Chapter 145 Neurologic Monitoring Techniques

Abstract

Patients with nocturnal spells present a unique challenge to the sleep specialist and the sleep laboratory. Although standard polysomnography (PSG) provides valuable information about the stage of sleep from which spells emerge and the time of the spell relative to sleep onset, the characterization of these spells is enhanced by video and extended electroencephalogram (EEG; 12 or more channels, and sometimes 21 or more). Addition of video and an extended EEG to the standard PSG is essential for precise definition of nocturnal spells, including epileptic seizures, rapid eye movement (REM) sleep behavior disorder, and arousal disorders. The video component provides information on the behavioral and motor manifestations of the nocturnal spell.

The EEG might show a rhythmic evolving discharge characteristic of an epileptic seizure, or it might show interictal epileptiform discharges that occur apart from epileptic seizures. Lack of an EEG change, however, does not exclude some types of epilepsy, particularly those of frontal lobe origin. The movement artifact usually associated with nocturnal seizures can also obscure the EEG. A stereotyped series of movements on the video is highly suggestive of epileptic seizures, even in the absence of EEG changes.

Pitfalls in the interpretation of recorded events are common; the inexperienced interpreter might mistakenly identify an artifact as EEG activity characteristic of seizures or parasomnias. A clinician who is in doubt about an EEG-PSG pattern should consult a trained electroencephalographer. Apart from artifacts, many other normal physiologic variants may be mistaken for epileptic activity.

Sophisticated digital video and EEG-PSG computer systems facilitate the acquisition, review, and storage of large quantities of data, with synchronization of behavioral manifestations and the EEG-PSG. Depending on the clinical situation, a daytime EEG, an ambulatory EEG, daytime short-term monitoring, 1 or 2 nights of polysomnography, or long-term (several days and nights) monitoring may be indicated.

Nocturnal spells often present diagnostic problems in sleep medicine because the history alone might not provide sufficient information to differentiate among the various diagnostic possibilities. Standard polysomnography (PSG) is helpful in defining the state and stage of sleep from which such nocturnal spells emerge, but it has limitations in diagnosis because behavioral analysis often is not included and the number of channels devoted to electroencephalography (EEG) is limited. These shortcomings are especially pertinent to the evaluation of suspected nocturnal epileptic seizures, which are defined by behavioral and motor manifestations in addition to EEG criteria.¹ Both behavioral and EEG analyses are critical for characterizing epileptic seizures and for distinguishing them from parasomnias.

The behavioral and EEG manifestations of nocturnal spells caused by parasomnias, neurologic disorders, and psychiatric disorders can be characterized more precisely by combining standard PSG with video recordings and extensive (12 or more channels) EEG montages.² This chapter emphasizes the methodology and indications for video-EEG PSG (VPSG) in the diagnosis of nocturnal events. The methodology and roles of routine EEG, short-term continuous video-EEG monitoring (STM), long-term continuous video-EEG monitoring (LTM), and ambulatory monitoring are addressed.

Methodology

Technical Aspects of Electroencephalography

The EEG measures the difference in electrical potential between pairs of electrodes placed on the scalp. These signals, reflecting synchronous postsynaptic potentials in large groups of neurons, are amplified and filtered to produce an analog or digital recording.³ The international 10-20 system of EEG electrode placement is customarily used, in which the 10-20 refers to 10% and 20% of the distances between standard cranial landmarks (Fig. 145-1).⁴^,⁵ Each electrode site is identified with a letter representing the underlying region of the brain and with a number indicating a specific position above that region, with odd numbers indicating the left hemisphere and even numbers indicating the right hemisphere (e.g., T3 represents a left midtemporal electrode).

Figure 145-1 Standard international 10-20 electrode placement: Electrodes are placed at 10% or 20% of the distances between standard cranial landmarks

(From Keenan SA. Polysomnographic technique: an overview. In: Chokroverty S, editor. Sleep disorders medicine. Boston: Butterworth-Heinemann; 1994. p. 84).

Each recording channel is derived from the signals from a pair of electrodes, and several pairs of electrodes, or derivations, are combined to form a montage. Montages may be either referential or bipolar. In referential montages, one of the electrodes in each pair is connected to a common electrode (e.g., channel 1: Fp1-A1; channel 2: F7-A1; channel 3: T3-A1; channel 4: T5-A1; channel 5: O1-A1). In bipolar montages, there is no common electrode. Bipolar montages are usually arranged in a chain with the same electrode in adjacent derivations (e.g., channel 1: Fp1-F7; channel 2: F7-T3; channel 3: T3-T5; channel 4: T5-01).

