- Email: [email protected]
Evoked potentials and their clinical application
ARTICLE IN PRESS Current Anaesthesia & Critical Care (2004) 15, 392–399
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FOCUS ON: MONITORING
Evoked potentials and their clinical application S.M. Mason Medical Physics Department, Queen’s Medical Centre, Nottingham NG7 2UH, UK
KEYWORDS Auditory evoked potentials; Visual evoked potentials; Somatosensory evoked potentials; Clinical application; Intraoperative monitoring
Summary Electrical activity evoked in a sensory pathway by an external stimulus is termed an evoked potential (EP). EPs are one class of investigations within electrophysiology which also includes areas such as electromyography and electroencephalography. The methodology of recording an EP is well-documented and primarily relies on techniques to detect and extract small responses from a somewhat noisier background signal. The activity being recorded from suitably sited electrodes, typically surface scalp electrodes. The three main modes of stimulation in clinical practice are auditory, visual and somatosensory and each provides a valuable, objective means of investigating the functioning of their respective pathways and diagnosis of pathology. EPs also play a major part in the intraoperative monitoring of surgical procedures. The practical application of EPs will be discussed both in their diagnostic role and as monitoring tools in the operating theatre. & 2005 Elsevier Ltd. All rights reserved.
Introduction Electrical activity evoked in a sensory pathway by an external stimulus is classed as an evoked potential (EP). The three main modalities of stimulation in clinical practice are auditory, visual and somatosensory and each of these provides a valuable objective means of investigating the functioning of their respective pathways. EPs play an important role in both the diagnosis of pathology and as an intraoperative monitoring tool during surgical procedures.1–3 The methodology of recording an EP is well-established and primarily relies on techniques to detect and extract small responses from a somewhat noisier background signal. This is achieved by filtering and averaging the signal which Corresponding author. Tel.: +44 115 9249924x43455.
E-mail address: [email protected].
improves the signal-to-noise ratio and enables visualization and analysis of the final averaged response waveform. The response is recorded from the patient using suitably positioned electrodes, typically surface scalp electrodes. EPs are one class of investigations within electrophysiology which also includes areas such as electromyography and electroencephalography. The practical application of the three main modalities of EPs—auditory, visual, and somatosensory—will be discussed both in their role in the clinic and as monitoring tools in the operating theatre.
Practical application The characteristics of the stimulus and data collection parameters are optimized within each
0953-7112/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cacc.2004.12.003
ARTICLE IN PRESS Evoked potentials and their clinical application modality so as to evoke an EP that is diagnostic at different levels of the sensory pathway. The majority of EPs are recorded from surface electrodes attached to the scalp. However, some techniques require more specialized electrodes such as an electrode positioned on the front of the eye to record retinal responses (electroretinography) or an epidural electrode to record spinal cord responses during monitoring in spinal surgery. The electrode forms the all important interface between the patient and the recording equipment and is a key component in achieving good quality EPs. For this reason it is essential to achieve a stable, low impedance contact with the electrode (typically less than 5 kO) which helps to reduce the level of interference on the signal baseline, particularly 50 Hz mains. EPs range in amplitude from less than 1 mV for surface recordings of the auditory brainstem response (ABR) up to 200–300 mV for an electroretinogram. The majority of EPs are small when compared to the level of background noise and techniques are employed to enhance the signal to noise ratio of the final recording of the response. Firstly, the frequency content of the signal picked up by the electrodes is controlled by a filter where the lower and upper cut-off frequencies (bandwidth) are set so that the response is passed through but as much of the background noise is excluded. Secondly, time synchronized signal averaging is employed to enhance the signal-to-noise ratio. A train of stimuli is presented and time locked responses from individual stimuli add together, whereas background noise is generally random and tends to cancel out. EPs are often called objective investigations. However, it is important to remember that they are only objective as far as the patient is concerned, because the recorded waveform in the majority of clinical applications still requires the skilled interpretation of an experienced professional. Passive cooperation of the patient is required in order to achieve a quiet signal baseline with minimal contamination with movement artefacts and myogenic activity. High levels of noise can easily mask out a small amplitude response. These recording conditions may be difficult to achieve in a young child and may require the use of sedation or even a general anaesthetic.
Interpretation of evoked potential waveforms The two main characteristics of the response waveform that typically define where or not it lies
393 within the normal range are its amplitude and latency with respect to the stimulus. The overall response waveform often consists of a number of different components, as shown in Fig. 1 for the ABR. Measurement of the amplitude and latency of these components is essential when using the ABR as a diagnostic tool. The diagnostic ability of an EP relies on the pathology affecting the relevant sensory pathway and thereby disrupting the generation and transmission of response activity.
