The presence of a photic driving response indicates that some of the visual pathways are preserved. Spread of the occipital response induced by photic stimulation, with the production of abnormal sharp or paroxysmal waves, provides evidence of abnormal cortical excitability (Fig. 2-7D). Seizure patterns may be produced during this type of EEG testing, accompanied by gross myoclonic jerks of the face, neck, and limbs (photomyoclonic response), by electrographic seizure activity that outlasts the photic stimulation (photoparoxysmal response), or by a convulsion (photoconvulsive response). Such effects occur often during periods of withdrawal from alcohol and other sedative drugs.
Children and adolescents are more sensitive than adults to all the activating procedures previously mentioned (see Blume and colleagues). It is customary for children to develop delta waves (3 to 4 Hz) during the middle and latter parts of a period of hyperventilation. This EEG activity, referred to as “breakdown,” or “buildup,” disappears soon after hyperventilation has stopped. The frequency of the dominant rhythms in infants is normally about 3 Hz, and they are very irregular. With maturation, there is a gradual increase in frequency and regularity of these occipital rhythms; an alpha rhythm appears by age 6 years and the adult frequency is reached by the age of 10 to 12 years (see Chap. 27 for further discussion of maturation of the brain as expressed in the EEG). The interpretation of records of infants and children require considerable experience because of the wide range of normal patterns at each age period (see Hahn and Tharp, Scher and Painter, and also Ebersole et al). Nevertheless, grossly asymmetrical records or seizure patterns are clearly abnormal in children of any age. Normal patterns in the fetus, from the seventh month onward, have been established. Certain changes in these patterns, as described by Stockard-Pope et al and by deWeerd, are indicative of a developmental disorder or disease.
Types of Abnormal Recordings
Localized regions of greatly diminished or absent brain waves are seen overlying large area of cerebral infarction, traumatic necrosis, tumor, or extensive clot. These findings allow gross localization of the abnormality—but, of course, the nature of the lesion is not disclosed. Two types of abnormal waves, already mentioned, are of lower frequency and higher amplitude than normal. Waves below 4 Hz with amplitudes from 50 to 350 mV are called delta waves (Fig. 2-7E); those with a frequency of 4 to 7 Hz are called theta waves. Fast (beta) activity tends to be prominent frontally and usually reflects the effects of sedative drugs or, if focal, an immediately underlying skull defect called a “breach rhythm” (bone normally filters the abundant fast activity of the cortex).
Spikes are transient high-voltage waveforms that have a pointed peak at recording speeds and duration of 20 to 70 ms; a sharp waves is a similarly configured transient with a duration of 70 to 200 ms (Fig. 2-7F). Spikes or sharp waves that occur interictally are referred to as epileptiform discharges. At times, it is possible to infer localization from the reversal of the polarity of a sharp transient or spike in the EEG record. This “phase reversal” between two channels implies that the electrical activity originates near the site of the position of the common electrode (Fig. 2-7G, H).
The paroxysmal interruption of normal background EEG activity by a run of fast or slow waves is suggestive of seizures. When this paroxysmal discharge is composed of spikes and sharp waves, it signifies a seizure with greater certainty. The electrical discharges associated with absence seizures have a more stereotyped pattern of 3-per-second spike-and-wave complexes that characteristically appear abruptly in all leads of the EEG simultaneously and disappear almost as suddenly at the end of the seizure (Fig. 2-7I).