Auditory Evoked Potential PDF

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Auditory Evoked Potential...

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FYI 22: AUDITORY EVOKED POTENTIAL AUDIOLOGY Auditory Evoked PotentialsAuditory = related to hearing Evoked = we create nerve signals that we are going to record Potential = an electro-chemical signal that comes from the nerves is detected at the surface of the skull with measuring electodes Electrophysiology- refers to measuring the electrical activity of the brain and body Electrophysiological measures- refers to the auditory brain stem response; auditory evoked potentials and otoacoustic emissions testing

These tests are based on the auditory central nervous system layout- there is not just one nerve running from ear to brain; nerves synapse as they ascend and there is electrical activity generated by each of the nerve pathways. Principals of operations: sound creates a nerve impulse; the nerve signal travels up the nerve pathways over time; the neural activity is recorded for the time in which the nerve signal is traveling. Thousands of stimuli are presented and the computer “picks out” and “cancels out” what was the same, each time. This leaves the auditory potential peaks of interest. For example, many clicks (like snapping fingers) are presented into the earphones and the patient is hooked up to a signal-averaging computer (removes unrelated brain activity and background noise so auditory evoked potential is revealed). This computer does the recording. With the exception of the ECoG, electrode placement is remote (meaning that the electrodes are placed away form the source, such as the vertex or top of the skull and behind the ears or on the earlobes). The tests (ABR, MLR LLR) are recorded as far-field responses. Four AEP tests: 1. EcoG (electrocochleogram)• Measures cochlear potentials and the distal portion of the VIII nerve response, which occurs within the first 5 msec of stimuli presentation (used primarily in diagnosis of Meniere’s disease) • Obtained near-field (electode is close to the source) • Invasive procedure- electrode placement is done through the TM and onto the promontory of the temperal bone or placed in the outer ear canal close to the TM • Effective for monitoring cochlear function in the operating room 2. ABR (auditory brain stem response)• Measures a series of 5 waves, labeled with Roman numerals, occurring within the first 10 msec of stimulus presentation. Waveforms are actually generated from different sites along the neural pathway from the cochlea up to the brain stem, to the base of the brain. Waveforms are repeatable and occur at predictable times, so a delay suggests retrocochlear pathology; click rate increases the “stress” on the CNS. • It can provide for threshold estimation because a click stimulates the cochlear partition between 1000 to 4000 Hz and the click can be made loud or soft.

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We can track the response of the VIII nerve from loud to soft and then estimate hearing sensitivity levels. This test is used to detect retrocochlear lesions and estimate hearing thresholds and slope of loss, particularly in a very young child/infant whose behavioral testing responses would be in question or unachievable. This test is used successfully to determine the existence of hearing loss in infants.

3. MLR (middle latency response)• Characterized by two successive positive peaks, occurring within the first 50 msec of stimulus presentation. Middle latency comes from the upper brain stem and primary auditory cortex, but does not reflect thought processes. • Uses include central auditory processing evaluation • It is the most difficult potential to record 4. LLR (late latency response)• Characterized by a negative peak followed by a positive peak, occurring within the first 250 msec of stimulus presentation. It reflects activity of the primary auditory and association areas of the cerebral cortex (most are thought reflected) • Cannot be acquired in sleep or sedation, so the best results are obtained when fully awake and attentive to sound presentations. • Used for central auditory processing evaluation. Abnormal or absent LLR responses in older children (>10 years) or adults are associated with CAPD. Summary: Four major uses of AEP measurements: 1. Prediction of hearing sensitivity 2. Neonatal hearing screening 3. Diagnostic assessment of central auditory nervous system functions 4. Monitoring of auditory nervous system function during surgery Grouped into categories based on the latency ranges over which the potentials are observed: 1. Earliest evoked potentials, occurring within the first 5 msec following signal presentation is EcoG, and reflects activity of the cochlea and VIII n. 2. Most commonly used evoked potential is the ABR which occurs within the first 10 msec following signal onset, reflects neural activity from the VIII n to midbrain. 3. MLR occurs within the first 50 msec following signal onset and reflects activity at or near the auditory cortex. 4. LLR occurs within the first 250 msec following onset and reflects activity of the primary auditory and association areas of the cerebral cortex. Neonatal Screening/Testing: The goal is to categorize auditory function as either normal or abnormal; to identify those who have significant sensorineural hearing loss. Those with normal cochlear function are eliminated from further consideration. ABR is the choice for evoked potential method, using surface electrodes to record, which can be affixed to the infant’s scalp. Click stimuli are presented at a fixed intensity level (usually between 30-40 dB) to determine if a reliable response can be recorded.

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Neonatal testing• If ABR is present- the child is likely to have normal or nearly normal hearing sensitivity in the 1000 to 4000 Hz frequency range. The underlying assumption is that such hearing is sufficient for speech and oral language development and that children who pass the screening are at low risk for developing communication disorders due to hearing loss. • If ABR is absent- it is concluded that the child is at risk for significant sensorineural hearing loss and further testing is needed. • Once only done in the NICU, but only half of the hearing problems were detected. So now we are looking for ways to screen all newborns (Universal Infant Hearing Screenings). Two conditions can interfere with correct identification of these infants: 1. Presence of middle ear disorder that is causing a conductive loss 1. Neuromaturational delays where some children’s brain stem function has not matured to the point (or is disordered to the point) that it cannot be measured accurately to provide an estimate of hearing sensitivity and must be referred for more testing. 2. Failure to identify a child with significant hearing loss with ABR will usually occur when the child has a reverse slope or rising loss (low frequency hearing is poorer than high frequency hearing). In such a case, it is believed that it will have minimal impact on communication development, as consonants, which give meaning to our language are found in the higher frequencies range of the speech spectrum.

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