hence these factors are a major focus of this chapter.

COPYRIG

HTED M

ATERIAL

2 Paediatric Audiological Medicine

DEFINITIONS USED IN EPIDEMIOLOGICAL STUDIES

Epidemiology is the study of how often diseases occur in different groups of people and why. When talking about research into hearing impairment, Sancho et al.14 use the term ‘epidemiol-ogy’ to refer to ‘the study of the distribution and determinants of hearing disorders in a popula-tion, and the application of the knowledge obtained to the prevention and amelioration of hearing problems’. A population study is the primary methodology for gathering information. The word ‘population’ in this case refers to the whole collection of units from which a sample may be drawn, but not necessarily to a population of people. For example, it may be a collec-tion of hearing aid clinics or schools for the deaf. The sample is intended to give results that are representative of the population as a whole. A cohort is that component of a population born during a particular period and identifi ed by period of birth, so that its characteristics (such as prevalence of childhood hearing impairment or age at fi rst hearing aid fi tting) can be ascer-tained as it enters successive time and age periods. If an epidemiological study follows a cohort and studies the group at several different intervals, the project is called a cohort study. A cohort study can be a follow-up study, a prospective study or a longitudinal study. It is essential for understanding change over time and the impact of services.

Another key term associated with epidemiology is incidence. This refers to the number of new instances of a specifi c condition (such as hearing impairment from meningitis) occurring during a certain period in a specifi ed population. The incidence rate is the rate at which this occurs per standard population, for example 10 new cases per year per 100,000 children. The term prevalence is often confused with incidence. However, these are not the same thing. Prevalence is the total number of instances within a given population at a specifi c time in which a specifi c condition (for example, Pendred syndrome) is present. In the case of hearing impairment, prevalence may be described as ‘the proportion of individuals with a defi ned type of hearing impairment in a specifi ed population cohort’.14 Accordingly, the prevalence rate is the number of individuals who have the condition or attribute divided by the population at risk at a point in time.

When attempting a prevalence study, if there are n children with hearing impairment in the study and the whole population is N, then the prevalence rate is (n × 100/N)%. In this case we must be sure that the n hearing-impaired children really come from all the birth cohorts of children represented by the population of N and that there is a coterminosity of n and N in terms of geographical boundaries. It is quite common to either underestimate n (because not all children with a given condition have been found) or to confuse populations (often because of migration of children into or out of particular districts).

THE DIFFICULTIES IN ESTIMATING PREVALENCE

Accurate estimations for the prevalence of childhood hearing impairment worldwide are hin-dered by the great diffi culty in interpreting the data; perhaps leading to the variability in prevalence rates seen from study to study. These variations may be thought of as arising from three factors: how cases of hearing impairment are defi ned; how cases of hearing impairment are found; and the population from which the cases come. The importance of having agreed defi nitions for epidemiological studies, such as the ones outlined in the previous section, can be seen to be of paramount importance. The lack of agreed prevalence rates hinders investiga-tion of possible risk factors and aetiologies, in turn, having implications for the planning of

Epidemiology of permanent childhood hearing impairment 3

service provision. Boxes 1.1 and 1.2 present the commonly used defi nitions for the various types of hearing impairment.

The term ‘deaf’ is generally associated with the most extreme form of hearing impairment, in which there is no response to auditory stimuli in excess of 120–125 dB at any frequency. This condition is practically never seen and is considered very rare. Hearing impairment, on the other hand, primarily refers to a series of descriptive terms that defi ne the decibel level at which an individual responds to sound (see Box 1.2). Hearing impairment is also defi ned by the frequency range the person can hear. That is, a low-frequency range is <500 kHz; a mid-frequency range is 500 to 2,000 kHz; a high-frequency range is 2,000 to 8,000 kHz; and an extended high-frequency range is >8,000 kHz. The pattern of the frequencies is also important with some fairly self-explanatory terms, such as u-shaped, low-frequency ascending, fl at and high-frequency sloping, used as descriptors of the responses plotted on an audiogram.

Box 1.2 Defi nitions of hearing impairment in dB levels.

Type of impairment Defi nition

Average hearing level The level of the thresholds (in dB HL) measured in the better hearing ear at 0.5, 1, 2, 4 kHz

Mild Average hearing level 20–39 dB HLModerate Average hearing level 40–69 dB HLSevere Average hearing level 70–94 dB HLProfound Average hearing level +95 dB HL

Box 1.1 Defi nitions of the various types of hearing impairment.

Type of impairment Defi nition

Sensorineural Related to disease/deformity of the inner ear/cochlear nerve with an air–bone gap less than 15 dB averaged over 0.5, 1 and 2 kHz

Conductive Related to disease or deformity of the outer/middle ears. Audiometrically there are normal bone conduction thresholds (less than 20 dB) and an air–bone gap greater than 15 dB averaged over 0.5, 1 and 2 kHz

Mixed Related to combined involvement of the outer/middle ears and the inner ear/cochlear nerve. Audiometrically greater than 20 dB HL in the bone conduction threshold together with greater than or equal to 15 dB air–bone gap averaged over 0.5, 1 and 2 kHz

Sensory A subdivision of sensorineural related to disease or deformity in the cochlea

Neural A subdivision of sensorineural related to a disease or deformity in the cochlear nerve

Central Sensorineural hearing loss related to a disease or deformity of the central nervous system rostral to the cochlear nerve


Given the variety of types of hearing impairments presented in Box 1.1, it can easily be understood why there may be some confusion when attempting to defi ne prevalence and/or incidence. However, the problem is compounded even further when various generalised cate-gories for the course of the hearing impairment and the pattern of the hearing impairment are taken into consideration. Hearing impairment can be congenital, meaning to be present and detectable using appropriate tests at or very soon after birth, or acquired. However, there can be a difference in the meanings of these terms when considering aetiology as well as prevalence – as the cause of hearing impairment may be present at birth, but problems in hearing appear later in life. Temporary hearing impairment (usually, but not always, a conduc-tive hearing impairment) can be treated and corrected by medical or surgical intervention. Such an impairment is often short-lived and of a mild nature. On the other hand, permanent hearing impairment cannot be readily treated by surgical or medical intervention. Both tem-porary and permanent hearing impairments can be unilateral (one ear only has either a greater than 20 dB hearing impairment through 500, 1,000 and 2,000 kHz or one frequency exceeding 50 dB, with the other ear normal) or bilateral (a greater than 20 dB hearing impairment through 500, 1,000 and 2,000 kHz or one frequency exceeding 50 dB in both ears). A unilateral situa-tion is, of course, asymmetrical. However, in studies of hearing, the term asymmetrical hearing impairment specifi cally refers to a greater than 10 dB difference between the ears in at least two frequencies, with the pure-tone average in the better ear exceeding 20 dB HL. Finally, both temporary and permanent hearing impairments can be progressive – that is, there is a deterioration greater than or equal to 15 dB in the pure-tone average within a 10-year period.

Traditionally, studies have tended to be cross-sectional and based on retrospective ascertain-ment. A selection of these studies is shown in Table 1.1. It can be seen that estimates for prevalence of PCHI vary up to 10-fold (0.58 per 1,000, Baille et al.,15 to 6.59 per 1,000, Parving and Hauch16) depending on defi nition, but most found levels of between 1.1 per 1,000 and 1.7 per 1,000 for their broadest defi nition.

