Sphenoidal Recordings Research Articles

Sphenoidal Electrodes Significantly Change the Results of Source Localization of Interictal Spikes for a Large Percentage of Patients With Temporal Lobe Epilepsy

Although scalp EEG is a very useful tool for presurgical evaluation in epilepsy, the 10-20 system of electrodes in many cases fails to accurately localize the source of the epileptic seizures. One suggested solution to this problem is to use additional electrodes. Sphenoidal electrodes especially have been suggested to be helpful in identifying the irritative and seizure onset zones in patients with temporal lobe epilepsy. However, the value of these electrodes has been debated, and in many epilepsy centers they are not used. In this study, we investigate the impact of sphenoidal electrodes by comparing the results of EEG source localization with and without sphenoidal recordings. We retrospectively selected patients with temporal lobe epilepsy based on their clinical semiology and electrophysiologic data. For each patient, a prototype spike was used as a template for an automatic pattern search to find similar activities. The identified spikes were then averaged and analyzed by fitting a dipole to the data. The recordings from sphenoidal electrodes were then excluded and the analysis was repeated. It was found that in more than half of the patients inclusion of sphenoidal electrodes resulted in a shift of more than 2 cm in the location of the fitted dipole, and in some cases moved the dipole from the frontal lobe or the insula to the temporal lobe. Our results suggest that sphenoidal electrodes are helpful in the analysis of the EEG recordings of patients suspected of having temporal lobe epilepsy.

Sensitivity of recordings at sphenoidal electrode site for detecting seizure onset: evidence from scalp, superficial and deep foramen ovale recordings

Objectives: Some authors have recently stressed that the position of the tip of sphenoidal electrodes plays a crucial role in their efficacy in detecting ictal onset. An opportunity to test this hypothesis is provided by recordings from the most superficial contacts of foramen ovale (FO) electrode bundles because these contacts are located at the FO, in a position equivalent to that of optimally located sphenoidal electrodes. To simplify wording, recordings obtained by superficial FO electrodes will hereafter be called sphenoidal recordings, although they have not been obtained with standard sphenoidal electrodes. The sensitivities of simultaneous scalp and sphenoidal recordings for detecting ictal onset have been compared with each other, and with a ‘gold standard’ provided by simultaneous deep intracranial FO recordings from the mesial aspect of the temporal lobe. Methods: Three hundred and fourteen seizures obtained from 110 patients under telemetric presurgical assessment for temporal lobe epilepsy have been studied. Scalp electrodes included anterior temporal placements. All scalp electrodes were considered when identifying seizure onset but the anterior temporal electrodes were most frequently involved. Results: Ictal onset time at sphenoidal and scalp recordings: initial ictal changes appeared simultaneously in scalp and sphenoidal recordings in 123 seizures (39.2%). Initial changes occurred earlier in sphenoidal recordings in 63 seizures (20.1%), whereas they were seen earlier on the scalp in 76 seizures (24.2%). Artefacts prevented the comparison between sphenoidal and scalp recordings in 16 seizures (5.1%) and no ictal changes were seen on the scalp and/or sphenoidal recordings in 36 seizures (11.5%). In most of the 63 seizures where ictal changes appeared earlier in sphenoidal recordings, a delayed ipsilateral scalp onset was seen as the signal amplitude increased or scalp changes could be identified retrospectively on the scalp with an onset which appeared simultaneous and ipsilateral to the initial sphenoidal changes. Sphenoidal recordings supplied additional information when compared to scalp recordings in only 22 seizures (7%): in 5 seizures with artefacts on the scalp, in 6 seizures with no changes on the scalp and in 11 seizures with discrepant laterality at onset. Congruence in laterality with respect to deep intracraneal FO recordings: of the 61 seizures with unilateral onset on the scalp, onsets at sphenoidal recordings and deep FO electrodes were ipsilateral in most cases. In only 3 of these 61 seizures (4.9%), sphenoidal recordings lateralized ipsilateral to the deep FO electrodes in the presence of a contralateral onset on the scalp. In 14 among the 122 seizures (11.5%) with bilateral asymmetrical onset on the scalp, sphenoidal recordings lateralized seizure onset ipsilateral to the deep FO electrodes in the presence of a contralateral scalp onset. Thus, when compared with scalp EEG, sphenoidal recordings increased laterality congruence with respect to deep FO electrodes in 17 seizures (5.4%). Conclusions: Extracranial electrodes located next to the FO at the sphenoidal electrode site yield an improvement over suitable surface electrodes in the identification of ictal onset in only 5.4–7% of seizures. Such improvement derives from the fact that the low amplitude signals often seen at seizure onset may show higher amplitude on sphenoidal than on scalp recordings.

