Utility of Sphenoidal Electrodes for Review of Ictal Tracings

Abstract

To the Editor: I read with interest the article of Kanner et al. 1 regarding the advantages of fluoroscopically placed sphenoidal electrodes (SEs) over anterior temporal scalp electrodes (ATEs) for ictal recordings in temporal lobe epilepsy. Our recently published study 2 suggests that the standard “blind” placement of SEs appears to offer little or no useful information for ictal recordings. It would appear that the precise placement of the electrodes is responsible for the increase in yield. Or is it? An alternative explanation, discussed in our manuscript 2, is also suggested by Fig. 1 of the article of Kanner et al. 1. That figure shows an ictal tracing in which the region of onset is more easily identified by using SEs than by using ATEs, but several things must be pointed out regarding this figure. First, although the discharge is more obvious in the bipolar montage with SEs, it is still identifiable with ATEs. Second, the differences between SEs and ATEs are far less pronounced in the referential channels, suggesting that much of the advantage of SEs was owing simply to the greater distance from neighboring midtemporal electrodes. Third, and most important, this study was performed by using paper printouts, a method that does not conform to contemporary neurophysiologic practice. In our study, using digital review with ad lib adjustment of filters and sensitivity, differences between SEs and ATEs were almost nil. Thus whereas SEs offer greater electrode separation and have been found by some (although not by us) to provide a higher signal-to-noise ratio 3, these limitations can be easily overcome by using post hoc adjustment of sensitivity. This does not change the signal-to-noise ratio, but does change the absolute difference between signal and noise, making the signal much easier to perceive above background. Had Kanner et al. 1 used this method of review, increasing the sensitivity likely would have resulted in easy identification of the right temporal ictal onset with ATEs for the recording pictured, and perhaps for other recordings as well. As mentioned in our article 2, this limitation also is seen in other published figures from other investigations of this nature 4. It would be very interesting to see this study repeated with ad lib digital review of tracings. Performing this in a blinded manner is a logistic challenge (our own study was unblinded), but undertaking such studies with printed paper records cannot fully the answer the question of whether SEs are truly useful to modern practice. The answer may be that SEs had genuine utility in the era of analog EEG, but offer little or no benefit when EEGs are reviewed digitally.