This module deals with the questions where the neonatal hearing screening is performed, which methods are applied and which consequences it has when the baby is diagnosed as hearing impaired.
Authors: Peter Böttcher und Barbara Bogner
Introduction
Aims of the Module
With regard to the development of children who have a congenital hearing disorder, it can justifiably be claimed that, nowadays – in contrast to the situation in our parents’ and grandparents’ generation, when the available possibilities were far more limited – it is no longer the impairment in and of itself that is the actual problem, but the timing of its detection and treatment. The earlier these disorders are brought to light, the brighter the outlook for treating them. Present-day advances made, in terms of diagnostics, assistive hearing devices and therapy, enable the affected children to develop just as well as their hearing counterparts. Although a hearing disorder cannot generally be healed, the existing deficiencies can be well compensated for from an early stage, so that the child need not be disadvantaged by his or her impairment.
The aim of this Module is to raise awareness of the great importance of newborn hearing screening. We wish to provide good arguments as to why parents should not forego this screening. The Module will highlight why early detection of hearing disorders is such a priority, as well as the prospects associated with an early diagnosis and, subsequently, early habilitation. The test procedures used will be outlined, along with the other conditions that must be fulfilled if newborn hearing screening is to be successful.
Learning rationale
Through self-study, readers should acquire a basic understanding of newborn hearing screening which they should be able to communicate in parent guidance work. The main learning goals are set out at the start of each chapter. Readers will also have the opportunity to assess for themselves how much they have learned by working through review questions for each chapter.
Why the early detection of hearing disorders is so important
The main learning goals are to:
- appreciate the risk factors for hearing disorders in early childhood;
- be aware that that newborn hearing screening is an effective and up-to-date means of early detection for hearing disorders present at birth;
- recognize that the auditory system matures in response to the input stimuli it receives;
understand that early diagnosis can minimize the impact of hearing disorders.
Risk factors and the frequency of hearing disorders in early childhood
Every year, between around one and two thousand children (otherwise born healthy) in Germany are either born with hearing loss or acquire it during the newborn phase. In some cases, the underlying causes cannot be accounted for. In around half the infants involved, certain risk factors for the occurrence of the hearing disorder are present. These may occur before birth (prenatal), during birth (perinatal) or after birth (postnatal). Potential risk factors are (Hildmann 2008):
- genetic defects that can lead to clustering of hearing disorders in particular families;
- genetic syndromes;
- malformations of the head, face and ears;
- infections during pregnancy (such as rubella, toxoplasmosis and cytomegalovirus infection);
- extreme immaturity in the event of premature birth (i.e. where birth weight < 1,500 g);
- long-term mechanical respiration (i.e. for longer than eight hours);
- critical hyperbilirubinaemia (i.e. pronounced neonatal jaundice);
- the use of ototoxic medication (i.e. drugs that damage the inner ear);
- meningitis;
- all diseases or conditions that necessitate a spell of 48 hours or more in a neonatological intensive-care unit.
Where one of these risk factors is present, it is assumed that at least one to three children in every 100 may (in addition to other medical conditions or disabilities) have hearing impairment.
Sensitive phases in the maturation of the auditory pathway
Hearing is not fully developed at birth. In order that its full function can emerge, the auditory pathway needs to mature after the child is born. These processes of maturation take place within particular timeframes, the sensitive periods, the length of which differs for various processes. They proceed unhindered only if appropriate stimulation is provided during these phases and stabilization of the valuable neuronal connections occurs by means of sound stimuli (Klinke 2008). There is thus only a narrow window of opportunity for treating hearing disorders in early childhood; therapy needs to have taken place by no later than the end of the sensitive main phase of auditory-pathway maturation (roughly corresponding to the completion of the first year of life) if the child is not to be left with irreversible damage. If hearing disorders in early childhood are not treated in sufficient time (i.e. within the first 6 months of life) by means of hearing devices or surgery to improve hearing, and if professional early intervention is not set in motion without delay, then the result may be problems with speech and language development, learning difficulties (with emotional, psychosocial and cognitive repercussions) and an adverse impact on school education, employment prospects and the family of those affected.
