Indian J Pediatr 1992; 59 : 361-366
Auditory Brainstem Evoked Responses in Infants and Children Ramesh C. Deka
77~e Evoked Potential Laboratory, Department of Otodffnola~.,ngolo~,, All India b~stitute of Medical Sciences, New Delhi During the last decade auditory brainstem evoked responses (ABR) have been used extensively in clinical medicine. It has greatly contributcd in two major areas viz, audiologic evaluation in infants and children, especially in those with risk factors and ncuroaudiologic evaluation of the 8th ncrve and brainstcm function in variety of ncurologic conditions. Since Hecox and Galambos ~ first reported in their paper about the successful application of ABR in the audiologic evaluation in children, several articles on this have appeared, rio Results of the use of ABR in difficult-to-test children for auditoryevaluation were gratifying in providing early diagnosis and institution of rehabilitation measures, s Presently, besides its use in evaluation of hearing function at the earliest, several other neurologic conditions have attracted its application, T M eg. sudden infant death syndrome (SIDS) and near miss for sudden infant deaths episode (NMSID). "~'~6Recent evidences suggest that S1DS is due to failure of central respiratory control mechanisms during sleep. ~'t7 The proximily of these respiratory centres to the Reprint requests : Dr. R.C. Deka, Additional Professor, Department of Otolaryngology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi-I l0 029.
auditory brainstem pathways is the basis of application of ABR in its early detection in suspect apenic infants. ABR thus can provide the objective evidence of an underlying brainstem lesion in apenic conditions. Wave Generator Site Although Sohmer and Feinmesser TM first in 1967, described electrical recordings of the 8th nerve, it was Jewetd 9 who successfully demonstrated seven upward deflections of submicrovolt amplitude during the first 10 msec period following click stimulation. Such minute electrical rcsponses were extracted with the help of the computer at the scalp from the background electroencephalogram activity. Thc electrical potcntlals generated at the cochlear nerve and the brainstem, spread through the conducting media of the brain, 'meninges, spinal fluid and skull-bones and appear at the scalp as evoked potentials. With the newer generation of computers today, it is possible to have variety of far-field scalp averaging system for clinical use. Today we have enormous animal and human experimental t as well as clinical5 7.11,19data on this subject. Clinical studies ~3'~ have corroborated the location of the brainstem generator-sites, described as follows :
361
~2
TIlE INDIAN JOURNAL OF PEDIATRICS
1. Auditory nerve: Wave I--Sth nerve action potential 2. Brainstcm level: Wave ll-Cochlear nucleus Wave Ill-Superior olivery nucleus Wave IV-Lateral Leminiscus, Wave V-Inferior collieulus Wave VI & VII-Not well understood. PATHOPHYSIOLOGICAL ASPECTS Latency Abnormalities Peripheral defect. In peripheral hearing loss, absolute latency of Wave I increases but the interpeak latencics (IPL) of wave I-III, wave III-V and wave I-V remain normal. Central conduction defects. Segmental demyelination, axonal and neuronal loss and degenerative changes occur as a result of diseasc or pressure effect on and/or due to poor blood supply to the 8th nerve and brainstem auditory pathways. lnterpeak Latency Abnormality (IPL) Wave I-Ill IPL abnormality. When wave l-III IPL values increase by more than mean + 2 SD, this suggests a conduction defect between the 8th nerve close to the cochlea and the lower pontine region, cg. acoustic nerve turnout.
Vol. 59, No. 3
Absence of wave IV and/or V. Brainstem lesions such as tumour or degenerative conditions like anoxaemia, kernicterus t3 etc., can produce such ABR abnormality ipsilateraliy as well as contralatcrally. Amplitude Abnormality
Loss of amplitude. The loss of amplitude is assumed to be due to less number of fibres conducting the volley and desynchronisation of the volley, secondary to widely different velocitics. Degeneration due to kernicterus and anoxia and brainstem turnout 13,14 often produce amplitude abnormalities in wave IV and/or V. Amplitude Ratio Wave V : I amplitude ratio. It is usually more than 1 in normal condition but its alternation occurs as a result of degeneration, pressure effect and/or poor supply, m4 to the brainstem.
