CERA v ABR

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A comparison of ABR & Cortical ERA as threshold estimation tests

 

Since both the auditory brainstem response (ABR) and Cortical ERA can be used for threshold estimation, it is worth briefly highlighting the pros and cons of the two techniques.

 

ABR responses are generally less variable, more robust and essentially immune from the patient's mental state and can conveniently be recorded in sleep, under general anaesthesia or with the patient physically relaxed. However, the presence of excess myogenic (muscle) activity makes accurate threshold estimation unlikely in awake patients. Cortical ERA tests are much less sensitive to muscle activity but are affected by mental arousal level, making them most suitable for alert adults and passively co-operative older children. ABR tests require short duration stimuli which carry restricted frequency-specific information and makes low frequency tests especially difficult whereas cortical tests can use longer, highly frequency-specific stimuli of any frequency, allowing an audiogram to be constructed, assuming one has the time and inclination to do so.

 

The following table summarises the main pros & cons of the two tests as implemented on a standard ERA system when testing adults or older children (manual collection, replication & manipulation of data):

Issue

ABR

Cortical ERA

Age of Patient

All ages

Adults & children > 8 years

Requirements of patient

Low muscle activity

Reasonably alert

Patient Conditioning

Lying down, eyes closed, relaxed

Sitting, reading or watching a video

Frequency Specificity

Using clicks: almost none

Using tone pips: about 30dB per octave maximum audiogram slope

Almost ideal, capable of resolving audiometric notches

Frequency range

0.5 - 8 kHz;

250 - 8000 Hz

Accuracy of threshold in individuals

Clicks: typically ±10dB

Pips: depends on frequency: 10-15dB at 2 - 4 kHz; increasingly worse at lower frequencies

Typically ±10dB

Accuracy is poorer in a small (~5%) percentage of cases

Typical test duration (assuming 3-5 levels)

8-10 minutes per threshold

8-12 minutes per threshold

Calibration of Stimuli

ISO 389-6 (2007) but no bone tone reference values yet

Uses ISO / ANSI audiometric pure tone calibration standards

 

Summary of ABR -v- Cortical ERA comparison

 

The two techniques are similar in many respects (test time and accuracy of threshold prediction) yet offer different advantages and limitations. ABR can used in neonates and young children; Cortical ERA is highly frequency specific, allows testing down to low frequencies, accesses a greater portion of the auditory pathway and uses the same calibration reference data as used for conventional audiometry. Perhaps the situation is best summarised by Stapells (2002): Cortical ERA is "the (threshold estimation) measure of choice for most older children and adults”. “It is unfortunate that especially in the United States, the P1-N1-P2 slow cortical response is underused, having been replaced by the ABR".

 

Just a very brief note here on the emerging use of Steady-State techniques: the pros & cons appear very similar to those of the ABR but with (possibly) better frequency specificity and better developed objective assessment tools. However, Steady-State techniques have an even more stringent requirement for patient relaxation than does ABR and thus a greater degree of patient co-operation is needed. This may limit its utility in non-organic and medico-legal client groups.