Auditory evoked potentials It is possible to record a range of different auditory EPs originating from progressively higher structures along the pathway from cochlea to cortex as described in Hall:1 1. Electrocochleography (ECochG): cochlear hair cells and peripheral auditory nerve. 2. Auditory brainstem response (ABR): neural pathways in the brainstem from auditory nerve to the inferior colliculus. 3. Middle latency response (MLR): upper brainstem, thalamus and primary auditory cortex. 4. Auditory cortical response (ACR): primary, secondary and associated cortical areas.
Auditory stimuli There are three types of transient stimuli routinely employed in the generation of these auditory EPs:
Click—a short duration, fast rise time stimulus which has good synchronization properties for generation of EPs in the cochlea and brainstem pathways (ECochG and ABR). Acoustically this is a wideband stimulus with poor frequency specificity. Tone pip—a slightly longer, slower stimulus than the click, consisting of a few cycles of a pure tone. The slower rise time of this stimulus improves its acoustical frequency specificity but this is achieved at the expense of degradation of the EP from the cochlea and brainstem pathways. However, this stimulus is employed widely with the ABR and ECochG but to a lesser extent with the MLR and ACR. Tone burst—a longer burst of a pure tone with a relatively slow rise time. This stimulus has excellent frequency specificity equivalent to the stimuli used in subjective pure tone
ARTICLE IN PRESS 394
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Figure 1 The characteristics and origins of the auditory brainstem response.
audiometry. It is an ideal stimulus for evoking the ACR but cannot be used for ECochG and the ABR as there is insufficient synchronization of neural activity to enable identification of the response. In addition to these transient stimuli, an amplitude and frequency modulated tonal stimulus can be used to evoke an auditory steady-state response (ASSR) from the auditory system as reviewed by Jerger.4 The modulation frequency of the stimulus determines primarily where the recorded response originates from; 80 Hz predominantly auditory brainstem and 40 Hz mainly middle latency components.
Clinical application Auditory EPs have a major diagnostic role in a wide range of clinical applications in the disciplines of audiology and neurology. The most widely studied and applied EP is the ABR3,5 followed by the ACR and ECochG and to a lesser extent the MLR. The ABR is used for the objective assessment of hearing in babies and young children when behavioural testing is not reliable. It is also a valuable otoneurological tool for investigation of auditory nerve tumours and brainstem pathology although over recent years MRI has become the primary investigation in many of these cases. However, ABR is still required in some patients due to long waiting lists
ARTICLE IN PRESS Evoked potentials and their clinical application for MRI, patients with claustrophobia or obesity, and in specific types of pathology. The ACR is a valuable clinical tool for objective assessment of hearing thresholds in older children and adults who are unwilling or unable to comply with subjective tests of hearing. It assesses the whole of the auditory pathway and the measurement of threshold is frequency specific. However the ACR is dependent on the subject being awake and alert and this highlights the advantage of ABR in young subjects as it is resistant to the effects of sleep, sedation and anaesthesia. Newborn hearing screening Since 2002 newborn hearing screening programmes (NHSP) have been introduced in the UK across an increasing number of centres following a review by Davis et al.14 The ABR is an integral part of a hearing screening programme. It is used at the early screening stage where automated ABR systems (AABR) provide a completely objective pass or refer result for the screening test that does not require interpretation by a skilled observer. This objective result is determined by a computer scoring algorithm that detects the presence or absence of an ABR at a fixed screening level of 45 dBnHL. The AABR is a rare example of where the subjectivity of interpretation has been removed from the investigation. EPs are ripe for wider application of these objective scoring techniques in the future. The ABR is also heavily involved in the early follow-up of babies that are referred following the screening test. In striving to achieve more frequency specific objective information about hearing loss there has been an increased emphasis on the use of tone pip stimuli with the ABR rather than a wideband click stimulus. Recordings of the ASSR also offer the possibility of an alternative method of measuring frequency specific thresholds. Recently this technique has been bolstered by the availability of commercial ASSR systems which apply objective statistical methods for detection of the presence of a response. Although this technique of estimating hearing thresholds shows considerable promise, its consistency and reliability still needs to be established in routine clinical use, particularly when testing very young babies. Cochlear implantation In the last 10–15 years cochlear implantation has been increasingly used as a means of restoring some perception of sounds and speech in patients with profound hearing loss. Auditory EPs play an essential role in the management of these patients and particularly in young children.7,8 An example of
395 this is the ABR which can be evoked using electrical stimulation presented to the auditory nerve by the cochlear implant rather than conventional acoustical stimulation.9 The electrically evoked ABR (EABR) can used to assist with tuning of the implant which involves setting an appropriate dynamic range for the electrical stimulus. A recent development in cochlear implant technology is a technique for recording the electrically evoked, compound auditory nerve action potential from the electrode array within the cochlea. One electrode on the array is used as a stimulating electrode and an adjacent electrode as the recording site. Information is passed to and from the implant using a radio frequency telemetry link. On the Nucleus cochlear implant supplied by Cochlear Europe Ltd. this technique is known as neural response telemetry (NRT). An example of recordings acquired using the NRT software is shown in Fig. 2.
