Fig. 24.1
10–20 electrode placement. The 10–20 system assigns a number to further specify the location in the left or right hemisphere. Location “z” is used to indicate that the location of the electrode is in the midline or “zero” meaning that it is neither left hemisphere nor right hemisphere. The electrode placed at Cz is said to be the “Vertex” meaning that it is the mid central or at the top of the head. Fpz and Oz are used in achieving the other measurements and can be used as additional electrode placements for localization of activity. Fpz may be used as the location of the COM (common) or ground electrode placement
Fig. 24.2
Updated AASM-recommended derivations for recording the EEG. Electrode locations as recommended in 2007 by the American Academy of Sleep Medicine. Key: Fp frontopolar or prefrontal, F frontal, C central, T temporal, P parietal, O occipital, A ear or mastoid, F3 left mid-frontal, P3 left parietal, T4 right temporal, A1 right ear, and Cz vertex. Modified from Kryger [8]
The electrooculogram (EOG) signals measure changes in the electric potential of the positive anterior aspect of the eye, the cornea, relative to the negative posterior aspect, the retina. The “Recommended” EOG derivations are E1-M2 and E2-M2 with E1 placed 1 cm below the outer canthus of the left eye and E2 placed 1 cm above the outer canthus of the right eye. An alternative “Acceptable” derivation is placing E1 1 cm below and lateral to the outer canthus of the left eye and E2 1 cm above and lateral the outer canthus of the right eye as demonstrated in Fig. 24.3. During any eye movement, the cornea moves toward one electrode, while the retina moves away. When the eye is not moving, the change in relative position is zero, and the eye leads do not record a signal. Slow rolling eye movements (SREM) occur during drowsiness and light sleep and are recorded as long gentle waves with the initial deflection lasting longer than 500 ms (Table 24.1). In contrast, rapid jerking eye movements are represented by sharply contoured fast waves with the initial deflection lasting less than 500 ms. Blinking of the eyes produces rapid vertical movements.
Fig. 24.3
Derivation of eye movements. E1 Left outer canthus eye electrode (previously LOC). E2 Right outer canthus eye electrode (previously ROC). M1 Left mastoid electrode location. M2 Right mastoid electrode location. The eye can be envisioned like a battery with the positive pole at the cornea and the negative pole at the retina. The EOG consists of a bipolar linkage from the ROC electrode 1 cm lateral and 1 cm superior to one outer canthus to the LOC electrode 1 cm lateral and 1 cm inferior to the other outer canthus. The electrode toward which the eyes move becomes relatively positive, the other relatively negative. As the eyes move during sleep, they produce corresponding changes in the electrical field producing a correlating potential change in the EEG electrodes. This can be verified by noting corresponding movements in the EOG channels. From: Avidan and Barkoukis [9]
Table 24.1
Key brain wave frequencies and landmarks used in sleep staging
Sample | Label | Definition |
|---|---|---|
 | Alpha activity | 8–13 Hz rhythm, usually most prominent in occipital leads. Thought to be generated by cortex, possibly via dipole located in layers 4 and 5. Used as a marker for relaxed wakefulness and CNS arousals |
 | Theta activity | 4–8 Hz waves, typically prominent in central and temporal leads. Sawtooth activity is a unique variant of theta activity (containing waveforms with a notched or sawtooth–shaped appearance) frequently seen during REM sleep |
 | Vertex sharp waves | Sharply contoured, negative-going bursts that stand out from the background activity and appear most often in central leads placed near the midline |
 | Sleep spindle | A phasic burst of 11–16 Hz activity, prominent in central scalp leads; typically last for 0.5–1.5 s. Spindles are a scalp representation of thalamocortical discharges; the name derives from their shape (which is spindle-like) |
