BAEP (Brainstem Auditory Evoked

Potentials)

BY: SYED IRSHAD MURTAZA

TECHNOLOGIST

NEUROPHYSIOLOGY DEPT

AKUH KARACHI

DATE: 04-11-2013

Brainstem Auditory Evoked Potentials (BAEPs)

BAEPs are responses of the auditory nerve, brainstem, and, perhaps, higher sub-cortical structures to acoustic nerve stimulation. It assess function of auditory pathways.

Most of its components appear to arise from multiple sources, preventing a simple one-to-one correspondence between potential generators and individual BAEP waves.

BERA, CONT’DThe term brainstem auditory evoked potentials,” is

somewhat inappropriate in that

(1) the first component of the “brainstem auditory evoked potentials” does not arise in the brainstem but in the auditory nerve; and

(2) the latest components may or may not originate, at least in part, above the brainstem.

• Although the pathways is mainly along the brainstem and some of the potentials are assumed to be generated from it, so called BAEP.

BERA, The Electrically elicited potentials

• All BAEP waveforms represent far-field potentials, caused by activity of deep gray and/or white matter structures, relatively far from the surface recording electrodes, except wave I, which is considered to be the near-field potential, since it is generated within the distal part of cochlea and near to the recording point.

• (Ref: Electroencephalography: Basic Principles, Clinical Applications, and Related Fields by: Ernst Niedermeyer)

The Auditory system

• In order to know about the potentials of BAEPs one must be aware of the anatomy and physiology of auditory system which is mainly about the following two components.

• The Peripheral Auditory System (The EAR)

• The Central Auditory System (The Brain)

1. The Peripheral Auditory System (The EAR).

The components are,

Outer Ear: it consists of pinna, ear canal & ear-drum(tympanum. The folds of cartilage surrounding the ear are called the pinna, which amplifies the sounds .

Middle Ear: The sound wave information travels across the air-filled middle ear cavity via a series of delicate bones:

• Malleus (Hammer), Incus (Anvil) and Stapes (Stirrup) (MIS) which are mainly responsible to convert the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval (or elliptical) window.

II. Middle ear

Three main parts of middle ear are

(1)Three Ossicle bones:

- Malleus(1)/(A), Incus(3) (B), Stapes(6) (C)

Function) Impedance matching

(2) Two muscles

- Stapedial muscle(5) - Tensor tympani(9)

Function) Protection (3) Eustachian tube(8)

Function) Equalizer of air pressure

Anatomy of Peripheral auditory system (EAR)

Inner Ear• The inner ear consists of the cochlea and several non-auditory structures,

including Organ of Corti, which is located at the scala media (cochlear duct ) and transforms mechanical waves to electric signals in neurons.

• Hair cell :• Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at

the top, for which they are named. There are two types of hair cells. Inner hair cells are the mechanoreceptors for hearing: they transduce the vibration of sound into electrical activity in nerve fibers, which is transmitted to the brain. Outer hair cells are a motor structure which serve to amplify sound vibrations in a frequency specific manner.

• Neurons (Hair cell neural connection)• Afferent neurons innervate cochlear inner hair cells, at synapses where the

neurotransmitter (glutamate) communicates signals from the hair cells to the dendrites of the primary auditory neurons.

INNER EAR• TwO parts of inner ear• 1) Cochlea (Hearing) • - Scala vestibuli • - Scala media• - Scala tympani• 2) Vestibular system

(balance)

• Major function of inner ear

• 1) Hearing (It transmits sound to neural impulse and gives resonant frequency)

• 2) Balance

HAIR CELLS

VIDEO CLIP

How_the_human_ear_works.avi

BAER WAVES

Outer ear common disorders (1) Pinna: Cases of abnormal pinna

• Anotia (No ear/pinna) • Microtia (Grade I)

• Microtia (Grade II) • Microtia (Grade III)

Types of AEPs• Short latency AEP• Middle latency AEP• Long latency AEP• The short latency AEP include peak of up to 1-10 msec

and amplitude of about 0.2uv, they are generated in brainstem.

• The middle latency AEP have several variable peaks with latency of 10-50 msec and with amplitude of about 1 uv, they probably reflect early cortical excitation.

• The long latency AEPs beginning after 50sec and having peak of 1-10uv, represent later cortical excitation.