International studies have shown that, without newborn hearing screening, the diagnosis tends to be made when the child is around 21-47 months old. Hearing disorders in early childhood are therefore usually not subject to technical intervention until after the sensitive phase of maturation has been completed. There is a good deal of evidence that – for children fitted early with hearing devices – the earlier they receive them, the better they progress in various aspects of their development. There are sufficient resources available from an early stage that enable the development of the auditory and language system in hearing-impaired children to match that of their hearing counterparts (Yoshinaga-Itano 2003).
Summary
Congenital hearing impairment in early childhood is relatively frequent in comparison with other inborn conditions affecting children. Like all people, hearing-impaired children have an innate system for acquiring hearing; they can thus learn to hear, as any child can. The prerequisite for this is that hearing impairment is reliably diagnosed within the first 6 months of life and that the child is provided with suitable therapy.
Review questions
How frequently do hearing disorders occur in newborns?
- Rarely; around one to two children in every 10,000 are affected.
- Occasionally; around one to two children in every 3,000 are affected.
- Frequently; around one to two children in every 2,000 are affected.
- Very frequently; around one to two children in every 1,000 are affected. If certain risk factors are present, the number may increase to three children in every 100.
Are children born with a fully mature and functional auditory system?
- Yes; the way in which the processing of sound sensations takes place in newborns is the same as in adults.
- Yes; which is why newborn hearing screening is a very good way of distinguishing between normal-hearing children and those with hearing disorders.
- No; the auditory system continues maturing after birth. Deficiencies must, however, be detected at an early stage and compensated for before certain critical phases are completed, otherwise the child may be left with irreversible damage.
- No; the auditory system undergoes different phases of maturation up until the age of about 14 or 15. A lack of auditory training in a child’s early years can be well compensated for by means of therapeutic measures later in life.
What is formed during the sensitive phases of auditory-pathway maturation?
- The cochlea with its sensory cells (i.e. hair cells).
- The neuronal links between the nerve cells of the auditory pathway and the auditory cortex.
- The connection between the outer and inner ear in infants.
- The nervous pathways between the right and left cochlea.
When must hearing disorders be detected and treated?
- As early as possible, but no later than the sixth month of life, as the auditory pathway begins to mature in utero and children with hearing disorders already have a ‘training shortfall’ at birth.
- By the time the child is 1 year old or thereabouts, as this phase of maturation is then completed.
- When the child is about 3 years old, as children of nursery school age have greater demands placed upon their auditory systems.
- At around 18 months, as the child can already say their first words and can play an active part in ‘training’ their own auditory system.
What is newborn hearing screening?
The most important learning goals are:
- to understand the purpose of newborn hearing screening;
- to know which testing procedures are used;
- to be familiar with possible weaknesses in the procedural arrangements;
- to be able to advise on suitable places to go for a confirming diagnosis.
Newborn hearing screening is a test that is simple to conduct and which can, using automated procedures, demonstrate that the ear is functioning normally in the majority of the population. Screening involves systematic testing, the aim of which is to obtain an indication of whether hearing is normal – with as much certainty as possible, yet with a minimum of cost, time and effort. Newborn hearing screening cannot, therefore, provide any information about any functional disorders that may or may not be present. Just because a child fails their hearing screening does not necessarily mean their hearing is impaired.
There are only three possible outcomes to screening: no abnormality detected, test interrupted or not conducted, or an outcome requiring further investigation. The process can, therefore, never identify a specific condition; it can only demonstrate that, in most individuals, the ear is working normally and without any problems. That there may be a functional disorder of the hearing is only one of several possible reasons why screening may require further investigation; it may also be because the screen was not conducted properly, or due to systematic and random testing errors. If screening is passed, the outcome is designated PASS; otherwise, the terms used are REFER or CONTROL.
Testing procedures
Two different testing procedures for newborn hearing screening have become established in the last two years. These involve measuring either transitory evoked otoacoustic emissions (TEOAE) or automated auditory brainstem response (AABR).
