Time from ictal subdural EEG seizure onset to clinical seizure onset: An electrocorticographic time factor associated with temporal lobe epileptogenicity

Long-term subdural video/electroencephalographic (EEC) monitoring was performed in a series of patients with medically intractable complex partial seizures, in a study of diagnostic accuracy, to test the hypothesis that the time from ictal subdural EEG seizure onset to clinical seizure onset (ECOT) is correlated with temporal lobe epileptogenicity and confirm measures of validity of ECOT for predicting seizure-free outcome following anterior temporal lobectomy and amygdalohippocampectomy (ATL/AH). In 34 patients with refractory temporal lobe epilepsy, subdural EEG monitoring localized the ictal epileptogenic focus to a single temporal lobe. In each patient, ECOT was analysed for correlation with temporal lobe epileptogenicity as measured by seizure interval in hours. Patients in whom ECOT was equal to or less than the mean (i.e. subdural EEG seizure onset preceding clinical seizure onset by at least 11.7 seconds) had a significantly greater likelihood of becoming seizure-free following ATL/AH compared to patients in whom ECOT was greater than the mean (i.e. subdural EEG seizure onset preceding clinical seizure onset by less than 11.7 seconds) (x²=5.78, p<0.05). The validity of ECOT for predicting seizure-free outcome following ATL/AH is confirmed to have sensitivity of 55.0%, specificity of 85.7%, false positive rate of 15.4%, false negative rate of 42.9%, diagnostic value of 84.6% and diagnostic accuracy of 67.6%. In addition, a significant correlation, described by a second order polynomial relationship, was found between the natural exponential function of ECOT and seizure interval [f(x)=0.415x²-25.554x + 267.036, r=0.731, df=32, t=6.05, p<0.001, where f(x)=e^ECOT and x=seizure interval). This result provides the epileptologist with a quantitative tool capable of predicting seizure interval based on ECOT. The capability of ECOT to predict seizure interval may allow the patient and epileptologist to anticipate future seizure onset based on ECOT, potentially facilitating accurate timing of ictal seizure focus localization techniques and clinical intervention to abort seizure onset using various available central and peripheral nervous system stimulation therapeutic strategies. The results suggest a relationship between ECOT and seizure interval. Fundamental pathophysiologic processes involved in the transition from ictal EEG to clinical seizure onset may be responsible for temporal lobe epileptogenicity.