Another method of cross-sectional study uses results from screening. This has the advantage of including cases that have not yet been diagnosed and works best for generating epidemio-logical data when the impairment is mild (and common) or where the whole population is screened. There are three common types of screen for hearing impairment: newborn; infant distraction test; sweep test.23 A ‘sweep’ test asks a child to respond to low-intensity pure tones at three or four set frequencies, and has been done by school nurses and others. It has been used routinely on school entry in the UK since 1955, but protocol and implementation vary around the country, and there have been few attempts at measuring outcomes until recently. The infant distraction test assesses children as young as seven months by testing their behav-ioural response to noise, but it has the potential to miss serious cases, and refer many infants with no hearing problem. Again, despite being routine in the UK, sensitivity, specifi city and outcome were not monitored. In the 1990s, technology became available to provide proxy measures of hearing in even newborn babies. Transient evoked otoacoustic emissions identify the presence or absence of outer hair cell activity from the inner ear, and auditory brainstem response (ABR) identifi es the presence or absence of electrophysiological activity from the early auditory pathway. Automated equipment is now available for each of these tests, which can be used by trained screeners in the NICU, at the mother’s bedside or in the community soon after birth. This has led to programmes of screening either high-risk babies or offering the test universally to every newborn baby. Universal newborn hearing screening (UNHS) has been has been recommended by the Joint Committee on Infant Hearing since 2000,24 and by

Tab

le 1

.1

Sele

ctio

n of

stu

dies

sho

win

g ho

w p

opul

atio

n, d

efi n

ition

of

case

and

met

hod

of a

scer

tain

men

t ca

n af

fect

pre

vale

nce.

Stu

dy

Pop

ula

tion

Defi

nit

ion

of

case

Meth

od

of

dete

ctio

nP

reva

len

ce p

er

1,0

00 c

hild

ren

Thre

shol

d*C

onge

nita

l or

acq

uire

dC

ause

Russ

et

al.

2002

17A

ged

six

livin

g in

Vic

toria

, A

ustr

alia

bor

n 19

89≥4

0 dB

all

with

he

arin

g ai

ds

(incl

udin

g un

ilate

ral)

Con

geni

tal

All

Hea

ring-

aid

clin

ic1.

242.

09

Myt

ton

and

Mac

kenz

ie

2005

18

Varia

ble

age

livin

g in

Old

ham

, U

K bo

rn 1

991–

2002

All

raci

al o

rigin

s

≥40

dBBo

thA

llA

udio

logy

and

ed

ucat

iona

l sou

rces

2.39

Asi

an o

rigin

4.64

Parv

ing

and

Hau

ch

2001

16A

ged

<1–1

0 liv

ing

in

Cop

enha

gen,

Den

mar

k bo

rn

1990

–199

9

>20

dB e

ither

ea

r at

any

fr

eque

ncy

Both

All

Surv

eilla

nce

prog

ram

me

of a

ll he

arin

g im

paire

d

2.91

Age

d 11

–20

livin

g in

C

open

hage

n, D

enm

ark

born

19

80–1

989

6.59

Fort

num

and

Dav

is

1997

19 a

nd

Fort

num

et

al.

2001

20

Age

d 5–

10 li

ving

in T

rent

, U

K,

born

198

5–19

90A

ged

5–10

livi

ng in

UK,

bor

n 19

88–1

993

Age

d 3–

8 liv

ing

in U

K, b

orn

1985

–199

0

≥40

dB

>40

dB

Both

All

Aud

iolo

gy s

ervi

ces

Aud

iolo

gy a

nd

educ

atio

n se

rvic

es

with

adj

ustm

ent

for

unde

r-as

cert

ainm

ent

1.33

1.63

1.44

Baill

e et

al.

1996

15U

p to

age

nin

e, li

ving

in F

ranc

e,

born

197

6–19

85>7

0 dB

Both

All

Adm

inis

trat

ive

depa

rtm

ents

0.58

Mac

And

ie e

t al

. 20

0321

Varia

ble

age

livin

g in

Gla

sgow

, U

K bo

rn 1

985–

1994

≥40

dBBo

thA

llEd

ucat

iona

l aud

iolo

gy

data

base

1.23

Con

geni

tal

1.09

Nek

ahm

et

al.

2001

22Va

riabl

e ag

e liv

ing

in T

yrol

, A

ustr

ia b

orn

1980

–199

4≥4

0 dB

Both

All

Med

ical

rec

ords

1.23

Sens

orin

eura

l1.

11

Thre

shol

d: P

ure

tone

thr

esho

lds

mea

sure

d in

dB

HL

(hea

ring

leve

l). M

ost

stud

ies

aver

aged

acr

oss

thre

shol

ds a

t 0.

5, 1

, 2

and

4 kH

z.Be

tter

ear

unle

ss s

tate

d.


2005, approximately 95% of newborn infants in the United States were screened for hearing loss before they were 1 month old.12 UNHS has been piloted in the UK since 2001 and became standard in England in 2006. Unlike its predecessors, it was accompanied by an IT system and rigorous quality control (see Bamford et al.23 and the programme website http://hearing.screening.nhs.uk).

With the implementation of UNHS in some areas, more data are becoming available for estimating prevalence. Results from studies of UNHS express a ‘rate’ of hearing impairment detected per baby screened. Depending on the coverage the sensitivity of the test and the con-fi dence of the diagnosis, prevalence of congenital hearing impairment can be made with increasing confi dence. Uus and Bamford25 gave the rates for the newborn hearing screening programme (NHSP) in England based on the 21 pilot sites around the UK between February 2002 and June 2004. A total of 169,487 babies were screened and amongst the children referred from the screen, a confi rmed permanent bilateral hearing loss of moderate or greater severity was found in 169. This leads to a rate of 1.00 (95% confi dence interval 0.78–1.22) per 1,000 babies screened. The programme achieved 96% coverage, and 90% of those babies who needed further tests were followed up. Yields of PCHI outside the UK have ranged from 0.68 per 1,000 in Western Australia26 to 4.4 per 1,000 in Jackson, Mississippi, USA.27 Trying to look for true geographical differences is once again diffi cult due to defi nition of a case, which sometimes includes unilateral and mild impairments, and other differences between studies. Some hospitals excluded results from NICU babies when reporting and other hospitals were tertiary referral centres handling very ill babies. Some programmes / studies found that effec-tiveness was limited by poor rates of screening or attending for follow-up. Vohr et al.28 in Rhode Island, US, found that those with traditional Medicaid insurance were less likely to be screened or re-screened, whilst Prince et al.29 in Hawaii found that low birth weight babies and those born to women who had not completed high school were twice as likely not to complete follow-up.

Prevalence of hearing impairment in the UK

Various studies have been carried out in the UK to ascertain accurate prevalence rates; however, there has been considerable disagreement between the rates established. Variation in sample populations, hearing levels included in the study, the fl uctuating numbers of children with hearing impairment, and no easy way of ensuring complete ascertainment of cases were all factors that led to such variation in prevalence fi gures. With no agreement on numbers, there was uncertainty about the extent of the problem that extended into the 1990s. For example, in Nottingham, prevalence of PCHI was estimated at 0.55/1,000 by Pabla et al.30 but 1.2/1,000 by Davis and Wood.31

An extensive study of epidemiology of PCHI was carried out for the Trent Regional Health Authority by Fortnum and Davis.19 The aim was to include all children with a permanent hearing impairment of 40 dB HL average or greater in their better ear, who had been born between 1 January 1985 and 31 December 1993 and were living within the boundary of Trent Regional Health Authority at the time of data collection (June–September 1995). Sources of information included the Education Database, the Community Audiology and Child Health Database, the Neonatal Screening Database, audiology, medical records and hearing aid records. The data collected were divided into two main groups: congenital hearing impairment and acquired hearing impairment. The congenital group consisted of those children presumed to have had a prenatal or perinatal hearing impairment. The acquired


group included those whose hearing impairment came later in life due to disease, progressive hearing impairment or late-onset hearing impairment where there was evidence that the child may have been able to hear at an earlier stage. Prevalence rates of 1.3/1,000 for both acquired and congenital permanent hearing impairment were reported. For congenital hearing impair-ment alone, the prevalence rate was 1.1/1,000. Taking the prevalence estimates derived from the Trent region, it was possible to estimate that there would be approximately 1,000 children with a hearing impairment of at least moderate severity in the UK per annual birth cohort, around 84% having a congenital hearing impairment. These numbers undoubtedly contributed to a decision by the government of the UK to develop a newborn hearing screening programme.

