Blume's Atlas of Pediatric and Adult Electroencephalography, 1st Edition

Chapter 1

Introduction

Electrode Placement

 

Fig. 1-1. International 10–20 electrode placement system (Jasper, 1958). Electrode placements indicated in this atlas conform to this system. *Mandibular notch electrode(Sadler & Goodwin, 1989).

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Fig. 1-2. Various waveforms. “Classical” appearance of several types of waveforms that appear in this atlas.

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Fig. 1-3. Morphologies of superimposed sine waves of several frequencies. Sine waves of several frequencies usually appear together in the clinical EEG to produce complex waveforms. This feature should be recognized in assessing such phenomena. This collage illustrates a simple 10-Hz sine wave that is then combined with those of other frequencies. The 10 + 2 resembles alpha activity with an underlying pulse artifact. The 10 + 4, 10 + 6, 10 + 8 are reminiscent of some posterior phenomena such as posterior slow of youth. The 10 + 9, 10 + 11, 10 + 12 illustrate “alpha beating” resulting from narrowly separate alpha rhthyms.

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Polarity Primer

Amplifiers for electroencephalographic (EEG) recording are differential; that is, each channel records the difference in voltage between Input 1 and Input 2. Moreover, the EEG signal records the change of voltage (ordinate) as a function of time (abscissa). When Input 1 is more negative than Input 2 (either Input 1 is negative or Input 2 is positive), the output signal deflects upward. A downward deflection, therefore, indicates that Input 1 is more positive than Input 2 (either Input 1 is positive or Input 2 is negative). No deflection indicates equipotentiality: equally negative, equally positive, or no potential. Several definitions apply: electrode position = any member of the 10/20 system + mandibular notch; a derivation = a pair of inputs (e.g., Fp1–F3); a montage = a combination of derivations.

Bipolar Recording

A bipolar montage uses multiple electrode pairs without having any one electrode as an input of every channel. Usually, bipolar electrode pairs, called derivations, are arranged so that the electrodes are aligned in straight lines (an array), and adjacent channels have an electrode in common.

Referential Recording

In this system, a different scalp electrode is connected to Input 1 of each amplifier, but a single common electrode is connected to Input 2 of each amplifier. It is hoped that the common electrode, the referential lead, will be minimally involved in the EEG event under scrutiny. Therefore, the output signal should represent the polarity and amplitude of the event of Input 1.

The following lists advantages and disadvantages of commonly used montages and analyzes the same right anterior mesial temporal spike focus using several of these.

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Fig. 1-4. Right anterior mesial temporal spikes. Longitudinal bipolar montage. Patient's age, 42 years. Awake. Eyes closed. Very typically, these electronegative spikes cancel in the F8–T4 derivation. Note the occasional parasagittal spread. These spikes are analyzed further in the following five montages. Calibration signal 1 s, 70 µV.

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Fig. 1-5. Right anterior mesial temporal spikes. Same illustration as previous using coronal montage with sagittal leads. As the previous montage indicates that these spikes reside in the right hemisphere, the downward deflections principally in the fourth and eighth channels indicate negativity as opposed to left hemisphere positivity. Note the virtually equal involvement of the right mandibular notch (M2) electrode, indicating its anterior mesial location. T6 is minimally involved. Calibration signal 1 s, 70 µV.

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Fig. 1-6. Right anterior mesial temporal spikes. Same illustration as previous using ipsilateral ear (A1,2) referential montage. As the previous montage indicates A2 involvement, other leads (F8–T4) that virtually cancel with A2 are also involved. On the other hand, large deflections indicate noninvolvement of inputs 1. Note the slight cancellation at the FP2–A2 derivation. Unknown is whether the downward deflection at the T3–A1 electrode represents positivity at T3 or negativity at A1: the latter would represent cross-firing from A2 to A1. Calibration signal 1 s, 70 µV.

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Fig. 1-7. Right anterior mesial temporal spikes. Same illustration as previous using coronal bitemporal montage. This montage illustrates spike field in coronal and anterior-posterior manners. Note the variable principal spike field involving T4 and A2. Calibration signal 1 s, 70 µV.

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Fig. 1-8. Right anterior mesial temporal spikes. Same illustration as previous using common average reference (CAR) montage. Unclear is whether the downward deflections represent diffuse electropositivity or simply instrumental involvement of the CAR. A noncephalic lead to the CAR could have solved this dilemma. Calibration signal 1 s, 70 µV.

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Fig. 1-9. Right anterior mesial temporal spikes. Same illustration as previous using coronal montage without sagittal leads. In circumstances where a sagittal focus has been excluded, this coronal run relates several electrode positions to A1, A2. In this instance, equal involvement of A2, F8 and T4 is demonstrated. For ease of visual assessment, A2 is linked to A1 in the 11th channel. Calibration signal 1 s, 70 µV.

