Usefulness of EEG-mapping of Renyi's entropy in patients with typical absences

Experimental and clinical data demonstrate that absence seizures depend on a dysfunction of the thalamocortical circuitry. It is unknown if such a dysfunction is steady or confined to the paroxysmal events. The Renyi’s entropy evaluate the degree of “order” into a system and may be employed to the EEG analysis. We studied the value of EEG-mapping of the Renyi’s entropy as a tool to reveal abnormalities not detected by conventional EEG in patients with typical absences. We analyzed 35 EEG recorded at 19 channels: 15 patients (mean age 7.5, range 5-14), with absence of childhood and adolescence; 20 EEG of normal subjects (mean age 15.5, range 6-20). The diagnosis was made following the criteria of the ILAE (1989) The EEG duration was 47.2 minutes. We analyzed ictal and interictal EEG separately. The Renyi’s entropy was then calculated on non-overlapped windows of 1 sec. duration and mapped in order to create a movie. Low entropy was found in all patients, suggesting high order (hypersynchronization) in frontal and temporal regions, bilaterally. Notably, no quantitative or regional differences were observed between ictal and interictal EEGs. In the control group the entropy was random distributed on the scalp. These findings suggest a steady dysfunction on the fronto-temporal regions in patients with absences, and open the way for future studies.