P3 latency jitter assessed using 2 techniques. II. Surface and sphenoidal recordings in subjects with focal epilepsy

We compared the latency variability in auditory P3s of 13 subjects with unilateral temporal lobe epilepsy (TLE) to that of normal controls. We predicted that increased latency jitter would occur in TLE subjects, particularly on the epileptic side. ERPs were recorded from scalp and sphenoidal sites relative to a balanced non-cephalic reference. Signal-to-noise ratios (SNRs) were calculated for each subject. Data were excluded if SNRs fell below 0.4. P3 latency jitter was estimated using 2 methods: Woody's algorithm and the maximum likelihood technique (MLT), a novel method of jitter assessment. SNRs were significantly higher in controls and were maximal posteriorly for both groups. P3 peak amplitude was significantly smaller in TLE subjects at temporal sites. Latency jitter (MLT method) was greatest in posterior sites and mirrored the jitter profiles of controls. Latency jitter was significantly higher in TLE subjects in bilateral frontal and temporal sites, but was not higher on the side of the focus and could not be attributed to lower SNRs. The increased bilateral latency jitter in these patients may be related to effects of anticonvulsants or the more extensive nature of the underlying epileptic disorder.

Yield of Sphenoidal Recording in Sleep-Deprived Outpatients

We retrospectively reviewed the results of sphenoidal recordings performed after sleep deprivation in 101 consecutive outpatients whose prior scalp EEGs failed to show epileptiform discharges (EDs). These patients were suspected of having seizures of temporal lobe origin. Blinded reviews of the recordings by two electroencephalographers showed a tendency for sphenoidal recording to detect EDs at a higher rate than scalp recording, but the difference did not reach statistical significance (p = 0.06). However, EDs appeared exclusively at sphenoidal electrodes in 11 patients and exclusively at scalp electrodes in 3. The probability of detecting EDs with combined scalp and sphenoidal recordings was greater when spells were characterized by focal symptoms or signs. Except in three patients, initial EDs occurred during the first 30 min of recording. We recommend that post-sleep-deprivation recording be performed initially with scalp and other noninvasive electrodes (such as anterior temporal, ear, or cheek electrodes). If no EDs occur after 30 min, sphenoidal electrode insertion and recording may then be considered, especially in patients with spells characterized by focal signs or symptoms.

Long-term EEG-video-audio monitoring: computer detection of focal EEG seizure patterns

Twelve individuals with medically refractory partial seizures had undergone EEG-video-audio (EVA) monitoring over 1–15 (mean 10.5) days. We selectively reexamined available 15-channel EEGs (video-cassettes) totaling 461 h and containing 253 EEG focal seizures. Computer analysis (CA) of these bipolar records was performed using a mimetic method of seizure detection at 6 successive computer settings. We determined the computer parameters at which this method correctly detected a reasonably large percentage of seizures (81.42%) while generating an acceptable rate of false positive results (5.38/h). These parameters were adopted as the default setting for identifying focal EEG seizure patterns in all subsequent long-term bipolar scalp and sphenoidal recordings. Factors hindering or facilitating automatic seizure identification are discussed. It is concluded that on-line computer detection of focal EEG seizure patterns by this method offers a satisfactory alternative to and represents a distinct improvement over the extremely time consuming and fatiguing off-line fast visual review (FVR). Combining CA with seizure signaling (SS) by the patients and other observers increased the correct detections to 85.38%. CA is best used in conjuncton with SS.