Laterality of ABR Abnormality Unilateral or bilateral changes. Unilateral changes at ABR are usually more prominent as a result of unilateral diseases. Even in intrinsic brainstem lesion, unilateral ABR changes may be more discernible. Abnormality seen at ABR are however not etiologically specific, but does give indications about the site and extent of the lesion. Clinicians have to employ, detailed clinical history and examination, other investigation like ENG, attdiometry, imaging radiology and related neurological investigations, besides ABR to arrive at a diagnosis.
Wave IlI-V IPL abnormality. If wave IlI-V 1PL values increase by more than mean + 2 SD, this suggests a conduction defect between the lower pons and the midbrain. Such defcct occurs usually in brainstem lesion including tumour, or as a result of pressure effect on the brainstem by cerebellar tumour. Brainstem gliomas, ~5 anEQUIPMENTS AND MEASUREMENT oxaemia and/or kernicterus ~3 and SIDS 3'16 TECHNIQUE also produce such central conduction defect in the brainstem auditory pathways. The basic aspect of equipments, operational
D E K A : A B R IN C t l I L i ) I U 2 N
363
methods, stimulus types, response-evaluation and its clinical interpretation can be. read elsewhere.'.~2v~'~9 Stimulus. For audiologic cvaluation most commonly used stimuli are (a) tone-pips~ and (b) filtered clicks5-7 and for ncurologic conditions it is the clicks or the filtered clicks at 4 KI-Iz?-13
conventional pure-tone-audiometry. Hence, a factor of correction by about 10-15 dB is required in interpreting hearing function with ABR in infants and childrcn.4~':3
Stimulus rate. Most commonly employed rate is 10-20/sec. 5-7 However for audiologic purposes in infants and children, rate can be increascd upto 30-50/secr ~ which also provide better resolution. Polarity. In noisy situation, an alternate condensation and rarefaction stimulus may help to avoid noise stimulus artifact,sTa3 Sound proof room. For purely audiologic purposes, adequate sound-proof room is needed; otherwise ABR can be performed in quiet room or even at bed side in the hospital. Infant and new born testing. In infant testing, the head phone should bc kept near the side of the ear under test since it is painful to tightly keep a head-phone. This may result in reduction of about 5-10 dB intensity of the sound stimulus at the drum. s,~3 Resolution ofwaveform. To get better resolution of wave-forms stimulus, repctition may be upto 2000, and stimulus intensity may be reduced from 80 dBSL to 60 dBSL. Interpretation of latency and amplitude. Beh)rc making analysis of wave latcncy (both peak and intcrpeak latencics) and amplitude, atlcast two consistent readings must be obtained and only then data interprctcd and analysed. It must be remembered while asscssing hearing threshold in children that the ABR threshold in terms of wave V appearance in response to a minimum intensity-stimulus, is higher by about 20 dB than the one at
The cochlea and the 8th nerve, are fully developcd at birth, although myelination of the nerve and maturation of the auditory brainstem pathways are yet to be completed. By 6 months of age, thc myelinatitm becomes evident and its maturation progrosses till the age of 2 years when it bccomcs complete and comparable to adult statc (Figure 1). Sound stimulates the process of brain maturation, and dcprivation of sound in early period of life before 2 years may prevent maturation of auditory brainstem pathways and also prevent other related brain function? -~.~ Neonatal period. The ABR has been extcnsivcly used in early asscssment of hearing defect and other possible neurologic handicaps in neonates and infants in the neonalal intensive carc unit. ~~ Incidence of sensorineural loss is very high in this group of children. 5's'I~ Hence all infants in the intensive care unit should be given an ABR test. For screening purposes, a 40 dB HL, 2 and 4 KHz filtcrcd click stimulus can be givcn monoaurally and it normally shows the wave V after 6 or 7 msee depending on age of the child. Longitudinal studies a-s have provcd the value of ABR in predicting hearing threshold in at risk infants. Audiologic evaluation for early detection of hearing loss is important, since it influences the development of speech and auditory skills. On early detection, adequate rehabilitation measures like hearing aids, auditory training and spcech
PEDIATRIC APPLICATION OF ABR l)evelopment o1"Auditory Pathways
364
Vol. 59, No. 3
"FILE I N D I A N J O U R N A l . O F P E D I A T R I C S
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Auditory Brainstem Evoked Responses in Infants and Children Ramesh C. Deka
77~e Evoked Potential Laboratory, Department of Otodffnola~.,ngolo~,, All India b~stitute of Medical Sciences, New Delhi During the last decade auditory brainstem evoked responses (ABR) have been used extensively in clinical medicine. It has greatly contributcd in two major areas viz, audiologic evaluation in infants and children, especially in those with risk factors and ncuroaudiologic evaluation of the 8th ncrve and brainstcm function in variety of ncurologic conditions. Since Hecox and Galambos ~ first reported in their paper about the successful application of ABR in the audiologic evaluation in children, several articles on this have appeared, rio Results of the use of ABR in difficult-to-test children for auditoryevaluation were gratifying in providing early diagnosis and institution of rehabilitation measures, s Presently, besides its use in evaluation of hearing function at the earliest, several other neurologic conditions have attracted its application, T M eg. sudden infant death syndrome (SIDS) and near miss for sudden infant deaths episode (NMSID). "~'~6Recent evidences suggest that S1DS is due to failure of central respiratory control mechanisms during sleep. ~'t7 The proximily of these respiratory centres to the Reprint requests : Dr. R.C. Deka, Additional Professor, Department of Otolaryngology, All India Institute of Medical Sciences, Ansari Nagar, New Delhi-I l0 029.
