Diagnostic Value of Electroencephalography with Ten Electrodes in Critically Ill Patients

Abstract

Background: In critical care settings, electroencephalography (EEG) with reduced number of electrodes (reduced montage EEG, rm-EEG) might be a timely alternative to the conventional full montage EEG (fm-EEG). However, past studies have reported variable accuracies for detecting seizures using rm-EEG. We hypothesized that the past studies did not distinguish between differences in sensitivity from differences in classification of EEG patterns by different readers. The goal of the present study was to revisit the diagnostic value of rm-EEG when confounding issues are accounted for.

Methods: We retrospectively collected 212 adult EEGs recorded at Massachusetts General Hospital and reviewed by two epileptologists with access to clinical, trending, and video information. In Phase I of the study, we re-configured the first 4 h of the EEGs in lateral circumferential montage with ten electrodes and asked new readers to interpret the EEGs without access to any other ancillary information. We compared their rating to the reading of hospital clinicians with access to ancillary information. In Phase II, we measured the accuracy of the same raters reading representative samples of the discordant EEGs in full and reduced configurations presented randomly by comparing their performance to majority consensus as the gold standard.

Results: Of the 95 EEGs without seizures in the selected fm-EEG, readers of rm-EEG identified 92 cases (97%) as having no seizure activity. Of 117 EEGs with "seizures" identified in the selected fm-EEG, none of the cases was labeled as normal on rm-EEG. Readers of rm-EEG reported pathological activity in 100% of cases, but labeled them as seizures (N = 77), rhythmic or periodic patterns (N = 24), epileptiform spikes (N = 7), or burst suppression (N = 6). When the same raters read representative epochs of the discordant EEG cases (N = 43) in both fm-EEG and rm-EEG configurations, we found high concordance (95%) and intra-rater agreement (93%) between fm-EEG and rm-EEG diagnoses.

Conclusions: Reduced EEG with ten electrodes in circumferential configuration preserves key features of the traditional EEG system. Discrepancies between rm-EEG and fm-EEG as reported in some of the past studies can be in part due to methodological factors such as choice of gold standard diagnosis, asymmetric access to ancillary clinical information, and inter-rater variability rather than detection failure of rm-EEG as a result of electrode reduction per se.

Keywords: Abbreviated EEG; Continuous EEG monitoring; Electroencephalography (EEG); Neuroemergencies; Non-convulsive status epilepticus; Seizure prediction.

Fig. 1

Full montage and reduced montage electroencephalogram construction. Electrodes used to construct bipolar anterior-posterior montages for conventional full montage (A) and reduced montage (B) electroencephalography (EEG), referred to as fm-EEG and rm-EEG, respectively. A sample of rm-EEG (C) showing burst suppression activity is shown, indicating the electrodes used to construct the montage. Readers could adjust display epoch time, scale, and high- and low-pass filters while reviewing EEGs in Phase I, but were not allowed to re-montage the EEG. Readers could not adjust any display settings in Phase II

Fig. 2

Flowchart of findings from Phase I and II. BSup burst suppression, EEG electroencephalography, NC no consensus, PD/ES periodic discharges or epileptiform spikes, SL/NL slow or normal activity, SZ seizure. Of note, 6 cases of burst suppression from Phase II were rated in fm-EEG review as either PD/ES (N = 2), SL/NL (N = 2), or reached no consensus (N = 2)

Fig. 3

Agreement between fm-EEG and rm-EEG majority consensus diagnoses during initial 4 h of monitoring. Phase I (top) and Phase II (bottom) diagnostic tables using either seizures as the sole pattern of interest (A) or both seizures and epileptiform discharges as patterns of interest (B). Concordant cases indicated by green cells; discordant cases indicated by yellow cells with bolded red text. Diagnostic concordance and intra-rater agreement are shown in (C). BSup burst suppression, NC no consensus, PD/ES periodic discharges or epileptiform spikes, SL/NL slow or normal activity, SZ seizure

Fig. 4

Samples of seizure activity diagnosed on fm-EEG, but classified as non-seizure on rm-EEG. Reduced EEG channels indicated by the blue box. The first sample contained generalized activity that was classified as seizure by the majority of reviewers using fm-EEG (top left); no majority consensus was achieved using rm-EEG (bottom left); however, the expert epileptologist diagnosed this activity as seizure using both fm-EEG and rm-EEG. The second fm-EEG sample (top right) shows focal parasagittal seizure activity that is not visible on rm-EEG (bottom right) and was interpreted as epileptiform spikes by the majority of reviewers. EEGs are shown in the fm- and rm-configurations shown in Fig. 1. EKG shown in pink