Monitoring The Central Nervous System

Electrophysiological Monitoring of the Central Nervous System

Brian McNamara & Simon J. Boniface

Introduction 69

Physiology 69

Practical Aspects 69

The EEG in Normal Patients 69

Evoked Potentials in Normal Subjects 71

Processing of the EEG 72

The EEG and General Anaesthetics 73

Preoperative Assessment in Epilepsy Surgery 75

Neurophysiological Monitoring and Intraoperative Care 76

Electrophysiological Monitoring in the Intensive Care Unit 77

EEG and Specific Diagnoses 77

Continuous Monitoring 77

The EEG in Status Epilepticus 79

Neurophysiology and Predicting Outcome 80

Summary 83

References 83

Introduction

The techniques of clinical neurophysiology provide a non-invasive and inexpensive means of assessing brain function. The EEG and evoked potentials respond rapidly to changes in cerebral physiology and are therefore a useful means of monitoring cerebral function during surgery and in the postoperative period.

The electroencephalogram represents the electrical activity of the brain as recorded from the scalp surface. Most of the activity recorded from the scalp surface is generated by cortical neurones. Pyramidal neurones are particularly important in generating the EEG as they are vertically orientated with regard to the cortex. Potentials arising in subcortical nucleii or in cells orientated horizontal to the cortex contribute little to the EEG.12 The scalp potentials recorded during EEG are probably due to the summation of excitatory or inhibitory postsynaptic potentials from many pyramidal neurones.34 The generation of the rhythmicity of the EEG is not well understood but is believed to be due to a combination of the inherent rhythmicity of some pyramidal neurones and the effect of pacemaker nucleii, with the reticular thalamus and thalamus thought to be particularly important in this respect.56

Evoked potentials (EPs) are the potentials elicited by physiological stimulation of receptors or electrical stimulation of peripheral nerves. EPs are generated in the cerebral cortex, subcortical nucleii, brainstem and spinal cord.7 The potential recorded from the scalp is probably due to the summation of excitatory postsynaptic potentials from many neurones. Visual evoked potentials (VEP) are the cortical response to visual stimulation with either a flashing light or reversing checkerboard pattern. Brainstem auditory evoked potentials (BAEP) are the potentials produced by brainstem structures in response to an auditory stimulus. Somatosensory evoked potentials (SEP) are the response from the brain or spinal cord in response to electrical stimulation of a peripheral sensory nerve.7

Practical Aspects

The EEG is normally recorded from the scalp surface using metal disc electrodes (usually silver) attached using a viscous conductive paste, usually containing silver chloride.8 The electrodes are arranged on the scalp surface using an internationally recognized system of electrode placement; most EEG laboratories use the 10: 20 electrode placement system9 (Fig. 5.1). The recorded signal is amplified and filtered to remove unwanted electrical activity. Most modern EEG machines can record from eight to 24 channels of electrical activity. These channels record the distribution of electrical potential on the scalp surface by interconnecting the electrodes in two different ways. The potential difference between pairs of electrodes can be recorded (the bipolar derivation) or the potential difference between each electrode and a common reference point can be recorded (the referential derivation). Using the bipolar montage, the EEG can be recorded from chains of electrodes which run from anterior to posterior or transversely along the scalp surface. The electrical signal is then recorded over time using either a pen and paper system or digitally converted and the EEG stored and reviewed using a personal computer.8

Evoked potentials are quite small when compared with background EEG activity. To eliminate background EEG and allow the evoked potentials to be studied in greater detail, the evoked potential is averaged by recording the response to successive sensory or electrical stimuli. The degree of averaging depends on the modality of the evoked potential: for VEPs, 100 stimuli are normally sufficient; for brainstem responses over 1000 stimuli may be required.7

The EEG in Normal Patients

The EEG is normally interpreted according to the following criteria.

• Frequency of the background rhythm. For convenience, the rhythm is classified into one of the following frequency bands: 8 1-4 Hz, 0 4-7 Hz, a 8-13 Hz and p <13 Hz.

Physiology

Figure 5.1 The international 10: 20 system of electrode placement.

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Figure 5.1 The international 10: 20 system of electrode placement.

• Amplitude of the EEG. Most normal EEG activity is between 20 and 200 |V.

• Paroxysmal activity. This refers to any bursts of transient activity.