For a more accurate calculation of prevalence across the whole of the United Kingdom, Fortnum et al.20 approached the health professionals and the education professionals responsi-ble for hearing-impaired children around the country, requesting details on every child with PCHI under their care. A total of 486 professionals replied, with over 26,000 sets of details. Many of these overlapped if the child was known to education and health services and the child’s details were provided by both. The fact that there was no total overlap implies that there was some under-ascertainment. This can be adjusted for with a capture–recapture method – thus records for 17,160 children suggested there were around 21,500 children aged 3 to 18 in the UK with a permanent bilateral hearing impairment more than 40 dB. The inclusion of such a large number in the study allowed a more accurate breakdown into subgroups. It was shown that the observed prevalence increased with age until reaching a plateau at age 9, and that this was present at all three severities studied (41–70, 71–95, >95 dB HL). The adjusted prevalence at age 3 was around 1.1 per 1,000, rising to 2.1 per thousand at ages 9–16, a rise of 92%. This signifi cant rise in prevalence during early childhood could be highly relevant for the planning of audiology and support services for secondary prevention of complications of hearing impairment, but this cross-sectional study is not ideal to confi rm changes over time – because the change could arise either from the age of the cohort or the year in which the cohort was born.

Better ideas of change over time come from longitudinal studies, such as those carried out in the East London borough of Waltham Forest.32 The relevant cohorts were born between 1992 and 2000 and numbered around 33,000. The numbers of children with PCHI, the method of identifi cation and audiological data were collected from educational and audiology services. These children had UNHS, some had the infant distraction test, and they all had a school-entry ‘sweep’ screen. Newborn screening identifi ed 1.58 per 1,000 children as having PCHI. More babies with PCHI were later identifi ed due to concerns raised by parents or health visitors before the children were 12 months old in a further 0.24 cases per 1,000. A further 1.30 per 1,000 children were identifi ed as having permanent hearing loss before they entered school at age 5, mainly due to parental concern. Finally, 0.34 per 1,000 were identifi ed by the school-entry screen. This gives a combined total prevalence of 3.47 per 1,000 children by primary school age identifi ed as having PCHI – of which 43% were of a moderate or greater severity bilateral hearing impairment, 35% mild bilateral and 22% unilateral (mild or above). This increase came partly from people moving into the area, but also from children who had not been offered, who had declined or who had failed to complete the screening process. Around 10% of the later identifi ed children had a history of meningitis, 15% a family history of hearing impairment, and 30% had some other developmental abnormality (especially craniofacial). Thus, it seems there is a real increase in prevalence of PCHI as a cohort ages, and therefore a need for services to identify and manage this impairment.


Prevalence of hearing impairment in the United States

Early detection and hearing intervention (EDHI) programmes were legally required in 41 states by 200733 to help improve outcomes for children with hearing impairments. In order to design these programmes and their predecessors, an estimate of prevalence is essential. The Metro-politan Atlanta Developmental Disabilities study was partly set up to help develop methods of surveillance of children with special needs such as hearing impairment. The study collected data on the prevalence of mental retardation, cerebral palsy, hearing loss, vision impairment and epilepsy in children aged 10 years living in fi ve counties in metropolitan Atlanta. Cases were actively sought from records at a number of sources, educational and medical, public and private, to maximise ascertainment. A hearing loss was defi ned as a permanent impairment of 40 dB HL averaged across thresholds at 0.5, 1 and 2 kHz in the better ear. Drews et al.34 report on the prevalence of hearing loss in the cohort born during 1975, 1976 and 1977; who were age 10 in 1985–87. One hundred of the 10-year-old children had been identifi ed and confi rmed with PCHI out of a population of 89,534. This gives a prevalence of 1.1 (CI 0.9–1.4) per 1,000. Van Naarden et al.35 looked at children aged 3 to 10 in 1991–1993, fi nding 411 cases, giving a prevalence of 1.10 (CI 1.00–1.20) per 1,000 children aged 3 to 10. The preva-lence varied with age, being lowest at 0.67 per 1,000 3-year-olds and rising steadily to 1.38 per 1,000 10-year-olds. The latter number shows an apparent increase in prevalence from the previous study of 10-year-olds. Both studies found that about 30% of the children had another disability, the most common being mental retardation. They also found that the prevalence was around 20% higher amongst black residents than white. An accompanying paper36 gives evidence for much of this difference being due to differing birth-weights: more babies with low birth weight are born to black mothers, and their outcome is less favourable than babies of low birth weight born to white mothers.

Another large population study to include childhood hearing impairment was the Third National Health and Nutrition Examination Survey (NHANES III), which used a 40,000-person sample with characteristics representative of the US population as a whole over 1988–1994. Niskar et al.37 report on children aged 6 to 19 who were asked about their hearing status and screened using pure-tone audiometry in a mobile examination centre. Self-report of ‘hearing diffi culties’ (not necessarily permanent) was 34 per 1,000 children. They screened using frequencies representing speech (0.5, 1 and 2 kHz) and at higher frequencies (3, 4 and 6 kHz), and defi ned hearing loss as average thresholds for either frequency above 15 dB HL. A hearing loss in at least one ear was present in 149 per 1,000 children (14.9%), most of which was unilateral and slight in severity, and some of which was likely to have been temporary or fl uctuating. However, by extrapolating from their data, the authors estimate that there are 7 million US children at any one time that may need extra help in the classroom due to a hearing impairment. An accurate count of how many children nationwide are getting extra funding for special education can be obtained from the reports of the Individuals with Disabilities Educa-tion Act Program (IDEA-B). Data for the 2005 report are broken down by disability and age for children aged 3 to 16.38 The number of children aged 3 to 16 for whom special education is funded due to a hearing impairment alone (there will also be children with PCHI in ‘deaf-blind’ or ‘multiple disabilities’ categories) was 70,702 – ranging from 2,174 aged 3 to 6,269 aged 12.

Universal hearing screening was taken up at differing rates across the United States, and a number of groups have published results from the early years of screening in individual hos-pitals and states – a selection of which is shown in Table 1.2. Once again, there is great diffi -


culty comparing between studies, and further diffi culty in generalising from these yields to prevalence across the country since these pioneering programmes were likely to have taken place in large, well-resourced or well-motivated hospitals, many of whom had a large propor-tion of babies in NICU.39 As part of EHDI, data are being collected by the Centers for Disease Control and Prevention (CDC). The annual data for 200540 show that of nearly 4 million births in states who were screening, 91.5% of babies were screened. Unfortunately, of the 64,421 babies referred for further screening or investigation, outcomes are only known for 43.5%, the majority of the rest being lost to follow-up or lost to documentation. The incidence of unilateral or bilateral PCHI of any severity reported to CDC is 0.92 per 1,000 babies screened, but this seems likely to be a vast underestimate of the actual rate.

Norton et al.48 and Johnson et al.49 have both re-screened babies at high risk of hearing impairment to test the sensitivity of newborn screening protocols. They found that although they are very good at detecting hearing loss of moderate severity or above, they miss a large proportion or slight and mild impairments. This must be clear to parents, professionals and those who plan provision for children with hearing loss.

