The absence of EEG activity, or “electrocerebral silence,” is a component of brain death but may be simulated by deep sedation with drugs or by profound hypothermia (Fig. 2-7J). Artifacts of various types should be apparent as the amplifier gains are increased; if not, there is a risk that the leads are not properly connected to the machine or of another technical fault. In the absence of nervous system depressants or extreme degrees of hypothermia, a record that is isoelectric (<2 uV except for artifacts) over all parts of the head is almost always a result of profound cerebral hypoxia, ischemia, massive cerebral hemorrhage or of trauma and raised intracranial pressure. Such a patient—without EEG activity, brainstem reflexes, and spontaneous respiratory or muscular activity of any kind—is said to be brain dead as discussed in
Chap. 16.
Epileptic seizures (see Chap. 15) are almost by definition associated with some abnormality in the EEG provided that it is being recorded at the time of the seizure. Rare exceptions are seizure states that originate in deep temporal, medial, or orbital frontal foci, from which the discharge fails to reach the scalp in sufficient amplitude to be seen against the normal background activity of the EEG. Most often, a completely normal EEG during a convulsion indicates a “pseudoseizure” (a psychogenic nonepileptic seizure, or “nonepileptic behavioral event”).
Some of the different types of seizure patterns are shown in Fig. 2-7F and I are associated with particular clinical syndromes in Chap. 16. The absence, myoclonic, and grand mal EEG patterns correlate closely with the clinical seizure type and may be present in milder form in the EEG during periods between clinically evident seizures (interictally). Seizures appear as generalized discharges throughout the cerebrum, or as localized to one region.
Between seizures, a single EEG recording will show a normal pattern in as many as 30 percent of patients with absence seizures and 50 percent of those with generalized tonic-clonic (grand mal) epilepsy (this percentage is less with repeated recordings). Antepileptic therapy may mask interictal EEG abnormalities but the extent to which this occurs is not known. The records of 30 to 40 percent of those with epilepsy, although abnormal between seizures, are nonspecifically so; consequently, the diagnosis of epilepsy can be made only by the correct interpretation of clinical data in relation to the EEG abnormality.
Focal Brain Lesions (Brain Tumor, Abscess, Subdural Hematoma, Stroke, and Encephalitis)
In a high proportion of patients, intracranial mass lesions are associated with focal or localized slow-wave activity (usually delta, as in Fig. 2-7E) or, occasionally, seizure activity. EEG is of considerable value in the diagnosis of herpes simplex encephalitis in which periodic high-voltage sharp waves and slow-wave complexes at intervals of 1 to 3 per second in the temporal regions are characteristic. The other encephalitides are also often associated with sharp or spike activity, particularly if there have been seizures. The EEG is particularly helpful in the diagnosis of prion disease as also noted below. Figure 2-7K shows the characteristic pattern of almost periodic sharp waves seen in Creutzfeldt-Jakob disease.