• Location. Some indication of the source of EEG or evoked potentials can be gained by studying the distribution of the electrical activity over the scalp surface.10

The EEG in normal patients depends on the age and state of arousal. In normal awake adults, the dominant rhythm has a frequency of about 9 Hz (a rhythm), is symmetrical, is located in the posterior two quadrants, is inhibited by eye opening and has an amplitude of 50-100 |V (Fig. 5.2). In young adults, an a frequency rhythm known as a |-rhythm may be seen over central and parietal regions. Sharp transients called X waves may be seen over occipital regions when subjects have their eyes open. As subjects become drowsy the a rhythm becomes intermittent and the amount of 8 and 8 activity increases. In light sleep the a rhythm disappears. In stage 2 sleep sharp transients known as vertex sharp waves are seen over the vertex in the midline. In addition, complex waveforms known as K-complexes appear (Fig. 5.3). Bursts of rhythmical activity known as sleep spindles may also be seen. In stage 3 and stage 4 sleep the K-complexes and spindles become less evident and the EEG becomes dominated by high-amplitude 8 and 8 activity. REM sleep is usually associated with a mixed frequency recording containing variable amounts of p ,a, 8, and a activity.11

The EEG evolves considerably during childhood. At one year of age the dominant rhythm has a frequency of 5-6 Hz, increasing to 8 Hz at three years and reaching adult frequency by 15 years. In addition, a varying amount of 8 and 8 activity may be seen over the frontocentral and occipital regions as the child matures (Fig. 5.4).12

Figure 5.2

EEG from a normal awake adult. The EEG is dominated by an a rhythm (approximately 10Hz) located in the posterior quadrants-channels 4, 8, 12 and 16.

Figure 5.2

EEG from a normal awake adult. The EEG is dominated by an a rhythm (approximately 10Hz) located in the posterior quadrants-channels 4, 8, 12 and 16.

Figure 5.3

EEG in stage 2 sleep. Both vertex sharp waves (V) and K-complexes (K) can be seen. Evoked Potentials in Normal Subjects

Figure 5.3

EEG in stage 2 sleep. Both vertex sharp waves (V) and K-complexes (K) can be seen. Evoked Potentials in Normal Subjects

Evoked potential waveforms are named according to whether they are positive (designated P) or negative (designated N) with reference to a reference zero voltage and according to the latency with which they are generated after application of the sensory stimulus (normally indicated by a subscript). For example, the positive waveform seen approximately 100ms after application of the visual stimulus in VEP testing is called the P100.

SEPs are produced by electrical stimulation of a peripheral nerve. The electrical stimulus used activates fast-conducting group Ia and group II sensory afferents. The different components of the SEP are generated by sequential activation of neural generators by the ascending volley. An example of the SEP recorded from the scalp surface is shown in Figure 5.5. The first component (N9 or P9 with median nerve stimulation, Pl8 with posterior tibial nerve stimulation) is produced by the volley of action potentials reaching the brachial or sacral plexus. The next component (with a latency of about l2ms following median nerve stimulation) is produced by the sensory volley passing through the dorsal column. A slightly later wave can sometimes be detected due to the activation of the dorsal grey matter of the spinal cord. These early spinal components are most easily recorded by electrodes placed over the neck or thoracolumbar spine. The later components are for the most part cortically generated and are recorded best at the scalp surface. The most important of these is the N20/P22 complex (N38/P38 complex after posterior tibial nerve stimulation). These are generated by the somatosensory cortex (N20 and N38/P38) and motor and premotor cortex (P22). There are also later components (P27, N30, P45 and N60). The P27 is generated by the parietal cortex, the N30 is generated by the supplementary motor area and the exact source of the P45 and N60 has not been clearly described.13

BAEPs are produced by applying a simple auditory stimulus, which is normally a click. A typical BAEP is shown in Figure 5.6. Wave I originates from the

Figure 5.4

EEG from a normal six-year-old. As in the adult, there is an a rhythm located in the posterior quadrants. However, there is also underlying 0 and 8 activity in the frontal and central regions.

Figure 5.4

EEG from a normal six-year-old. As in the adult, there is an a rhythm located in the posterior quadrants. However, there is also underlying 0 and 8 activity in the frontal and central regions.

peripheral portion of the cochlear nerve. Wave II represents stimulation of the proximal portion of the cochlear nerve and postsynaptic responses from cochlear nucleus cells. Wave III is generated by the pontine portion of the auditory pathway. The source of wave IV is not clearly established but it is probably due to propagation of action potentials in the lateral lemniscus. Wave V is probably generated by a combination of the action potentials in the lateral lemniscus and postsynaptic responses in the midbrain auditory nucleii.14

Most VEP paradigms used in clinical practice employ a checkerboard with alternating black and white squares as a stimulus. The VEP waveform contains three peaks: N75, P100 and N145. The most consistent of these is P100, which is generated in the visual cortex.15

Processing of the EEG

The routine EEG results in a large amount of data requiring specialist interpretation. To facilitate the continuous EEG monitoring, a number of techniques have been developed to simplify and summarize the data. The most commonly used methods are based on analysis of the frequency of the EEG signal over time or amplitude over time.