TEOAE testing
TEOAE procedures measure the inner ear’s response to sound stimuli, or more precisely the response signals from the outer hair cells in the inner ear. With their active properties, these in turn generate a sound signal themselves, which is transmitted via the middle ear to the external ear canal, where it can be measured using a microphone. This generally involves the ear being exposed to sound waves from a probe tightly sealed within the ear canal. If emissions are received, it can be assumed that the hearing loss does not exceed 30 dB. As most functional disorders of the inner ear involve impairment of the outer hair cells’ function (see Module 2), otoacoustic emissions cannot be measured in these cases. There may, however, be other reasons why no emissions are recorded; the probe may not, for example, be placed sufficiently deep within the external ear canal, or the emissions received by the microphone on the probe may be subject to interference from other sound sources (i.e. the environment is too loud).
The test can be conducted when the child is asleep; if they are awake, they must be settled (Fig. 1). Using modern screening devices under favourable testing conditions, the duration of testing is between around 3 seconds and 1 ½ minutes.
Figure 1: Measuring otoacoustic emissions (OAE) in newborn hearing screening (Mack Medizintechnik)
AABR testing
AABR procedures measure the brainstem’s response to sound stimuli. Differences in potential on the skin surface are measured using electrodes placed on the skull. The ear is exposed to sound waves via a probe placed in the external ear canal or using either ‘shell’ or plug-in headphones. The sound is transmitted, via the eardrum-ossicular system in the middle ear, to the cochlea, from where it is converted into action potentials by the hair cells. These are passed on via the auditory nerve and the adjacent auditory pathway towards the auditory cortex (see Module 2). On their way there, synchronous discharges from many cells are generated at various neuronal synapses, which can be measured at the skull using electrodes. If there is no response from the brainstem, this may indicate a disorder in the outer ear, middle ear, inner ear (i.e. the outer and inner hair cells) or in the auditory nerve as far as the brainstem. AABR testing thus looks at a larger portion of the auditory system than TEOAE assessment does. However, the causes of most of the treatable childhood hearing disorders originate in the peripheral part of the ear and, in most cases, can be adequately recorded using TEOAE alone.
Modern AABR screening devices use a stimulus level of 35 decibels (hearing level) for the test and are thus able to detect a hearing threshold of 35 dB and above. The preparation time required for measuring brainstem potentials is longer than that for registering otoacoustic emissions. Depending on the device used, either three disposable electrodes are attached to the head, or permanent electrodes are placed on the skin at the preferred recording sites. Both preparation and subsequent testing should ideally by carried out when the child is asleep, or at least settled. If readings are taken with the child conscious – especially if they are unsettled – this generally necessitates a longer testing period and may even lead to an outcome that requires further investigation, as muscle artefacts or other neural events may cause interference with the acoustically triggered responses of the brainstem and make it harder to detect them. Here too, therefore, failure to pass screening need not therefore indicate hearing impairment; it may have been caused by failure to follow correct procedures, or by poor testing conditions. Where modern screening devices are used, the test duration (under favourable testing conditions) is between 30 seconds and three minutes.
Figure 2: Automated brainstem audiometry (AABR) used in newborn hearing screening (Mack Medizintechnik)
TEAOA and AABR
Both methods – TEOAE und AABR – ensure a sufficiently accurate distinction between ears for which no abnormality is detected (and which thus tend to be healthy) and those which do not pass screening and require further investigation. TEOAE testing is quicker and easier to use, and the equipment is less costly. It is also more sensitive when it comes to detecting mild hearing loss, and enjoys widespread use worldwide. Its disadvantage consists in the fact that the results are more susceptible to being affected if the skin of the external ear canal is, after birth, still covered with the vernix that protected the baby in utero, or if the middle ear is coated with amniotic fluid, as is frequently the case during the first 48 hours of life. With regard to conduction hearing loss, the sensitivity of TEOAE is generally greater than that of AABR. The pass rate for AABR tests within the first 48 hours of life is therefore higher.
In recent years, combined TEOAE/AABR procedures for newborn hearing screening have grown in importance worldwide. The latter involves the testing of OAE and (if an abnormality is detected after one or several tests) following this by AABR testing. A procedure of this kind combines the benefits of OAE testing – namely the short test duration, a low detection threshold (30 dB) and low costs for consumables – with those of AABR assessment, namely higher specificity and the fact that neural or central auditory pathologies may also be detected because a larger portion of the overall auditory system is investigated.
