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Guidelines for Cochlear Microphonic Testing
سال ارائه مقاله : 2011// سال پذيرش : 2011// نوع مقاله : Guide Line دسته بندي :
The cochlear microphonic (CM) is a pre-neural response from the cochlear outer hair cells which is thought to follow the waveform of the stimulus - it is as though the cochlea is acting as a microphone, hence the term. Like the oto-acoustic emission (OAE), when reliably present, it can be taken as evidence of hair cell function but cannot be used to estimate hearing threshold. Auditory Neuropathy Spectrum Disorder (ANSD) is characterised by an absent or abnormal auditory brainstem response (ABR) in the presence of evidence of outer hair cell function. This is thought to arise when there is a failure to transmit hair cell activity to the auditory nervous system or when there is abnormally poor temporal synchronisation of the signals. Either an OAE or CM may be taken as evidence of outer hair cell function. The presence of an OAE or a CM (using a stimulus at or below a level that does not evoke a recordable ABR) is usually suggestive of ANSD. A reliably present OAE is evidence of outer hair call function and CM testing is usually not necessary. However, an OAE may be absent for a number of reasons (e.g. a conductive component) so it is important to consider CM testing when the OAE
is absent and the ABR is absent or abnormal as defined below. An absent OAE cannot exclude ANSD. The CM is known to be less vulnerable to the effects of a conductive component than the OAE (since a conductive component affects sound both on the way into the ear and on the way out). Where the AC 4kHz tpABR threshold is significantly raised ( >75dBeHL) and there are no recordable BC 4kHz tpABR responses for that ear it is recommended to switch to ckABR (i.e. a broad band stimulus) to determine if any AC responses can be recorded up to the maximum recommended stimulus level available. This is because absent tpABR responses at 4kHz cannot exclude an island of better hearing which could generate an ABR and CM in a “conventional” cochlear hearing loss. However in some cases it may be possible to record a low frequency (e.g. 1kHz) tpABR and this must be considered when interpreting the CM (see the notes in the section on interpretation, below). In practical terms therefore if CM testing is being considered it is necessary to first perform a ckABR up to the maximum permitted stimulus level (if required) in addition to tpABR. ANSD should be considered when a click ABR is not present at the maximum permissible stimulus level or is present but abnormal at or above 75dBeHL. ABR waveforms should be considered abnormal if they have unexpected (even for a baby with hearing loss) latencies, amplitude or morphology (e.g. missing peaks). It is worth noting that the label ANSD merely indicates the pattern of results, not a single pathology: accordingly among these cases will be babies with auditory maturational delay which may resolve, as well as those with a permanent condition.