Visual evoked potentials EPs can be recorded which represent electrical activity originating from the retina, optic nerve and visual cortex 1. Electrooculography (EOG): pigment epithelium layer of the retina; 2. Electroretinography (ERG): rod and cone receptors and the inner and outer nuclear layers; and 3. Visual evoked potential (VEP): response activity from the visual cortex.
Visual stimuli Two important characteristics associated with the visual stimuli are contrast and luminance and are incorporated in the main types of visual stimuli used in clinical practice.
Plain flash of light—a low contrast stimulus that can be presented at high and low levels of luminance. Pattern onset—a black and white checkerboard stimulus that is flashed on and off the screen with an overall constant luminance. The field size of the stimulus is typically 201 with individual check sizes ranging from 15 to 60 min of arc subtended at the eye. Pattern reversal—a checkerboard stimulus where the individual black and white checks are reversed on successive stimuli.
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Figure 2 Recording of the electrically evoked compound auditory nerve action potential (ECAP) using the NRT software. In this example the electrical stimulus has been applied to electrode 10 of the 22-electrode array and the ECAP response recorded from electrode 12.
ERG and VEPs are subdivided depending on the characteristics of the stimulus presentation and whether the investigations are carried out in the dark adapted (scotopic) or light adapted eye (phototopic). Recordings of the ERG and VEP are standardized to the recommendations of the International Society for the Clinical Electrophysiology of Vision (ISCEV). For example, the standardized ERG to a full field flash stimulus includes a range of measurements that specifically investigate differential function of the rods and cones. The ERG can also be evoked by a checkerboard pattern stimulus (pattern ERG). The response components of the PERG arise from more central structures and are selectively affected in macular disease and ganglion cell dysfunction. The full field flash ERG and the pattern ERG complement each other in the differential diagnosis of peripheral retinal involvement and more central dysfunction. A more recent development of the ERG and the VEP is the use of a multifocal stimulus where a twodimensional array of small stimulus elements is used to evoke response activity from specific small regions of the visual field. A detailed discussion of the multifocal technique is described in Hood.10 A typical recording of the multifocal ERG in a normal subject is shown in Fig. 3.
Clinical application Visual electrodiagnostic investigations of the functioning of the retina, optic nerve pathways and visual cortex will complement, and often supplement, clinical examination.11,12 Typical examples of their role, either individually or in combination, are as follows:
Identification of the location and possible nature of dysfunction along the visual pathway, for example, a delayed response from the visual cortex (VEP) due to inflammation of the optic nerve (optic neuritis). Involvement of particular retinal cell types such as in cone dystrophy or more gross retinal involvement as in cases of retinitis pigmentosa where there is reduction in all components of the ERG. Involvement of a particular subpopulation of visual nerve fibres or processing system such as in cases of macular degeneration (e.g., Stargardt’s disease). The pattern ERG is a valuable clinical tool in the assessment of macular dysfunction. In non-organic aetiology (e.g., hysterical amblyopia) the VEP evoked by different stimulus
ARTICLE IN PRESS Evoked potentials and their clinical application
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Figure 3 A multifocal ERG in a normal subject showing the array of responses and a 3D representation of response activity (courtesy of Colin Barber and Yaqin Wen).
check sizes is a valuable tool to objectively assess visual function. Historically, visual electrodiagnostic investigations have been extensively studied and documented in adults but more recently their value in paediatric work has been highlighted. Practical application of these tests in a young child or baby is a challenge. It requires special techniques to address issues such as compliance with the procedure and need for fixation of the stimulus by an awake child. This is in contrast to auditory EPs where testing can often be carried out with a child asleep or sedated. The multifocal techniques for ERG and VEP are rapidly becoming essential clinical tools in the visual electrodiagnostic investigation of patients. Retinal functional losses due to regional disorders in outer retinal layers can be described in detail with the multifocal ERG. In macular disease there are decreased or absent central ERG components surrounded by normal ERG activity. In diseases of the outer retina the pattern of distribution of
multifocal ERG activity is similar to the pattern of the visual field defect. However this relationship is dependent on whether the disease primarily affects the outer retina (retinitis pigmentosa) or the ganglion cells (glaucoma) or optic nerve (ischemic optic neuropathy and optic neuritis). It is important to measure not only the amplitude of the response but also the timing as some diseases affect one of these measures and not the other. There is currently a big drive in documenting and comparing disease-related findings in both multifocal ERG and VEP.
