Reproducibility and clinical relevance of quantitative EEG parameters in cerebral ischemia: A basic approach
Introduction
The progression from ischemia to infarction is a dynamic, rapidly evolving process with irreversible changes occurring within a few minutes to a few hours. EEG may reflect changes in cerebral blood flow and metabolism within seconds as these are directly reflected in the neuronal rhythms. Although the spatial resolution of the EEG is low compared to structural imaging modalities, the high temporal resolution of the EEG may permit a rapid, inexpensive and sensitive evaluation of instantaneous brain functioning (Niedermeyer, 1993, Jordan, 2004). Moreover, quantitative EEG from a standard number of electrodes could assist in outcome prediction (Finnigan et al., 2007, Cuspineda et al., 2007, de Vos et al., 2008). Cuspineda et al. (2007) found absolute power of alpha and theta band for short-term outcome and absolute power of delta band for long-term outcome to be highly reliable in 28 patients with subacute ischemic stroke. Delta/alpha power ratio and relative alpha power were significantly correlated with 30-day NIHSS score in 13 patients with subacute ischemic stroke (Finnigan et al., 2007). Poststroke EEG delta activity was shown to be significantly reduced after thrombolysis with consequent reperfusion of cerebral tissue prior to any significant improvement in clinical symptoms (Finnigan et al., 2006). Moreover, using sLORETA (Pascual-Marqui et al., 2002), sources of pre-thrombolysis delta activity were shown to be co-localized with ischemic brain regions, particularly those which were shown to be reperfused after thrombolysis as indicated by perfusion weighted MR imaging (Finnigan et al., 2006). de Vos et al. (2008) demonstrated that the change in EEG brain symmetry as quantified with the revised Brain Symmetry Index during intravenous thrombolysis correlated significantly with the change in NIHSS score.
Clinical sensitivity and specificity of quantitative EEG (qEEG) are related to the stability and reliability of qEEG upon repeat testing (Thatcher, 2006). qEEG is reported to be highly reliable and reproducible in healthy controls and in young patients with psychiatric disorders (Hughes and John, 1999, Lund et al., 1995, Hamilton-Bruce et al., 1991, Salinsky et al., 1991, Pollock et al., 1991). However, reproducibility measures for qEEG in patients with ischemic cerebrovascular conditions have not been reported so far. A basic issue that needs to be addressed is the reproducibility of visual artifact rejection on the raw EEG selected for quantitative analysis.
Furthermore, persons labeled as normal, healthy controls do not necessarily exhibit EEG signals containing normative data 3 (Guggenheim and Karbowski, 1979, Logar et al., 1993).
This raises the question which EEG segment is representative of the underlying condition for which correlative EEG measures are determined. The aim of this study was to examine intra-record, intra-rater and inter-rater reproducibility and clinical relevance of EEG power and asymmetry in patients with ischemic cerebrovascular disease – i.e. TIA or stroke – during a single recording in routine clinical setting. We included a control group consisting of age-matched subjects who were free of clinically overt central neurologic dysfunction.
Section snippets
Study population
The study is part of the Middelheim’s Interdisciplinary Stroke Study (MISS), which is a project on the clinical, biochemical, neuroimaging, neuropsychological and electrophysiological evaluation of patients with ischemic stroke or transient ischemic attack (TIA) at ZNA Middelheim hospital, Antwerp. EEG was recorded in 41 patients. Ten patients, aged 55 years or more, with subjective memory complaints without evidence of cognitive decline on formal neuropsychological evaluation, normal brain MRI
Results
Fig. 1 illustrates the sensitivity of pdBSI based on the ratio of spectral power density of homologous channel pairs as a function of the amplitude ratio in a typical range of interest of 2.
As the left/right – amplitude ratio increases, pdBSI increases reflecting its sensitivity to lateralized asymmetry in the amplitude domain.
Fig. 2 illustrates the sensitivity of pdBSI as a function of varying frequency at one channel compared to its contralateral homologous channel in a typical range of
Discussion
Recent reports indicate a promising diagnostic and/or predictive value of qEEG parameters in acute ischemic stroke (Finnigan et al., 2004, Finnigan et al., 2007, van Putten and Tavy, 2004, Burghaus et al., 2007, Cuspineda et al., 2007, de Vos et al., 2008). Sensitivity and specificity of qEEG parameters are related to the reproducibility of qEEG upon repeat testing (Thatcher, 2006). Visual reading of the raw EEG by a trained electroencephalographer is considered a crucial and indispensable
Conclusions
In addition to the previously described BSI (van Putten and Tavy, 2004) and rsBSI (van Putten, 2007) which measure global hemispheric asymmetry, we now propose the pdBSI, which measures asymmetry between homologous channels. As we have shown, pdBSI allows a reliable analysis of brain symmetry changes in acute ischemic stroke after visual artifact rejection.
Based on this exploratory methodological analysis, reliable EEG parameters with discriminatory features, obtained in a naturalistic setting,
Conflict of interest
None.
Acknowledgements
We thank the anonymous reviewers for their valuable comments.
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