What is involved in newborn hearing screening
Procedures for newborn hearing screening have not yet been standardized across different countries. In Germany, universal newborn hearing screening (i.e. involving the screening of all neonates) has been in place since 1 January 2009, and is paid for by the statutory health insurance providers as a standard service. Both ears are individually tested by means of TEOAE or AABR, with screening to be performed by the third day of life. Under the arrangements, the testing procedure used for at-risk children involves AABR only. Children born prematurely should be tested no later than the originally calculated date of birth; where the child is ill or has multiple disabilities, these additional problems must be taken into account and necessary clinical measures undertaken by the age of 3 months. If the result of the initial TEOAE or AABR test is abnormal, a repeat AABR screen should be carried out for both ears, preferably on the same day, but no later than the 10th day of life. If an abnormality is detected by this repeat AABR screen, then extensive diagnostic testing by paediatric audiologists is required by the 12th week of life to confirm or rule out hearing impairment. Suitable therapy (i.e. fitting with assistive hearing devices and early intervention) should be initiated before the child is 6 months old.
General framework for the screening programme
Newborn hearing screening is a highly complex overall system and process, involving many different components. It can be fully effective only if all procedures are reliably coordinated with a minimum of delay. If hearing screening is to be universal (i.e. to include each and every child), then consideration must also be given not only to the screening technique itself, but also to the external framework governing the complete screening programme for a given region or country. This will enable the actual objective of the screening to be met: namely, to bring forward the time at which therapy for hearing disorders in early childhood commences (compared to that in a population without systematic screening).
The time of diagnosis cannot always be brought forward, however, even where newborn hearing screening is in place, because many different factors may conspire against an early start to therapy. For example, if the first screen necessitates it, the child may need to have repeat screening after a few days. Where is the child to undergo appropriate testing? When can an appointment for this be obtained? Will the parents be accompanying this child for this appointment? Is there any confidence at all in the results of the first screening? What happens if the repeat screen again produces an abnormal finding? Who can diagnose hearing disorders in children to a sufficient level of accuracy? When can therapy begin, and how good will it be?
Necessarily, the screening process can be regarded as completed only when a hearing disorder has either been ruled out or has, unequivocally and comprehensively – specifying the nature of the therapeutic intervention and the time at which it will start – been diagnosed.
Educating the parents
Providing parents with written information resources, and educating them with regard to the benefits of hearing screening, will play a big part in whether they indeed participate. The more firmly established screening is within a given region, the more often parents will actively request a hearing test for their child. The ‘refusers’ constitute so small a group – vanishingly small, in fact – that they do not warrant any further attention here. A much bigger category is that of the ‘indifferent’, who do not expect screening of this kind and will not take action unless their awareness of it is raised ‘from outside’.
If the first test necessitates a repeat screen, parents need to be briefed on the possible reasons for this. In the first days of life, sound transmission problems caused by the presence of vernix or residual amniotic fluid may be responsible; this, however, need give no cause for concern, because in about 90 % of cases the child passes the repeat screening a few days later. In babies born prematurely, brainstem potentials are often detectable only once the child has reached their originally predicted date of birth. Equally, however, the result may indeed indicate a functional disorder of the auditory system. Whatever happens, therefore, dogmatic diagnostic statements are to be avoided following screening, as is speculation or ‘leaning’ in a particular direction. Depending on the screener’s level of knowledge, they may seek refuge in excuses such as “Sometimes the equipment doesn’t work” or “It’s caused by the amniotic fluid”. The problem here is obvious: in seeking to ‘take the easy way out’ and play down the result’s possible significance, the screener may produce exactly the same effect in the parents. The parents may now feel a repeat screen to be unnecessary, as specific reasons have been given for the initial outcome. If parents have received this kind of input, it is very difficult to persuade them that their child does indeed require repeat screening. A good screener will be able to advise parents of the full range of possibilities and explain the necessity of having a properly conducted second screen as soon as possible.
