November 28, 2016.
By: Prajakta Bhogte
Ninety percent of babies with hearing loss are born to parents with normal hearing. It is therefore not surprising that the majority of these parents want their children to grow up to listen and talk, like other children their age. Therefore, when measuring the hearing loss of these babies and young children, it becomes critical that we obtain accurate information on their responses to spoken language or speech sounds, not just tones. Further, it follows that the measures used to evaluate hearing loss must be the result of a battery of tests; no single test conveys all the information parents and professionals seek. These test results when compiled must give a complete picture of how the baby or young child hears.This information must be co-related to determine how much of spoken language the child hears.
There are two types of tests for measuring hearing loss (hyperlink to Tests of Hearing): Subjective tests of hearing and Objective tests of hearing. Subjective tests of hearing require the participation of the child/adult in terms of a specific response, as in Behavioural Audiometry (hyperlink to page). Objective tests of hearing do not need the child/adult being tested to participate. Objective tests of hearing are also referred to as Electro-physiological tests of hearing. Auditory Brainstem Response (ABR) or Brainstem Evoked Response Audiometry (BERA) is an example of the objective tests of hearing.The ABR or BERA is commonly administered at hospital centres; it may also be done at specific audiological facilities.
The ABR or BERA is administered in order to determine:
i) how much the child or adult’s hearing loss is (known as Threshold ABR)
ii) the site of lesion or where the hearing loss occurs ( known as Diagnostic ABR) (i.e. whether hearing loss is cochlear or retro-cochlear which means beyond the level of cochlea)
The ABR or BERA is also done for children and adults who may have additional difficulties which make it difficult-to- test them behaviourally.
The Click stimulus is used to adminster the BERA.These click stimuli contain only high frequency information. It is for this reason that the information conveyed by BERA or ABR testing is about the child or adult’s hearing in the high frequencies. This information is general in nature. In order to understand what a child or adult hears, we need specific information related to each frequency in the speech spectrum (ie. 250 Hz to 6000 Hz). This information is conveyed by the child’s audiogram that is the result of Behavioral Testing (hyperlink).The audiogram is then compared to the results of the BERA to check that information across tests is consistent.
In order for the ABR to be administered, the baby or young child needs to be sleeping. Children for whom a natural sleep-state is not attainable during testing, may need sedation. The audiologist then places electrodes or stickers on the baby’s or young child’s head and behind the ears. Sounds are presented to the child or adult via ear inserts or via head phones and responses to these click stimuli are picked up by the ABR system and recorded. These responses depend on the maturation of the baby or child’s neural structures. If the baby’s or very young child’s neural structures are not mature enough, then the ABR system is unable to pick up auditory responses, even though there is normal hearing peripherally(i.e. the structures of the ear are normal). Therefore, ABR or BERA cannot be viewed in islolation. Hearing aids must not be fitted on the basis of the results of a BERA alone.
The audiogram conveys important information that enables the audiologist to programme the child or adult's hearing aids or set levels at each speech frequency, so that speech is heard optimally, given the technology. Hearing aids must be fitted and cochlear implantation recommended, after considering the results of both Behavioral Audiological Testing and Electro-physiological testing in babies, young children and adults.