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Identification of hearing loss in infants
The United States National Institutes of Health issued a consensus statement
in March 1993 in which, amongst other things, universal screening for infants
within the first three months of life was recommended. In addition, the panel
recommended screening prior to discharge for all infants admitted to the
neonatal intensive care unit, together with continuing surveillance of all
children throughout infancy and early childhood. The preferred model for
neonatal screening they suggested was an evoked otoacoustic emissions (OAE)
test, followed by auditory brainstem response (ABR) testing for all infants
failing the evoked otoacoustic emissions test.
This has proved a controversial recommendation with opinions both
criticising and in support of the proposals being reported in the literature
subsequently.
Where does New Zealand stand on this issue?
New Zealand has had a national hearing screening programme in place for many
years. The two main strategies for identifying permanent hearing loss in young
children involve ABR testing of high risk infants, together with use of a
questionnaire regarding speech, language and hearing impairment, "Can Your Child
Hear?". Children are also screened for hearing loss at school entry, as part of
the National Vision and Hearing Screening Programme, which functions as a
backstop for the early childhood programme.
Over the years, other strategies for detecting early onset sensorineural
hearing loss have included:
- behavioural screening of 9-month infants by well-child care workers
- behavioural screening of high-risk 9-month infants by audiologists
- behavioural screening of neonates (at National Women's Hospital)
- automated behavioural screening of neonates - the Crib-o-gram (at
Middlemore Hospital)
All of these strategies have been rejected and are no longer used because of
unacceptably high false positive and false negative rates.
Although hard evidence is scarce on the ground, there is little disagreement
among people working with deaf and hearing impaired children that early
identification is vital so that language (whether oral, signed or a combination
of the two) can be effectively introduced during the child's critical period for
language acquisition.
A database of deaf and hearing impaired children identified in New Zealand
has been kept by the National Audiology Centre since 1980. The criteria for
inclusion are that the child must have a permanent bilateral hearing loss
exceeding 30 dB on average in the better ear. While the database is incomplete,
it nevertheless has provided useful information in evaluating the effectiveness
of the national policies. For example, in 1994, only two of the 78 children
notified were reported to have been identified by use of the "Can Your Child
Hear?" questionnaire.
Also available from the database is information on the etiology of the
hearing loss. In 63% of cases, there was no known causal or risk factor present.
Family history was the largest risk factor, accounting for 19% of notifications.
The impact of rubella has decreased over the years and is now a factor in only
2% of the total database.
Parents are normally the first to suspect deafness (45%). Next were
paediatricians (referring because of high risk) at 15%, followed by vision
hearing testers, who detected 12% of the children during screening of school
entrants. In all cases, the children identified by the vision hearing testers
had mild or high frequency hearing losses. In 88% of all cases of hearing loss,
there was no other handicap.
For children with at least a moderate degree of hearing loss notified during
1994, the mean age of identification was 26.8 months. Children who were high
risk for deafness were identified earlier (22 months) than those with no risk
factor (29 months). The difference between these two figures can be interpreted,
according to the prevailing point of view, as a measure of the efficacy of the
at risk testing policy, or, alternatively, as an indictment of our relatively
poor record in identification of children not known to be at risk of deafness.
One risk factor which is associated with a particularly high risk of deafness
and which is frequently asymptomatic in the neonatal period is cytomegalovirus (CMV).
Efforts to improve identification of CMV could well bear fruit in terms of
improving identification of children currently thought of as not at risk of
deafness.
Ethnic data have only been collected since 1992. Of deaf and hearing impaired
children notified in 1994, 36% were Maori. This compares with the 14% of New
Zealand children who are Maori, showing that Maori are over-represented among
the deaf and hearing impaired children by a factor exceeding two. No obvious
difference in causation between Maori and other children was evident.
Some ethnic differences in the age of identification of hearing loss were
also evident. Maori children tended to be identified later than other children
(mean age for those with at least moderate hearing loss: 39 months for Maori cf
24 months for other children).
Another area included in the database is the time delay between suspicion and
confirmation of deafness. The factor most commonly associated with a delay in
identification of hearing loss was the coexistence of a conductive hearing loss
- the child's ear pathology tended to be the focus of attention, with
identification of the permanent hearing loss occurring only at the end of a
protracted period. This occurred in 14% of all cases. Vigilance on the part of
general practitioners to refer children for significant hearing problems (as
opposed to ear pathology) for audiological assessment is clearly an issue.
Currently, we are a long way from achieving the aim of identifying children
at risk of hearing loss by the age of 6 months. Examination of the mean age of
identification of hearing loss over the last two decades shows that a plateau
seems to have been reached. The implication is that a change in policy will be
necessary if we are to make any real advances in this area.
Should we embrace a universal screening policy? It is certainly a major
undertaking for a small yield rate: permanent hearing loss or deafness is a rare
condition, with an incidence of about 2.3 per 1000 births. While New Zealand's
performance in identification of deafness is nowhere near the optimum, reports
of better performance, especially at a national level, are rare. If we were to
consider widening the population screened, what screening methodology should be
employed?
The gold standard for the assessment of infant hearing is unquestionably ABR
which measures evoked electrical responses to sound at the level of the
brainstem. However, in a screening context, ABR is expensive in terms of time,
particularly in the need to achieve adequate electrical contacts. OAE - the
sounds produced by the hair cells during the hearing process, have emerged as a
faster technique, with initial reports claiming excellent identification of
cochlear hearing loss exceeding 40 dB. Some types of pathology (eg anoxia,
hyperbilirubinemia) may cause damage at higher levels of the auditory system,
and it remains to be seen whether OAE identify these children.
Currently a research programme is underway at National Women's Hospital to
evaluate the feasibility of introducing neonatal screening by OAE.
The study has shown that, although noise is a factor in obtaining reliable
OAE readings, screening by OAE is able to be carried out satisfactorily in the
mother's room. This finding is fortuitous because it minimises the time to
perform the test, and reduced the amount of disturbance for both mother and
baby. Noise produced by the infant's movement seems to be more of a problem than
external noise. It has been found that it is most efficient to test infants
while they are sleeping quietly.
A major limitation in introducing OAE as a neonatal screening test is the
finding that the occurrence of OAE is low in the hours after birth, and may not
stabilise until approximately 48 hours afterwards. This is increasingly an issue
as mothers opt for home births, or for early discharge following a hospital
birth.
The reason for this delay in appearance of OAE is still not clear. One factor
may be a delay in amniotic fluid draining from the middle ear following birth.
Another possibility is that this is a reflection of switch-on followed by rapid
maturation of the system following birth. It has been suggested that those
infants who fail to show OAE after 48 hours post-partum, but who do not have
evidence of cochlear hearing loss, may be at risk for developing chronic otitis
media throughout early childhood
Any hospital-based screening programme has the challenge of home births as
well as early discharges to contend with if universal coverage is the aim. A
combination of hospital and community testing within the neonatal period would
clearly be necessary to achieve high coverage.
If there is to be further progress made in early identification of hearing
loss and deafness in childhood, solutions to these problems must be found. Now
that there is the option of a cochlear implant for children with severe or
profound hearing loss who do not respond to hearing aids, there is further
impetus for early identification, with implantation being recommended at two
years of age. Children fitted with hearing aids, and profoundly deaf children
whose parents opt for reliance on sign language, have the same need for early
exposure to language (in its widest sense), to optimise their educational and
social development, so that they can become fully functioning members of
society.
Anne Greville
NZ Med J, 9 February 1996, No 1015, Vol 109, pages 21-2
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