Other ‘developed’ countries

Martin et al.50 performed an ascertainment study of hearing-impaired children in the European Community (nine countries) who were born in 1969, were eight years old at the time of the study and who had a hearing impairment of at least 50 dB HL. They found a prevalence rate of 0.9 per 1,000. They also reported that 29% of the children had additional disabilities. These fi gures agree well with similar studies in the United States.34 More recently, epidemiologists from European countries have compared prevalence of hearing loss from cohorts of children in the 1980s with the data from the Trent study in the UK.19 In Denmark, Davis and Parving51 reported on the prevalence of bilateral sensorineural or mixed PCHI of at least moderate severity (average threshold >40 dB HL), and the prevalence is shown by severity profi le in

Table 1.2 Selected results from US screening programmes showing large variation in defi nition and rates.

Citation Site Defi nitionRate per 1,000 screened

Barsky-Firsker and Sun 199741

Tertiary referral centre, New Jersey

Sensorineural. Bilateral or unilateral. Not including babies from NICU.

2.1

Vohr et al. 199842 Rhode Island (statewide) Sensorineural and permanent conductive

2.0

Mason and Herrmann 199843

University hospital, Honolulu, Hawaii

Bilateral loss requiring amplifi cation

1.4

Finitzo et al. 199844 Texas (multi-site) Detectable permanent. Bilateral or unilateral

3.14

Dalzell et al. 200045 New York (statewide) Bilateral or unilateral 2.0Stewart et al. 200046 Kentucky (multi-site) Sensorineural 2.7Mehl and Thomson

200247Colorado (multi-site) Sensorineural or permanent

conductive. Bilateral1.39

Connolly et al. 200527

Tertiary referral centre, Jackson, Mississippi

Detectable permanent. Bilateral or unilateral

4.4


Table 1.3 where it is compared with fi gures generated from the Trent study.19 Approximately 90% of hearing impairment reported was congenital in both studies. It can be seen that there were signifi cantly more severely and profoundly hearing-impaired children in Denmark than in England. When risk factors were investigated it was found that signifi cantly more congeni-tally hearing-impaired children had an NICU history in England (33%) than in Denmark (17%), whereas more hearing-impaired children had a family history of hearing impairment in Denmark (40%) than in England (27%). Further work by Uus and Davis,52 centred around the same issues in Estonia, reported that the prevalence of hearing impairment in Estonia (1.72 per 1,000) was higher than that of England (1.32 per 1,000) and Denmark (1.45 per 1,000).

Data from universal screening in parts of some countries are also available. Results from groups in Paris53 and Siena54 show rates of bilateral permanent hearing impairment >40 dB of 1.4 and 1.42 per 1,000 babies screened, respectively, which are slightly higher than the rate of 1.0 per 1,000 from the pilot sites in England, despite the fact the Paris study did not include babies from NICU. In Western Australia, the yield was found to be 0.68 per 1,000.26 Out of 28,708 babies screened over 7 months, only nine babies were diagnosed with permanent bilat-eral hearing loss and eight of these had known risk factors for PCHI. This was seen to represent a poor detection rate, and universal screening was subsequently stopped, with a return to tar-geted screening. In Asia, the yield of bilateral PCHI detected per 1,000 children screened was 2.8 in a university hospital in Hong Kong55 and 2.0 in a university hospital in Japan.56 It is not immediately clear whether the difference in yields between countries, has to do with the per-formance of the screening programmes, the prevalence of risk factors or some specifi c envi-ronmental / genetic infl uence on the population. In Taiwan, two studies looked at the feasibility of screening in two environments: a hospital in Taipai57and a community-based screen in Tainan.58 They achieved similar yields of 1.3 and 1.5 confi rmed bilateral cases of hearing loss per 1,000 babies screened, but the lack of babies from NICU and the need for parents to choose to pay for the test makes the fi gures diffi cult to compare with those from England.

It thus may not be possible to generalise the fi ndings of one study regarding prevalence to other geographical areas or, for that matter, to other birth cohorts. For that reason, epidemio-logical studies at the local level should be considered necessary to determine needs when planning for service provision.

Prevalence rates in disadvantaged countries

In order to obtain local data from developing countries, there have been some attempts to screen children, mainly of school age, in their communities, see Table 1.4. These have gener-ally taken the form of pure-tone audiometry with or without otoscopy and tympanometry. Once again, the inconsistencies between the studies makes it diffi cult to compare them, and diffi cult to compare estimated prevalence with estimates from studies in developed countries. Neverthe-

Table 1.3 Prevalence of all (congenital and acquired) permanent sensorineural and mixed childhood hearing impairments per 1,000 children in the birth cohort 1982–1988 for Denmark and children living in Trent Region, England, 1985–1993.

Country 40–130 dB HL 70–130 dB HL 95–130 dB HL

Denmark51 1.45 0.86 0.54England19 1.32 0.59 0.31


Table 1.4 Selected results from child hearing screening studies in disadvantaged countries showing the differences in defi nitions and results.

Study and location Population Defi nition of case Cases per 1,000

Abdel–Rahman et al. 200761

Ismailia, Egypt

Secondary school children

Sensorineural hearing loss ascertained by Rinne and Weber tests

222

Sobhy 199862

Alexandria, EgyptSchool children Excluding wax occlusion and

OMEBilateralAverage thresholds >25 dB

1.17–2.59

Seely et al. 199563

Pangama, Sierra LeoneChildren Bilateral

Average thresholds >40 dB6.5

Olusanya et al. 200064

Lagos, NigeriaSchool children

(mainstream school)

Conductive and sensorineuralUnilateral or bilateral

139

Hatcher et al. 199565

Kiambu, KenyaSchool children Bilateral

Average thresholds >30 dB22

Westerberg et al. 200566

ZimbabwePrimary school

childrenSensorineural onlyUnilateral or bilateralAverage thresholds >30 dB

10

Swart et al. 199567

SwazilandFirst year school

childrenSensorineural onlyBilateral

2.1

Swart et al. 199567

SwazilandFirst year school

childrenMiddle-ear disease with

hearing loss (conductive, mixed or sensorineural)

Unilateral or bilateral

22

Minja et al. 199668

Rural Dar es Salaam, Tanzania

Primary school children

Sensorineural only 141

Minja et al. 199668

Urban Dar es Salaam, Tanzania

Primary school children

Sensorineural only 77

Elahi et al. 199869

Rural areas, PakistanChildren Sensorineural or permanent

conductiveBilateralAverage thresholds >30 dB

39

Rao et al. 200270

Rural south, IndiaFirst year school

childrenSensorineural or mixedUnilateral or bilateralAverage thresholds >30 dB

32

Liu et al. 200171

Sichuan, ChinaChildren <15 y Unilateral or bilateral

Average thresholds >30 dB2.6

Mencher and Madriz Alfaro 200072

Costa Rica

School children BilateralPermanent

1.50–1.63

less, there is a consensus that the levels of PCHI are greater in underdeveloped countries, with Davidson et al.59 estimating that sensorineural loss is twice as common. Evidence also seems to point towards a higher rate of hearing impairment amongst disadvantaged communities in richer countries,18,34 with Niskar et al.37 fi nding that children from families with incomes at or below the national poverty line were signifi cantly more likely to have a hearing impairment when screened. The World Health Organization in its report on chronic diseases60 views the


process from poverty to chronic diseases as ‘interconnected in a vicious cycle’, as poor people have greater exposure to risks and decreased access to health services.

