ABR and Hearing Aid Fittings
In certain cases where behavioral thresholds cannot be attained, ABR thresholds can be used for hearing aid fittings. New fitting formulas such as DSL v5.0 allow the user to base the settings in the hearing aid on the ABR thresholds. Correction factors do exist for converting ABR thresholds to behavioral thresholds, but vary greatly. For example, one set of correction factors involves lowering ABR thresholds from 1000–4000 Hz by 10 dB and lowering the ABR threshold at 500 Hz by 15 to 20 dB. Previously, brainstem audiometry has been used for hearing aid selection by using normal and pathological intensity-amplitude functions to determine appropriate amplification. The principal idea of the selection and fitting of the hearing instrument was based on the assumption that amplitudes of the brainstem potentials were directly related to loudness perception. Under this assumption, the amplitudes of brainstem potentials stimulated by the hearing devices should exhibit close-to-normal values. ABR thresholds do not necessarily improve in the aided condition. ABR can be a inaccurate indicator of hearing aid benefit due to difficulty processing the appropriate amount of fidelity of the transient stimuli used to evoke a response. Bone conduction ABR thresholds can be used if other limitations are present, but thresholds are not as accurate as ABR thresholds recorded through air conduction.
Advantages of hearing aid selection by brainstem audiometry include the following applications:
- evaluation of loudness perception in the dynamic range of hearing (recruitment)
- determination of basic hearing aid properties (gain, compression factor, compression onset level)
- cases with middle ear impairment (contrary to acoustic reflex methods)
- non-cooperative subjects even in sleep
- sedation or anesthesia without influence of age and vigilance (contrary to cortical evoked responses).
Disadvantages of hearing aid selection by brainstem audiometry include the following applications:
- in cases of severe hearing impairment including no or only poor information as to loudness perception
- no control of compression setting
- no frequency-specific compensation of hearing impairment
Read more about this topic: Auditory Brainstem Response
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