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SD Detection on Strip vs. Depth Electrode Arrays

For assessment of ECoG recordings for SD activity by expert and non-expert reviewers, 5 strip electrode data files and 5 depth electrode data files were created. Each individual file contained data from only one electrode type. Recordings from each channel were presented in the following formats for review: (a) unfiltered, (b) band pass filtered (0.5 to 50 Hz) and (c) leaky power integrals of band-pass filtered data. The SPC associated with spreading depolarization is best visualized on unfiltered data channels whereas band pass filtering and integral calculations are used to assess the depression of the amplitude of high- frequency spontaneous activity. Data files additionally included accelerometer data. Data files were anonymised, randomized and stripped of all other information before presentation to 2 expert assessors and 4 non-expert assessors. Both expert assessors (MF and JAH) were fully trained and experienced clinical or basic neurophysiologists. Non-expert assessors had all attended a 2-day training seminar on interpretation of ECoG data conducted by one of the expert assessors (MF). All assessors were given an illustrated guide on identification of SD in ECoG data to which they could refer during data analysis.

All assessors were provided with instructions on how to complete the analysis with standardized score sheets. Recordings were scored in 30-min time epochs using a 1-10 scale for confidence that an SD event began within the epoch. Score ≥7 indicated positive SD identification. Scores recorded by each participant were collated by an independent researcher who did not serve as an assessor. Results were compared to a gold standard for presence or absence of SD events in each epoch. Following their independent scoring of the test files, the expert assessors generated this gold standard by considering concurrent data from both depth and strip electrode arrays and discussing their findings with each other to resolve any discrepancies.

Following scoring, the data was compiled into contingency tables and descriptive statistics performed. Categorical outcomes of interest were compared using Fisher’s exact test. All tests were 2 sided and p-values less than 0.05 were considered statistically significant.

CSD detection on depth vs subdural electrode arrays in humans

A gold standard scoring of the data was decided on by the two expert reviewers with access to complete data files showing concurrent strip and depth electrode data in addition to data from the movement transducer. This scoring was undertaken after blinded scoring of the study files and finalized after discussion and joint analysis. The total number of SD events identified from these five patients was 35. All of the events seen on depth electrode data were also seen co-occurring on strip data but four events seen on strip were not identified on depth data. The majority of events (3 of the 4 events) confirmed on strip but not depth were associated with movement artifact partially or completely obscuring the SPC waves on depth data. Movement artifact was also seen in strip data but events are observed over a longer time period due to spread occurring along the strip electrodes so events can be identified with greater confidence in strip data. The bi-polar referencing of strip recordings also served to reduce movement artifact when compared to depth recordings referenced in a unipolar fashion to a scalp sited reference electrode.

Expert review

Expert reviewers did not identify all of the 35 events seen in the gold standard review of the data. Expert one identified 30 events on strip data and 29 events on depth data. Expert 2 identified 28 events on strip data and 29 events on depth data. These differences likely reflect a greater caution to assign a designation of SD to strip events by the second expert reviewer. All assessors (expert and non-expert) commented on the ease of identification of depth events due to a more stereotyped, coherent and consistent appearance when compared with strip events.

Non-expert review

All of the non-expert reviewers were able to identify SD events on both strip and depth data. Non-expert reviewers identified a mean of 22 events on strip data and 24.5 events on depth data. The null hypothesis, that there was no significant difference in accuracy of SD detection between expert and non-expert reviewers, was rejected (P = <0.001).

Depth vs. Strip

A significant difference in detection rates between the two data modalities was found for non-expert assessors (P = <0.045) but not for expert assessors (P = 0.07). When all assessors were considered together a significant difference in detection was found between the two modalities (P = 0.01).

To determine the nature of these differences in SD detection, 2 x 2 tables were compiled for detection of SD events in strip data and depth data for expert and non-expert assessors. Positive predictive value (PPV), negative predictive value (NPV), sensitivity (Sens.) and specificity (Spec.) were calculated for the four groups. In the expert group the PPV and Sens. of depth data was superior to that of the strip data (PPV = 1 vs. 0.91, Sens. = 0.94 vs. 0.83). The NPV and Spec were effectively the same in the two groups (NPV = 0.99 vs. 0.97, Spec. = 1 vs. 0.99).

In the non-expert group there was a similar detection advantage seen with depth data (PPV = 0.85 vs. 0.73, Sens. = 0.76 vs. 0.64). The NPV and Spec. were effectively equal between the two groups (NPV = 0.97 vs. 0.95, Spec. = 0.99 vs. 0.97).

In summary, all assessors were able to identify SD events from both strip and depth data. More SD events were identified in strip data on the gold standard review. However, the depth electrodes were associated with a greater sensitivity and positive predictive value than the strip electrodes in both the expert and non-expert reviewer groups when the two data sources were compared.

As expected there was a decreased positive predictive value and sensitivity to epochs containing SD events in the non-expert assessor group when compared to the expert reviewers.

Depth vs. Strip

Experts and non-experts can identify SD events on both strip and depth electrode data, and the sensitivity and specificity of depth electrodes for the identification of SD is superior to that for strip electrodes. We also found that the total number of events identified by all assessors was greater on strip data than the number of events identified on depth but this difference was not statistically significant. Any true difference might reflect the greater surface area covered by the strip electrode (5 cm) compared to the depth (single location), as sampling at multiple points across the cortex allows a greater opportunity to capture an SD if it fails to propagate widely throughout the region. However, propagation of SD waves is known to traverse the depth of sulci, thus appearing at a surface subdural strip to spread to an adjacent gyrus with appropriate delay, i.e. beyond the confines of sulcal boundaries (1). This robust propagation would suggest minimal advantage of strip electrodes over depth electrodes and may explain the near-equivalence of the methods evidenced in our data.

1. Strong AJ, Anderson PJ, Watts HR et al. Peri-infarct depolarizations lead to loss of perfusion in ischaemic gyrencephalic cerebral cortex. Brain 2007;130:995-1008.