Somatosensory evoked potentials (SSEP) The most common form of stimulus used to evoke an SSEP is an electrical pulse applied to a peripheral afferent nerve such as the median nerve at the wrist or the posterior tibial nerve in the lower limb. The resultant EP can be recorded from electrodes positioned at various sites along the
ARTICLE IN PRESS 398 pathway where the nerve becomes superficial or at cortical level. The amplitude of the SSEP varies considerably across subjects, and the interpretation of clinical diagnostic studies is based primarily on component latencies rather than amplitude. They have diagnostic role in peripheral neuropathy and more central degenerative disorders through measurement of the conduction time of the afferent nerves. However, the development and easier access of imaging such as MRI have had an impact on the usage of SSEPs in clinical practice and fewer studies are now performed.
Intraoperative monitoring Implementation of intraoperative monitoring requires a somewhat different approach when compared to recording EPs in the outpatient clinic. The environment and practical procedure is more challenging from the point of view of electrical interference, lack of access to the patient during the procedure, time pressures and a requirement to provide immediate feedback of information on which the surgeon can act if necessary. A team approach is required involving the surgeon, anaesthetist, and the monitoring staff. The use of EPs in intraoperative monitoring of surgical procedures can provide valuable feedback about the status and potential compromise of important neural pathways and vascular structures2,8,13
S.M. Mason surgery. The most susceptible EPs to the effects of anaesthesia are those arising from more central structures and particularly cortical responses whereas more peripheral responses are sparred. In general the longer the latency of a response component, the more synapses there are between the stimulation site and the neural generator, the greater the degree of effect of the anaesthetic agent. This susceptibility of the EP to anaesthetic agents is exploited in a technique to monitor the depth of anaesthesia. It has been demonstrated by Davies et al.6 that components of the auditory MLR show changes in latency during transitions between consciousness and unconsciousness. Analysis of these changes may provide an indicator of potential awareness during anaesthesia.
Summary EPs have a wide ranging role in assisting with the diagnosis and management of patients both in the clinic and during surgery. They can provide objective information about the functioning of sensory pathways which is difficult to acquire using other techniques. Some recordings are well-established and form part of standard clinical practice whereas others have an exciting future ahead such as developments with multi-focal ERG and VEP, electrical EPs associated with cochlear implantation, and the ASSR.
References
Monitoring the status of the auditory pathway during cerebello-pontine angle (CPA) surgery where the aim is to prevent avoidable postoperative hearing deficit during tumour removal Recording EPs related to monitoring of spinal cord function during spinal surgery, such as correction of scoliosis deformatives. Monitoring cranial/vascular procedures such as endartectomy and aneurysm surgery using somatosensory EPs. Monitoring the functional status of a cochlear implant and stimulation of the auditory pathways during implant surgery.
In addition to the surgical procedure there are other factors that can affect the EP recorded intraoperatively such as tissue temperature, blood pressure and anaesthetic agents. These need to be taken into consideration when reporting potential changes in the EP as a result of the
1. Hall III JW. Handbook of auditory evoked potentials. MA: Allyn & Bacon; 1992. 2. Misulis KE. Essentials of clinical neurophysiology. Boston: Butterworth-Heinemann; 1997. 3. Mason SM. Electric response audiometry. In: McCormick B, editor. Paediatric audiology 0–5 years. 2nd ed. London: Whurr Publishers Ltd; 2004. p. 188–264. 4. Jerger J. The auditory steady state response: parts 1 and 2. J Am Acad Audiol 2002;13:numbers 4 and 5. 5. Hood LJ. Clinical applications of the auditory brainstem response. San Diego, CA: Singular Publishing; 1998. 6. Davies FW, Mantzaridis H, Kenny GN, Fisher AC. Middle latency auditory evoked potentials during repeated transitions from consciousness to unconsciousness. Anaesthesia 1996;51(2):107–13. 7. Cullington HE. Cochlear implants: objective measures. London: Whurr Publishers; 2003. 8. Mason SM. Electrophysiological and objective tests. In: McCormick B, Archbold S, editors. Cochlear implants for young children. 2nd ed. London: Whurr Publishers; 2003. p. 162–216. 9. Mason SM. The electrically evoked auditory brainstem response. In: Cullington H, editor. Cochlear implants:
ARTICLE IN PRESS Evoked potentials and their clinical application objective measures. London: Whurr Publishers Ltd; 2003. p. 130–59. 10. Hood DC. The multifocal technique: topographical ERG and VEP responses. Documenta Ophthamologica: 100, No 2/3; 2000. 11. Halliday AM, editor. Evoked potentials in clinical testing. 2nd ed. London: Churchill Livingstone; 1993. 12. Fishman GA, Birch DG, Holder GE, Brigell MG. Electrophysiologic testing in disorders of the retina, optic nerve, and
399 visual pathway. 2nd ed. Mongraph published by American Academy of Ophthalmology. 2001. 13. Jones SJ, Boyd S, Hetreed M, Smith NJ, editors. Handbook of spinal cord monitoring. Dordrecht: Kluwer Academic Publishers; 1994. 14. Davis A, Bamford J, Wilson I, Ramkalawan T, Forshaw M, Wright S. A critical review of the role of neonatal hearing screening in the detection of congenital hearing impairment. Health Technol Assess 1997;1(10):i–iv 1–176.