2. The Central Auditory System (The Brain)• The sound information, re-encoded, travels down the

vestibulo-cochlear nerve, through intermediate stations such as the cochlear nuclei and superior olivary complex of the brainstem and the inferior colliculus of the midbrain, being further processed at each waypoint. The information eventually reaches the thalamus, and from there it is relayed to the cortex. In the human brain, the primary auditory cortex is located in the temporal lobe. Associated anatomical structures include

• Cochlear nucleus• Trapezoid body• Superior olivary complex

• :

• The cochlear nucleus is the first site of the neuronal processing of the newly converted “digital” data from the inner ear. This region is anatomically and physiologically split into two regions, the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus (VCN).

• Trapezoid body•  The Trapezoid body is a bundle of decussating fibers in the

ventral pons that carry information used for binaural computations in the brainstem.

• Superior olivary complex•  The superior olivary complex is located in the pons and

receives projections predominantly from the ventral cochlear nucleus, although the posterior cochlear nucleus projects there as well, via the ventral acoustic stria.

Cochlear nucleus

Lateral lemniscus • The lateral lemniscus is a tract of axons in the brainstem

that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain.

• Inferior colliculi

• The Inferior colliculi (IC) are located just below the visual processing centers known as the superior colliculi. The central nucleus of the IC is a nearly obligatory relay in the ascending auditory system, & most likely acts to integrate information (specifically regarding sound source localization from the superior olivary complex & dorsal cochlear nucleus) before sending it to the thalamus and cortex.

Medial geniculate nucleus•  The medial geniculate nucleus is part of the thalamic relay

system• Primary auditory cortex• The primary auditory cortex is the first region of cerebral

cortex to receive auditory input.• Perception of sound is associated with the left posterior

superior temporal gyrus (STG). The superior temporal gyrus contains several important structures of the brain, including Brodmann areas 41 and 42, marking the location of the primary auditory cortex, the cortical region responsible for the sensation of basic characteristics of sound such as pitch and rhythm

GENERATORS OF WAVEFORMS FROM 1-VII

Waves Site of Neural Generator (peripheral portion of cranial nerve VIII)

I Cochlear nerve (distal end)

II Cochlear nerve ( proximal end)

III Superior Olivary Complex/Nucleus

IV Lateral Leminiscus

V Inferior Colliculus

VI & VII Presumed to be generated by the medial geniculate body and the thalamocortical pathways respectively

ByIM

STANDARDS AND GUIDELINES FOR/OF BAEP ACCORDING TO ACNS:

• The ACNS guidelines for the standard parameters settings and techniques for the Brain stem Auditory-Evoked Potentials (BAEPS) are as followings.

• I. Stimulus • It is recommended that “broad-band” clicks, the acoustic energy of

which is spread over a wide range of audio frequencies, be used for the neurologic applications of auditory evoked potentials. These clicks should be generated by driving with a 100usec rectangular pulse (single monophasic square wave), a standard audiometric earspeaker having a relative flat frequency spectrum.

• Many other types of acoustic stimuli are used for eliciting BAEPs, such as tone bursts, tone pips, filtered clicks, single-cycle clicks, etc.

•

II. Stimulus PolarityThe polarity of the first and most prominent wave of the acoustic waveform of

the click determines whether a negative or positive pressure is applied in front of the ear speaker diaphragm. Those clicks in which the first and major acoustic wave applies negative pressure in front of the ear-speaker diaphragm are referred to as rarefaction clicks.

• Those clicks in which the first and most prominent acoustic wave applies a positive pressure in front of the ear-speaker diaphragm are referred to as condensation clicks.

• In certain pathologic conditions associated with severe, steep high-frequency hearing loss, BAEPs elicited by rarefaction clicks may differ in latency and, to a degree, in morphology from BAEPs evoked by condensation clicks. In these circumstances, using clicks of alternating polarity results in poorer resolution of the response than using either rarefaction or condensation clicks alone. This problem is obviated by using rarefaction only, condensation only, or separate rarefaction and condensation clicks.

III. Stimulus Rate • Stimulus rates employed vary widely from 5 to 200/s. depending on

test applications. Waves I,II, VI, and VII are particularly reduced in amplitude at rates higher than 10/s. Thus, stimulus rates of 8-10/s are especially suited to resolve these peaks.

