Early acoustic evoked potentials

Early acoustic evoked potentials (FAEP) are a subgroup of acoustic evoked potentials . They have their electrical source in the auditory pathway between the cochlea , the auditory nerve and various core areas in the brain stem ( upper olive nucleus (nucleus olivaris superior), lateral loop ( lemniscus lateralis ), lower quadruple nucleus ( colliculus inferior )). The FAEP signal consists of 5 waves, which according to the first descriptors Jewett and Williston (1971) are counted with JI to JV. Clinically, the origin of each wave is assigned to a core area in the brain stem, although intraoperative measurements do not support this interpretation.

The FAEP and its measurement are also known as BERA ( brainstem-evoked response audiometry ; German: Hirnstammaudiometrie), ABR ( auditory brainstem response ) or AEHP (acoustically evoked brainstem potentials ).

Areas of application

FAEPs have two basic areas of application:

Diagnostics of the hearing system (so-called "audiological" indication, then they are usually called BERA or ABR),
Diagnostics of the nerve tracts (so-called "neurological" indication, then the designation is usually FAEP or AEHP).

The medical fields of application thus overlap between the specialist areas especially ENT , phoniatrics - pediatric audiology on the one hand and neurology on the other.

Topodiagnostics: Diseases of the auditory nerve can be examined by comparing the running time with standard value tables and by intra-individual side comparison . After sudden hearing loss , dizziness or tinnitus , FAEPs are routinely performed to rule out acoustic neuromas , but are increasingly being replaced by imaging procedures ( MRI ). Degenerative or inflammatory diseases such as B. Multiple sclerosis as an early sign of auditory nerve involvement leading to changes in terms and potentials.

Hearing threshold determination : Especially in pediatric audiology , but generally with uncooperative patients, FAEPs are used to determine the hearing threshold, as they can also be derived in sedation or anesthesia . The stimulus level at which the potential JV can no longer be detected corresponds to the hearing threshold. Frequency-specific measurements can also be carried out using stimulus filter methods (previously mostly notched noise methods , more recently chirp stimuli).

Newborn hearing screening : An automated derivation of the FAEP (so-called Automated Auditory Brainstem Response, AABR measurement) is made possible by an evaluation algorithm of the devices. If correct potentials are recognized at the defined stimulus level, a "pass" is displayed as the result, otherwise a "refer" is displayed in the event of a fault or missing potentials. With combination devices, the diagnostic mode can be switched to immediately after the AABR.

Neuromonitoring : especially in neurosurgery .

stimulation

By default, a click stimulus (also tone burst) is used via headphones with a capsule. Insertion earphones with a tube or loudspeaker are used less often because the longer duration of the stimulus to the eardrum can impair the synchronization in the auditory nerve. A frequency-specific hearing threshold determination can be achieved via notched noise stimulation (notched noise, the stimulus is covered by noise with the target frequency omitted) or with chirps . Usual measuring frequencies are 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz.

Lead / electrode position

The active brain area, the “source”, should be located between the lead electrodes (1) and (2): (1) hair swirls on the head (Cz in the 10-20 system; “vertex”); (2) behind the ear ("mastoid"). A third electrode (3) is used as a reference (“ground”), usually forehead or neck.

evaluation

Bera of a patient with normal findings in the left ear but atypical findings in the right ear (amplitudes decreased, latency prolongation I-V)

The audio track

The Hörbahn generates the following responses:

Wave I - the peripheral portion of the cranial nerve VIII
Wave II - the central portion of the vestibulocochlear nerve
Wave III - cochlear nucleus
Wave IV - the core complex of the nucleus olivaris superior or the lemniscus lateralis
Wave V - Lemniscus lateralis or Colliculi inferiores

Lemniscus lateralis (red) and Lemniscus medialis (blue)

The shape of the five FAEP waves (JI to JV) depends on the volume of the stimulus (stimulation level). Latency is the time between the start of the stimulus ("onset") and the wave maximum, while amplitude is the difference between the maximum and the subsequent minimum.

For the audiological BERA, absolute latencies are measured (stimulus to wave maximum, designated LV, LIII or LI) and plotted against the stimulation level (level-latency diagram, PLD). By comparing the individual PLD with standard curves, one can deduce the hearing threshold - and in the case of hearing loss - the location of the hearing loss.

For the neurological FAEP, the measurement curves of the right and left ear are compared with a fixed level. If there are side differences or changes in the shape of the waves, relative latencies are measured (e.g. LV LI, so-called “brain stem transit time”).

Measurement results from different laboratories can only be compared with one another if the measurement parameters are identical.

