Interictal spikes

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)...

A seizure is accompanied by a burst of spikes in the EEG. Between these so-called iclal phases are solitary EEG interictal spikes. Each of them represents the field potential associated with a burst of action potentials in a group of neurons within the epileptic focus (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... 

Ayala, G. F., Dichter, M., Gumnit, R. L, Matsumoto, H. and Spencer, W. A. Genesis of epileptic interictal spikes. New knowledge of cortical feedback systems suggests a neurophysiological explanation of brief paroxysms. Brain Res. 52 1-17,1973. 

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. 

Jawad, S. et al., The effect of lamotrigine, a novel anticonvulsant, on interictal spikes in patients with epilepsy, Br. J. Clin. Pharmacol, 22 191-193, 1986. 

Besides ictal events, BOLD signal is also measured in human interictal discharges (Benar et al., 2002). It is unknown how this period of relative ictal/interictal ischemia impacts neuronal function or causes permanent damage. However, in humans, interictal spikes may be associated with transient negative impact on cognition (Shewmon and Erwin, 1988 Binnie and Marston, 1992). 

Shewmon, D.A., and Erwin, R.J. 1988. The effect of focal interictal spikes on perception and reaction time. I. General considerations. Electtoencephalogr Clin Neurophysiol 69(4) 319-337... 

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. 

Fig. 2 MEG with simultaneous EEG recording, (a) MEG recording in left temporal area showed 1 Hz auditory evoked PLEDs (black arrow) (b) Simultaneous EEG recording (A1 reference) showed no PLEDs or interictal spike...

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