Abstracts

Rationale: The objective of this study is to apply independent component analysis (ICA) to subdurally recorded occipital seizures in order to gain additional information about the underlying generators of them and to demonstrate the usefulness of ICA as a supplementary tool in the analysis of these seizures with respect to the composition of the epileptogenic zone and propagation patterns from the epileptic focus. Methods: Twenty-seven subdurally recorded occipital seizures from eight patients were studied. Patient characteristics are shown in Table 1. ICA was performed on the subdural electroencephalographic (EEG) data, and the resulting independent components (ICs) were compared with respect to their power, frequency, degree of spread within the epileptogenic zone, and propagation patterns. Results from all 27 seizures were categorized into: simple focal seizures (SFS), complex partial seizures (CPS), or generalized tonic-clonic seizures (GTCS). The ictal ICs were further subdivided into propagating and non-propagating types. For those patients who underwent resective surgery, post-operative outcome was defined by the Engel classification. Results: ICA provided information over and above that supplied by standard visual EEG analysis, confirming that all of the seizures originated in the occipital lobe, and affording additional knowledge regarding the nature of the seizure onset zone. Each seizure was composed of multiple ICs, some of which propagated while others remained within the epileptogenic zone. There was no statistical difference between the propagating and non-propagating ICs with respect to power or frequency. However, propagating ICs involved a significantly greater number of recording electrodes at their onset when compared to the non-propagating ICs (Figure 1). Surgery was offered to all 8 patients; 5 individuals declined, while 3 patients accepted and underwent occipital lobe surgery. Conclusions: The propagation likelihood of ICs in occipital seizures is independent of their power or frequency, but it is dependent on the volume of brain giving rise to that signal; ICs that are generated by a greater volume of brain within the epileptogenic zone are more likely to propagate. Despite the complexity of OLE and the limited accuracy of the current state of visual analysis in subdural EEG display, ICA of occipital seizures resulted in clinically relevant information that could not otherwise be obtained. The application of this information from ICA, in combination with the diagnostic output from subdural EEG, may be used to improve the surgical outcome of patients with medically intractable epilepsy.