Ernst Rodin

Source analysis of generalized spike-wave complexes.

SummaryDipole source analysis was carried out in three adults and four children with 3 c/s generalized spike-wave (SW) discharges and absence seizures. Eighty-seven SW complexes were investigated. When one regional source was used the equivalent location was always near the baso-frontal area close to the midline. It explained on the average 89% of the variance in the adults and 82% in the children. When two regional sources were used and initially constrained for symmetry, the residual variance (RV) was reduced to 7% in the adults and 9% in the children. The sources remained in the frontal areas and were on the average 2 cm from the midline. When the mirror source was freed it remained contralateral in 76% of the complexes but moved ipsilaterally to various locations in the rest. To reduce the RV to between 2 and 3% one or two additional regional sources were required. These were located in the temporal or parietal areas in the adults and occasionally one occipital region in the children. The study showed that widespread and repetitive EEG events, like the generalized SW, can lend themselves to spatio-temporal multiple dipole modeling. More precise anatomic correlates may become possible with an expanded electrode array and the verification of their locations against the patients MRI scan.

Seizure onset determination.

Digital electroencephalography has greatly expanded the opportunities for data analysis. Although commercial software packages are available they seem not to be used as widely in the preoperative work-up of epilepsy patients as might be warranted. This review will demonstrate that seizure onset can be better defined by judicious use of post hoc filter settings, expanded electrode coverage, and special electrode montages. In scalp recordings, ictal baseline shifts and infraslow activity (ISA) can be evaluated with conventional EEG systems by opening the high-pass filter to 0.01 Hz; in intracranial recordings high-frequency activity (>60 Hz; HFA) can be observed in addition. Inasmuch as ISA and HFA have considerably smaller electrical fields than the conventional frequencies they may better define seizure onset than might be possible otherwise. It is recommended that to determine the clinical value of ISA and HFA for epilepsy surgery, retrospective analyses of seizure data, which include assessment of ISA and HFA, be performed from patients who have undergone surgical resections of epileptogenic tissue. These may yield information as to whether or not the epileptogenic areas of ISA and HFA had been included in the resected tissue and their relationship to surgical outcome can then be determined.

Magnetoencephalographic spikes not detected by conventional electroencephalography

OBJECTIVE To investigate some of the reasons why magnetoencephalographic (MEG) spikes are at times not apparent in conventional electroencephalograms (EEG) when the data are co-registered, and to explore to what extent modern EEG analysis methods can improve the yield. METHODS Seventy seconds of MEG-EEG co-registration on a 122 channel Neuromag system were studied in a 10-year-old boy with Landau-Kleffner syndrome. Twenty-six EEG channels were originally recorded with a left ear reference. The EEG data were subsequently reformatted (BESA) to a variety of montages for the 10-20 and 10-10 electrode array. A 10 s data epoch was compared in detail for concordance between MEG and EEG spikes. To detect the characteristics of hidden low voltage EEG spikes, MEG spikes were averaged and compared with the concomitant averaged EEG spike. RESULTS While there was an abundance of EEG as well as MEG spikes on the left; definite right-sided spikes were not visible in the EEG. Right hemispheric MEG spikes were, however, plentiful with an average strength of 757 fT. When the individual MEG spikes from the right hemisphere were compared with the corresponding EEG events their amplitude ranged between 24 and 31 microV and were, therefore, indistinguishable from background activity. The majority of them became visible, however, with further sophisticated data analysis. CONCLUSIONS When the relative merits of MEG versus EEG recordings for the detection of epileptogenic spike are investigated the 10-20 system of electrode placement and conventional methods of EEG analysis do not provide optimal data assessment. The use of the 10-10 electrode array combined with modern methods of digital data analysis can provide better concordance with MEG data.

