chronic suppurative otitis media

EVOKED RESPONSE AUDIOMETRY 79
2. The summating potential. 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.
3. The action potential. This is the depolarization of the cochlear nerve and is similar in many respects to
any neural depolarization. It has a threshold, is independent of signal polarity and exhibits adaptation.
Clinical indications
High-resolution computerized axial tomography and MRI have superseded many older otological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 may be used for:
1. Threshold testing. ECochG is the most accurate of the electrical response audiometric techniques for
threshold testing and can predict to within 5–10 dB of the psychoacoustic threshold at 3–4 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.
2. Investigation of suspected Menière’s disease. Typically there is an increase in the summating potential
with a normal action potential in the affected ear.
3. Intraoperative monitoring during surgery around the inner ear and internal meatus.
Brainstem electrical response audiometry
Brainstem electrical response audiometry (BERA) records the signals produced in the brain stem detected
by electrodes placed over the mastoid, forehead and vertex.
Technique
The patient reclines on a bed or couch. The electrodes are surface electrodes. The active electrode is
attached to the vertex, the reference electrode to the ipsilateral (test ear) mastoid process and the ground
electrode to the contralateral mastoid process. The hardware and test signals used (wideband clicks and
high-frequency tone bursts) are identical to those used for EcochG, but the filter and time window settings are
altered. The signals are usually presented using headphones to allow monaural testing. As the evoked
responses are so small they are easily masked by other neuromuscular signals. It is therefore important that
the patient stays as still as possible and, because of their size, several thousand responses are analysed as
opposed to hundreds in ECochG. The results are analysed by looking at the absolute values for various
wave latencies, the so-called I-V latency, and comparing results between the two ears. The accuracy of the
I-V latency has been improved by combining ECochG with BERA to aid the detection of wave I.
Physiology
The signal recorded by BERA is made up of a five-wave complex, which is thought to represent successive
synapses in the auditory pathway as follows (Jewett classification):