• IV. Stimulus Intensity • It is recommended that click intensity be acoustically calibrated in

“decibels peak-equivalent sound pressure level” (dB pe SPL). Stimulus intensities employed generally range between 40 and 120 dB per SPL.( sound pressure level). Intensity of stimulus should always be well defined to ensure :

Maximum release of neurotransmitters from the hair cells, so the nerve fires promptly.

• - Less intensity will result in increase in the latency of Wave I l

V. Monaural Versus Binaural Stimulation • Click should be delivered monaurally, i.e., to one

ear at a time. • Contralateral Masking • It is Although not necessary in every situation, it is

recommended that contralateral masking be included in the routine test protocol to avoid its inadvertent omission when it is required.

• recommended that the contralateral (nonstimulated) ear be masked by white noise at 60 dB SPL to eliminate “crossover” responses, i.e., bone-conducted responses originating in this ear.

VI. Recording.System Bandpass • The recommended system bandpass for BAEP recording is

10-30 to 2,500-3,000 Hz . Whenever this test is performed in the presence of irreducible EMG and mechanical artifacts, the low-frequency cutoff may be raised to 100-200 Hz.

• VII. Stimulus Artifact • The use of properly electrostatically and

electromagnetically shielded stimulus delivery systems is suggested to attenuate or eliminate the stimulus artifact, especially when using rarefaction-only or condensation-only clicks.

VIII. Analysis Time• An analysis time of 10-15 ms from stimulus onset is

suggested. An analysis time of no less than 15 ms is sometimes required to demonstrate extremely delayed responses in certain pathologic conditions. Analysis times of 15 ms are also essential for neonatal and intraoperative recordings.

• IX. Filters Setting • Low frequency filter setting is 10-30Hz but may be

increased to 100Hz.BAEP consist of multiple high frequency components reaching a frequency close to 1000Hz.Thus the high frequency filter should not be less then 2000Hz.

X. Number of Trials to be Averaged

• It is suggested that about 1,000-4,000 individual trials be averaged until good waveform resolution has been achieved. Two or more responses must be obtained and superimposed to demonstrate replicability or lack of replicability of their components.

• XI. Electrode Placement • It is recommended that recording electrodes be placed as follows:

(1) on the scalp at the vertex (Cz position of the 10-20 International System of EEG electrode placement) and

• (2) over the left and right earlobes (auricular) A1 and A2 positions of the 10-20 System) or the left and right mastoid processes (M1 and M2).

• The ground electrode may be placed anywhere on the body. For convenience, it is recommended that it be placed on the head, for instance, on the scalp in a midline frontal location (position Fz of the 10-20 System). Electrode impedances must be < 5 KOhms.

XII. Montage• A montage consisting of the following derivations is suggested for

BAEP recording: • Channel 1: Vertex-ipsilateral earlobe or mastoid (Cz-Ai or Mi) • Channel 2: Vertex-contralateral earlobe or mastoid (Cz-Ac or Mc)• XIII. State of Consciousness • BAEPs can be obtained during either wakefulness or sleep. Sedation

may occasionally be indicated with very young or tense patients, but now requires special provisions in most facilities. In recording patients who are comatose or are undergoing surgery, consideration must be given to the fact that hypothermia may produce BAEP alterations indistinguishable from those caused by structural lesions of the auditory pathways

• XIV. Analysis of Results • Records are analyzed primarily for the presence of waves I, III, and V.•

Technical modifications to improve waveforms identification.• 1. If Stimulus artifact is too large and obscures wave I.• Decrease the impedance of recording and ground electrodes• Adjust the location of input cables and stimulus cables separate them• Decrease the stimulus intensity• Replace the ear phone• Change the polarity • 2. If Wave I is not identified:• Increase stimulus intensity• Change click polarity• Decrease stimulus rate• Use ear canal electrodes• 3. If Wave V is difficult to distinguish from wave IV• Decrease stimulus intensity• Use contra lateral ear reference recording• 4.Wave V is difficult to differentiate from wave IV or VI:• Decrease stimulus intensity. When stimulus intensity is progressively decreased , wave V

is the last wave to remain.

Technical modifications• Not all normal recording contain all BAEP peaks.• Wave V present most often.• Wave I and III can usually also be identified.

Wave II is often absent and wave IV may merge more of less completely with wave V.• Wave I may be enhanced by increasing the stimulus intensity and decreasing the

stimulus rate.