 | K complex | Recently redefined in the AASM manual as an EEG event consisting of a well-delineated negative sharp wave immediately followed by a positive component standing out from the background EEG with total duration ≥0.5 s, usually maximal in amplitude over the frontal regions |
 | Slow waves | High-amplitude (≥75 µV) and low-frequency (≤2 Hz) variants of delta (1–4 Hz) activity. Slow waves are the defining characteristics of stage N3 sleep |
 | REM | Rapid eye movements are conjugate saccades occurring during REM sleep correlated with the dreamer’s attempt to look at the dream sensorium. They are sharply peaked with an initial deflection usually <1/2 s in duration |
 | SEM | Slow eye movements are conjugate, usually rhythmic, rolling eye movements with an initial deflection usually ≥1/2 s in duration |
The electromyographic (EMG) signals are muscle twitch potentials, which are used in sleep studies to distinguish between sleep stages based on the fact that electromyographic activity progressively diminishes during deepening stages of sleep. Specifically, during rapid eye movement (REM) sleep, muscle activity is minimal. There are occasional intrusions of EMG artifact into the record, some of which may be expressed as yawns, swallows, and teeth grinding (bruxism). Three electrodes are placed to record chin EMG: (1) midline 1 cm above the inferior edge of the mandible, (2) 2 cm below the inferior edge of the mandible and 2 cm right of midline, and (3) 2 cm below the mandible and 2 cm left of the midline. The standard chin EMG derivation utilizes one electrode below the mandible referred to the electrode above the mandible.
Scoring Stages of Sleep
Epochs of polysomnography are broken into 30-second sequential epochs with a sleep stage assigned to each epoch based on scoring rules that will be discussed below. If two or more stages coexist during a single epoch, the stage occupying the greatest portion of the epoch will be assigned. When scoring a record for stage of sleep, it may be helpful to scroll through the entire record quickly to evaluate the quality of the recording. He or she should observe the specific shape of the features that represent the stages in that particular individual and to gain an overall picture of the cycles for that record. Specifically observe for sleep spindles, K complexes, slow waves, and rapid eye movements.
EEG cortical activity can be characterized by their specific frequencies. Frequency is defined as the number of times a repetitive wave recurs in a specific time period (typically one second). Frequency is noted as cycles per second (i.e., Hertz, Hz). EEG activity has been divided into four bands based on the frequency and amplitude of the waveform and are assigned Greek letters (alpha, beta, theta, and delta) and is summarized in Tables 24.1 and 24.2.
Table 24.2
Definitions and examples of sleep figures encountered on an EEG
EEG rhythm | Characteristics | Best seen | Examples |
|---|---|---|---|
Posterior dominant rhythm (PDR) | 8–13 Hz | Occipital |  |
Slow waves | 0.5–2 Hz; amplitude ≥75 µV | Frontal |  |
Spindle | 11–16 Hz; duration ≥0.5 s 8–12 Hz | Central |  |
K complex | Diphasic; large amplitude, duration ≥0.5 s | Frontal |  |
The following convention is used to define EEG frequencies as per the AASM scoring manual:
Beta is greater than 13 Hz;
Alpha is between 8 and 13 Hz;
Theta is between 4 and less than 8 Hz; and
Delta is the slowest activity at less than 4 Hz.
Beta activity originates from the frontal and central regions and can be present during wakefulness or drowsiness. Beta activity typically declines during deeper stages of sleep but may reemerge during REM sleep. Use of sedative hypnotics (such as benzodiazepines) may cause enhanced beta activity (termed pseudo-spindles or drug-spindles) as depicted in Fig. 24.4, with frequencies faster than true bona fide physiological spindle activity (Table 24.3).