Alberti73 estimated that half of all disabling hearing loss worldwide was preventable by primary means, from vaccination to better protection from noise exposure. Consanguinity is a common risk factor in some communities.69 There also seems to be an increased prevalence of middle-ear disease in disadvantaged communities and this can be aggressive, becoming chronic suppurative otitis media (CSOM) or leading to cholesteatoma.74 The presence of recur-rent or chronic middle-ear disease is highly correlated with a permanent hearing loss in this population because of the reduced access to effective treatment.69,75

There is some impetus for an increased effort of identifi cation of PCHI in developing countries, and this seems to be backed by the opinion of mothers.76 Trials of UNHS at immuni-sation clinics have been undertaken in Nigeria and South Africa and were successful in terms of coverage, but the attendance at follow-up was poor. Swanepoel et al.77 report that out of 68 subjects (14% of screened sample), only 40% returned for the second follow-up and 44% for the third follow-up. Some argue that primary prevention strategies should take priority, as the current high prevalence would overwhelm the capacity for early intervention.78 Others would argue that with facilities available for deaf and hearing-impaired children throughout the world, children worldwide should be identifi ed to take advantage of those facilities.79

RISK FACTORS

Risk in this context refers to an increased probability that an event will occur; in this case, that a child will have a hearing impairment. Factors that increase the likelihood can be non-specifi c, i.e. affecting a whole population but not by much, or can be specifi c to the child. The former is important to know for planning services, and examples might be poverty or being aged less than 9 years old. This section will concentrate more on the latter. Specifi c risk factors – the most notable being a family history of permanent hearing impairment present since childhood in a parent, sibling, grandparent, great-grandparent, aunt, uncle, nephew, niece or cousin or a lengthy stay in NICU – can be used for targeted screening during universal screen-ing. The practice of targeted screening in babies was common between the invention of the technology for newborn screening and the infrastructure being put into place for UNHS. An example of this was in the Redbridge District of London, where there was a targeted newborn hearing screen for 10 years between 1990 and 2000.80 From the 32,890 babies born, 3.5% were identifi ed before discharge from hospital as high-risk using the appropriate JCIH guidelines at the time, and screened using ABR. The yield was 1.6 per 1,000 babies screened for bilateral impairment 40 dB HL (17 children, or 0.52 per 1,000 live births). By the time these children and their peers started primary school, they made up only 40% of all cases of bilateral PCHI 40 dB HL; 18% had risk factors at birth but had not been screened and 42% had no obvious risk factors. This is a compelling argument for universal over-targeted screening.

Some risk factors have come from understanding the aetiology of PCHI, conversely the aetiology has sometimes been worked out after observational studies showed something as a risk factor. This is a continuing reason for studying risk factors – in order to understand more about what might be causing hearing loss. Other reasons are that the high risk may extend beyond the neonatal period, indicating the need for further observation of a child as he or she develops, and to help parents who encounter one of these risk factors understand the increased risk their children may face.


A frequently quoted list of risk factors is published by the Joint Committee on Infant Hearing.12 Some are highlighted as particularly relevant when thinking about progressive or delayed-onset cases, and they recommend that any child who has these risk factors is seen by an audiologist before 30 months old if the newborn screen is clear. The Newborn Hearing Programme (NHSP) in the UK publish their own guidelines on the management / surveillance of high-risk individuals.81 It is recommended that any neonates with meningitis are referred straight to audiology without a screen, and children who recover from meningitis be offered an audiology appointment within 4 weeks of discharge from hospital. Babies born with cranio-facial abnormalities (including cleft palate) or Down syndrome should be screened again at eight months. Other babies who should be offered an assessment at eight months and at inter-vals throughout their childhood are those with: a family history of PCHI; assisted ventilation in NICU for >5 days; neonatal jaundice to a level needing exchange transfusion; congenital infection with toxoplasmosis, rubella, cytomegalovirus (CMV) or herpes; and developmental delay associated with a neurological disorder. They recommend audiological testing for babies who have had high levels of ototoxic drugs and caution strongly against their use if there is a family history of hearing loss after antibiotics.

Weichbold et al.82 examined the histories for 23 9-year-old children who had developed bilateral PCHI after a clear newborn hearing screen. Eleven children had risk factors (as defi ned by JCIH 2000): three had a family history of hearing loss; two had recovered from meningitis; two had a cranio-facial malformation; one had persistent pulmonary hypertension; one had a congenital CMV infection; one received extracorporeal membrane oxygenation; and one had recurrent otitis media with effusion. They also found that fi ve children had received ototoxic therapy (not on the list of risk factors at the time) and two had been born before the 33rd gestational week (one child had a combination of the last two). Six children (26%) showed no risk indicators for post-natal hearing loss.

AETIOLOGY

The major aetiological classifi cation system suggested by Davidson et al.59 has been used in most recent studies. The categories are:

● genetic;● prenatally acquired;● perinatally acquired;● post-natally acquired;● cranio-facial anomalies; and● other.

Unfortunately, it is common for a large percentage of children in epidemiological studies to have an unknown aetiology, referred to as ‘missing’. In a selection of recent studies stating the aetiology of PCHI for different populations, there are reports of 16 to 55% of unknown origin (see Table 1.5).19,21,34,83–85

Several studies have looked at ways of fi nding the underlying aetiology in missing cases, both as a way of improving epidemiological data and for clinical reasons. In the Trent study,19 41% of children did not have an identifi able aetiology. Nevertheless, it was possible to impute aetiology from other data such as medical notes, and this reduced the percentage of people who

Tab

le 1

.5

Sele

ctio

n of

cro

ss-s

ectio

nal c

ohor

t st

udie

s w

ith p

erce

ntag

e of

cas

es f

rom

eac

h ae

tiolo

gica

l cat

egor

y, s

how

ing

diffe

renc

es a

cros

s tim

e an

d st

udy.

Firs

t a

uth

or

(yea

r p

ub

lish

ed

)D

rew

s (1

994)3

4P

arv

ing

(1993)8

3Fo

rtn

um

(1

997)1

9M

acA

nd

ie

(2003)2

1Fo

rtn

um

(2002)8

5B

illin

gs

(1999)8

4

Coh

ort

(N

)

Atl

an

ta,

born

1977–1

979

(100)

Cop

en

ha

gen

, b

orn

1980–1

990

(228)

Tren

t, b

orn

1985–1

993

(653)

Gla

sgow

, b

orn

1985–1

994

(130)

UK

, b

orn

1980–1

995

(17 1

60)

Bost

on

, d

iag

nosi

s 1993–1

996

(301)

Cat

egor

yG

enet

ic13

3645

4330

23*

Pren

atal

716

4 3

42

Perin

atal

–14

1715

818

Post

-nat

al24

† 5

6 7

79

CFA

1–

312

–7

Oth

er–

– 2

– 2

9**

Mis

sing

5527

2516

4932

* Inc

ludi

ng ‘

know

n sy

ndro

me’

(12

) ‘fa

mily

his

tory

’ (1

1) *

* ‘co

ngen

ital a

bnor

mal

ity,

othe

r’† I

nclu

ding

13

case

s of

Hib

men

ingi

tis.


had no aetiological information to approximately 25%. Taking this one step further, Parker et al.86 report investigating 82 children from the Trent study using a questionnaire, home visit and genetic test for the most common genetic mutation causing hearing impairment (Connexin 26 35delG, see Genetic hearing impairment–non-syndromic below). They found eight children had a genetic syndrome not previously assigned and seven further cases had the Connexin 26 35delG mutation. Parving87 found that aetiology was signifi cantly more likely to be found if a child with PCHI had a non-audiological examination in addition to a standard audiological exam (37/61 vs. 61/117). Peckham et al.88 suggested that congenitally acquired CMV might be responsible for a large proportion of children for whom no other obvious cause is found for PCHI. Their study found that such children were twice as likely to have CMV excreted in their urine than children with normal hearing (13% vs. 7%). There are a number of guidelines now available for clinicians investigating the cause of hearing loss in individual children89 with core investigations that all children with PCHI should receive; additional tests are suggested depend-ing on the circumstances (Table 1.6).

