Insights to absence seizures have been obtained from animal models of bilaterally synchronous 3-per-second high-voltage spike-and-wave discharges. The spike-and-wave complex, which represents brief excitation followed by slow-wave inhibition, is the type of EEG pattern that characterizes the clonic (inhibitory) phase of the focal motor or grand mal seizure. By contrast, this strong element of inhibition is present diffusely throughout an “absence” attack, a feature that perhaps accounts for the failure of excitation to spread to lower brainstem and spinal structures (tonic-clonic movements do not occur). Earlier mentioned is the work by Blumenfeld’s group, suggesting that the interruption of consciousness in this syndrome can be linked with electrophysiologic changes in the thalamus, comparable to what is described for types of generalized seizure.
Of theoretical importance is the observation that a seizure focus may establish, via commissural connections, a persistent secondary focus in the corresponding cortical area of the opposite hemisphere (mirror focus). The nature of this phenomenon is obscure; it may be similar to the “kindling” phenomenon mentioned earlier in animals, where a repeated nonconvulsive electrical stimulation of normal cortex induces a permanent epileptic focus. No morphologic change is visible in the mirror focus, at least by light microscopy. The mirror focus may be a source of confusion when trying to identify the side of the primary discharging lesion by EEG. However, there is only limited evidence that mirror foci related to the kindling phenomenon produce seizures in humans (see Goldensohn).
The origins of EEG activity of an epileptic focus and the generalization of seizures are discussed in Chap. 2 and earlier in this chapter. The EEG provides confirmation of Hughlings Jackson’s concept of epilepsy—that it represents a recurrent, sudden, excessive discharge of cortical neurons. The EEG is the most sensitive, indeed indispensable, tool for the diagnosis of epilepsy; but like other ancillary tests, it must be used in conjunction with clinical data. In patients with idiopathic generalized seizures, and in a high proportion of their relatives, interictal spike-and-wave abnormalities without any clinical seizure activity are common, especially if the EEG is repeated several times or taken over long periods. By contrast, a proportion of epileptic patients have a perfectly normal interictal EEG. Using standard methods of scalp recording, the EEG may even be normal during the experiential aura of a simple or complex partial seizure. Furthermore, interpretation of EEG abnormalities must take into account that a small number of healthy persons (approximately 2 to 3 percent) show paroxysmal EEG abnormalities.
A single EEG tracing obtained during the interictal state is abnormal to some degree in 30 to 50 percent of epileptic patients; this figure rises to 60 to 70 percent if patients are subjected to several recordings. Many EEG patterns are possible in seizures. One consistent observation, however, has been that the region of earliest spike activity corresponds best to the epileptogenic focus, a rule that has come to guide epilepsy surgery. The postseizure or postictal state also has an EEG correlate, taking the form of random generalized slow waves after generalized seizures and focal slowing following partial seizures. With clinical recovery, the EEG returns to normal or to the preseizure state.
A higher yield of abnormalities and a more precise definition of seizure types can be obtained by the use of several special EEG procedures, as described in Chap. 2. Here it is restated that activating procedures such as hyperventilation, photic stroboscopic stimulation, and sleep increase the yield of EEG recordings. EEG recording during sleep is particularly helpful because focal abnormalities, particularly in the temporal lobes, may become prominent in slow-wave and stage II sleep. Sphenoidal leads have been used to detect inferomedial temporal seizure activity, but they are uncomfortable and probably add little more information than can be obtained by the placement of additional subtemporal scalp electrodes. Nasopharyngeal electrode recordings are too contaminated by artifact to be clinically useful.