Auditory Evoked Potentials (AEPs)

Auditory Evoked Potentials (AEPs)

• Acoustic stimuli reaching the inner ear are converted into a series of neuroelectric events, transmitted to the brain, not as "sound" but as electrical signals.
• Electrical potentials are generated by the cochlea, auditory nerve, brainstem, and auditory cortex.
• AEPs measure how the ears respond when stimulated with sounds.

Electrode Sites and Labels

• Electrode site selection is critical for recording a well-formed response versus observing no response.
• Electrode sites are defined by the 10-20 International System (Jasper, 1958).
• The first letter of the electrode label indicates the brain region (F = Frontal, T = Temporal, O = Occipital, P = Parietal).
• Right side electrode placements are identified with even numbers, left side with odd numbers.

Electrode Placements

• Some auditory evoked responses can be recorded using only 3-4 electrode sites.
• Cortical auditory evoked responses are sometimes recorded with 20 to 30 electrodes on the scalp.

Factors influencing AEPs

• Subject factors: Age, gender, medication, attention, body temperature, muscle artifact, peripheral hearing status, and neural status
• Stimulus Intensity will affect AEPs, these are measures in: dB sound pressure level (dB SPL), dB peak-equivalent SPL (peSPL_, dB hearing level (dB HL), dB sensation level (dB SL), dB normal hearing level (dB nHL): used in estimation of hearing thresholds

Transducers

• Transducers convert energy from one form to another, typically acoustic energy into electrical impulses.
• For most auditory evoked responses, the stimulus is acoustic, delivered via earphone/inserts.
• There are exceptions to this.
• Three main types of transducers are insert earphones, supra-aural earphones, and bone conduction oscillators.
• Soundfield speakers can also be used.
• Exceptions where acoustic stimulus isn't used, electrically elicited auditory evoked responses are recorded most often in cochlear implant patients.
• Air-conduction acoustic stimuli can be presented via loudspeaker, often used in cortical evoked responses in children and patients with hearing aids or cochlear implants.
• Bone conduction will elicit an electrical signal due to mechanical oscillation transmitted to inner ear fluids and hair cells.

Supplies for Preparing Electrode Sites

• Supplies to prepare electrode sites includes abrasive liquid substance such as NuPrep & gauze pads to scrub skin
• Conductive gel like Ten20 gel is used (for re-usable electrodes)
• A cotton tip applicator and abrasive gel
• Cloth medical tape for securing reusable electrodes

Electrode Types

• Includes disc electrodes using metals gold, silver, or silver chloride.
• Ear clip electrodes.
• Disposable electrodes.
• Ear canal electrodes.

Patient Set Up

• It's important to place electrodes on the skin as close as possible to the neural generator of the response.
• The signal then passes from Electrodes to the pre-amplifier and then to into the evoked response system
• Clinicians are usually looking for Electrode impedance: measuring opposition to alternating current (AC) flow.
• Impedance is measured by passing a small 10-Hz current between 2 or more electrodes and measuring the opposition to the flow of this current

Myogenic vs. Neurogenic Responses

• Myogenic responses are produced by changes in electrical potential in muscles on the scalp and behind the ear.
• Neurogenic responses are produced by electrical potentials in nerve units within the auditory pathways.

"Subjectively Objective" Responses

• Auditory evoked response measurement is referred to as "objective".
• Behavioral subject response is not required, but the analysis of response waveform is often dependent on subjective analysis
• Analysis is Influenced by clinical skill and experience.

AEP Classifications

• AEPs are generally described in milliseconds (ms).

• Early-evoked responses (10-15ms): Electrocochleography (ECOG) and Auditory brainstem response (ABR)

• Middle latency responses (15-60 ms): Auditory middle latency response (AMLR) and Auditory steady-state response (ASSR).

• Auditory late-latency evoked response (ALR) occurs later in the time window (50-200), where later responses are considered event-related evoked potentials.

• Exogenous: related to external factors

• Endogenous: having an internal cause or origin

• Microvolts (μvolts): one-millionth of a volt, or one-thousandth of a millivolt

Electrocochleography (ECOG/ECochG)

• Response from the cochlea and/or early portions of the auditory nerve. and is Used in hearing/balance assessment

The Auditory Brainstem Response (ABR)

• Objective measurement of auditory neural pathway function from the auditory nerve to the brainstem.
• Tests synchronous neural function.
• Estimates hearing sensitivity thresholds in individuals who are unable to tolerate traditional behavioral audiometry.
• The ABR is characterized by five to seven waves occurring within the first 10-15 milliseconds after stimulus presentation.
• Waves I, III, and V are most important components.
  • Wave I: Originates from the distal portion of the VIII nerve.
  • Wave II: Generated by the proximal portion of the VIII nerve as it enters the brainstem
  • Wave III: Arises from cochlear nuclei.
  • Wave IV: Arises from the superior olivary complex (SOC).
  • Wave V: Arises from the inferior colliculus (contralateral to the stimulated ear).
  • Waves VI and VII: Suggested site of origin is the medial geniculate body.

