Patients are usually examined with their eyes closed and while relaxed. Consequently, the ordinary EEG represents the electrocerebral activity that is recorded under restricted circumstances, usually during the waking or sleeping state, from several parts of the cerebral convexities during an almost infinitesimal segment of the person’s life.
In addition to the resting record, a number of activating procedures are usually employed. First, the patient is asked to breathe deeply 20 times a minute for 3 min. Hyperventilation, through a mechanism yet to be determined, may activate characteristic seizure patterns or other abnormalities. Second, a powerful strobe light is placed about 15 inches from the patient’s eyes and flashed at frequencies of 1 to 20 per second with the patient’s eyes open and closed. In a healthy subject, the occipital EEG leads show waves corresponding to each flash of light (photic driving, Fig. 2-7C).
The EEG is recorded while the patient is drowsy and after the patient is allowed to fall asleep naturally or following the administration of sedative drugs. The drowsy state and the transition to and from deeper stages of sleep can reveal abnormalities.
Many abnormalities associated with sleep are more evident with long-term, continuous EEG monitoring (hours to days) as described in Chap. 18. EEG activity can be synchronized with videographically recorded seizure activity in order to characterize the nature of a seizure. EEGs recorded by small digital devices or telemetry from freely moving ambulatory patients may be similarly useful in cases of suspected seizure disorders. Chapter 15 discusses these techniques in detail. Chapter 18 contains information on the use of EEG to analyze disorders of sleep (polysomnography).
Certain preparations are necessary if electroencephalography is to be most useful. The patient should not be sedated (except as noted above) and should not have been without food for a long time, for both sedative drugs and relative hypoglycemia may modify the normal EEG pattern and caffeine should be avoided if a sleep EEG study is planned. When dealing with patients who are suspected of having epilepsy and are already being treated for it, most physicians prefer to record the EEG while the patient continues to receive antiepileptic medications. During inpatient monitoring, these drugs are often withdrawn for a day or two in order to increase the likelihood of recording a seizure discharge but this requires careful clinical monitoring.
The interpretation of EEGs involves the recognition of several characteristic normal and abnormal patterns and background rhythms (in accordance with the age of the patient), the detection of asymmetries and periodic changes in rhythm, and, importantly, the differentiation of artifacts from genuine abnormalities (see Goldenshohn ES and Hughes JR).
The normal record in adults shows slightly asymmetrical 8to 12-per-second 50-mV sinusoidal alpha waves in both occipital and posterior parietal regions. These waves wax and wane in amplitude spontaneously and are attenuated or suppressed completely with eye opening or mental activity (see Fig. 2-7B). In contrast, the frequency of the alpha rhythm is almost invariant for an individual patient, although the rate slows with aging. Waves faster than 12 Hz and of lower amplitude (10 to 20 mV), called beta waves, are normally recorded from the frontal regions symmetrically. If benzodiazepines or other sedating drugs have been administered, an increase in the fast frequencies is typically observed. When the normal subject falls asleep, the alpha rhythm slows symmetrically and characteristic waveforms consisting of vertex sharp waves and sleep spindles appear (see Fig. 18-1). A small amount of theta (4to 7-Hz) activity may normally be present over the temporal regions, somewhat more so in persons older than 60 years of age. Delta (1to 3-Hz) activity is not present in the normal waking adult.