www.elsevier.com/locate/cacc
FOCUS ON: MONITORING
Evoked potentials and their clinical application S.M. Mason Medical Physics Department, Queen’s Medical Centre, Nottingham NG7 2UH, UK
KEYWORDS Auditory evoked potentials; Visual evoked potentials; Somatosensory evoked potentials; Clinical application; Intraoperative monitoring
Summary Electrical activity evoked in a sensory pathway by an external stimulus is termed an evoked potential (EP). EPs are one class of investigations within electrophysiology which also includes areas such as electromyography and electroencephalography. The methodology of recording an EP is well-documented and primarily relies on techniques to detect and extract small responses from a somewhat noisier background signal. The activity being recorded from suitably sited electrodes, typically surface scalp electrodes. The three main modes of stimulation in clinical practice are auditory, visual and somatosensory and each provides a valuable, objective means of investigating the functioning of their respective pathways and diagnosis of pathology. EPs also play a major part in the intraoperative monitoring of surgical procedures. The practical application of EPs will be discussed both in their diagnostic role and as monitoring tools in the operating theatre. & 2005 Elsevier Ltd. All rights reserved.
Introduction Electrical activity evoked in a sensory pathway by an external stimulus is classed as an evoked potential (EP). The three main modalities of stimulation in clinical practice are auditory, visual and somatosensory and each of these provides a valuable objective means of investigating the functioning of their respective pathways. EPs play an important role in both the diagnosis of pathology and as an intraoperative monitoring tool during surgical procedures.1–3 The methodology of recording an EP is well-established and primarily relies on techniques to detect and extract small responses from a somewhat noisier background signal. This is achieved by filtering and averaging the signal which Corresponding author. Tel.: +44 115 9249924x43455.
E-mail address: [email protected].
improves the signal-to-noise ratio and enables visualization and analysis of the final averaged response waveform. The response is recorded from the patient using suitably positioned electrodes, typically surface scalp electrodes. EPs are one class of investigations within electrophysiology which also includes areas such as electromyography and electroencephalography. The practical application of the three main modalities of EPs—auditory, visual, and somatosensory—will be discussed both in their role in the clinic and as monitoring tools in the operating theatre.
Practical application The characteristics of the stimulus and data collection parameters are optimized within each
0953-7112/$ - see front matter & 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cacc.2004.12.003
ARTICLE IN PRESS Evoked potentials and their clinical application modality so as to evoke an EP that is diagnostic at different levels of the sensory pathway. The majority of EPs are recorded from surface electrodes attached to the scalp. However, some techniques require more specialized electrodes such as an electrode positioned on the front of the eye to record retinal responses (electroretinography) or an epidural electrode to record spinal cord responses during monitoring in spinal surgery. The electrode forms the all important interface between the patient and the recording equipment and is a key component in achieving good quality EPs. For this reason it is essential to achieve a stable, low impedance contact with the electrode (typically less than 5 kO) which helps to reduce the level of interference on the signal baseline, particularly 50 Hz mains. EPs range in amplitude from less than 1 mV for surface recordings of the auditory brainstem response (ABR) up to 200–300 mV for an electroretinogram. The majority of EPs are small when compared to the level of background noise and techniques are employed to enhance the signal to noise ratio of the final recording of the response. Firstly, the frequency content of the signal picked up by the electrodes is controlled by a filter where the lower and upper cut-off frequencies (bandwidth) are set so that the response is passed through but as much of the background noise is excluded. Secondly, time synchronized signal averaging is employed to enhance the signal-to-noise ratio. A train of stimuli is presented and time locked responses from individual stimuli add together, whereas background noise is generally random and tends to cancel out. EPs are often called objective investigations. However, it is important to remember that they are only objective as far as the patient is concerned, because the recorded waveform in the majority of clinical applications still requires the skilled interpretation of an experienced professional. Passive cooperation of the patient is required in order to achieve a quiet signal baseline with minimal contamination with movement artefacts and myogenic activity. High levels of noise can easily mask out a small amplitude response. These recording conditions may be difficult to achieve in a young child and may require the use of sedation or even a general anaesthetic.