Tracking
The purpose of tracking is to keep tabs on children following discharge from the birthing centre until such time as they may need a repeat screen or to be taken to a referral appointment for diagnosis. It is thus addressed at children who have left the birthing centre without having (binaurally) passed screening (a monaural screening strategy no longer being pursued in the majority of cases). Tracking is necessary as parents differ in the extent to which they are committed to this process. This state of affairs is understandable up to a point, as ‘bad-news messages’ tend not to be well tolerated during the process of parent-child bonding. The fact that, without tracking, only around 50 % of parents take their child for repeat screening, speaks volumes about the widespread ‘denial’ surrounding this issue following discharge from the birthing centre.
The child’s parents are advised, by means of written reminders and phone calls, to have the child’s ears tested as soon as possible at a suitable follow-up centre. This necessitates the availability of regional centres at which systematic procedures are followed.
Follow-up
‘Follow-up’ refers to examination procedures, making use of diagnostic means, which are conducted subsequent to screening outcomes that require further investigation. Specialist centres provide different investigative procedures for this purpose and are especially geared towards diagnosing hearing disorders in children. The quality of both diagnosis and therapy depends greatly on how qualified and experienced those providing it are. It is here that therapeutic measures tend to be commenced, and aims set.
These ear, nose and throat specialists or paediatric audiologists, who are specially qualified to diagnose children, apply procedures to arrive at a definitive diagnosis that confirms or rules out hearing impairment. As children are unable to actively cooperate in the diagnostic process, several testing procedures are used here to determine the nature, the location and the degree of the hearing disorder (see also Module 2). The term ‘paediatric audiology’ is appropriate when – in addition to audiometric methods – the underlying causes and treatment are brought into consideration, along with medical care, hearing-aid fitting and language intervention for hearing-impaired children. As a rule, children’s hearing needs to be tested by experienced examiners in order that both false positive and false negative results are avoided. It should be noted that the most important method of examination is testing of otoacoustic emissions using diagnostic procedures, frequency-specific brainstem audiometry, both high- and low-frequency tympanometry, and (in infants) reflex audiometry (see Module 2). Differential diagnostics also requires the possible causes of the condition (i.e. its etiology) to be identified, a prognosis to be made and a therapy indication to be proposed. Follow-up can and should take place shortly after a screening that requires further investigation. The diagnosis can also be made during the first weeks of life. Most therapies are available from an early stage. It is vital that all children be diagnosed by the age of three months.
Starting therapy
Technical intervention and therapy for childhood hearing disorders have undergone a radical change as medical and technological advances have been made. Assistive hearing devices, especially the cochlear implant (CI), provide access to the world of hearing even for severely hearing-impaired children.
Surgery to improve hearing, or hearing-aid fitting, can generally be carried out from the age of around 2 months onwards. Operations of this nature merit consideration especially for children with congenital facial clefts, such as cleft palates involving the jaw and lips. These children often need to be fitted with grommets (tiny tubes that ventilate the middle ear) at an early stage, as the impaired function of the palate prevents normal ventilation of the middle ear. Bilateral blockage of the ear canals caused by ear malformations is not surgically corrected until later in childhood, and necessitates fitting with bone conduction hearing aids.
Hearing-aid fitting in children so young places exacting demands on hearing-aid acousticians, early-years practitioners and medical staff. If – despite fitting with hearing aids – both hearing and language fail to develop at all, or show only rudimentary development, then it may be necessary to fit the child with an electronic inner-ear prosthesis called a cochlear implant. This device, implanted under the scalp, enables the auditory nerve to be directly stimulated by means of electrical impulses, from which the brain (after a certain period of training) generates auditory sensations – even if the child has never heard before. The prerequisites are that the cause of their virtual deafness lies in the inner ear (which tends to be the case) and that the auditory nerve is intact. The earlier this therapeutic intervention – which enables children who are practically deaf to hear – begins, the more successful it is (see Module 3).
