ARTICLES Dipole Density of Low-Frequency and Spike Magnetic Activity: A Reliable Procedure in Presurgical Evaluation of Temporal Lobe Epilepsy Alberto Ferna ´ndez,* Rafael G. de Sola,† Carlos Amo,* Agustı ´n Turrero,‡ Pilar Zuluaga,‡ Fernando Maestu ´,* Pablo Campo,* and Toma ´s Ortiz* Abstract: Conventional visual analysis and dipole density analysis of magnetoencephalographic data for both spike and low-frequency magnetic activity were compared for presurgical evaluation in tem- poral lobe epilepsy (TLE) in a sample of 26 drug-resistant operated TLE patients. A series of logistic regression analyses were per- formed. Dipole density sensitivity was superior to visual localization analysis. Three separated logistic models were calculated for inter- ictal spikes, low-frequency magnetic activity, and the combination of both measures. A combined interictal spike/low-frequency mag- netic activity model predicted correctly the operated temporal lobe in all patients. Clear-cut criteria for the probability model are proposed that are valid for 92.3% of cases in the sample. The quantitative approach proposed by this study is an evidence-based model for presurgical evaluation of temporal lobe epilepsy, which improves previous magnetoencephalographic investigations and es- tablishes working clinical criteria for patient evaluation in TLE. Key Words: Temporal lobe epilepsy, Magnetoencephalography, Interictal, Low-frequency magnetic activity. (J Clin Neurophysiol 2004;21: 254 –266) Presurgical evaluation of patients with complex partial sei- zures of temporal lobe origin, which is needed to accurately localize the epileptogenic area, is an expensive and time- consuming process. Several noninvasive or minimally inva- sive diagnostic techniques, including EEG, single photon emission computed tomography (SPECT), positron emission tomography, MRI, and magnetic resonance spectroscopy, in addition to neuropsychological evaluation, are typically used in the presurgical evaluation process (Engel, 1997; Olivier, 1988; Spencer, 2002a). Magnetoencephalography (MEG) is another noninvasive technique that can reliably localize the epileptogenic zone in patients with temporal and extratem- poral epilepsy (Baumgartner et al., 2000; Iwasaki et al., 2002; Knowlton et al., 1997; Mamelak et al., 2002). MEG offers some advantages over EEG recordings: unlike EEG, mag- netic fields are not distorted by different conductive ratios of brain and skull tissues, and no reference point is needed in magnetic recordings (Elbert et al., 1998). Nevertheless, MEG has some inherent limitations, primarily due to limited sen- sitivity for radial dipoles and decreasing resolution with depth within the brain, which has particular relevance in mesial temporal lobe epilepsy (TLE) (Lewine and Orrison, 1995). In addition, MEG is typically limited to interictal recordings because body movements during ictal episodes interfere with the recordings. As Brockhaus et al. (1997) noted, interictal spike activity provides less accurate localization of the mesial temporal epileptogenic area in comparison with ictal record- ings. In addition, interictal MEG spiking is typically recorded in 65% to 95% of TLE patients undergoing MEG studies (Iwasaki et al., 2002; Kirchberger et al., 1998; Knowlton et al. 1997). Consequently, if we base our interpretation only on interictal epileptic findings, not all patients can benefit from MEG studies during the presurgical investigation. Different approaches have been proposed to overcome this limitation, including pharmacologic activation (Brock- haus et al., 1997; Kirchberger et al., 1998), spike averaging (Chitoku et al., 2003; Leal et al., 2002), and two-dimensional inverse imaging (Moran and Tepley, 2000). An alternative to conventional MEG spike source localization based on the analysis of MEG slow-wave activity has been proposed. Quantitative evaluation of slow-wave activity may be rele- vant for presurgical evaluation because it represents a com- mon sign of focal brain damage (Gloor et al., 1977). EEG slow-wave activity has been shown to provide reliable later- alization in approximately 70% of TLE patients (Geyer et al., 1999; Quesney et al., 1988), and it represents a steady *Centro MEG Dr. Pe ´rez Modrego, Universidad Complutense de Madrid; †Servicio de Neurocirugı ´a, Hospital Universitario de la Princesa; and ‡Departamento de Bioestadı ´stica e I.O., Facultad de Medicina, Univer- sidad Complutense de Madrid, Madrid, Spain Address correspondence and reprint requests to Dr. Toma ´s Ortiz Alonso, Centro MEG Dr. Pe ´rez-Modrego, Facultad de Medicina, Pabello ´n 8, Universidad Complutense de Madrid; Madrid, Spain; e-mail: cmeg@rect.ucm.es. Copyright © 2004 by Lippincott Williams & Wilkins ISSN: 0736-0258/04/2104-0254 Journal of Clinical Neurophysiology • Volume 21, Number 4, August 2004 254