EEG and neuroimaging localization in partial epilepsy

We have studied cortical localization provided by surface and sphenoidal electroencephalograms (EEG) and that of computed tomography (CT), magnetic resonance imaging (MR) and single photon emission tomography (SPECT) in 58 patients with partial epilepsy. Each patient had EEG, MR and SPECT during a hospitalization period of 1–2 weeks. CT scans were obtained either during the same period or had been performed in the preceding year. EEG evaluation consisted of 3–5 days of continuous monitoring including video-telemetry and ambulatory recordings as well as conventional EEGs with special electrode placements. Additionally 33 of 58 patients (55%) who were potential surgical candidates had sphenoidal recordings. All patients had an abnormal EEG which showed evidence of epileptic hyperexcitability. EEG abnormality was localized in 43 patients (74%). Neuroimaging studies were focally abnormal in 38 patients (66%); 12 CT (21%), 29 MR (50%) and 24 SPECT (41%). Thirty four of 43 patients with localized EEG had at least 1 focally abnormal neuroimaging study (79%), whereas 4 of 15 (27%) patients with non-localized EEG did so. Twenty-eight of 29 patients with focal MR (97%), 11 of 12 patients with focal CT (92%) and 20 of 24 patients with focal SPECT (83%) had a concordant focal EEG. EEG and neuroimaging localization agreed in all 15 patients in whom both MR and SPECT disclosed a concordant focal abnormality. This study demonstrates a significant ( P < 0.005) correlation between surface/sphenoid EEG and neuroimaging localization in partial epilepsy.

Long‐Term Ambulatory Cassette EEG with Sphenoidal Recording in Complex Partial Seizures

Long-term ambulatory cassette EEG with sphenoidal recording (A/EEG-SP) was performed in 51 patients. Group A comprised 31 patients with a clinical diagnosis of complex partial seizures (CPS), and group B comprised 20 patients suspected of having CPS. In group A, detection of temporal spikes was 27% by routine EEG (R/EEG), 72% by sphenoidal EEG (SP/EEG), 27% by A/EEG with temporal chain montage, and 91% by A/EEG-SP. The highest yield achieved by A/EEG-SP as compared with other EEG recordings was documentation of spontaneous seizures of temporal lobe origin. The detection rate of spontaneous seizures by A/EEG-SP was approximately 30%, and the focal or regional onset of seizures could be determined in 60% of patients. In group B, detection of temporal spikes was 11% by R/EEG, 18% by SP/EEG, and 30% by A/EEG-SP. Spontaneous seizures were recorded by A/EEG-SP in two patients (10%). Thus, A/EEG-SP was more effective than A/EEG in detecting interictal and ictal temporal discharges, and A/EEG-SP was superior to SP/EEG in documenting spontaneous seizures of temporal lobe origin.

Distribution of temporal spikes in relation to the sphenoidal electrode

Sphenoidal EEG recordings were performed in 111 patients with epilepsy, who showed antero-mesial temporal epileptiform discharges. In 6, a multipolar sphenoidal electrode showed a shallow potential gradient between the standard sphenoidal site and turface. In 17 patients a superficial electrode at the site of entry of the sphenoidal wire recorded all discharges seen at the sphenoidal. Out of 165 foci, in only 2 instances were less than 90% of sphenoidal discharges recognisable on the surface. In 39 patients who underwent surgery, lesions confined to mesial temporal structures were found to be associated with inter-ictal discharges maximal at the sphenoidal electrode. A mid-temporal maximum was always associated with diffuse non-specific, or lateral temporal pathology. It is concluded that sphenoidal recording offers recording offers no advantage over suitably placed scalp contacts for detecting inter-ictal epileptiform discharges. It may be of some value for differentiating between mesial and lateral temporal foci.