One of the most commonly used frequency-based methods is power spectral analysis. A fixed period or epoch of EEG signal is digitized and mathematically manipulated using a fast Fourier transform. The distribution of EEG frequencies in each epoch is then plotted in a spectral array. An EEG with a normal a rhythm would therefore be summarized as a single plot with a peak at 9-10 Hz. To simplify the data even further, the frequency spectrum can be presented as a single number which summarizes the distribution of frequencies in that spectrum. Two descriptors commonly used in clinical practice are the spectral edge frequency (SEF) and the median frequency (MF). The spectral edge frequency is defined as the frequency below which 95% of the power in the EEG lies (SEF95). The median frequency is the frequency below which 50% of the power in the EEG spectrum is

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Figure 5.5

An example of the SEP (following left median nerve stimulation) recorded from left Erb's point (B) and right central region (A). Components generated by the brachial plexus (N9), spinal cord (N13) and cerebral cortex (N20, P22 and N30) can be clearly seen with left median nerve stimulation.

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Figure 5.5

An example of the SEP (following left median nerve stimulation) recorded from left Erb's point (B) and right central region (A). Components generated by the brachial plexus (N9), spinal cord (N13) and cerebral cortex (N20, P22 and N30) can be clearly seen with left median nerve stimulation.

found. Each time period or epoch analysed in this way generates a spectrum and spectra for succeeding epochs can be stacked on top of one another to show how the distribution of frequencies varies with time in a montage termed a compressed spectral array.16

Aperiodic analysis is utilized by commercially available monitors. It maps each waveform in terms of its frequency amplitude and time of occurrence. The EEG signal is subdivided into three components: 1-8 Hz, 9-30 Hz and a composite signal which can detect spikes. The computer can then display a summary of wave amplitude and frequency on a graphical display.17

One of the simplest methods of processing the EEG is the cerebral function monitor (CFM) first developed by Maynard et al.18 This processes only one channel of EEG data, normally from a pair of parietal electrodes. This signal is filtered, compressed and rectified to produce a single trace which is dependent on both the amplitude and frequency of the underlying EEG (Fig. 5.7). Although a great deal f information is lost in summarizing complex data in this manner, this single trace on the CFM monitor trace can be easily interpreted with a minimum of training. A more sophisticated version of the CFM monitor is the cerebral function analysing monitor (CFAM) which displays three amplitude traces, showing the 10th centile, mean and 90th centile of the amplitude of the processed EEG signal. There is also a frequency display which shows the percentage of EEG power in each of the four major frequency bands.19

The EEG and General Anaesthetics

As a general rule, the EEG alters in a predictable manner as the depth of anaesthesia increases. The initial phase is dominated by the appearance of p activity over the frontal regions, followed by gradual disappearance of the a rhythm. As anaesthesia deepens, the amount of 8 and 8 activity increases. Eventually a burst suppression pattern is attained consisting of periods where the EEG is isoelectric or of low voltage alternating with periods of highamplitude activity (Fig. 5.8).20 Some anaesthetic agents can alter the amplitude and latency of evoked potentials. Inhaled Agents Nitrous Oxide

Nitrous oxide can produce characteristic fast (34Hz) oscillatory activity in humans. It can also reduce the amplitude of auditory and visual evoked potentials without affecting the latency.

Halothane

Halothane produces a progressive slowing of the EEG frequency. It also decreases SEP amplitude in a dosedependent fashion. It also increases VEP latency and reduces amplitude.

Hede]ec/Teca E.

Hede]ec/Teca E.

Figure 5.6 BAEP recording showing waves I-V.

Enflurane

Enflurane causes progressive slowing of the EEG and eventual burst suppression. It can also reduce SEP amplitude with slight effects on latency.

Isoflurane

This initially produces fast activity, followed at higher levels of anaesthesia by an increasing amount of highamplitude S activity, eventually culminating in burst suppression at approximately 2 MAC. Isoflurane decreases SEP amplitude and high concentrations can completely suppress the scalp SEP.21

Intravenous Agents

Barbiturates at low doses cause an increase in p activity while higher doses successively produce EEG slowing and burst suppression and very high doses cause elec-

Figure 5.7

CFM tracing. The EEG data from a single channel is summarized as a tracing which is dependent on the frequency and amplitude of the EEG signal. The top tracing (A) was obtained from a patient post-head injury. The bottom tracing was obtained in the same patient after barbiturate established burst suppression.

Figure 5.7

CFM tracing. The EEG data from a single channel is summarized as a tracing which is dependent on the frequency and amplitude of the EEG signal. The top tracing (A) was obtained from a patient post-head injury. The bottom tracing was obtained in the same patient after barbiturate established burst suppression.

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