The BAAP give several reasons for investigating the cause of hearing loss:

● To try to answer parents who ask ‘why is my child deaf?’● To help identify, monitor, treat or prevent associated medical complications in some patients.● To help prevent further deterioration of hearing loss in some patients.● To enable better-informed genetic counselling.● To inform epidemiological research.● If the diagnosis is known, then the doctor can provide better advice to parents, such as

assisting the family in making decisions about the most appropriate communication mode, about educational placement and about cochlear implantation.

Table 1.6 The recommended history and examination for a child with PCHI – this may help discover aetiology.

Core Additional

Personal history: pre- and perinatal problems, general development, general health and head injury.

Genetic tests: Chromosomal examination (karyotyping) if developmental delay or dysmorphic features; Connexin 26 and 30 gene testing for common mutations if PCHI severe or greater; testing for other mutation, including mitochondrial, as suggested by the history.

Family history: looking back three generations, including congenital and acquired hearing loss.

Renal ultrasound: If syndrome with multi-system abnormalities suspected, or if family history of renal problems.

Imaging of head and neck: Computerised Tomography (CT) or Magnetic Resonance Imaging (MRI) scans.

ECG, as some syndromes are associated with dangerous cardiac conduction abnormalities.

Infection screen: for CMV and rubellaOphthalmology: may show changes due to a

syndrome such as Usher or congenital rubella. Also important for ascertaining extra needs for children with hearing impairment.

Infection screen: toxoplasmosis and syphilis tests if indicated.

Thyroid function: usually done at birth. Blood tests and urine examination: if syndromes involving kidneys are suspected, such as Alport or Alstrom syndromes.


The presence of one possible aetiology does not exclude other causes. For example, it is increasingly recognised that some mutations do not in themselves cause hearing loss, but lower than the threshold for environmental insults pre, peri and post-natally.90,91 Such mutations include the A1555G mitochondrial gene mutation, which predisposes to hearing loss when a child takes aminoglycoside antibiotics, such as gentamicin. There is also a controversy over whether perinatal problems, often cited as the cause of congenital defects, are actually the effect of pre-existing developmental anomalies.92 Children with a sensorineural hearing loss can be more at risk of conductive problems such as chronic otitis media,93 something that can potentially be treated to improve hearing.

As might be expected for a condition with such a variety of causes, the frequency of occur-rence of some causes of PCHI varies over time and geographical areas. Parving and Hauch94 looked at the ascribed causes of hearing loss in children attending the School for the Deaf in Copenhagen in 1993–1994 in comparison to causes evaluated 10 and 40 years previously. They found that the frequency of congenital inherited hearing impairment increased steadily with time, whilst between 1953 and 1983 there had been a signifi cant increase in prenatal infections, which then declined between 1983 and 1993. Admiraal and Huygen95 in the Neth-erlands found a similar decrease in prenatal infectious causes from 1988 to 1998, whilst the proportion of PCHI thought to have a perinatal cause had increased.

The changes in the developed world over the last few decades have shown the success of primary prevention. Measles, mumps, rubella and meningitis are all implicated in PCHI, and all have been the subject of immunisation programmes. Secondary prevention has also helped, with better nutrition and treatment leading to better outcomes from infections such as measles and meningitis. Meanwhile there has been a rise not just in the proportion of genetic cases but in the actual numbers. In some cases this is due to better neonatal care leading to the survival of babies with life-threatening syndromes, in others it is due to the increase in prevalence of particular mutations. Nance and Kearsey96 suggest that the frequency of PCHI caused by Con-nexin 26 or 30 mutations may have doubled in the last 200 years due to the establishment of a ‘Deaf community’* leading to healthier hearing-impaired adults. These adults go on to have children and this decreases genetic selection for the unmutated forms of the Connexin gene.

In contrast, Dunmade et al.97 looked at the aetiologies of sensorineural hearing loss in chil-dren in Nigeria, comparing aetiologies of hearing loss in 1980 and 2000, and found there had been no signifi cant decrease in infectious causes. The fi gures for the 115 children studied in 2000 showing some common causes were febrile illness (18.3%), measles (13.9%), meningitis (8.7%) and mumps (6.9%). Saunders et al.98 offered the Connexin 26 35delG genetic test to children with PCHI in an audiology clinic in Jinotega in Nicaragua and found that despite a family history of hearing loss in 33%, this mutation, so common in the UK, was not present in any of their children. Another difference he found was in the unmonitored use of ototoxic antibiotics, which are cheaper than their alternatives.99

GENETIC HEARING IMPAIRMENT

At least half of all cases of PCHI are known to have a genetic cause.100,101 However, despite signifi cant advances in the understanding of the molecular basis of hearing loss, identifying

* The use of the capital D indicates the community of deaf people who use BSL as their language and identify with other deaf people who share their language, culture and history.


the precise genetic cause in an individual remains diffi cult. Using systematic investigation, such as that described in Table 1.6, will increase the chances of fi nding the aetiology, but it is estimated that a mutation in one of between 300 and 500 genes (around 1% of the total number of genes) can cause hearing loss.102 Approximately 120 of these genes have been identifi ed so far – around 80 causing syndromes that include hearing loss and over 40 respon-sible for ‘non-syndromic’ hearing loss. Most of these genes are located on the autosomal chromosomes, up to 20% on the X-chromosome and up to 20% in the maternally inherited mitochondrial DNA. This confi rms the fi ndings from the questionnaire section of the Parker et al.86 study based on the Trent cohort: the families of 526 hearing-impaired children (aged 4–13) were sent questionnaires asking about any family history of hearing loss, the results pointing towards different genetic disorders with autosomal dominant, autosomal recessive and sporadic inheritance.

Syndromic PCHI

If hearing loss is one of several clinical fi ndings, the disorder is described as a syndrome. Approximately 30% of genetic hearing impairment is syndromal.100,101 Over 400 syndromes featuring PCHI have been described and many of the genetic abnormalities responsible identi-fi ed. Syndromal hearing impairment can be sensorineural or conductive, due to structural anomalies of the auditory system. McClay et al.103 report that the presence of any congenital syndrome signifi cantly increased risk of an abnormality of the temporal bone involving the cochlear or vestibular system visible on a CT scan. This risk was found to be elevated regard-less of the presence of PCHI, but higher still if PCHI was present. The presence of a genetic syndrome in children with PCHI should not be overlooked as it can be important in determin-ing prognosis and intervention measures – as well as for estimating the recurrence risks in the family.104

Chromosomal syndromes may occur either during meosis or mitosis, resulting in too much or too little genetic material, and many increase the risk of PCHI. Two of the most common syndromes caused by chromosomal abnormalities are Down and Turner syndromes. Maatta et al.105 studied 129 individuals (mainly children) with Down syndrome, and found that one-third of the sample had hearing impairment or recurrent ear infections. Overall, the risk of sensory impairments increased with increasing levels of intellectual disability.

Genetic syndromes caused by mutations, deletions or additions on the autosomal chromo-somes can be inherited in a recessive or dominant manner. The majority of syndromal genetic hearing impairments are inherited in an autosomal recessive way and are detectable at birth. Recessive inheritance occurs when both parents – who may not necessarily exhibit the trait – carry a mutated gene that may cause a genetic syndrome. If both parents carry one normal copy of the gene and one mutated copy of the gene, there is a 25% chance of the child inherit-ing both of the mutated genes (one from each parent) and manifesting the genetic disorder. There is also a 50% chance that the child will inherit one of the mutated genes and become a carrier for that disorder but not manifest the syndrome. Such disorders include Usher syn-drome, Cockayne syndrome, Pendred syndrome, Jervell and Lange-Nielsen syndrome, Hurler syndrome and Alstrom syndrome.106 Usher syndrome is one of the most studied of these syn-dromes. It was formally classifi ed into three clinical types and was expected to be caused by three corresponding mutations. However, recent work reported by Cohen et al.107 suggests that there are more than three genetic causes of Usher syndrome, each having different potential effects in different individuals with very little evidence for phenotypic–genotypic correlations.


