The sphenoidal electrode: myth and reality.

It was Herbert Jasper who mentioned the greater wing of the sphenoid bone as a possible electrode recording site in the evaluation of patients with seizures of anterior temporal lobe origin and quoted the prior work of Kristensen and Reyes in developing such an approach. But it was D.P. Jones, working out of the Maida Vale Hospital, who first reported on the use of the ‘‘sphenoidal’’ electrode at the annual meeting of the EEG Society in 1950 and later detailed his experience with it [1]. He used a fine, insulated needle electrode and described the approach, under light local anesthesia, through the notch in the mandible just under the zygomatic arch that is still used today for the ‘‘anterior sphenoidal’’ electrode. He noted that the electrode was at the ‘‘base of the skull lateral to the foramen ovale.’’ He recorded the EEG through this electrode in the coronal plane and felt that he could see interictal and ictal activity that was either not seen at all with other scalpbased electrodes or that was more localizable than activity seen with other basal electrodes that were in use at that time. He concluded that this electrode recorded activity that had ‘‘probable origin’’ from the basal and mesial temporal areas. His description over 50 years ago of the placement of this electrode, the ideal recording montage, the region of interest that it sees, and the comparison with other electrodes in the same region remains an active topic for discussion and controversy today. Researchers at the Maudsley Hospital [2] followed up on these earlier observations and performed an extensive study on the effectiveness of this electrode in the interictal evaluation of patients with known or presumed epilepsy. They, too, used an insulated needle placed under local anesthesia and with a similar neuroanatomical direction as Jones. Additionally, in many of their cases, they used zygomatic, chin, frontal, and nasopharyngeal electrodes for comparison purposes. They felt that the sphenoidal electrode was much more likely to show clear interictal markers for epilepsy, i.e., focal spikes or spike–wave activity, than did the surface-scalpbased electrodes. The frequency of detection of interictal discharges was also observed to be greater from the sphenoidal electrodes than from the surface-scalp electrodes. They observed that rarely was there synchrony between the sphenoidal discharges and the surfacescalp-based discharges. These two observations led them to conclude that the two electrodes were looking at physiologically different events originating in different areas of the brain. Additionally, they noted that electrodes near the surface entry region for the sphenoidal electrodes did occasionally have synchronous activity with the deeper contact but they seldom showed activity when the deeper electrode did. They also felt that the deeper electrode was recording sources different from the more superficial electrode, although there were some common sources. Marshall [3] was the first to introduce a flexible piano wire instead of the rigid needle and thus allowed physicians the option of recording patients electroencephalographic activity over longer periods. This opened the door for studies that compared the various electrodes ability to record seizure onsets, i.e., ictal activity. Rovit et al. [4,5], in a two-part paper, looked systematically at the ability of the sphenoidal electrode to do just that. He defined a variation on the standard sphenoidal electrode placement allowing for two different locations, one just anterior to the pterygopalatine fossa and the other inferior and lateral to the foramen ovale. These results were combined for both interictal and ictal recordings. He also concluded that often the basal sphenoidal electrodes showed abnormalities that were not seen on surface or nasopharyngeal electrodes. When surface Epilepsy & Behavior 4 (2003) 192–197