Interpretation of evoked potential waveforms The two main characteristics of the response waveform that typically define where or not it lies
393 within the normal range are its amplitude and latency with respect to the stimulus. The overall response waveform often consists of a number of different components, as shown in Fig. 1 for the ABR. Measurement of the amplitude and latency of these components is essential when using the ABR as a diagnostic tool. The diagnostic ability of an EP relies on the pathology affecting the relevant sensory pathway and thereby disrupting the generation and transmission of response activity.
Auditory evoked potentials It is possible to record a range of different auditory EPs originating from progressively higher structures along the pathway from cochlea to cortex as described in Hall:1 1. Electrocochleography (ECochG): cochlear hair cells and peripheral auditory nerve. 2. Auditory brainstem response (ABR): neural pathways in the brainstem from auditory nerve to the inferior colliculus. 3. Middle latency response (MLR): upper brainstem, thalamus and primary auditory cortex. 4. Auditory cortical response (ACR): primary, secondary and associated cortical areas.
Auditory stimuli There are three types of transient stimuli routinely employed in the generation of these auditory EPs:
Click—a short duration, fast rise time stimulus which has good synchronization properties for generation of EPs in the cochlea and brainstem pathways (ECochG and ABR). Acoustically this is a wideband stimulus with poor frequency specificity. Tone pip—a slightly longer, slower stimulus than the click, consisting of a few cycles of a pure tone. The slower rise time of this stimulus improves its acoustical frequency specificity but this is achieved at the expense of degradation of the EP from the cochlea and brainstem pathways. However, this stimulus is employed widely with the ABR and ECochG but to a lesser extent with the MLR and ACR. Tone burst—a longer burst of a pure tone with a relatively slow rise time. This stimulus has excellent frequency specificity equivalent to the stimuli used in subjective pure tone
ARTICLE IN PRESS 394
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Figure 1 The characteristics and origins of the auditory brainstem response.
audiometry. It is an ideal stimulus for evoking the ACR but cannot be used for ECochG and the ABR as there is insufficient synchronization of neural activity to enable identification of the response. In addition to these transient stimuli, an amplitude and frequency modulated tonal stimulus can be used to evoke an auditory steady-state response (ASSR) from the auditory system as reviewed by Jerger.4 The modulation frequency of the stimulus determines primarily where the recorded response originates from; 80 Hz predominantly auditory brainstem and 40 Hz mainly middle latency components.
Clinical application Auditory EPs have a major diagnostic role in a wide range of clinical applications in the disciplines of audiology and neurology. The most widely studied and applied EP is the ABR3,5 followed by the ACR and ECochG and to a lesser extent the MLR. The ABR is used for the objective assessment of hearing in babies and young children when behavioural testing is not reliable. It is also a valuable otoneurological tool for investigation of auditory nerve tumours and brainstem pathology although over recent years MRI has become the primary investigation in many of these cases. However, ABR is still required in some patients due to long waiting lists
ARTICLE IN PRESS Evoked potentials and their clinical application for MRI, patients with claustrophobia or obesity, and in specific types of pathology. The ACR is a valuable clinical tool for objective assessment of hearing thresholds in older children and adults who are unwilling or unable to comply with subjective tests of hearing. It assesses the whole of the auditory pathway and the measurement of threshold is frequency specific. However the ACR is dependent on the subject being awake and alert and this highlights the advantage of ABR in young subjects as it is resistant to the effects of sleep, sedation and anaesthesia. Newborn hearing screening Since 2002 newborn hearing screening programmes (NHSP) have been introduced in the UK across an increasing number of centres following a review by Davis et al.14 The ABR is an integral part of a hearing screening programme. It is used at the early screening stage where automated ABR systems (AABR) provide a completely objective pass or refer result for the screening test that does not require interpretation by a skilled observer. This objective result is determined by a computer scoring algorithm that detects the presence or absence of an ABR at a fixed screening level of 45 dBnHL. The AABR is a rare example of where the subjectivity of interpretation