23 Evoked Response Audiometry
In response to sound stimulation, neural electrical potentials are produced by various parts of the auditory system from cochlea to cortex. Evoked response audiometry (ERA) is the technique designed to measure these signals. No conscious response is required from the patient, thus these tests can be considered as more objective measures of the auditory system (there is usually a subjective evaluation of the waveforms obtained but some equipment utilises an objective measure of the probability of the waveform being present). ERA is ideal where patient cooperation is not forthcoming, such as small children, suspected malingering adults, etc.
The evoked potentials may be evoked by an external stimulus (exogenous) where the response is related to the nature of the stimulating signal or by a cognitive response to a signal (endogenous) related more to the psychological significance of that signal.
Other than electrocochleography, these potentials are recorded using scalp electrodes and are, by their nature, of small magnitude. Therefore, efforts need to be made to amplify the signal and reduce the background signal ‘noise’.
Signal detection is optimised by appropriate electrode placement with recording electrodes at opposite positions so that a positive potential in one corresponds to an equal and opposite negative potential in the other. Background noise is reduced by having the subject in a quiet, dimly lit room and a comfortable position to avoid distractions and eye, and other muscle, movements. The signal-to-noise ratio is improved by amplification, filtering, removal of random noise (picked up by the ground electrode) and by repetition and averaging.
The amplitudes and latencies (time delay) of the various potentials differ at various parts of the auditory system and this information can be used appropriately for amplifier and filter settings, to allow a more focused analysis of the various potentials.
Furthermore, the nature (click/chirp/tone burst), frequency and volume of the stimulus also affect amplitude and latency. So, for example, louder sounds, by virtue of synchronously stimulating more neural elements, produce shorter latencies and larger amplitudes.
Finally, the majority of measured responses are designated as transient. In other words, the interval between successive stimuli is sufficient to allow the auditory system to return to its resting state before the next signal occurs. However, if the interval between stimuli is shortened so that the response to the previous signal has not died away before the next signal is received, then the resulting response is referred to as a ‘steady-state response’ (SSR).
There are a number of different measurable responses, shown in Fig. 23.1, and includes cochlear, brainstem, middle-latency, long-latency and cognitive potentials. Middle-latency potentials, at around 25 ms, lend themselves to SSRs and are the most studied of this phenomena.
2. Auditory nerve and brainstem-evoked potentials.
3. Cortical electrical response audiometry.
Electrocochleography (ECochG) aims to measure the signal produced by the cochlea and cochlear nerve in response to acoustic stimulation.
The patient lies comfortably in a soundproof room. A ground electrode is attached to the patient’s forehead and a reference electrode to the ipsilateral mastoid. The active electrode is usually a transtympanic needle placed on the promontory (tympanic membrane or extratympanic canal electrodes may be used but give a less satisfactory signal) after preparation with local anaesthetic (EMLA). The test signal can be produced using a loudspeaker or headphones (especially if acoustic conditions are less than ideal). Wideband clicks and high-frequency tone bursts are the usual stimulating test signals.
The signal recorded by ECochG is described as a compound action potential (Fig. 23.2). It is diphasic at threshold and is made up of three parts: (a) The cochlear microphonic (CM): This signal is produced by the hair cells and resembles the pattern of the basilar membrane vibration. It has no threshold and increases in amplitude with the stimulus intensity. Its polarity follows that of the test signal. (b) The summating potential (SP): This complex potential is derived from a variety of sources but, in essence, is an alteration of the electrical potential baseline (usually negative) in response to a sound stimulus. It is also produced by cochlear hair cells and does not adapt in response to high stimulation rates. (c) The action potential (AP): This is the depolarisation of the cochlear nerve and is similar in many respects to any neural depolarisation. It has a threshold, is independent of signal polarity, and exhibits adaptation.
23.4 Clinical Indications
High-resolution computerised axial tomography and magnetic resonance imaging (MRI) have superseded many older otological investigative techniques and consequently removed many of the indications for EcochG, particularly the search for an acoustic neuroma. In current practice, ECochG is rarely used but may be useful for the following:
1. Threshold testing. ECochG is the most accurate of the electrical response audiometric techniques for threshold testing and can predict to within 5 to 10 dB of the psychoacoustic threshold at 3 to 4 kHz. Unfortunately, it gives little low-frequency information (< 1 kHz) but has the advantage of being a monaural test technique and is relatively resistant to minor muscular contractions which would preclude brainstem response audiometry and is unaffected by general anaesthetic. It is therefore particularly useful in very young children or those with neurological disorders.