Magnetoencephalography and Magnetic Source Imaging in Epilepsy

Magnetoencephalograpy (MEG) and Electroencephalography (EEG) provide physicians with complementary data and should not be regarded as mutually exclusive evaluative methods of cerebral activity. Relevant to this edition, MEG applications related to the surgical treatment of epilepsy will be discussed exclusively. Combined MEG/EEG data collection and analysis should be a routine diagnostic practice for patients who are still suffering seizures due to the failure of drug therapy. Clinicians in the field of epilepsy agree that a greater number of patients would benefit from surgery than are currently referred for pre-surgical evaluation. Regardless of age or presumed epilepsy syndrome, all patients deserve the possibility of living seizure-free through surgery. Technological advances in superconducting elements as well as the digital revolution were necessary for the development of MEG into a clinically valuable diagnostic tool. Compared to the examination of electrical activity of the brain, investigation into its magnetic concomitant is a more recent development. In MEG, cerebral magnetic activity is recorded using magnetometer or gradiometer whole-head systems. MEG spikes usually have a shorter duration and a steeper ascending slope than EEG spikes, and variable phase relationships to EEG. When co-registered spikes are compared, it is apparent that EEG and MEG spikes differ. There is agreement among investigators that more interictal epileptiform spikes are seen in MEG than EEG. When MEG is co-registered with invasive intracranial EEG data, the detection rate of interictal epileptiform discharges depends on the number of electrocorticographic channels that record a spike. When patients have a non-localizing video-EEG recording, MEG pinpoints the resected area in 58–72% of the cases.

Dipole sources of the human alpha rhythm

SummaryDipole sources were investigated in 22 normal subjects with a variety of strategies available through the BESA program. When all the data were summed one regional source, located near the midline in the basal portions of the occipital lobe, explained 92% of the variance. Two regional sources, initially constrained for symmetry but subsequently freed from constraint placed them also in the occipital regions near the midline and reduced the residual variance to 4%. Pooled data obscure, however, the marked individual differences especially in regard to lateralization. In the individual case the major source was also always in one occipital area but its location, especially the degree of separation from the midline depended upon alpha distribution and the strategy used in the workup of the data. The orientation of the major components of the regional sources was usually in the posterior-anterior direction, fairly parallel to the midline and while the other one pointed to the upper convexity. Because of the considerable variability of the alpha rhythm in given subjects and even within the same individual a model which requires symmetry constraints is not optimal for all instances, even when constraints are lifted thereafter. The study demonstrated the feasibility of distinguishing predominantly mesial sources from those which are bihemipheric with more lateral origins but several different models may have to be used to reach the most realistic conclusions.

EEG infraslow activity in absence and partial seizures.

It is currently assumed that for recording of infraslow activity (ISA) DC-coupled amplifiers are required. This report will demonstrate that this may not be the case and presents some data about its potential clinical usefulness. Archived EEGs of 29 seizures from 6 children with absence attacks, accompanied by 3 Hz classical spike-wave discharges (SW), were compared with 20 partial seizures from 10 adult patients. The data from the children were acquired on a Bio-logic system, those from the adults on a Grass-Telefactor instrument. In the children the original 30-minute routine EEG was used while in the adults stored videomonitored data were excerpted to provide 20-minute segments which included the preictal, ictal and postictal state. All data were analyzed with the BESA software package. The seizures were evaluated separately on conventional filter settings, full band of 0.01-to the upper limit of the instrument, and 0.01–0.1 Hz (infraslow activity, ISA). Filter settings of 0.01–0.1 Hz provided a better assessment of ISA than when the full band was evaluated. Absence seizures showed bilateral essentially synchronous ISA with a negative positive sequence in the frontal areas and opposite polarity in the posterior head regions. It is concluded that ISA can be recovered from conventional EEG recordings and may be helpful not only in determining the area(s) of seizure onset but can also differentiate truly focal seizures from those where an additional generalized seizure tendency is present. This is likely to be important when epilepsy surgery is performed in absence of a demonstrable structural lesion In partial seizures when seizure onset was clearly lateralizeable from conventional frequency settings ISA corresponded to that hemisphere, but the electrode position could be displaced to a neighboring one from the one which was maximally involved on conventional settings. Topographic analysis showed two types of ISA: one with focal spread only and the other where there was in addition an element of synchrony especially in the frontal areas.