auditory brainstem pathways is the basis of application of ABR in its early detection in suspect apenic infants. ABR thus can provide the objective evidence of an underlying brainstem lesion in apenic conditions. Wave Generator Site Although Sohmer and Feinmesser TM first in 1967, described electrical recordings of the 8th nerve, it was Jewetd 9 who successfully demonstrated seven upward deflections of submicrovolt amplitude during the first 10 msec period following click stimulation. Such minute electrical rcsponses were extracted with the help of the computer at the scalp from the background electroencephalogram activity. Thc electrical potcntlals generated at the cochlear nerve and the brainstem, spread through the conducting media of the brain, 'meninges, spinal fluid and skull-bones and appear at the scalp as evoked potentials. With the newer generation of computers today, it is possible to have variety of far-field scalp averaging system for clinical use. Today we have enormous animal and human experimental t as well as clinical5 7.11,19data on this subject. Clinical studies ~3'~ have corroborated the location of the brainstem generator-sites, described as follows :
361
~2
TIlE INDIAN JOURNAL OF PEDIATRICS
1. Auditory nerve: Wave I--Sth nerve action potential 2. Brainstcm level: Wave ll-Cochlear nucleus Wave Ill-Superior olivery nucleus Wave IV-Lateral Leminiscus, Wave V-Inferior collieulus Wave VI & VII-Not well understood. PATHOPHYSIOLOGICAL ASPECTS Latency Abnormalities Peripheral defect. In peripheral hearing loss, absolute latency of Wave I increases but the interpeak latencics (IPL) of wave I-III, wave III-V and wave I-V remain normal. Central conduction defects. Segmental demyelination, axonal and neuronal loss and degenerative changes occur as a result of diseasc or pressure effect on and/or due to poor blood supply to the 8th nerve and brainstem auditory pathways. lnterpeak Latency Abnormality (IPL) Wave I-Ill IPL abnormality. When wave l-III IPL values increase by more than mean + 2 SD, this suggests a conduction defect between the 8th nerve close to the cochlea and the lower pontine region, cg. acoustic nerve turnout.
Vol. 59, No. 3
Absence of wave IV and/or V. Brainstem lesions such as tumour or degenerative conditions like anoxaemia, kernicterus t3 etc., can produce such ABR abnormality ipsilateraliy as well as contralatcrally. Amplitude Abnormality
Loss of amplitude. The loss of amplitude is assumed to be due to less number of fibres conducting the volley and desynchronisation of the volley, secondary to widely different velocitics. Degeneration due to kernicterus and anoxia and brainstem turnout 13,14 often produce amplitude abnormalities in wave IV and/or V. Amplitude Ratio Wave V : I amplitude ratio. It is usually more than 1 in normal condition but its alternation occurs as a result of degeneration, pressure effect and/or poor supply, m4 to the brainstem.