Technical details of the measuring apparatus

The table shows the differences between the individual technical parameters of the measuring equipment for the two areas of application:

	FAEP	BERA
Filter type	Butterworth	Bessel
Filter limits	150/1500 Hz	30/3000 Hz
Interstimulus Interval (ISI)	100 ms	20-50 ms
Min. Number of averages	1000	2000
Acoustic stimulator	Headphones, insert phones	calibrated headphones
Stimulus form	Click (not standardized)	Click according to IEC 645-3
Stimulus level	70 - 90 dB HL	10-100 dB HL
Electrodes	Surface electrodes vertex mastoid	Surface electrodes vertex mastoid
Electrode impedance	5 kΩ	3 kΩ

ASSR

ASSR stands for Auditory steady-state responses (translated: continuous responses to listening stimuli).

Acoustically evoked brainstem potentials are usually transient (“volatile”) potentials of a brainstem response to a tone or click stimulus with a latency of 1–20 ms after the stimulus. ASSR are triggered by clicks, but preferably by amplitude ( AM ) and frequency-modulated ( FM ) stimuli (mostly chirps ), which are offered with such a high repetition rate that the transient responses in the brain stem overlap and what appears to be a permanent response. By statistical measurements, the ASSR from the be signal-to-noise ratio (SNR, signal-to-noise ratio) is calculated. Usually the amplitude of the stimulus response is compared with the background noise of the EEG or its phase variability. One way to "measure" the ASSR is to subject a number of responses, or an average response, to an FFT to examine amplitudes and phase . If the amplitude of the ASSR shows a significant SNR, the answer is considered “present”. In contrast to the classic FAEP, the assessment of the ASSR does not require a visual, subjective curve assessment by the examiner. In the case of stimulus presentation of chirps of different frequencies with slightly different duration of the repetition rate, both ears can be measured simultaneously with several frequencies, which means a clear time advantage compared to the conventional frequency-specific FAEP derivation. A frequency-specific audiogram can be "calculated" from the ASSR using device-specific standard value curves.

literature

James W. Hall: New Handbook of Auditory Evoked Potentials . Pearson, Boston, Mass. 2006, ISBN 0-205-36104-8 .
Konrad Maurer, Nicolas Lang, Joachim Eckert: Practice of evoked potentials . Steinkopff-Verlag, 2005, ISBN 3-7985-1500-X .

Web links

Henning Joachim Stegmann: BERA screening methods in use with high-risk children. (PDF; 2.2 MB). Inaugural dissertation. Faculty of Medicine at the Philipps University of Marburg, 2009.

Individual evidence

^ DL Jewett, JS Williston: Auditory far fields averaged from the scalp of humans. In: Brain. 94, 1971, pp. 681-696.
↑ AR Møller: Evoked potentials in intraoperative monitoring. Williams & Wilkins, Baltimore 1988, ISBN 0-683-06115-1 .
↑ JI Benito-Orejas, B. Ramírez, D. Morais, A. Almaraz, JL Fernández-Calvo: Comparison of two-step transient evoked otoacoustic emissions (TEOAE) and automated auditory brainstem response (AABR) for universal newborn hearing screening programs. In: Int J Pediatr Otorhinolaryngol. 72 (8), Aug 2008, pp. 1193-1201.
↑ E. Stürzebecher et al.: Frequency-specific hearing threshold measurement using "notched noise" BERA in children . In: ORL Nova . No. 5 , 1995, p. 300–306 , doi : 10.1159 / 000313227 ( karger.com [PDF; accessed October 8, 2018]).
↑ James W. Hall: New handbook of auditory evoked responses. Pearson, Boston 2007, ISBN 978-0-205-36104-5 .

[1] DL Jewett, JS Williston: Auditory far fields averaged from the scalp of humans. In: Brain. 94, 1971, pp. 681-696.

[2] AR Møller: Evoked potentials in intraoperative monitoring. Williams & Wilkins, Baltimore 1988, ISBN 0-683-06115-1 .

[3] JI Benito-Orejas, B. Ramírez, D. Morais, A. Almaraz, JL Fernández-Calvo: Comparison of two-step transient evoked otoacoustic emissions (TEOAE) and automated auditory brainstem response (AABR) for universal newborn hearing screening programs. In: Int J Pediatr Otorhinolaryngol. 72 (8), Aug 2008, pp. 1193-1201.

[4] E. Stürzebecher et al.: Frequency-specific hearing threshold measurement using "notched noise" BERA in children . In: ORL Nova . No. 5 , 1995, p. 300–306 , doi : 10.1159 / 000313227 ( karger.com [PDF; accessed October 8, 2018]).

[5] James W. Hall: New handbook of auditory evoked responses. Pearson, Boston 2007, ISBN 978-0-205-36104-5 .