Interictal and ictal activity recorded with subdural electrodes during preoperative evaluation for surgical treatment of epilepsy

A total of 94 subdural strip electrodes were implanted in 22 patients during preoperative EEG evaluation for surgery of epilepsy. Eighteen patients had temporal lobe seizure onset, three had frontal lobe seizure onset, and one had occipital lobe seizure onset. Most electrodes (total, 83) were localized over the temporal lobe cortex, but in four cases additional strip electrodes (total, 11) covered the frontal, parietal, and occipital lobe cortexes. The electrodes were left in place for up to 28 days. No complications occurred. Interictally, focal spiking was recorded subtemporally, mostly without being seen in electrodes recording from the lateral temporal cortex. In three patients studied with simultaneous subdural and sphenoidal wire electrodes, spiking recorded from subdural electrodes was often not seen in the sphenoidal recording. There were 151 seizures recorded (with or without simultaneous video monitoring). The mean number of seizures per patient was 6.7 (range, 0–21). The seizures were classified as having focal (80 seizures) or local (71 seizures) onset. It is concluded that subdural electrodes are safe and have a sufficient selectivity with regard to localization of interictal spiking and seizure onset in patients with mesial temporal epileptic lesions. In such cases, electrodes have to be placed subtemporally. Other cortical areas may also be explored with these electrodes.

P300 in sphenoidal recordings and depth recordings in temporal lobe epilepsy.

P300 in sphenoidal recordings and depth recordings in temporal lobe epilepsy.

Temporal lobectomy for partial complex seizures: evaluation, results, and 1-year follow-up.

Twenty-six patients were evaluated for temporal lobectomy. Fourteen underwent initial monitoring with electrodes in the amygdala and hippocampus bilaterally. Twelve had initial monitoring with scalp and sphenoidal electrodes. Four had conclusive localization without depth electrodes. Twenty-three patients underwent lobectomy. At 1-year minimum follow-up, 15 were seizure free. Five had greater than 90% reduction in seizure frequency. Complications of depth electrodes were one hemorrhage and one abscess. One patient developed impaired memory following surgery. Temporal lobectomy is effective in well-selected patients. Depth electrodes localize seizure onset from mesial temporal structures. Scalp and sphenoidal recording may be sufficient in some cases.

Techniques and applications of sphenoidal recording.

Sphenoidal electrodes were introduced in the late 1940s and early 1950s to record EEG activity from the inferior mesial temporal region. They are inserted percutaneously beneath the zygomatic arch to rest in the vicinity of the foramen ovale. They are safe, easy to insert, and well tolerated for up to 3 weeks. Artifacts are less prominent than with other types of basal electrodes. Complications are rare. Sphenoidal electrodes are superior to scalp electrodes in detecting interictal epileptiform discharges and ictal EEG patterns from the inferior mesial temporal lobe. Clinical indications include: documentation of epileptiform activity in patients with partial complex seizures in whom scalp recording has been inconclusive; diagnosis of episodes of uncertain mechanism; and localization of an epileptogenic focus in patients being evaluated for temporal lobectomy.

Surgical aspects of temporal lobe epilepsy. Results and problems.

The Montreal Neurological Institute's experience with cortical resection for medically refractory temporal lobe epilepsy had grown to 1,102 patients as of the end of 1978, 932 in the non-tumoral category and 170 patients with tumors, including a few major vascular malformations. Follow-up data, (complete to date or to the patients' death in over 80% of the patients) shows that 70% of those with 2 or more years of adequate follow-up data (median period 11 years) have experienced a complete or nearly complete reduction of the seizure tendency. Since the attack pattern frequently does not give evidence as to the lateralization of the epileptogenic area, special EEG techniques are often required to select those patients apt to benefit from temporal lobectomy. Sphenoidal recordings with chronically implanted fine wire electrodes have been particularly useful. Telemetry has enhanced the capability of recording a large number of epileptic spikes during sleep and various daytime activities and of recording spontaneous attacks. An automatic spike detection gadget gives promise of major help in analyzing these lengthy records. The addition of a video tape system, correlated with the EEG telemetry system, aids in determining the side of origin of the seizures. A small number of patients in whom this could not be determined with these various EEG techniques, were studied with stereotactically implanted chronic depth electrodes employing a modification of the Talairach-Bancaud technique. Over 3/4 of these patients were shown to have the onset of their seizures to be sufficiently well lateralized to warrant temporal lobectomy.