• Recording a BAEP to condensation clicks in addition to the BAEP rarefaction clicks may help to distinguish wave I from mechanical and electrical stimulus artifact.

• Wave III may be normally splits into two peaks, its latency is then measured to the first peak or to the middle between the two peaks. splitting may disappear if the condensation clicks are used instead of rarefaction clicks and vice versa.

• Wave IV normally fuse with wave V.

• Wave V is the most reliable peak. It may be identified by its low threashold, its persistance during repetative stimulation up to 100/sec and by large negativity that commonly follow it.

Application of BEAPS in Specific Disorders

BEAPs are widely used for evaluation of• Acoustic neuromas• Degenerative diseases• Brain tumor and stroke• Multiple sclerosis• Reversibility of comas• Hearing assessment in children

Why Wave V is used for Interpretation

• Wave V is used as the indicator in HTT because it has,

1. lowest threshold for stimulation

2.highest amplitude of BAEP waves

3.Actual & consistent later wave in all subjects which indicates the integrity of the pathway (peripheral to central).

Analysis of Results • Measurements must include the following: • (1) wave I peak latency; • (2) wave III peak latency; • (3) wave V peak latency; • (4) I-III interpeak interval; • (5) III-V interpeak interval; • (6) I-V interpeak interval; • (7) wave I amplitude; • (8) wave V amplitude; and • (9) wave IV-V/I amplitude ratio.

Criteria for Clinically Significant Abnormality

• Abnormal BAEP measures do not necessarily imply altered retrocochlear function. At present, criteria for retrocochlear dysfunction include the following.

• 1. Absence of all BAEP waves I through V. unexplained by extreme hearing loss determined by formal audiometric testing.

• 2. Absence of all waves following waves I, II, or III. • 3. Abnormal prolongation of I-III, III-V. and I-V

interpeak intervals. I-III or III-V intervals can sometimes be abnormally prolonged even in the face of a normal I-V interval

Minimal Test Protocol

• It is recommended that, for neurologic applications, minimal BAEP testing should consist of responses to rarefaction, condensation, or summated separate rarefaction and condensation clicks delivered monaurally at intensities of 90—120 dB pe SPL, preferably 115 or 120 dB pe SPL and at rates preferably below 25/s. The contralateral ear should be masked by white noise at 60 dB SPL.

Recording at High Stimulus Rates • Recording BAEPs at stimulus rates of 50—70/s

facilitates the rapid identification of wave V in screening studies of neonates and infants as well as adults.

• 4. Abnormal diminution of the IV-V/I amplitude ratio, especially when accompanied by other abnormalities.

• 5. Abnormally increased differences between the two ears (interaural differences) as regards the I-III, III-V, and I-V interpeak intervals, when not explained by unilateral or asymmetric middle and/or ear dysfunction determined by appropriate audiometric tests.

Common Abnormal Finding

• Normal absolute latency of wave I but delayed absolute latencies of corresponding waveform

• Prolonged interpeak latencies • Prolonged absolute latency of wave I with

prolonged corresponding absolute latencies of the remaining waveform but normal interpeak latencies

Abnormal BEAP Tracing

Laboratorywise and rationale criteria of abnormality in adult (BERA EVALUATION)

• Absolute latencies of wave I, III, and V should not exceed 1.9, 4.08 and 5.99 msec respectively

• Interpeak latencies of wave I-III, III-V and I-V should be less than 2.54, 2.37 and 4.58 msec respectively.

• Amplitudes are variable. Amplitudes of wave I, and V should be atleast 0.16 and 0.18uv respectively.

• Hemispheric absolute peak latency difference of wave I, II, III, IV and V should be less than 0.2, 0.3, 0.2, 0.3 and 0.2 msec respectively.

• Hemispheric interpeak latency difference of wave I-III, III-V and I-V should not exceed 0.4, 0.3, and 0.4msec respectively.

Abnormal BEAP Tracing

BEAP responses in Acoustic Neuroma

ReferenceS: • 2008 American Clinical Neurophysiology Society• Coats and Martin 1977• Chiappa K, Gladstone KJ, Young RR. Brainstem

auditory evoked responses• Chatrian GE, Wirch AL, Lettich K, Turella G,

Snyder J.M. Click-Evoked Human Electrocochleogram

• Thoru Yamad and Elizebith, Basic concepts of neurophysiological testing