The significance of the interictal spike discharge: a review

This review deals with the significance of the interictal spike discharge (ISD) and the evidence that it is more than an epiphenomenon or an EEG curiosity. The ISD has a close but complex relationship to ictal activity, as exemplified by the epilepsia partialis continua that may appear with some periodic lateralized epileptiform discharges. The ISD correlates with the locus of the hypometabolic zones in positron emission tomography scans and a decreased rate of oxygen consumption, but there is an increase in glucose utilization and regional blood flow, in addition to changes in echo pulsation. The discharges may also be associated with cardiac arrhythmias, changes in free amino acids, and changes in prolactin level, and the discharges also may serve as a stimulus for a conditioned reflex. The effects of a cortical ISD may be long lasting and can be found in subcortical or other cortical regions, especially in the contralateral homologous area, where T1 spin echo changes can be seen, with effects also on spinal field potentials. In addition to behavioral changes, cognitive effects are clear, especially in determining a nonresponse, a long latency of response, or other types of transitory cognitive impairment, in which a given stimulus presented at the time of the discharge within the appropriate responding cortical area will impair the perception of that stimulus. An important example of the significance of these discharges is the impaired ability to operate an automobile, which may appear at the moment that the ISD occurs. Thus, the evidence is clear that the ISD is more than an EEG curiosity.