Spectral properties of EEG fast activity ictal discharges associated with infantile spasms

Objective: The aim of this study was to evaluate the characteristics of the ictal EEG event accompanying infantile spasms. Methods: Quantitative analysis was used, based on the application of a bivariate autoregressive (AR) parametric model; autospectra, coherence, phase functions and inter-hemispheric time differences were estimated on homologous EEG channels in 18 infants presenting with either cryptogenic or symptomatic West syndrome. Results: The AR analysis of the 500 ms EEG epochs preceding spasm onset revealed the presence of a short discharge of fast activity restricted to a narrow frequency band in 13 of the 18 cases included in the study. The fast discharge peaked at 17.5±2.1 Hz, with rather low inter-hemispheric coherence values (0.52±0.17) and asymmetric amplitude on homologous EEG derivations. It persisted briefly after spasm onset, reaching a higher coherence value (0.71±0.16). The inter-hemispheric time difference, estimated in those cases with the coherence values significantly different from zero, ranged from 9.1 to 14.3 ms (11.4±1.9) in the epoch preceding spasm onset. Conclusion: The data obtained from the analysis of the ictal EEG events, compared with clinical and interictal EEG features, indicate that an asymmetric EEG pattern (mainly consisting of a rhythmic burst of fast activity) consistently preceded both symmetric and asymmetric spasms, thus suggesting a localized cortical origin of the ictal discharge giving rise to the spasms.