How can we identify ictal and interictal abnormal activity?

Abstract

The International League Against Epilepsy (ILAE) defined a seizure as "a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain." This definition has been used since the era of Hughlings Jackson, and does not take into account subsequent advances made in epilepsy and neuroscience research. The clinical diagnosis of a seizure is empirical, based upon constellations of certain signs and symptoms, while simultaneously ruling out a list of potential imitators of seizures. Seizures should be delimited in time, but the borders of ictal (during a seizure), interictal (between seizures) and postictal (after a seizure) often are indistinct. EEG recording is potentially very helpful for confirmation, classification and localization. About a half-dozen common EEG patterns are encountered during seizures. Clinicians rely on researchers to answer such questions as why seizures start, spread and stop, whether seizures involve increased synchrony, the extent to which extra-cortical structures are involved, and how to identify the seizure network and at what points interventions are likely to be helpful. Basic scientists have different challenges in use of the word 'seizure,' such as distinguishing seizures from normal behavior, which would seem easy but can be very difficult because some rodents have EEG activity during normal behavior that resembles spike-wave discharge or bursts of rhythmic spiking. It is also important to define when a seizure begins and stops so that seizures can be quantified accurately for pre-clinical studies. When asking what causes seizures, the transition to a seizure and differentiating the pre-ictal, ictal and post-ictal state is also important because what occurs before a seizure could be causal and may warrant further investigation for that reason. These and other issues are discussed by three epilepsy researchers with clinical and basic science expertise.

Fig. 1.1

Common epileptiform EEG patterns. Common patterns are shown for individuals with focal spikes, generalized spikes, spike-waves, and a seizure with focal onset (From Fisher, unpublished)

Fig. 1.2

Common EEG patterns at the start of seizures in patients with epilepsy (From Fisher, unpublished)

Fig. 1.3

Is this a seizure? Rhythmical brief epileptiform activity, illustrating the ambiguity involved in deciding whether an EEG event corresponds to interictal activity or a seizure (From Fisher, unpublished)

Fig. 1.4

Periodic lateralized epileptiform discharges (PLEDs) – are they ictal or interictal? PLEDs over the left central (C3) region are shown. Some electroencephalographers consider this pattern to be interictal and others ictal, while still others believe it depends upon particular circumstances (From Fisher, unpublished)

Fig. 1.5

Interictal-ictal disparity with spikes in the right hemisphere and seizures on the left. Interictal-ictal disparity in the same patient as Fig. 1.5, with interictal spikes over the right temporal region, but seizure onset from the left temporal region. Note different time scales for each segment (From Fisher, unpublished)

Fig. 1.6

Recordings with intracerebral stereo-EEG electrodes in a patient with focal epilepsy secondary to focal cortical dysplasia. Far left: The position of the recording electrodes is illustrated. Left: Interictal discharges recorded with intracerebral stereo-EEG electrodes in a patient with focal epilepsy secondary to focal cortical dysplasia. Right: Seizure onset is marked by the arrow. The slow spikes that precede the ictal low-voltage fast activity are different in location and morphology from the interictal spikes (Courtesy of Francione, Tassi and LoRusso of Claudio Munari Epilepsy Surgery Center, Niguarda Hospital, Milano)

Fig. 1.7

Intracerebral recording of a focal seizure with stereo-EEG electrodes (as shown in the upper right inset) in a patient with cryptogenic focal epilepsy during presurgical evaluation. Multi-contact electrodes are identified by letters. The EEG marked by an asterisk is expanded at the bottom. When the seizure begins (seizure onset, arrow) there is a reduction of background activity, appearance of fast activity, and subsequently there is a very slow potential (From Gnatkovsky, Francione, Tassi and de Curtis, unpublished)

Fig. 1.8

Seizures recorded in guinea pig entorhinal cortex. The upper trace was recorded in the in vitro isolated guinea pig brain after systemic application of 50 μM bicuculline. In the lower panel a seizure is shown, which was recorded in vivo 3 months after injection of kainic acid in the hippocampus. Both seizures are characterized by fast activity at the onset followed by irregular firing and late periodic bursting (From DeCurtis, unpublished)

Fig. 1.9

EEG characteristics in the normal adult rat. (a) Using 8 electrodes (shown in d), awake behaving rats were recorded in their home cage. During exploration, hippocampal electrodes exhibited theta oscillations. The area outlined by the box is expanded at the bottom. (b) During a spontaneous arrest of behavior, sharp waves (arrows) occurred regularly in the hippocampal EEG. (c) During sleep, the hippocampal EEG became active. (d) The recording arrangement included 4 epidural electrodes and 2 twisted bipolar electrodes in the dorsal hippocampus, one in each hemisphere. Grd ground; Ref reference. (e) A summary of a-c is shown. In three behavioral states there are large differences in the hippocampal EEG with sharp waves (arrows) in behavioral arrest and sleep. (f) During sharp waves, filtering in the ripple band (100–200 Hz) shows that a ripple occurs at the same time as the sharp wave (From LaFrancois and Scharfman, unpublished)

Fig. 1.10

Spike-wave discharges recorded from the normal adult hippocampus of the rat. (a–b) A recording from an adult Sprague-Dawley rat shows typical EEG activity during exploration and behavioral arrest. In behavioral arrest, there were spike-wave discharges. Animals were monitored during the recordings to be sure that artifacts related to grooming or chewing did not occur during spike-wave discharges. (c) Recordings in b are expanded (From Pearce and Scharfman, unpublished; see also [101])