AUDITORY BRAINSTEM RESPONSE ABR DR S H HASHEMI

AUDITORY BRAINSTEM RESPONSE (ABR) DR. S. H. HASHEMI 1

Auditory Assessment q Subjective tests: v Pure Tone Audiometry v Speech Audiometry q Objective tests: v Acoustic Immittance v Auditory Brainstem Responses (ABR) v Electrocochleography (ECoch. G) v Otoacoustic Emissions (OAE) v Auditory Steady-State Response (ASSR) 2

What is an auditory evoked potential ? q Electrical activity of the auditory system that occurs in response to an acoustic stimulus, which can be recorded by surface electrodes. 3

q Short latency response ( < 10 ms ) q Middle latency response ( 10 -100 ms ) q Long latency response ( > 100 ms ) 4

Introduction q What is an ABR? The Auditory Brainstem Response is the representation of electrical activity generated by the eighth cranial nerve and brainstem in response to auditory stimulation. ABR assesses the integrity of the peripheral auditory system and auditory brainstem pathway. q Jewett & williston ( 1971) q BSER (Brainstem Evoked Response) q BAER (Brainstem Auditory Evoked Responce) q BAEP (Brainstem Auditory Evoked potential) 5

Introduction. . . It’s a set of seven positive waves recorded during the first 10 msec after a click stimuli. They are labeled as I – VII. q q Waves labeled with Roman numerals 6

PHYSIOLOGY q ABR typically uses a click stimulus that generates a response from the hair cells of the cochlea, the signal travels along the auditory pathway from the cochlear nuclear complex to the inferior colliculus in midbrain generates wave I to wave V. 7

Origin of each wave Wave Origin I Distal cochlear nerve II Proximal cochlear nerve III Cochlear nucleus IV Superior olivary complex V Lateral lemniscus VI Inferior colliculus VII Medial geniculate body 8

VII VI V I II IV 9

Medial geniculate bodies VII Inferior colliculi Auditory cortex VI 10

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Electrode placement Ø Cz (at vertex) (recording electrode) Ø Ipsilateral ear lobule or mastoid process (reference electrode) Ø Contra lateral ear lobule (act as a ground) 13

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Normal values ü ABR should be done at around 80 d. B ü 1000 Hz – 4000 Hz Wave Latency I 1. 5 m. sec ü Latencies are the key measure II 2. 5 m. sec III 3. 5 m. sec IV 4. 5 m. sec V 5. 5 m. sec VI 6. 5 m. sec ü Peaks I, III, and V most useful ü Calculate the peak – interpeak latencies for the ABR waves ü Find out the interpeak latencies of I–III, III–V, I–V ü Disorders will produce delays 15

Interpretation üLatency üAmplitude üMorphology 16

Interpretation. . . ü Wave I : delayed or absent may indicate cochlear lesion. ü Wave V : delayed or absent may indicate upper brainstem lesion. ü I – III inter-peak latency: prolongation may indicate lower brainstem lesion. ü III – V inter-peak latency: prolongation may indicate upper brainstem lesion. ü I – V inter-peak latency: prolongation may indicate whole brainstem lesion. 17

Applications q Auditory threshold testing q Identifying the hearing loss q Differential diagnosis § Classification of type of deafness (conductive or sensorineural) q Neonatal hearing screening q Identification of retrochoclear pathology q Neurosurgical interoperative monitoring 18

Disadvantages q Not normally possible for awake children q Skill q Time consuming q Not frequency specificity ü Not sensitive to hearing loss < 1000 Hz or > 4000 Hz ü Cannot provide information about entire audiogram q ABR recorded just at 90 d. B ( cannot differentiate between sever and profound hearing loss ) 19

Factors affecting the ABR q Factors that not affecting ü ü ü Awake Sleep stages Level of arousal Degree of attention paid to the eliciting stimulus q Factors that affecting ü Age ü Gender § Females have slightly shorter wave III and V latencies than males. ü Pharmacological agents § No affect → sedatives, general anesthetics, neuromuscular blocking agents § Affect → phenytoin, lidocaine, alcohol ü Body temperature § ↓ Body temperature → ↑ latencies 20

Limitation q ABR assesses the integrity of the peripheral auditory system and auditory brainstem pathway, before sound is received by the cortex. ABR is generated subcortically, it does not truly measure hearing. q ABR will not be sensitive to lesions above the midbrain level. ü [Example: Central deafness (bilateral temporal lobe lesions), sever to profound pure tone hearing loss but ABR is normal ] 21

Auditory threshold testing ü Click : 1000 Hz – 4000 Hz ü Beginning at a 80 d. B and continuing down to lower levels until wave V is no larger seen ( 20 d. B). ü It is good practice to obtain two waveform at each stimulus level. ü In normals and conductive HL click threshold will be ~10 -20 d. B higher than the best audiometric threshold. ü In Sensory HL click threshold will be ~ 5 d. B higher than the best audiometric threshold. ü Because maximum output for click is limited to ~ 90 -100 d. B one will not be able to differentiate between sever and profound losses. Ø The most common cause of absence of ABR waves is sever to profound hearing loss. 22

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Latency-intensity function ( LIF ) 24

q The relation between LIF and hearing loss is quite complex (due to the myriad of possible audiometric contours). 25

Normal 26

Normal 27

Identifying the hearing loss 28

Conductive HL ü Latency-intensity function ( LIF ) slope → NL ü The latency of wave V at each intensity → prolonged ü Shifted to the right 29

Conductive HL 30

Sensory hearing loss ü Wave V latencies show NO or LESS prolongation compared with conductive HL. ü LIF lies close to normal curve 31

Sloping sensory hearing loss ü Slope of the LIF is typically steeper than normal. ü The region of the cochlea where the response is initiated (high-frequency fibers in the basal region are stimulated and longer when lower frequency fibers toward the apical region are stimulated ). 32

Low frequency sensory (cochlear) hearing loss with normal ABR 33

Sever high frequency sensory (cochlear)hearing loss with normal ABR 34

Mild-to-moderate, high frequency sensory (cochlear) hearing loss with normal ABR 35

Mild-to-moderate , high frequency neural (retrochochlear) hearing loss 36

Severe-to-profound hearing loss with absent ABR 37

Neonatal hearing screening ü Several clinical trials have shown ABR testing as an effective screening tool in the evaluation of hearing in newborns. ü sensitivity of 100% and specificity of 96 -98% ü Can be recorded as early as 28 wks gestational age. 38

Neonatal hearing screening. . . ü Wave I amplitude is larger than in adults § (Recording electrode is closer to the cochlea due to the smaller head size of infants. ) ü The latencies of the ABR waveform components are longer than adults § (Decreased throughout the maturation of the cochlea and brainstem. ) ü Should correct by 18 -24 months of age. 39

Intraoperative monitoring q IN SURGERY : Ø ABR is useful in the prevention of neurotologic dysfunction and the preservation of postoperative hearing loss. Ø For many patients with tumors of CN VIII or the cerebellopontine angle, hearing may be diminished or completely lost postoperatively, even when the auditory nerve has been preserved anatomically. 40

Identification of retrochoclear pathology q ABR is considered an effective screening tool in the evaluation of suspected retrocochlear pathology such as an acoustic neuroma or vestibular schwannoma. 41

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Acoustic neuroma 45

q Likely abnormal in: Ø Multiple Sclerosis and other demyelinating processes Ø Hyperbilirubinemia at levels requiring exchange transfusion 46