Electroencephalographic Monitoring

The international 10-20 system1,2 for electrode placement for electroencephalographic (EEG) monitoring is illustrated in Figure 27–1. Each electrode is represented by a letter that represents the underlying area or lobe of the brain (F_p = frontopolar; F = frontal; P = parietal; O = occipital; T = temporal) (Fig. 27–2) and numerical subscripts representing position. The odd subscripts are on the left and the even on the right, and the “z” subscripts refer to electrodes in the midline.^1,2 The left and right auricular (earlobe) electrodes are A₁ and A₂. In sleep monitoring, these are actually placed on the mastoids and termed M₁ and M₂.³ Figure 27–1 illustrates the new electrode nomenclature¹ in which T₇, T₈, P₇, and P₈ have replaced T₃, T₄, T₅, and T₆. In the new nomenclature, all electrodes in a given sagittal plane have the same subscript (F₇, T₇, P₇) and most electrodes in the same coronal plane have the same letter (P₇, P₃, P_z, P₄, P₈). However, many EEG laboratories still use the old electrode nomenclature as does the majority of the published literature on epilepsy. Of note, the 10-20 electrode position² refers to the fact that electrodes are positioned at 10% or 20% of the distance between landmarks such as nasion, inion, or the preauricular points (Fig. 27–3).

Bipolar Monitoring and Standard Montages

A derivation is the voltage difference between electrodes: for example, F_p1-F₃ is the voltage difference between electrodes F_p1 and F₃. By electroencephalography (EEG) convention, if F_p1 is more negative than F₃, the deflection is up.4,5 A set of derivations is called a montage. Montages are designed with a particular purpose in mind. Standard montages for sleep monitoring were illustrated in Chapter 1. Tables 27–1 and 27–2 show standard clinical EEG montages.⁶ Bipolar longitudinal montages (Fig. 27–4; see also Table 27–1) sequentially compare two adjacent electrodes in chains covering the head in an anteroposterior (AP) direction (“double banana”). In the most frequently used variant (LB-18.1), the chains start at the left temporal area and then progressively move toward the right. Bipolar transverse montages compare two electrodes in chains in the transverse directions (Fig. 27–5; see also Table 27–2). Referential montages compare electrodes with the ipsilateral auricular electrodes A₁ and A₂ (see Table 27–2). Different laboratories may display the electrodes in a given montage in different sequences. In modern digital EEG recording, all electrodes are usually recorded against a common reference. Then any two electrodes may be compared by digitally subtracting the signals (F₇-ref) − (P₇-ref) = F₇-P₇. Thus, one can change the display montage while recording or later during study review. Digital recording also allows one to visualize multiple time scales. The polysomnography (PSG) window to stage sleep is 30 seconds but the clinical EEG window is 10 seconds. The 10-second time window allows detection of brief, sharply contoured waveforms that may signify seizure activity.

If the capacity to add a few electrodes to traditional sleep monitoring exists, one can increase the ability to detect epileptiform activity. For example, one could add two electrodes (T₇, T₈). The derivations F₃-T₇, T₇-O₁, F₄-T₈, and T₈-O₂ would add coverage over much of the frontal and temporal areas. These areas are the predominant foci of seizures occurring mainly during sleep. Other clinicians have utilized the addition of F_z, C_z, F₇, and F₈ to the usual PSG electrodes.⁷ The addition of synchronized digital video monitoring to nocturnal EEG or PSG recording greatly enhances the ability of the clinician to diagnose nocturnal events.⁸ As is discussed later, many nocturnal seizure disorders are not associated with scalp EEG findings and the patient’s actions during the seizure (seminology) may be very helpful in determining whether an episode is likely a parasomnia or nocturnal epilepsy.⁸

Waveform and Seizure Terminology

The terminology and identification of epileptic seizures and interictal activity is challenging for physicians without extensive training in EEG (Table 27–3). A transient is any isolated wave or complex that stands out compared with background activity. A spike is defined as a transient with a pointed peak and a duration of 20 to 70 msec (Fig. 27–6). On a 30-second page, spikes look like a single vertical line. A sharp wave is a transient with a deflection of 70 to 200 msec.