The EEG montages used in a combined EEG-PSG study depend on the clinical indication and the number of channels available for recording (Table 145-1). The channels suggested in this text are in addition to those recommended by the American Academy of Sleep Medicine (AASM) for the scoring of sleep stages.⁶ Physicians and technicians involved in the use of EEG monitoring techniques require a solid knowledge of the principles of EEG recording and interpretation. Additional information regarding EEG methodology is available in a standard EEG text.⁷

Table 145-1 Sample Attended Electroencephalographic Montages

NUMBER OF AVAILABLE CHANNELS	MONTAGE
8	F7-T3, T3-T5, T5-O1, F8-T4, T4-T6, T6-O2, F3-C3, F4-C4
10	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, F3-C3, F4-C4
12	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, F3-C3, C3-P3, F4-C4, C4-P4
14	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, F3-C3, C3-P3, P3-O1, F4-C4, C4-P4, P4-O2
16	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, Fp1-F3, F3-C3, C3-P3, P3-O1, Fp2-F4, F4-C4, C4-P4, P4-O2
18	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, Fp1-F3, F3-C3, C3-P3, P3-O1, Fp2-F4, F4-C4, C4-P4, P4-O2, Fz-Cz, Cz-Pz
24	Fp1-F7, F7-T3, T3-T5, T5-O1, Fp2-F8, F8-T4, T4-T6, T6-O2, Fp1-F3, F3-C3, C3-P3, P3-O1, Fp2-F4, F4-C4, C4-P4, P4-O2, Fz-Cz, Cz-Pz, T1-T3, T3-C3, C3-Cz, Cz-C4, C4-T4, T4-T2

Computerized digital EEG-PSG systems facilitate the review of large amounts of EEG-PSG data by displaying scoring and event information in a format that allows the user to, for example, click on the stage or event of interest and bring up the corresponding EEG-PSG.

The recording may be viewed at a variety of display settings. Filters, sensitivities, and montages may be adjusted to characterize events of interest and to help distinguish abnormalities from artifacts or normal variants. For example, certain montages can more easily identify and distinguish artifacts from interictal epileptiform discharges (IEDs), defined as epileptic activity occurring between seizures. Digital EEG enhances the detection and review of IEDs by allowing for remontaging, changing the display settings that influence temporal resolution, and isolating specific channels for review (Fig. 145-2). For example, by altering the display settings, synchronous delta or theta activity characteristic of non–rapid eye movement (NREM) arousal disorders may be more easily distinguished from spike-wave activity or an evolving ictal (seizure) pattern characteristic of an epileptic seizure disorder.

Figure 145-2 Use of different montages to enhance the review of interictal epileptiform discharges (IEDs). A, Left temporally dominant IEDs (arrows) on an extended EEG montage that are not apparent in the central-to-ear channels (asterisk). B, Digital EEG allows the interpreter to select the relevant left temporal and parasagittal channels for review. The arrows highlight IEDs with phase reversals (asterisks) at T3 and F3, indicating maximal negativity at these electrodes, which defines the approximate location of the epileptic region. Both are 30-second epochs. Calibration symbol at bottom right, 100 µV.

Several digital EEG-PSG systems share a common platform with epilepsy monitoring systems, allowing data obtained during a PSG to be analyzed by IED detection programs. Conversely, a study performed in the epilepsy-monitoring unit can be enhanced via the addition of electrooculogram (EOG) and chin electromyogram (EMG) electrodes to score sleep and determine the stage of sleep that precedes a particular event. This can be diagnostic in the case of distinguishing dissociative events, which rarely occur during sleep,⁸ from epileptic seizures, which can occur during sleep or wakefulness.

Daytime Electroencephalography

Daytime EEG is used to look for IEDs, which support the diagnosis of a seizure disorder in many clinical settings.⁷ In addition to the electrodes listed earlier, central (Fz, Cz, Pz) and ear (A1, A2) electrodes are included. Nasopharyngeal electrodes, although used in the past, are not recommended because they are uncomfortable, are prone to artifact, and rarely provide additional information. The activating techniques of hyperventilation and intermittent photic stimulation are routinely performed and can bring out focal asymmetries or epileptiform activity.

