Interictal activity

Small circuits of hippocampal neurons grown in microisland cultures displayed both ictal and interictal epileptiform activity. Not only was both ictal and interictal activity seen in circuits with two excitatory neurons, but both types of activity were seen also in two-neuron circuits containing one excitatory neuron and one inhibitory neuron. The activity in these El microislands persisted even when inhibitory transmission was blocked pharmacologically, suggesting that epileptiform activity could be generated under these conditions by the single excitatory neuron. Studies of such sohtary neurons produced the simplest... 

Cohen, I, Navarro, V, Qemenceau, S, Baulac, M, Miles, R (2(X)2) On the origin of interictal activity in human temporal lobe epilepsy in vitro. Science, 298 1418-1421. 

However, other seizure-induced changes do not support seizure recurrence. The presence of interictal activity deCTeases the probabiUty of seizure occurrence in several in vitro and in vivo studies (Barbarosie and AvoU, 1997). If seizures induce seizures, seizure control would imply improvement in the natural history of epilepsy but this is not the case. 

Figure 2.14 Relation between the EEG recorded from an epileptic focus on the surface of the cerebral cortex (EEG) and the activity of a single cortical neuron recorded extracellularly (e.c.) and intracellularly (i.c.) during an experimental epilepsy induced by topical application of penicillin. Note that the large EEG excursions correspond to the large (synchronised) depolarisations of the neuron, not to action potential discharges. (Adapted from Brain Res. 52 Ayala, GF et al. Genesis of Epileptic Interictal Spikes. New Knowledge of Cortical Feedback systems suggests a Neurophysiological Explanation of Brief Paroxysms, 1-17 (1973) with permission from Elsevier Science)...

Figure 16.3 Changes in neuronal function required for the development of epileptic seizures. The factors that may control or induce the changes in neuronal function that turn a normal neuron into a focal one (A) recruit other neurons (focal epileptogenesis) to produce an interictal EEG spike (B) and ensure the spread of activity (general epileptogenesis) to full ictal activity (C) are discussed in the text. They include alterations to various ion channels, especially those for Na, a reduction in local inhibitory activity or an increase in local excitatory drive. The electrophysiological counterparts of some of the events involved are shown in Fig. 16.2...

From this survey it is clear that just as normal neuronal function requires appropriately balanced inhibitory and excitatory controls so the generation of interictal spikes depends on disturbances in both. Clearly activity cannot spread without the activation of excitatory circuits, in which NMDA receptors play an important role, but it will be much facilitated by reduced inhibition (Masukawa et al. 1989). These observations may help to explain the establishment of a focus and the development of the interictal spike, but why activity can only spread to seizure proportions, at certain times, is less clear. It will, however, again require overactivity of excitatory circuits inadequately controlled by inhibitory processes. Since these controls are mediated by... 

Incarcerated individuals have a two to four fold greater prevalence of epilepsy, however, a relationship between criminal acts and seizure activity is seldom demonstrated (Marsh and Krauss, 2000). Aggression in seizure-disordered patients is often directed in nature, occurring during the interictal period, in response to a social context, likely related to general brain dysfunction rather than to seizure activity. Therefore, consideration of the social context in which the behavior occurs and of the sociological and biological risk factors is important. 

Visioli F., Rodriguez de Turco E. B., Kreisman N. R., and Bazan N. G. (1994). Membrane lipid degradation is related to interictal cortical activity in a series of seizures. Metab Brain Dis. 9 161-170. 

Watts AD, Herrick lA, McLachlan RS, Craen RA, Gelb AW. The effect of sevoflurane and isoflurane anesthesia on interictal spike activity among patients with refractory epilepsy. Anesth Analg 1999 89(5) 1275-81. 

Once the precise epileptic focus was defined, the transitory electrodes were replaced by four contact depth brain stimulation electrodes (3789 DBS and IPG by Medtronic, Inc., Minneapohs, MN) (Figure 36.4) and connected to an independent IPG system that were placed in a subcutaneous subclavicular pocket on each side. The target of the electrode contacts was the site of maximal interictal and ictal activities. AH antiepileptic drugs were withdrawn to avoid any possible interference with the neuromodulation procedure [Velasco et al., 2000e] and were replaced with phenytoin. 

The kindling model is particularly well-suited to the study of epileptogenesis. Brain regions can be precisely activated by focal stimulation, the interictal, ictal and post-ictal periods are easily manipulated (Sato et al., 1990) and the seizure types are similar to those seen in naturally occurring TLE (Loscher, 2002). 

EEG signals are difficult to interpret since they represent the comprehensive activity of billions of neurons transmitted via the brain tissues, fluids, and scalp [18]. Nevertheless, certain features can be interpreted. In the waveform itself, it is possible to see interictal seizure spikes or a full seizure (such as a petit mal or a grand mal epilepsy) [18]. Analysis of the frequency specfrum of the EEG can reveal changes in the signal power at different... 

Datas were reviewed in parallel to identify spike activities. Auditory evoked 1 Hz unifocal PLEDs were found which evolved to rhythmic MEG sharp transient like activity continuously for 15 mins at the left temporal area (Fig 2a), but not found in motor, visual, somatosensory stimuli even in spontaneous MEG recordings. Simultaneous EEG recording did not show any PLEDs or interictal spike (Fig 2b). The patient showed no subjective or objective symptoms during recording. 

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