Study guidelines
- 1.
Be able to describe the origin of the recorded EEG, underlying rationale for the 10–20 recording system, and the significance of the letter/number combinations used to define electrode placements.
- 2.
Contrast the patterns that typify the waking EEG from that associated with sleep, both REM and NREM sleep.
- 3.
Discuss why the phenomenon of phase reversal is useful in the interpretation of an EEG abnormality.
- 4.
Describe the classification of epileptic seizures and how an epileptic seizure is different from epilepsy.
- 5.
List the characteristics of narcolepsy and its pathogenesis.
We suggest you review the concepts expressed in regards to transmitters and receptors in Chapter 8 before reading about drug therapy in the Clinical Panels.
Neurophysiologic basis of the electroencephalogram
Since its initial development, electroencephalography has remained a unique tool for the study of cortical function and a valuable supplement to history, physical examination, and information gained by radiologic studies.
| Motor | Movement of any part of the motor homunculus, sometimes with aphasia |
| Somatosensory | Contralateral numbness/tingling of face, fingers, or toes |
| Primary visual cortex | Flashes of light or patches of darkness in contralateral visual field |
| Basal occipitotemporal junction | Formed visual images of people or places, sometimes accompanied by sounds |
| Superior temporal gyrus (unusual) | Tinnitus, sometimes garbled word sounds |
When small metallic disc electrodes are placed on the surface of the scalp, oscillating currents of 20 to 100 μV can be detected and are referred to as an electroencephalogram ( EEG ) . Their origin is a direct consequence of the additive effect of groups of cortical pyramidal neurons being arranged in radial (outward-directed) columns. The columns relevant here are those beneath the surface of the cortical gyri. As the membrane potentials of these columns fluctuate, an electrical dipole (adjacent areas of opposite charge) develops. The dipole results in an electrical field potential as current flows through the adjacent extracellular space as well as intracellularly through the neurons ( Figure 30.1 ). It is the extracellular component of this current that is recorded in the EEG, and variations in both the strength and density of the current loops result in its characteristic sinusoidal waveform.
The oscillations of the EEG, measured in microvolts (μV), are thought to be generated by reciprocal excitatory and inhibitory interactions of neighbouring cortical cell columns.
Technique
After careful preparation of the skin of the scalp to ensure good contact, electrodes are affixed in a placement that is in conformity with the 10–20 International System of Electrode Placement (with the modified combinatorial nomenclature ), in which the scalp is divided into a grid in accordance with Figure 30.2 .
By defining a consistent placement of electrodes, direct comparison to follow-up studies is feasible, as is a method to compensate for differences in head size. Each electrode placement allows it to preferentially record over a cortical surface area of approximately 6 cm 2 . The nomenclature employed to define each electrode position combines a letter with a number, as shown in the figure.
Actual EEG recordings are made from all sites simultaneously. The potential difference between electrode pairs is recorded (as a rule), and this is displayed as a separate individual graph or channel. Often other physiologic recordings are performed at the same time (e.g. an electrocardiograph [ECG] and/or a surface electromyograph [EMG]).
If varying pairs of electrodes are used, the montage (output) is termed bipolar ( Figure 30.3A ). If they have one recording site in common (auricle, or mastoid area), it is called referential ( Figure 30.3B ).
Figure 30.4 provides a complete set of normal tracings.
Types of patterns
Normal EEG rhythms
Awake state EEG
The EEG demonstrates prominent changes both with the level of alertness and during the various stages of sleep. Each of these patterns is specific and is taken into account during EEG interpretation. A routine EEG study will usually take 30 minutes and will include recordings made during wakefulness and during early stages of sleep, because specific abnormalities (especially epileptiform ones) may only be detected during the sleep portion of the recording.
In the alert awake state ( Figure 30.5A ) the pattern is described as desynchronised because the waveforms are quite irregular. The background frequency is usually around 9.5 Hz. A β frequency of more than 14 Hz may be superimposed over anterior head regions.
In a relaxed state with the eyes closed, rhythmic waveforms called the α rhythm appear in the α frequency (8 to 14 Hz), notably over the parietooccipital area ( Figure 30.5B ).
Normal sleep EEG
Glossary
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Rapid eye movement (REM) sleep. Dreamy light sleep accompanied by REMs; also called paradoxical sleep because the EEG resembles that for the awake state.
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Non-REM (NREM) sleep. NREM sleep (stages 1–4); stage 3 and 4 are also called slow wave sleep . In a routine EEG, NREM sleep will usually be identified, but REM sleep patterns, because of the briefness of the EEG, would be unusual.
Sleep can be defined at both a behavioural level (e.g. immobility) and by patterns of neuronal activity of the brain (e.g. cortical neuronal firing patterns). People normally pass through three to five sleep cycles per night, the first within the first 90 minutes of sleep. The sequence of events is summarised in Figure 30.5 . α rhythm becomes more apparent (on occipital leads) during quiet rest with eyes closed.
By general agreement, proper sleep is associated with slow-wave patterns in the EEG and characteristic EEG patterns that allow sleep stages to be recognised. This begins with a rapid descent through stage 1, characterised by a steady θ rhythm, into stage 2, characterised by θ waves interrupted by sinusoidal waveforms called sleep spindles, and by occasional K complex spikes. stage 3 and 4 is characterised by slow δ waves—hence the term slow wave sleep for that stage ( Figure 30.6 ).