has been removed from the investigation. EPs are ripe for wider application of these objective scoring techniques in the future. The ABR is also heavily involved in the early follow-up of babies that are referred following the screening test. In striving to achieve more frequency specific objective information about hearing loss there has been an increased emphasis on the use of tone pip stimuli with the ABR rather than a wideband click stimulus. Recordings of the ASSR also offer the possibility of an alternative method of measuring frequency specific thresholds. Recently this technique has been bolstered by the availability of commercial ASSR systems which apply objective statistical methods for detection of the presence of a response. Although this technique of estimating hearing thresholds shows considerable promise, its consistency and reliability still needs to be established in routine clinical use, particularly when testing very young babies. Cochlear implantation In the last 10–15 years cochlear implantation has been increasingly used as a means of restoring some perception of sounds and speech in patients with profound hearing loss. Auditory EPs play an essential role in the management of these patients and particularly in young children.7,8 An example of
395 this is the ABR which can be evoked using electrical stimulation presented to the auditory nerve by the cochlear implant rather than conventional acoustical stimulation.9 The electrically evoked ABR (EABR) can used to assist with tuning of the implant which involves setting an appropriate dynamic range for the electrical stimulus. A recent development in cochlear implant technology is a technique for recording the electrically evoked, compound auditory nerve action potential from the electrode array within the cochlea. One electrode on the array is used as a stimulating electrode and an adjacent electrode as the recording site. Information is passed to and from the implant using a radio frequency telemetry link. On the Nucleus cochlear implant supplied by Cochlear Europe Ltd. this technique is known as neural response telemetry (NRT). An example of recordings acquired using the NRT software is shown in Fig. 2.
Visual evoked potentials EPs can be recorded which represent electrical activity originating from the retina, optic nerve and visual cortex 1. Electrooculography (EOG): pigment epithelium layer of the retina; 2. Electroretinography (ERG): rod and cone receptors and the inner and outer nuclear layers; and 3. Visual evoked potential (VEP): response activity from the visual cortex.
Visual stimuli Two important characteristics associated with the visual stimuli are contrast and luminance and are incorporated in the main types of visual stimuli used in clinical practice.
Plain flash of light—a low contrast stimulus that can be presented at high and low levels of luminance. Pattern onset—a black and white checkerboard stimulus that is flashed on and off the screen with an overall constant luminance. The field size of the stimulus is typically 201 with individual check sizes ranging from 15 to 60 min of arc subtended at the eye. Pattern reversal—a checkerboard stimulus where the individual black and white checks are reversed on successive stimuli.
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Figure 2 Recording of the electrically evoked compound auditory nerve action potential (ECAP) using the NRT software. In this example the electrical stimulus has been applied to electrode 10 of the 22-electrode array and the ECAP response recorded from electrode 12.
ERG and VEPs are subdivided depending on the characteristics of the stimulus presentation and whether the investigations are carried out in the dark adapted (scotopic) or light adapted eye (phototopic). Recordings of the ERG and VEP are standardized to the recommendations of the International Society for the Clinical Electrophysiology of Vision (ISCEV). For example, the standardized ERG to a full field flash stimulus includes a range of measurements that specifically investigate differential function of the rods and cones. The ERG can also be evoked by a checkerboard pattern stimulus (pattern ERG). The response components of the PERG arise from more central structures and are selectively affected in macular disease and ganglion cell dysfunction. The full field flash ERG and the pattern ERG complement each other in the differential diagnosis of peripheral retinal involvement and more central dysfunction. A more recent development of the ERG and the VEP is the use of a multifocal stimulus where a twodimensional array of small stimulus elements is used to evoke response activity from specific small regions of the visual field. A detailed discussion of the multifocal technique is described in Hood.10 A typical recording of the multifocal ERG in a normal subject is shown in Fig. 3.