Laterality of ABR Abnormality Unilateral or bilateral changes. Unilateral changes at ABR are usually more prominent as a result of unilateral diseases. Even in intrinsic brainstem lesion, unilateral ABR changes may be more discernible. Abnormality seen at ABR are however not etiologically specific, but does give indications about the site and extent of the lesion. Clinicians have to employ, detailed clinical history and examination, other investigation like ENG, attdiometry, imaging radiology and related neurological investigations, besides ABR to arrive at a diagnosis.
Wave IlI-V IPL abnormality. If wave IlI-V 1PL values increase by more than mean + 2 SD, this suggests a conduction defect between the lower pons and the midbrain. Such defcct occurs usually in brainstem lesion including tumour, or as a result of pressure effect on the brainstem by cerebellar tumour. Brainstem gliomas, ~5 anEQUIPMENTS AND MEASUREMENT oxaemia and/or kernicterus ~3 and SIDS 3'16 TECHNIQUE also produce such central conduction defect in the brainstem auditory pathways. The basic aspect of equipments, operational
D E K A : A B R IN C t l I L i ) I U 2 N
363
methods, stimulus types, response-evaluation and its clinical interpretation can be. read elsewhere.'.~2v~'~9 Stimulus. For audiologic cvaluation most commonly used stimuli are (a) tone-pips~ and (b) filtered clicks5-7 and for ncurologic conditions it is the clicks or the filtered clicks at 4 KI-Iz?-13
conventional pure-tone-audiometry. Hence, a factor of correction by about 10-15 dB is required in interpreting hearing function with ABR in infants and childrcn.4~':3
Stimulus rate. Most commonly employed rate is 10-20/sec. 5-7 However for audiologic purposes in infants and children, rate can be increascd upto 30-50/secr ~ which also provide better resolution. Polarity. In noisy situation, an alternate condensation and rarefaction stimulus may help to avoid noise stimulus artifact,sTa3 Sound proof room. For purely audiologic purposes, adequate sound-proof room is needed; otherwise ABR can be performed in quiet room or even at bed side in the hospital. Infant and new born testing. In infant testing, the head phone should bc kept near the side of the ear under test since it is painful to tightly keep a head-phone. This may result in reduction of about 5-10 dB intensity of the sound stimulus at the drum. s,~3 Resolution ofwaveform. To get better resolution of wave-forms stimulus, repctition may be upto 2000, and stimulus intensity may be reduced from 80 dBSL to 60 dBSL. Interpretation of latency and amplitude. Beh)rc making analysis of wave latcncy (both peak and intcrpeak latencics) and amplitude, atlcast two consistent readings must be obtained and only then data interprctcd and analysed. It must be remembered while asscssing hearing threshold in children that the ABR threshold in terms of wave V appearance in response to a minimum intensity-stimulus, is higher by about 20 dB than the one at
The cochlea and the 8th nerve, are fully developcd at birth, although myelination of the nerve and maturation of the auditory brainstem pathways are yet to be completed. By 6 months of age, thc myelinatitm becomes evident and its maturation progrosses till the age of 2 years when it bccomcs complete and comparable to adult statc (Figure 1). Sound stimulates the process of brain maturation, and dcprivation of sound in early period of life before 2 years may prevent maturation of auditory brainstem pathways and also prevent other related brain function? -~.~ Neonatal period. The ABR has been extcnsivcly used in early asscssment of hearing defect and other possible neurologic handicaps in neonates and infants in the neonalal intensive carc unit. ~~ Incidence of sensorineural loss is very high in this group of children. 5's'I~ Hence all infants in the intensive care unit should be given an ABR test. For screening purposes, a 40 dB HL, 2 and 4 KHz filtcrcd click stimulus can be givcn monoaurally and it normally shows the wave V after 6 or 7 msee depending on age of the child. Longitudinal studies a-s have provcd the value of ABR in predicting hearing threshold in at risk infants. Audiologic evaluation for early detection of hearing loss is important, since it influences the development of speech and auditory skills. On early detection, adequate rehabilitation measures like hearing aids, auditory training and spcech
PEDIATRIC APPLICATION OF ABR l)evelopment o1"Auditory Pathways
364
Vol. 59, No. 3
"FILE I N D I A N J O U R N A l . O F P E D I A T R I C S
[
iI]
V
I 2 ','ear
~10_ ~: 8..
I
'," V ,'7
,~
umsec
/ f "
~
50m~ck~/
102-msec
T 25 Year
%'6-
g4z_