Fig. 24.4
This 30 s PSG epoch in a 45-year-old woman who takes nightly benzodiazepines. “Psuedospindles,” or enhanced beta activity (identified by the blue bars), are noted during NREM sleep due to use of benzodiazepines
Table-24.3
Major differences between Rechtschafffen and Kales manual (R and K) and the AASM scoring manual from 2007
Differences | R and K manual | AASM scoring manual |
|---|---|---|
EEG Electrodes | Score sleep stages using central (C3 and C4) leads | Score using frontal, central, and occipital leads |
Major body movements | Movement time can be scored if more than half the epoch is obscured | No movement time staging exists |
Slow wave sleep | Consists of both stage 3 and stage 4 sleep with delta wave amplitude measured using central leads | Only recognizes stage N3 sleep with delta wave amplitude measured using frontal leads |
Terminology of stages | Stage 1, stage 2, stage 3, stage 4, and stage REM sleep | Stage N1, stage N2, stage N3, and stage R sleep |
Reference electrode | Left and right ear or mastoid termed A1 or A2 | Left and right mastoid termed M1 or M2 |
Scoring stage 2 (or N2) sleep | Three minute rule that states if greater than 3 min pass in between spindles or K complexes, then score stage 1 sleep | No 3 min rule exists |
Alpha activity, also known as the posterior dominant rhythm, originates in the parieto-occipital regions bilaterally and is normally symmetric over both cerebral hemispheres. The alpha rhythm is noted during quiet alertness with the eyes closed and disappears or decreases in amplitude when the eyes open (reactivity) (Fig. 24.5).
Fig. 24.5
This 30 s polysomnogram epoch showing the appearance of background alpha activity in the occipital leads when the patient is in quiet alertness and closes their eyes (red star). This rhythm is not apparent when the subject’s eyes are open during the first portion of the epoch
Stage Wake
Typically, the first several minutes of recording will consist of wake (W) stage. Stage W is scored when more than 50 % of the epoch has scorable alpha EEG activity over the occipital region. Submental EMG is relatively high tone and will reflect the high-amplitude muscle contractions and movement artifacts. The EOG channels will show eye blinks (0.5–2 Hz) (Fig. 24.6) or possibly reading eye movements if the eyes are open (Fig. 24.7). As the patient becomes drowsy, with the eyes closed, the EEG will show predominant alpha activity, while the EMG activity will become less prominent. The EOG channels may show slow rolling eye movements. The patient may enter stage N1 sleep briefly for one or two epochs and then return to stage W. From stage W, patients typically proceed to stage N1, but infrequently they may enter REM sleep or stage N2 sleep directly, if the pressure to do so is high (reflecting a state of pathological sleep deprivation).
Fig. 24.6
This 30 s polysomnogram epoch demonstrates stage W with alpha rhythm noted in the occipital leads (blue star), increased chin EMG tone (CHIN electrode), and rapid eye movements of wakefulness (LOC, ROC). The lower panel demonstrates the prominent alpha activity (orange star) primarily seen in the occipital leads
Fig. 24.7
This 60 s polysomnography epoch demonstrates stage W with alpha rhythm noted in the occipital leads, increased chin EMG tone (CHIN electrode), eye movements of wakefulness associated with rapid reading
Stage N1 Sleep
Stage N1 sleep is scored if the alpha rhythm is attenuated or replaced by low-amplitude, mixed-frequency activity (4–7 Hz) for more than half of the epoch (Fig. 24.8). Some individuals (approximately 10 %) do not generate an alpha rhythm upon eye closure and have similar occipital EEG activity during eye opening or closure. In these individuals, stage N1 is scored if there are vertex waves, slow rolling eye movements, or EEG activity in the range of 4–7 Hz (Fig. 24.9) with slowing of background frequencies by greater than or equal to 1 Hz from stage W. Vertex sharp waves (V waves) are sharply contoured waves lasting less than 500 ms that are maximal over the central regions (Fig. 24.9). Vertex sharp waves may be present but are not required to score stage N1. The EMG shows less activity than in wake, but the transition is gradual and of little assistance in scoring. In patients who have a background rhythm that is theta frequency due to a pathological state (encephalopathy, dementia, etc.), stage N1 sleep can be scored if there is further slowing of the background rhythm by greater than 1 Hz.
