This study investigated the relationship between slow waves and sharp waves (spikes) and also clinical seizures in 255 patients with 694 EEGs over a 25-yr period. Slow waves were quantified into five groups, sharp waves into three groups and clinical seizures also into three groups. In general, as clinical seizures increased, the number of patients with many discharges increased and the number with only rare discharges decreased. Also, as patients became seizure free, there was a decreasing incidence of many sharp waves and an increasing incidence of only rare sharp waves. No relationships could be found in patients with a typical number of discharges or a steady number of seizures. Thus, a relationship between sharp waves and seizures was found mainly when there were changes in these variables. For sharp and slow waves, rare discharges were most often associated with mild slowing, while many discharges were most often related to a marked degree of slow wave abnormality. The combination of slow and sharp waves together, compared with only one type of EEG abnormality, was associated with more clinical neurological symptoms, especially a mental status change. The EEG groups were divided into four types of findings of slow and sharp waves, appearing in at least a first and/or second record. The results from these four EEG groups allow for a prediction of the findings to be found in a second record, based on the results of the first EEG. In general, there was a relationship between more slow waves, more sharp waves and more seizures. Also minimal slowing and rare discharges on the first record tended to disappear in the next EEG.