A spike and wave complex is a spike followed by a slow wave (usually wide and often higher amplitude). Polyspike complexes often consist of multiple spikes superimposed on a slow wave. The term epileptiform activity literally means “EEG activity resembling that found in patients with epilepsy.” This is a somewhat circular definition. Interictal activity is defined as abnormal EEG activity that occurs between seizures. Epileptiform activity includes spikes, spike and waves, and polyspike complexes. Abnormal sharp waves are also considered epileptiform or interictal activity. Of course, sharp waves can be normal (e.g., vertex sharp waves). Artifacts can sometimes mimic spikes or spikes and waves. In general, true spike and wave complexes have a “field” (Box 27–1). That is, true spike and wave activity should be seen in derivations containing several contiguous electrodes. If a spike and wave is seen only in a single derivation, the activity is usually be an artifact.

Because seizures do not always appear during a given recording (see Box 27–1), the physician reading an EEG or PSG searches for spikes and/or abnormal sharp waves that may represent the interictal footprint of possible seizure activity. However, it should be noted that not all patients with spikes have seizures and not all patients with seizures have detectable interictal activity. Spikes represent abnormal brain activity that is seen as an area of negativity at the scalp. Spikes can be localized (negativity at the scalp over one area of the brain) or appear diffusely. Focal seizures usually, though not invariably, begin at the same location as the interictal spikes. The classic spike is followed by a slow wave. Although most common postictally, spike and sharp waves will occur sporadically at any time and may have a slight increase preictally. Using the usual 30-second time window, it may be difficult to appreciate spikes. A switch to a 10-second window is indicated to scrutinize any suspicious paroxysmal activity.

Phase Reversal

Localized EEG waveforms (including spikes and sharp waves) will show phase reversal if the bipolar chain crosses the area of the localized EEG activity.4,5 Phase reversal does not imply an abnormal waveform. For example, K complexes and vertex sharp waves show phase reversal in montages that cross the location of origin. Phase reversal may help differentiate epileptiform activity such as a spike from artifact. Epileptiform activity is recorded as an electronegative potential. For example, in Figure 27–7A, negative spike activity is seen under electrode T₇. This results in down-going deflections in F₇-T₇ because T₇ is more negative than F₇. This pattern reverses for T₇-P₇ because now P₇ is more positive than T₇. In this figure, “s” is for spike and “w” is for wave. If the spike focus is located nearly equidistant between two monitoring electrodes (F₈ and T₈) (see Fig. 27–7B), the derivation containing them may show little or no activity (F₈-T₈) and the derivations on either side will show phase reversal ([F_p2-F₈] to [T₈-P₈]).

Localization in Referential and PSG Montages

In referential montages, a spike will have greater activity in montages containing electrodes nearer to the location of the spike. In Figure 27–8, a spike focus is located between F₈ and T₈. On the bipolar montage, this results in the electrical activity being nearly equal in the two electrodes, resulting in minimal activity in F₈-T₈ but a phase reversal in adjacent bipolar pairs. However, in referential recording, the spike results in larger and nearly equal activity in F₈ and T₈ with less activity in adjacent electrodes. In the usual PSG montage, electrodes are referenced to the opposite mastoid.³ Therefore, a spike located near F₄ will result in greater deflection in F₄-M₁ compared with C₄-M₁ or O₂-M₁. It is also important to recall that the mastoid electrodes may actually be closer to spikes in the temporal area than the F₄, C₄, and O₂ electrodes. For example, a left temporal focus of activity may be nearer M₁ than F₃, C₃, and O₁. Thus, C₄-M₁ will actually show more activity from a left temporal focus than C₃-M₂ because M₁ is closer to the temporal area than C₃. In the case of a left temporal spike, one would expect to see epileptiform activity in most channels referenced to the left mastoid (M₁).

Normal Sleep Waveforms in the 10-Second Window

The standard waveforms and eye movements used to stage sleep have a different appearance when viewed in a 10-second window in a standard clinical EEG montage. With digital PSG, one can change the time base (10-second to 30-second window) and vice versa to help with waveform recognition.

Eye Movements and Bell’s Phenomenon

Bell’s phenomenon describes the reflex upward movement of the front of the eyeball when the eyelids close (or blink). Because the cornea is positive with respect to the retina, this causes a deflection in derivations containing electrodes placed near the eyes (F_p1, F_p2). Recall that if G₁ becomes positive with respect to G₂, the derivation G₁-G₂ shows a downward deflection. Thus, eyelid closure results in downward deflections in F_p1-F₇ and F_p2-F₈. This is illustrated in Figures 27–9 to 27–11.

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