Clinical application Visual electrodiagnostic investigations of the functioning of the retina, optic nerve pathways and visual cortex will complement, and often supplement, clinical examination.11,12 Typical examples of their role, either individually or in combination, are as follows:
Identification of the location and possible nature of dysfunction along the visual pathway, for example, a delayed response from the visual cortex (VEP) due to inflammation of the optic nerve (optic neuritis). Involvement of particular retinal cell types such as in cone dystrophy or more gross retinal involvement as in cases of retinitis pigmentosa where there is reduction in all components of the ERG. Involvement of a particular subpopulation of visual nerve fibres or processing system such as in cases of macular degeneration (e.g., Stargardt’s disease). The pattern ERG is a valuable clinical tool in the assessment of macular dysfunction. In non-organic aetiology (e.g., hysterical amblyopia) the VEP evoked by different stimulus
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Figure 3 A multifocal ERG in a normal subject showing the array of responses and a 3D representation of response activity (courtesy of Colin Barber and Yaqin Wen).
check sizes is a valuable tool to objectively assess visual function. Historically, visual electrodiagnostic investigations have been extensively studied and documented in adults but more recently their value in paediatric work has been highlighted. Practical application of these tests in a young child or baby is a challenge. It requires special techniques to address issues such as compliance with the procedure and need for fixation of the stimulus by an awake child. This is in contrast to auditory EPs where testing can often be carried out with a child asleep or sedated. The multifocal techniques for ERG and VEP are rapidly becoming essential clinical tools in the visual electrodiagnostic investigation of patients. Retinal functional losses due to regional disorders in outer retinal layers can be described in detail with the multifocal ERG. In macular disease there are decreased or absent central ERG components surrounded by normal ERG activity. In diseases of the outer retina the pattern of distribution of
multifocal ERG activity is similar to the pattern of the visual field defect. However this relationship is dependent on whether the disease primarily affects the outer retina (retinitis pigmentosa) or the ganglion cells (glaucoma) or optic nerve (ischemic optic neuropathy and optic neuritis). It is important to measure not only the amplitude of the response but also the timing as some diseases affect one of these measures and not the other. There is currently a big drive in documenting and comparing disease-related findings in both multifocal ERG and VEP.
Somatosensory evoked potentials (SSEP) The most common form of stimulus used to evoke an SSEP is an electrical pulse applied to a peripheral afferent nerve such as the median nerve at the wrist or the posterior tibial nerve in the lower limb. The resultant EP can be recorded from electrodes positioned at various sites along the
ARTICLE IN PRESS 398 pathway where the nerve becomes superficial or at cortical level. The amplitude of the SSEP varies considerably across subjects, and the interpretation of clinical diagnostic studies is based primarily on component latencies rather than amplitude. They have diagnostic role in peripheral neuropathy and more central degenerative disorders through measurement of the conduction time of the afferent nerves. However, the development and easier access of imaging such as MRI have had an impact on the usage of SSEPs in clinical practice and fewer studies are now performed.
Intraoperative monitoring Implementation of intraoperative monitoring requires a somewhat different approach when compared to recording EPs in the outpatient clinic. The environment and practical procedure is more challenging from the point of view of electrical interference, lack of access to the patient during the procedure, time pressures and a requirement to provide immediate feedback of information on which the surgeon can act if necessary. A team approach is required involving the surgeon, anaesthetist, and the monitoring staff. The use of EPs in intraoperative monitoring of surgical procedures can provide valuable feedback about the status and potential compromise of important neural pathways and vascular structures2,8,13
S.M. Mason surgery. The most susceptible EPs to the effects of anaesthesia are those arising from more central structures and particularly cortical responses whereas more peripheral responses are sparred. In general the longer the latency of a response component, the more synapses there are between the stimulation site and the neural generator, the greater the degree of effect of the anaesthetic agent. This susceptibility of the EP to anaesthetic agents is exploited in a technique to monitor the depth of anaesthesia. It has been demonstrated by Davies et al.6 that components of the auditory MLR show changes in latency during transitions between consciousness and unconsciousness. Analysis of these changes may provide an indicator of potential awareness during anaesthesia.
Summary EPs have a wide ranging role in assisting with the diagnosis and management of patients both in the clinic and during surgery. They can provide objective information about the functioning of sensory pathways which is difficult to acquire using other techniques. Some recordings are well-established and form part of standard clinical practice whereas others have an exciting future ahead such as developments with multi-focal ERG and VEP, electrical EPs associated with cochlear implantation, and the ASSR.
References
Monitoring the status of the auditory pathway during cerebello-pontine angle (CPA) surgery where the aim is to prevent avoidable postoperative hearing deficit during tumour removal Recording EPs related to monitoring of spinal cord function during spinal surgery, such as correction of scoliosis deformatives. Monitoring cranial/vascular procedures such as endartectomy and aneurysm surgery using somatosensory EPs. Monitoring the functional status of a cochlear implant and stimulation of the auditory pathways during implant surgery.
In addition to the surgical procedure there are other factors that can affect the EP recorded intraoperatively such as tissue temperature, blood pressure and anaesthetic agents. These need to be taken into consideration when reporting potential changes in the EP as a result of the
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