Fitting with hearing aids or cochlear implants is only part of the intervention required during the phase, lasting several years, during which hearing-impaired children (some of whom also have multiple disabilities) and their parents receive care and assistance. It is followed by a particularly intensive phase of auditory habilitation. As well as ongoing professional support, involving the parents in the therapeutic process also plays an important part (see Modules 4 & 5).
Summary
For a child with a congenital hearing disorder, newborn hearing screening improves the chances of obtaining an earlier diagnosis and suitable therapy. Especially in the European countries, newborn hearing screening programmes should definitely be mandatory. The medical and technical possibilities have been sufficiently explored and are continually advancing. Newborn hearing screening programmes are relatively cost-effective and can be made available to all children. If hearing disorders are detected and treated in early childhood, these children will have the same educational, career and development prospects as those who just happened to be born with normal hearing.
Review questions
Can newborn hearing screening be used to diagnose a hearing disorder?
- No; hearing screening merely provides an indication that a functional disorder may be present, which can be confirmed only by using proper diagnostic procedures. The time interval between screening and actual diagnosis should be kept short.
- Yes; as the majority of children pass the screening, the probability that those who do not (i.e. those whose test requires further investigation) indeed have a hearing disorder is very high.
- Yes, otherwise hearing screening of this nature could not be recommended. The screening procedure must, however, be suitable and the test results analysed automatically, in order that even inexperienced screeners can arrive at a reliable diagnosis.
- No, hearing screening is not a diagnostic procedure. Its purpose is to enable hearing disorders to be ruled out at a very early stage in the majority of children. For children who do not pass the test, it provides a reference point for monitoring their ongoing hearing and language development more closely.
Do we know, after hearing screening, whether a child can hear or not?
- Yes, the outcomes make a distinction between normal-hearing children and those with hearing disorders.
- No; it is well documented that these tests do not yet produce meaningful enough results.
- No; in particular, children who do not pass the first test need to undergo repeat screening soon afterwards.
- Yes, the result of the hearing screening provides a sufficiently reliable answer.
How many parents take their children for repeat screening (and in sufficient time) if the initial outcome requires it?
- Around 80 %.
- Around 75 %.
- Around 50 %.
- Around 25 %.
Tracking involves:
- keeping tabs on children with abnormal test results.
- writing reminders to the parents of children with abnormal test results.
- a responsibility to conduct screening and to keep track of the children until their repeat screening.
- keeping tabs on children with abnormal test results (or who have not undergone full screening, or have not been screened at all) until they obtain a conclusive outcome or, if necessary, until therapy begins.
Follow-up involves:
- a child who fails to pass the screening presenting for reassessment at a medical centre;
- checking an abnormal screening finding at the birthing centre or other medical centre;
- fitting of hearing aids at a specialist centre;
- undergoing diagnostic procedures at a specialist (i.e. paediatric audiology) centre to confirm or rule out impairment.
When should follow-up take place?
- Within the first few weeks of life, but before the child reaches 3 months of age.
- Not before the child reaches the age of 3 months, but no later than the 6th month of life.
- Not before the child’s first birthday.
- Not until the finding requiring further investigation has been confirmed by the treating paediatrician; after this, however, no more time should be lost (which requires efficient scheduling of patient appointments).
When should therapy for an early-childhood hearing disorder begin?
- Before the child turns 3 years of age (i.e. nursery school age).
- Before the child turns 6 months of age.
- Before the child turns 2 months of age.
- Before the child turns 2 years of age.
References
Hildmann A (2008): Frühkindliche Hörstörung – eine interdisziplinäre Aufgabe. Sprache Stimme Gehör 32, 2-5
Joint Committee on Infant Hearing (2007) Year 2007 Position Statement: Principles and Guidelines for Early Hearing Detection and Intervention Programs. Pediatrics 120,898-921 http://www.pediatrics.org/cgi/content/full/120/4/898 (07.02.2009)
Klinke R (2008): Hören lernen – die Bedeutung der ersten Lebensjahre. Sprache Stimme Gehör 32, 6-11
Yoshinaga-Itano C (2003): From Screening to Early Identification and Intervention: Discovering Predictors to Successful Outcomes for Children With Significant Hearing Loss. Journal of Deaf Studies and Deaf Education 8:1, 11-30
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