Interictal infraslow activity in patients with epilepsy

OBJECTIVE To evaluate if interictal infraslow activity (ISA), as obtained from a conventional EEG system, can contribute information about the epileptogenic process. METHODS The entire long-term intracranial monitoring sessions of 12 consecutive patients were evaluated on an XLTEK system for ISA. Three additional patients had long-term scalp recordings. RESULTS In intracranial as well as scalp recordings, the ISA background was consistently higher in the waking state than during sleep. From this background emerged intermittently focal changes, which could achieve in intracranial recordings millivolt amplitudes, while they remained in the microvolt range in scalp recordings. Although they were mainly contiguous between adjacent channels, this was not necessarily the case and intermittent build-up could be seen distant from the epileptogenic zone or radiographic lesion. CONCLUSIONS Interictal ISA can be detected in routine intracranial and scalp recordings, without the need for DC amplifiers, and can provide additional information. SIGNIFICANCE Since ISA is a separate element of the electromagnetic spectrum, apparently non-neuronal in origin, its assessment should be included not only in the pre-surgical evaluation of epilepsy patients but also in patients with other neurologic disorders and normal volunteers.

The clinical concept of epilepsy

This article reviews the history of clinical concepts of epilepsy and its classification, especially in the last 100 years. Throughout its recorded history of 3 to 4 millennia, epilepsy has always been defined by its most dramatic symptoms, for example, falling, motor activity or loss of consciousness, but separation from other causes of the same paroxysmal symptoms has always proved challenging. For over a century there has been some semantic confusion whether to call the various paroxysms fits, convulsions, seizures, or epilepsies. Since the middle of the 19th century a great unresolved debate has continued about whether recurrent seizures or epilepsy should be viewed as a separable symptom of underlying brain disease or as one or more idiopathic diseases or syndromes, with an inherent age‐related natural history; or indeed viewed as both a symptom and a disease. A major advance in the 20th century is that vascular theories of epilepsy, which reached their peak with Turner in 1907, have been replaced by electromagnetic discharges, based especially on the work of Todd, Jackson, Berger, Lennox, and the Gibbs, culminating eventually in new ILAE classifications of seizures (1981) and epilepsy syndromes (1989). However 21st century uncertainties about symptomatic versus idiopathic or cross‐sectional (seizures) versus longitudinal (epilepsy) approaches to the problem very much reflect similar divergences of view a century ago. More attention is now being directed at interseizure events and processes which may lead either to remission or intractability with associated cognitive and psychosocial consequences. The search for the elusive essence, diathesis or predisposition to epilepsy, including seizure threshold, continues.

Optimal Evaluation of Digital Electroencephalograms

Currently available digital EEG equipment provides considerably greater opportunities for clinical data analysis than is generally appreciated especially when appropriate software is used. Data from 7 different laboratories that had been obtained for routine diagnostic evaluations on 7 different EEG instruments and stored on compact disks were investigated. Since the instruments do not filter the data at input, ultra slow activity down to 0.01 Hz is currently being recorded but the attenuation factor is instrument dependent. Nevertheless, relevant clinical information is potentially available in these data and needs to be explored. Several examples in regard to epilepsy are presented. Determination of seizure onset may depend on the frequencies that are examined. The use of appropriate filter settings and viewing windows for the clinical question to be answered is stressed. Differentiation between simple and complex spike wave discharges, as well as spread of spikes, can readily be achieved by expanding the time base to 1 or 2 seconds and placing a cursor on the peak of the negative spike. Latencies in the millisecond range can then become apparent. EEGs co-registered with MEG should be evaluated with the same software in order to allow an adequate assessment of the similarities and differences between electrical and magnetic activity. An example of a comparison of EEG, planar gradiometers and magnetometers for an averaged spike is shown.

Paper Recordings of Ultrafast Frequencies in Experimental Epilepsy

Digital EEG technology has facilitated the recording of fast activity above 50 Hz, but previous work carried out in this field is, commonly, no longer referred to in the literature. This paper presents a summary of our experience in experimental epilepsy when frequencies above 100 Hz were recorded. It was shown that conventional recordings (1–70 Hz) do not correlate with the onset of clinical seizures and can actually lead to misleading neurophysiologic conclusions. Ultrafast activity (100–1500 Hz), on the other hand, showed excellent correlation with clinical behavior and pointed to the low brainstem for the origin of nonfocal tonic-clonic seizures. In the animal analogue of absence seizures the cortex, including cingulate gyrus, thalamus, and cerebellum, showed greater involvement than other brain structures. Ultrafast activity has a very limited electrical field, and evoked responses remain restricted to the corresponding sensory pathways. Focal penicillin administration led not only to spike generation but also to associated ultrafast frequency bursts in that area. It appears likely that ultrafast burst activity may be a better marker for focal cortical epileptogenesis than spikes or sharp waves, which can be transmitted from a distance.