ABR recordings are Interpreted via

• Latency
• Amplitude
• Morphology

ABR Clinical Applications

• Assessment of peripheral auditory dysfunction in young or difficult-to-test children, patients with non-organic hearing impairment, those that are lethargic patients or comatose persons who cannot give a valid behavior response
• Neurodiagnosis includes, eighth cranial nerve tumors and cerebello-pontine angle tumors.
• Assessing cochlear implant candidacy. The ABR is helpful for cochlear implant patients
• Newborn hearing screening: The ABR is sensitive test to be used in hearing screening in infants, with very low failure rate in healthy newborns.
• Intra-operative monitoring during acoustic neuroma surgery, during cochlear Implants surgery, facial nerve surgery.
• Should never be used in isolation to identify hearing loss
• Auditory neuropathy/Auditory dis-synchrony

Auditory Steady State Response, or ASSR:

• Is frequency specific and diagnostic only.
• Complete evaluation takes longer as each frequency is tested.
• Helps to approximate the configuration of hearing loss.
• Differentiates between severe and profound loss

Auditory Middle Latency Response (ALMR)

• Occurs between 15 and 60 milliseconds after signal presentation Reflects activity from the cortex.
• Patient setup for measurement of the AMLR is essentially the same as for ABR.
• Patients must be awake because the amplitudes of the responses are reduced during sleep.
• In testing It is necessary to average 500 to 1000 responses to obtain a clear AMLR.
• Provides the same accuracy in frequency-specific threshold estimation as the ABR does
• ABR and AMLR may be obtained simultaneously
• AMLR is also useful in the assessment of neurological function of the higher central auditory nervous system

Auditory Long Latency Responses (ALRs)

• Responses appear at least 60 milliseconds after signal presentation.

• A major advantage of measuring the later responses is that stimuli include frequency-specific stimuli, such as pure tones, as well as short segments of speech.

• Responses are considerably larger than the earlier waves

• They can be tracked closer to the individual's behavioral threshold.

• In contrast to ABR and ASSR, it is affected by the patient's attention and has limitations when used with children as they Depend of state of consciousness

• Significant degradation can occurs with sleep, whether natural or induced and for this reason, can be used for threshold estimation as well as speech-assessment procedures

• The first late response (P1) often occurs at approximately 60 milliseconds after stimulus onset for a moderately intense tonal or speech sound.

• P1 is followed by a series of negative (N) and positive (P) waves (N1, P2, N2) occurring between 100 and 250 milliseconds

Interpretation of the Auditory Late Responses

• May aid in the estimation of threshold for pure tones over a wide frequency range and in the assessment of neurological function
• If patients are are cooperative and voluntarily tested if patients are able to
• The P300 potential is called event related because it depends on discrimination of target stimuli by the listener
• P300 is related to the perception or processing of stimuli rather than to the mere activation of the auditory nervous system by a stimulus

Auditory P300

• At approximately 300 milliseconds, a larger (10 to) positive wave is seen
• This response has become is known as P300 or the auditory event-related response
• The amplitude of the P300 can be increased significantly by having the patient count the stimuli (one per second)
• Late-evoked, P300, and MMN responses offer great promise for practical applications in diagnosis of neurological disease and injury
• Measured changes in the P1, N1, and N2 responses and the MMN response may find utility in the documentation of the effects of training

Mismatch Negativity

• Is elicited in response to any change in auditory stimulation-frequency, intensity, rise time, duration, etc.

• A small negative electrophysiologic response using an oddball paradigm, "rare” stimuli at 2000 Hz presented 15

• Subjects are instructed to ignore both the rare and standard stimuli, which may both be present in the same ear or presented individually to different ears

• The greater negativity on the waveform in response to the "odd,” or infrequently presented, stimuli

• Different responses to between rare and standard stimuli are caused by the brain is noticing a difference, hence the term mismatch

Introperatrive Monitoring (IM)

• Electrophysiological testing can be used to monitor responses from the brain during delicate neurosurgical procedures
• Electrical responses can offer information of the physiological state of the patient
• Surgeons will request information from the patient to alert damage can occur during surgery
• This can help prevent loss of hearing and damage other structures in the brain

Auditory Evoked Potentials (AEPs) measure the ears' response to sound stimuli by tracking neuroelectric activity in the auditory pathway, from the cochlea to the cortex. Electrode site selection, guided by the 10-20 International System, is crucial for accurately recording these responses. Proper placement, indicated by labels defining brain regions, ensures effective data collection.