For dominant inheritance, only one mutated copy of the gene is required for a syndrome to be manifest. Usually, one parent will have the syndrome, and there is at least a 50% chance of the child inheriting the gene and manifesting the genetic disorder. If both parents exhibit the trait, there is a 75% chance of the child manifesting the disorder. Hearing impairment inherited in this way usually manifests itself after the neonatal period, either because it is congenital and progressive or because it is late-onset. Examples of autosomal dominant syn-dromes include Marshall-Stickler syndrome, Waardenburg syndrome and Treacher Collins syndrome.106

Syndromes carried on the X-chromosome affect males predominantly because they have only one X-chromosome. Females ‘carry’ the mutated syndrome-causing gene but are unaf-fected if they have a normal copy on their other X-chromosome. Any male children of a carrier will inherit the genetic material for their X-chromosome from their mother (their father con-tributing the Y-chromosome instead), with a 50% chance that this will include the mutated gene. If this occurs, since there is no copy of the gene on the Y-chromosome, the syndrome will be manifest. Examples of X-linked syndromes include Hunter syndrome, Alport syndrome and Norrie syndrome, all of which do not manifest at birth but develop in early infancy.106 It is a mutation in an X-linked gene that is responsible for ‘deafness with fi xation of the stapes’, which gives a progressive hearing loss of sensory and conductive types. Although this muta-tion is very rare, diagnosis is important because if this is not recognised, there can be further damage to hearing if surgical methods to release the stapes are not attempted.108

Mitochondria are small organelles located within the cytoplasm of the cell and have their own DNA (mDNA), which is independent of the nuclear DNA. Mitochondria are inherited from the mother only. Thus, a mother who has hearing loss from a mutation on her mDNA will pass this mutation onto her children of whatever sex, but a father with the same mutation will not pass it on. There are multiple copies of mDNA in each mitochondrion, and, therefore, expression of a syndrome-causing gene is not inevitable. Thus the clinical phenotype is extremely variable. An important syndrome to recognise is the MELAS syndrome, where permanent hearing loss may be the fi rst manifestation; and recognition allows better manage-ment of subsequent complications.109

Non-syndromic

Autosomal recessive non-syndromic hearing impairment is the most common form of genetic deafness, accounting for around 80% of all cases.110 Thus, it can be estimated to account for around 40% of all profound PCHI. Numerous non-syndromal recessive hearing impairment genes have been localised, with Petersen and Willems110 reporting 85 loci on 39 different genes. Autosomal dominant inheritance is thought to account for approximately 15% of the cases. X-linked inheritance accounts for approximately 2–3% of the inherited hearing impairments (but 5% of those affecting males).

Mutations in the GJB2 gene are responsible for as much as 50% of autosomal recessive non-syndromic PCHI. This gene codes for a protein called Connexin 26, a gap junction protein regulating the passage of ions in and out of the cell, and was identifi ed in 1997. As with the mutations responsible for Usher syndrome, it has become obvious that genotype–phenotype relationships are more complex than once thought.111,112 Green et al.113 studied the prevalence of mutations in the GJB2 gene in 52 people with congenital sensorineural hearing loss at a clinic in Iowa. Twenty-two were found to have GJB2 mutations, 19 of whom had a mutation on both chromosomes. Of the 41 abnormal copies of GJB2, 29 had the same mutation – 35delG.


The siblings of these 52 people were also screened, and it was found that all those who had two abnormal copies of the gene also had PCHI. A total of 560 unrelated children were also screened and there were 14 in whom one copy of the GJB2 gene had a mutation. This gives a carrier rate of 3.0% (probable range 2.5–3.6%). It is important to remember that this carrier rate will be specifi c for this particular population – mid-western United States. Pandya et al.114 searched the DNA of children from the Annual Survey of Deaf and Hard of Hearing Children and Youth, conducted at the Research Institute of Gallaudet University, and found that GJB2 mutations accounted for 22.2% of deafness in the overall sample but differed signifi cantly amongst Asians, African Americans and Hispanics. Ethnic differences are particularly marked where there is a small founder population, such as in some Jewish communities.115

PRENATAL FACTORS

Infections

Infections are considered to be the main cause of prenatally acquired hearing impairment. In the 1970s–1980s, congenital rubella was the single most common reported cause of sensori-neural hearing impairment in childhood, accounting for 16–22% of cases of hearing impair-ment in babies.94 If infected during the fi rst month, there is a 50% chance of the developing fetus being affected such that congenital rubella defects are detectable. This risk declines throughout pregnancy to an approximate 6% chance in the fi fth month and beyond. Problems associated with congenital rubella (CRS) include learning disability, heart disease, cataracts, microcephaly, hepatomegaly, splenomegaly, bone lesions, purpura, glaucoma and hearing impairment. Hearing impairment is the most common permanent manifestation and affects 68 to 93% of children with congenital rubella.116 The hearing impairment is usually severe to profound sensorineural hearing impairment and can be progressive.117

Congenital rubella was a devastating syndrome that became a major public health issue. A rubella vaccine was fi rst licensed in 1969. By 1999, 105 (49%) of the 214 countries and terri-tories reporting to WHO had introduced the rubella vaccine in their national immunisation programme.118 In the UK, the rubella vaccine was offered to schoolgirls in the United Kingdom from 1970, and post-partum susceptible women shortly after. Mass vaccination with MMR (measles–mumps–rubella vaccine) of babies was introduced in 1988. Schoolgirl vaccination was discontinued in 1996, although post-partum vaccination of susceptible women identifi ed through antenatal testing continues. Reported cases of CRS declined from about 50 a year in 1971–1975 to just over 20 a year in 1986–1990.119 About 40 infants with CRS were reported over all of the next 12 years. Women living in the UK who were born abroad have much higher rubella susceptibility rates than UK-born women, and two-thirds of the CRS cases since 1991 have been to mothers born outside the UK. The previously high coverage of children interrupted the epidemic transmission (which was mainly in children), but concerns over the safety of MMR have led to a decrease in immunity amongst children. If an epidemic of rubella occurred in the UK, women born in places without vaccination will be at increased risk of acquiring infection in pregnancy. The likelihood of importation of infection is high, as the developing world still has endemic rubella. Rittler et al.120 found 43 cases of CRS recorded from the records of 3,883,165 live births collected by the Latin-American Collaborative Study of Congenital Malformations, World Health Organization (WHO) Collaborating Centre for


the Prevention of Birth Defects (ECLAMC), which suggests a prevalence of CRS in Latin America of around 1 : 100,000 live births.

Another prenatal infection that causes congenital abnormalities is toxoplasmosis. Sever et al.121 studied 23,000 mothers and children from around 20 weeks gestation until 7 years old. Of these mothers, 38.7% had antibodies to toxoplasmosis during pregnancy, and children born to these mothers had double the risk of developing PCHI by age 7 (0.4% vs. 0.2%, p = 0.01).