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Montages: Advantages and Disadvantages

The following text lists the benefits and limitations of principal montages appearing in this atlas. Although bipolar runs better localize focal or regional features, the morphology of widespread phenomena appears best on referential montages. M1,2 can be considered approximately equivalent to A1,2.

Longitudinal Bipolar Montage, “Double Banana”

·   Best overall survey montage.

·   Adequately lateralizes and localizes most EEG phenomena.

·   If expanded to 18 channels, depicts sagittal events.

·   Omits anterior inferior temporal leads.

·   Potentials with longitudinal fields may cancel.

·   May not establish suprasylvian–infrasylvian relationships.

Fpl–F3
F3–C3
C3–P3
P3–O1
Fp2–F4
F4–C4
C4–P4
P4–O2

Fpl–F7
F7–T3
T3–T5
T5–O1
Fp2–F8
F8–T4
T4–T6
T6–O2

 

Coronal Bipolar Montage with Sagittal Leads

·   Effective when laterality is established by longitudinal bipolar montage.

·   Establishes suprasylvian–infrasylvian relationships.

·   Confirms anterior posterior topology.

·   Records anterior inferior temporal potentials.

·   Predicts involvement of ear references (A1,2) in subsequent referential recordings.

·   Appropriate initial sleep montage as it depicts V waves and spindles.

·   Detects sagittal events.

·   Unless expanded beyond 16 channels, omits frontopolar and occipital leads.

·   Unreliable for potentials of uncertain laterality.

F7–F3
F3–Fz
Fz–F4
F4–F8
T3–C3
C3–Cz
Cz–C4
C4–T4

T5–P3
P3–Pz
Pz–P4
P4–T6
F7–F8
T3–T4
A1–A2
T5–T6

 

Coronal Bipolar Montage without Sagittal Leads

·   Has same general advantages/disadvantages as coronal montage described earlier.

·   Provides better anterior inferior temporal coverage.

·   M2–M1 linkage in this manner and position facilitates visual assessment.

M1–F7
F7–F3
F3–F4
F4–F8
F8–M2
M1–T3
T3–C3
C3–C4
C4–T4
T4–M2

M2–M1
M1–T5
T5–P3
P3–P4
P4–T6
T6–M2

 

Coronal Bitemporal Bipolar Montage

·   Should be used only when significant extratemporal phenomena have been excluded.

·   Mandibular notch electrodes (M1,2) record anterior mesial temporal spikes as effectively as sphenoidal leads (Sadler & Goodwin, 1989).

·   Coronal component better detects temporal spikes with longitudinal fields.

Cz–C3
C3–T3
T3–M1
M1–M2
M2–T4
T4–C4
C4–Cz
F7–F8

Fp1–F7
F7–T3
T3–T5
T5–O1
Fp2–F8
F8–T4
T4–T6
T6–O2

 

Anterior coronal Bipolar Montage

·   May distinguish among frontopolar (Fp1,2), inferior frontal (F7,8), and anterior temporal (A1,2; M1,2) involvement.

·   Useful for assessments of frontal sagittal versus parasagittal fields.

M1–F7
F7–Fp1
Fp1–Fp2
Fp2–F8
F8–M2
F7–F3
F3–Fz
Fz–F4
F4–F8

M2–M1
M1–T3
T3–C3
C3–CZ
CZ–C4
C4–T4
T4–M2

 

Ear (A1,2) or Mandibular Notch (M1,2) Referential Montage

·   Depicts morphologies of widespread waveforms.

·   Reliably lateralizes most phenomena.

·   Assesses symmetry of normally bilateral phenomena such as alpha, beta, mu and spindles.

·   Moderate A1,2/M1,2 involvement distorts depiction of parasagittal potentials.

·   Smaller fields are difficult to appreciate visually.

·   Electrocardiographic (ECG) potentials may be prominent.

Fp1–A1
Fp2–A2
F3–A1
F4–A2
C3–A1
C4–A2
P3–A1
P4–A2
O1–A1
O2–A2

F7–A1
F8–A2
T3–A1
T4–A2
T5–A1
T6–A2

 

Average Reference (Also Known as Common Average Reference) Montage

·   Clearly depicts events with a restricted field.

·   Localizes spikes with multiple phases producing ambiguous fields on bipolar montages.

·   Events with widespread fields may “contaminate” a reference, either obscuring a localizing component or falsely localizing more diffuse ones.

Inputs 1–avg

Central Sagittal (Cz) Reference Montage

·   Useful for temporal events as reference external to field.

·   Heavily involved in V waves and other sleep potentials.

Inputs 1–Cz

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References

Jasper, HH. The ten-twenty electrode system of the International Federation. Electroencephalogr Clin Neurophysiol 1958;10:371–373.

Sadler RM, Goodwin J. Multiple electrodes for detecting spikes in partial complex seizures. Can J Neurol Sci 1989;16:326–329.