Use of EEG-fMRI for Standard Evaluations of Patients with Nonlesional Frontal Lobe Epilepsy

December 22, 2009


Friederike Moeller, MD, and Jean Gotman, PhD, discuss their paper "Adding to standard evaluations of patients with nonlesional frontal lobe epilepsy, "which was recently published in Neurology® (2009; 73:2023-2030). They spoke with José E. Cavazos, MD, PhD, FAAN. Please briefly describe your study.

Authors: Many patients with nonlesional frontal lobe epilepsy are not considered surgical candidates because the delineation of the epileptogenic zone is difficult. The aim of this study was to investigate whether using EEG-fMRI during the preoperative evaluation of these patients can help delineate the epileptic focus. By comparing EEG-fMRI results with standard clinical tests and postoperative results, we found that this was indeed the case. Clinicians often feel frustrated when diagnosing a nonlesional frontal lobe epilepsy (FLE) because of the limited options. Please tell us how often intractable partial-onset epilepsy presents as a nonlesional frontal lobe syndrome, and whether the prevalence of this condition varies across different age groups.

Authors: To our knowledge there are no epidemiological studies that clearly answer this question. From surgical series we know that frontal lobe resection is the second most common procedure performed after temporal-lobe resections to treat pharmacoresistant epilepsy. The percentage of nonlesional frontal lobe cases in these series varies across different centers, but may account for up to 25 percent of operated patients. The actual number of patients with nonlesional frontal lobe epilepsy, however, may be much higher, since many patients with FLE do not have surgery because of an unclear seizure focus. We are not aware of any study that investigated how the prevalence of nonlesional lobe epilepsy varies across different age groups. What are the technical requirements to do EEG-fMRI in the preoperative evaluation?

Authors: EEG-fMRI is a complex technique, and analysis of the data requires expertise in computer processing. To obtain a good quality EEG, MRI gradients artifacts and heartbeat artifacts need to be removed, and this can be done with commercial software packages. When an MRI scanner is available, minimal additional investments are needed to establish EEG-fMRI: nonmagnetic electrodes and an MRI-compatible amplifier, as well as software to record and analyze the data. It is possible to use a 1.5 T MRI for EEG-fMRI recordings; in fact, all early studies were performed at 1.5 T. However, higher magnetic field strengths show higher sensitivity. The gold standard for seizure localization is mapping electrically the zone of onset of seizures. Given that often this is impractical (i.e., patient would be waiting in the MRI scanner until a seizure occurs), what is your opinion on triggering the fMRI scanner by other interictal abnormalities such as high-frequency oscillations or by inducing epileptic events with photic stimulation?

Authors: EEG-fMRI is able to map ictal events if the seizures are not associated with head movements. Dynamic analyses of this kind of seizure may differentiate between seizure onset and propagation. But as shown in our study on nonlesional frontal lobe epilepsy, interictal events also provide useful information about the epileptogenic zone. The idea of analyzing high-frequency oscillations in an EEG-fMRI study is very interesting. However, until now high-frequency oscillations could not be detected on the scalp EEG. It is feasible to perform photic stimulation inside the scanner to trigger discharges. We conducted a study in which photic stimulation was used to detect networks associated with photoparoxysmal responses in the EEG. However, since photosensitivity is mainly found in patients with idiopathic generalized epilepsy—and is rare in focal epilepsy—we think that photic stimulation would not be helpful in the majority of the patients with focal epilepsy. You studied nonlesional frontal lobe epilepsy. Could you foresee problems extending this technique to lesional epilepsies or to other lobar localizations?

Authors: There are already a number of studies investigating this technology for the evaluation of patients with different types of epilepsy and different types of underlying pathologies. These studies show that EEG-fMRI is also feasible in lesional cases, but in patients with lesions, the identification of the seizure focus is not as challenging as in nonlesional cases. For this reason, EEG-fMRI seems to be less important in the presurgical evaluation in these patients. What is the main message of your study for clinicians?

Authors: We think that EEG-fMRI can help to delineate the epileptic focus in patients with epilepsy. Like any other clinical test, the results should be interpreted in the context of the entire clinical picture. EEG-fMRI may contribute to decision-making regarding intracerebral EEG implantations.

Author Disclosures

Dr. Moeller has nothing to disclose.

Dr. Gotman serves on the editorial boards of Epilepsia and the Journal of Clinical Neurophysiology. He has received speaker honoraria from General Electric and non-industry-sponsored activities, and he receives research support from the Canadian Institutes of Health Research in the form of Open Operating Grants (MOP). In addition, he is a major stockholder and CEO of Stellate (former CEO) and Lacerta Research (current CEO).

Within the past 24 months, Dr. Cavazos received personal compensation for his work as a scientific board advisor for UCB Pharma and Ortho-McNeil, and as a consultant for GXC Global. In the same period, he served as an editorial board member of Epilepsy Research and as Web Content chair of the American Epilepsy Society. Within the past five years, he received grant funding from NIH (NINDS) and VA (Merit review award).