Although seizures are not uniformly recorded during a routine EEG, focal IEDs or generalized spike-and-wave discharges may be observed and can assist in the classification of an epileptic syndrome as partial or generalized. The location of IEDs, which can be determined with the use of an extended EEG montage, can clarify the nature of the epilepsy syndrome and its prognosis.⁹ For example, benign epilepsy of childhood with centrotemporal spikes has an excellent prognosis (Fig. 145-3). In contrast, some temporal lobe IEDs may be refractory to medical treatment, and the patients can become candidates for epilepsy surgery.

Figure 145-3 Runs of interictal epileptiform discharges with a centrotemporal dominance in benign epilepsy of childhood with centrotemporal spikes. The bipolar montage readily demonstrates peak negativity at the C4 and T4 electrodes. Such localization is not possible with EEG montages commonly used with standard polysomnography (PSG). Calibration symbol, 500 µV, 1 sec.

(From Malow BA. Sleep and epilepsy. Neurol Clin 1996;14:774.)

Daytime studies performed while the patient is asleep for at least a portion of the recording increase the yield of finding abnormalities, because in many patients, IEDs are more common in drowsiness and NREM sleep. Stage N2 sleep is usually, but not always, recorded on the routine EEG, whereas stage N3 sleep and REM sleep are rarely recorded. When the routine EEG does not show IEDs and the physician has a high suspicion for seizures, a sleep-deprived EEG improves the yield of finding epileptiform activity, at least in part because sleep is more likely to be recorded. Analog EEG recordings use a variety of montages to display ongoing brain activity, whereas digital EEG recordings permit the viewing of a segment of EEG in a variety of montages and speeds, which can help distinguish an IED from an artifact.

Video-Electroencephalography Polysomnography

When the history does not allow the physician to diagnose nocturnal spells associated with complex movements and behavior, recording of the sleep-related event in question might allow definitive diagnosis. VPSG, which combines video recording with an extended EEG montage and with other standard PSG physiologic monitoring, is useful in characterizing unusual behavior and movements during sleep. Diagnostic considerations for patients with complex actions at night can include epileptic seizures, NREM arousal disorders, REM sleep behavior disorder (RBD), rhythmic movement disorder, or psychiatric disorders such as panic disorder or dissociative disorder. Episodes associated with these disorders have specific clinical features as discussed in Chapters 92 and 94 to 96. Events recorded with VPSG are reviewed to characterize the motor and behavioral manifestations of the event and the EEG-PSG features, including the stage of sleep preceding the event, the time of the event relative to sleep onset, and EEG patterns occurring during the event or between events.

Video recordings that are synchronized with digital EEG-PSG recordings allow review of the event in question by clicking on the epoch of interest. The time-locked video can then be reviewed second by second along with the EEG-PSG. Infrared cameras are useful for recording nighttime events. Movable cameras can be mounted in a patient’s room and display close-ups or full-body views. Double cameras are useful for focusing on the face while simultaneously monitoring the body. During recorded events, the technologist should interact with the patient to test for level of consciousness and ability to perform commands.

The stage of sleep from which the spells emerge and the time of the spell relative to sleep onset provide useful diagnostic information. For example, the actions accompanying the NREM arousal disorders arise from stage N3 or sometimes stage N2 sleep, usually in the first third of the sleep period (see Chapter 94), whereas those associated with RBD emerge from REM sleep, most commonly in the last third of the sleep period (see Chapter 95). Epileptic seizures are more common during NREM sleep than during REM sleep (see Chapter 92).¹⁰ Rhythmic movements associated with rhythmic movement disorder usually occur during sleep–wake transitions, and dissociative episodes emerge from wakefulness. Nocturnal panic attacks occur from NREM sleep, usually at the transition from Stage N2 to N3.¹¹

Specific EEG patterns associated with nocturnal spells are discussed in the section Relative Indications, Advantages, Disadvantages, and Limitations. If complex partial seizures are a diagnostic consideration, the EEG montage should emphasize the use of electrodes placed over the temporal lobes (e.g., F7, T1, T3, T5). If benign childhood epilepsy with centrotemporal spikes is a consideration, the montage should include the parasagittal region (e.g., C3, C4). The specific montage that is used depends on the number of channels available for EEG. Sample montages for 8, 10, 12, 14, 16, 19, and 24 channels are shown in Table 145-1.

The following montage of 16 electrodes in anterior-to-posterior chains provides excellent coverage for suspected seizures:

• Left temporal: Fp1-F7, F7-T3, T3-T5, and T5-O1

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