Auditory Evoked Potential (AEP)Testing Lecture 11

Outline Principles/Background Purpose/Features View video Clinical application/limitations ABR Interpretations Threshold Differential Diagnosis Other Evoked Potentials

Underlying Principles Normal auditory system Transmit acoustic stimuli into an electrical response Stimulus – triggers “Action Potentials” Response to the stimulus is extremely small Repeated stimuli –response patterns “averaged” over time result in a robust tracing that’s observed

Background Computerized test of hearing (cochlear) and auditory nerve (neurological) functioning Used in evaluation of: Hearing Integrity Neurologic Integrity First application 1971(Jewett and Williston) Hundreds of published clinical studies on applications Does not provide info about cortical areas

Purpose To convert acoustic stimuli into electrical stimuli and measure associated brain wave activity Brain wave activity is monitored by measuring computer-averaged changes in EEG activity Electrical responses recorded from the scalp in response to an auditory stimulus Time that it takes for sounds to travel can be measured on the acquired waveforms http://www.youtube.com/watch?v=8ZXnN9AVL0o

Features Objective test Noninvasive Ear specific BAER. BSER, BAEP,ABR Noninvasive Ear specific Performed in quiet or sleep state Performed with AC and/or BC Performed with frequency specific stimuli

Clinical Application of evoked potential testing Differential diagnosis (cochlear and retrocochlear) Screening procedure for newborns Diagnostic tool to ID HL in infants and children Estimate hearing in difficult to test population Intra-operative monitoring Prognostic indicator with head trauma

Generator sites of ABR Major peaks in waveforms: labeled by Roman numerals I-V Response originates in the VIIIth cranial nerve Wave I: VIII distal Wave II: CN Wave III: SOC Wave IV: LL Wave V: IC

Generator Sites Without exception, every anatomical structure on one side of the brain has an identical structure on the opposite side.

Synchronous activation Reflects synchronous activation (onset type neurons)

Synchronous activation

Frequency range of ABR response Response dependent on activation of basilar region of the cochlea in response to click (2K-4K)

Stimulus Click: brief duration signal, broad band signal ______through AC headphones Tone burst : provides more frequency specific info Rate of stimulus: _______/sec Intensity : Start w high intensity and decrease Lowest level at which a repeatable waveform observed (of wave _______) is called threshold

Preparation Application of recording electrodes Placement of earphones Vertex Forehead Earlobe/mastoid Placement of earphones Patient lying still or asleep 30 minutes or longer

Electrode Set up

ABR clinical measures (time) Two Metrics Inter-wave Interval Absolute latency value of wave

ABR clinical Measures (intensity) One Metric Absolute threshold

Factors that influence ABR Age of subject (<18 mos., >60 yrs) Longer latency values for older and younger clients Gender Not affected by most drugs (including sedatives) Movement Temperature

Clinical limitations Frequency range 2000-4000 Hz, most important to ABR Does not estimate hearing levels in lower frequency ranges ABR is NOT a test of hearing Response provides no information on the auditory system above the brainstem level

ABR mostly reflects higher freq hearing

Normal Hearing: Wave V threshold @ 20 dB: Suggests normal hearing I-V interval

ABR used to determine cochlear function II III IV V Lower intensity associated with increased latency values Try to ID the dB HL level where you still observe Wave V V V V

Threshold ABR - Moderate HL Wave V responses observed down to 55 dB HL Results suggest a possible moderate hearing loss

Threshold ABR: Severe HL What is threshold?

ABR to determine function of VIIIth nerve Multiple Sclerosis VIII nerve tumors Meniere’s Disease Auditory Neuropathy Calculate Interwave Interval

Calculate IWI Hearing: WNL Right sided tinnitus Intolerance to loud sounds

Acoustic Neuroma

Reporting results of ABR

What would ABR look like?

Differential Diagnosis

Examples of Differential Diagnosis

Screening ABR Prevalence of HL NICU: .5-5% Well baby <1% Conducted on newborns prior to discharge Test at 2 levels: 60 dB HL and 30 dB HL Factors influencing outcome: Neurologic abnormalities Poor health Transient conductive problems Muscle artifact Collapsing canals Earphone placement

Screening ABR 60 dB HL 30 dB HL

Cochlear Microphonic (CM) (Wever and Bray, 1930) Summary of classic research Electrode in the auditory nerve of a cat Speech through loudspeaker The resulting electrical activity when transduced back to sound (thru a telephone receiver) and amplified, was transmitted as clear speech Pure tones were reproduced accurately as well up to 3000 Hz.

CM defined An electrical response from the cochlea reflects a combination of IHC and OHC function in response to acoustic stimulation CM Mimics the form of the sound pressure waves that arrive at the ear … (aka a “stimulus following response” Reverses in phase with changes in the stimulus polarity from rarefaction (-)to condensation (+)(Ferraro & Krishnan, 1997). looks like the waves "flip" or invert

Cochlear Microphonic in AN AN is defined as absent or severely distorted ABR with preserved OAE’s and cochlear microphonics CM “reverses or flips” when the stimulus reverses polarity (from + to -) Can only be seen if ABR done with both + and – polarity

Other Auditory Evoked Responses Middle Latency Response (MLR) Auditory Late Response (ALR)

MLR Documentation of CNS dysfunction above brainstem through thalamus estimation of auditory sensitivity in older children / adults (malingerers) State of arousal

ALR Assesses higher cortical processing from A1 and A2 areas P300 – information processing Latencies slow with age Auditory processing

Calculate Interwave intervals