Cytomegalovirus CMV is a common chronic asymptomatic infection in adults, which can cross over the placenta to affect the developing fetus and child. Roizen117 has observed that CMV infection occurs in 2.2% of all newborns, making it the most common intrauterine infec-tion. Lipitz et al.122 report that from their sample of 18 babies with confi rmed CMV, four (22%) had neurological problems at birth. Fowler and Boppana123 summarised seven studies between 1982 and 2004, and found that the risk of PCHI was 22–65% in those babies symptomatic at birth and 6–23% in those asymptomatic at birth. Amongst those affected by PCHI, there were progressive, fl uctuating and delayed-onset cases. They were unable to identify any way of predicting which babies were more at risk of PCHI from asymptomatic CMV infection, and they fl ag up the fact that UNHS may miss many babies with PCHI due to CMV because of its variable course. It is not yet established how much CMV infection contributes to the overall prevalence of PCHI, as studies vary in their method of investigating infection. In the meta-analysis by Morzaria et al.124, the mean of cases reportedly due to CMV in the studies from 1990 to 2002 was 0.92% (s.d. 1.07) of the total, but Peckham et al.88 reported that 14% of those diagnosed with PCHI of unknown aetiology excreted CMV in their urine (compared with a base rate of 7%), and Barbi et al.125 reported that of 130 children with PCHI, 24.7% had CMV in blood retained from a sample at birth (base rate not given). Given the availability of an antiviral treatment for CMV,126 there is an argument for screening newborn babies for CMV.125,127

Maternal drug therapy

Maternal drug therapy during pregnancy can also contribute to congenital hearing impairment. Some substances may permanently injure or destroy the hair cells of the cochlea resulting in a sensorineural PCHI. For example, alcohol, streptomycin, quinine and chloroquine phosphate may destroy neural elements of the inner ear.128 The loss is usually triggered by the ingestion of ototoxic drugs during the fi rst trimester, with damage to the auditory system occurring especially in the sixth or seventh week after conception. Conductive PCHI can also result from ototoxicity, primarily as a result of ossicular malformations of the middle ear. Brent91 empha-sises the gene–environment interaction involved in teratogenic drugs.

Perinatal factors

Perinatal factors which may predispose to PCHI include prematurity, hyperbilirubinaemia (kernicterus), anoxia (including apnoea and cyanosis), severe neonatal sepsis, rhesus incompat-ibility, low birth weight and trauma.129 Some perinatal problems that were known to cause neurological damage have been much diminished in the modern maternity hospital, for example, the introduction of photosynthetic lights to reduce jaundice (hyperbilirubinaemia) to non-toxic levels and rhesus inoculation to prevent rhesus incompatibility in future pregnancies. On the


other hand, medical advances have ensured that more premature, anoxic and low-birth-weight (LBW) babies survive, leading to more babies graduating from NICU with a hearing impair-ment. Davis and Wood31showed that a baby admitted to NICU for any reason had a risk of developing PCHI by 3 years old that was seven times higher than those who had not. Razi and Das129 showed that even in children who had a hearing threshold within the normal range, the mean high-frequency threshold was higher in children who had experienced an adverse peri-natal event.

Prematurity is a risk factor for PCHI, but it is not clear whether this itself is the causal factor or whether the causes are factors associated with prematurity such as anoxia, hyperbili-rubinaemia, increased bacterial and viral infections, treatment with ototoxic drugs and/or LBW. Veen et al.130 concluded that in their study of 890 5-year-olds who had very LBW or had been very premature, the prevalence of sensorineural hearing impairment was 15 times higher than the average Dutch population of 5- to 7-year-olds. Van Naarden and Decoufl e131

studied 320,000 children born in Georgia US in 1991–1993 and found that by age 3, 169 children had developed PCHI, of which 17 had been amongst the 3,362 children who had been born weighing less that 1,500 g. This gives a relative risk of 13.9 (95% CI 8.2–23.4). In this group of very low weight babies, it is hard to identify the precise cause of hearing impairment due to the sheer numbers of possible complications those infants may experience.

Children with adverse perinatal events are also at risk of having other developmental dis-abilities, making them particularly high need. Van Naarden and Decoufl e131 estimated that for a child who was born weighing less than 1,500 g, the risk of developing a hearing impairment plus another disability was 27.8 times (95% CI 11.6–66.5) that of a child born weighing over 3,000 g. Davis and Wood31 found NICU babies with a hearing impairment were considerably more likely to have another disability (odds ratio 8.7 to 1). Yoon et al.132 suggest that UNHS may not pick up all of the NICU graduates who develop PCHI due to the high incidence of middle ear problems and delayed-onset sensorineural hearing loss.

Post-natal factors

It is possible for post-natal causes of acquired PCHI to be genetic due to delayed onset of hearing impairment, but most acquired cases are caused post-natally by infection, ototoxic agents or trauma.59 Otitis media should be included even though permanent hearing impairment secondary to otitis media is uncommon in the developed world, because it may delay the detection of permanent hearing impairment. Systemic and neurological infections that have been linked with PCHI are bacterial meningitis, measles, mumps, HIV133and CJD.134 Thanks to a successful vaccination programme and better general health, new-onset measles and mumps-related hearing loss is now rare in the developed world.

Bacterial meningitis is a serious infectious disease both in the neonatal period and through-out childhood. It can be caused by a variety of pathogens, including Haemophilus infl uenzae type b (Hib), Streptococcus pneumoniae (pneumococcus) and tuberculosis (TB). For children who survive meningitis, there are often sequelae, which include learning disabilities, hydro-cephalus, motor abnormalities, vestibular defi cits, psychosis, hyperactivity and visual and sensorineural hearing impairments. Reports have indicated that acquired hearing impairment represents 9.5% of total PCHI, with 6.5% of these cases being caused by meningitis.135 Men-ingitis-induced hearing impairment is often bilateral, severe or profound and rapid in onset. Clinical and experimental studies have shown that the loss results from direct damage to the


cochlea by the infection, but it may be exacerbated by additional cochlear damage resulting from any ototoxic drugs used to treat the disease.136 Children who have lost their hearing to meningitis are often considered to be amongst the best candidates for cochlear implants due to their previous experience with language and their total loss of any auditory neural function. The incidence of post-meningitic hearing impairment varies from 7 to 31%, depending on the type of meningitis and type of hearing impairment included.137–141 Wellman et al.140 and Kutz et al.141 also compared the complication rate between Hib and pneumococcus, fi nding the latter signifi cantly more likely to lead to hearing impairment. In 2004, a Hib vaccine was added to the routine childhood vaccination schedule in England, and from 2006, a vaccine against invasive pneumococcal disease was also added. It is hoped that this will reduce the incidence of acquired PCHI.

Children may be given a number of ototoxic treatments, for example, aminoglycosides (such as gentamicin) for severe infections or those resistant to penicillin; platinum-containing chemotherapy such as carboplatin for retinoblastoma a childhood cancer of the eye, and radio-therapy for tumours in the glands of the neck. Many of these treatments are the best available142 but often the adverse effects can be minimised by action such as co-administering aspirin with gentamicin,143 careful dosing of carboplatin144 and well-placed radio-opaque shields.145

CONCLUSION

Hearing impairment is the most frequent sensory impairment in humans, with signifi cant social and psychological implications. In the light of the impact that PCHI can have on children and their families, the importance of epidemiological studies cannot be underestimated. In devel-oped countries, around 1 in 1,000 babies is born with at a serious permanent bilateral hearing loss, and permanent hearing loss becomes more common as children grow older.

In developing countries, the prevalence may be higher and in some countries it may be considerably higher, but there is a lack of large-scale, robust epidemiological studies.

Epidemiologal data were at the forefront of public health and audiological arguments for universal hearing screening, and have also been used to plan and monitor primary prevention such as vaccination. This chapter explained the diffi culties in collecting data on incidence, prevalence and aetiology. Recent results of studies from throughout the world on the preva-lence and aetiology of deafness have been presented that show the changing nature of deafness throughout the world. Clearly, a greater emphasis on collecting routine data on the pattern, degree, aetiology and natural history of children with deafness is needed throughout the world. It is only by recording these data that we will understand the extent and nature of childhood deafness and propose realistic public health plans to provide support for these children and their families.

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