Bruce J. Fisch

The weighted average reference montage

In this article we describe the implementation of a linearly weighted average reference montage. This montage differs from the commonly used source derivation montage in that the reference includes all the scalp electrodes of the 10-20 system; and it differs from all other montages implemented thus far in that the weighting of the reference electrodes is based on directly measured interelectrode distances. Using EEG and computer generated signals we have been able to demonstrate that the weighted average reference montage combines topographic selectivity and accuracy in the display of both focal and regional background changes. In comparison to the source derivation montage, ectopic peaks and troughs of localized potential fields are less prominent and interhemispheric symmetry is better preserved. In comparison to the common average reference montage the frequent prominent ectopic displacement of high amplitude (e.g., vertex waves) or widespread potentials (e.g., alpha background activity) is suppressed. We believe that the spatial filtering characteristics of the weighted average reference montage, which are intermediate between those of the common average reference montage and the source derivation and Laplacian montages, will make it a useful alternative for topographic analysis. Our results also indicate that actual scalp measurements should be used to calculate reference electrode weighting factors because such measurements yield values that are substantially different from those derived from other methods of estimation presented thus far. A weighted average montage derived from pooled scalp measurements can be easily implemented using the weighting factors provided herein.

Interictal epileptiform activity: diagnostic and behavioral implications: 2002 ACNS presidential address.

Interictal epileptiform activity (IEA) is an electrographic term that describes the abrupt appearance of spikes or sharp waves, often associated with aftergoing slow waves that, unlike ictal epileptiform activity, are limited in duration and do not evolve in frequency and distribution over time. Although the term ’interictal’ would seem to imply the absence of a behavioral correlate, the definition of IEA does not address neuropsychological function. The implications of IEA have continued to evolve with research in: (1) the genetic and metabolic basis of electrophysiological events (Noebels, 2003; Pedley, 1997), (2) intracranial electrophysiology (Bragin et al., 2002a, b; Staba 2002), and (3) the relationship of electrophysiological events to neuropsychological function (Aarts et al., 1984; Browne et al., 1974; Shewmon and Erwin, 1988a, b, c). Accumulating information has begun to shift the older concept of IEA away from that of an electrographic event unassociated with cognitive or behavioral consequence to an electrophysiological event that is the phenotypic expression of a variety of cellular disorders (Noebels, 2003; Pedley, 1997). For the clinician, the study of IEA remains essential to the diagnosis and treatment of epilepsy. While preparing this discussion, I had the opportunity to discuss IEA with neurology residents and practicing neurologists. I also conducted informal surveys during the 2002 EEG Scholars Program for EEG fellows at the annual American Clinical Neurophysiology Society meeting and during the 2003 Clinical EEG course at the annual American Academy of Neurology meeting to get a better idea of how neurologists view IEA. Similar to the experience of Hughes (1989) and Goodin and Aminoff (1984) over a decade ago, I was surprised to find that that most neurologists do not have a good understanding of the diagnostic or potential neuropsychological implications of IEA. Perhaps, as suggested by Hughes (1989), this is because of an educational approach in which the EEG is regarded as a graphic entity with little relationship to actual biophysical cellular function. The adage “never treat a lab test” has undoubtedly been applied to EEG more often than to any other neurodiagnostic procedure, despite the fact that there are situations in which brain dysfunction is treated according to the cortical electrophysiological abnormality, i.e., the EEG. The clinical relevance of IEA depends on how the following questions are answered:

Lateralization of temporal lobe epilepsy and learning disabilities, as defined by disability-related civil rights law.

Summary:  Purpose: Epilepsy research has identified higher rates of learning disorders in patients with temporal lobe epilepsy (TLE). However, most studies have not adequately assessed complex functional adult learning skills, such as reading comprehension and written language. We designed this study to evaluate our predictions that higher rates of reading comprehension, written language, and calculation disabilities would be associated with left TLE versus right TLE.

Postictal EEG suppression and hippocampal atrophy in temporal lobe epilepsy.

Summary Postictal EEG suppression and slowing recorded with scalp electrodes in patients with partial epilepsy is often maximal over the cortical area of ictal onset. The aim of this study was to determine whether a quantitative relationship exists between immediate postictal EEG suppression and hippocampal atrophy. Immediate postictal EEG was analyzed in 31 scalp-recorded seizures obtained from 8 patients who underwent temporal lobectomy with seizure-free outcomes (2 left, 6 right). Quantitative EEG analysis was performed using a temporal power asymmetry index for each frequency band. The hippocampal asymmetry (left-to-right ratio) based on T1- and T2-weighted MR images was determined by hippocampal volumetric analysis. The relationship between the average temporal power asymmetry index and either T1 or T2 hippocampal asymmetry ratio was assessed for each frequency band using Pearson’s correlation coefficient. Only correlations of the temporal power asymmetry index with T1 hippocampal asymmetry were significant for the total bands (r = 0.768, P < 0.026) and &dgr;-bands (r = 0.728, P < 0.041). The findings suggest that a quantitative relationship exists between postictal EEG suppression in the &dgr;-frequency band and hippocampal atrophy in temporal lobe epilepsy.

Botulinum Toxin Injections for Simple Partial Motor Seizures Associated with Pain

Intractable epilepsy with painful partial motor seizures is a relatively rare and difficult disorder to treat. We evaluated the usefulness of botulinum toxin to reduce ictal pain. Two patients received two or four botulinum toxin (BTX) injections at one-to-two-month intervals. Patient 1 had painful seizures of the right arm and hand. Patient 2 had painful seizures involving the left foot and leg. Injections were discontinued after improved seizure control following resective surgery. Both patients received significant pain relief from the injections with analgesia lasting at least two months. Seizure severity was reduced, but seizure frequency and duration were unaffected. For these patients, BTX was effective in temporarily relieving pain associated with muscle contraction in simple partial motor seizures. Our findings do not support the hypothesis that modulation of motor end-organ feedback affects focal seizure generation. BTX is a safe and reversible treatment that should be considered as part of adjunctive therapy after failure to achieve control of painful partial motor seizures.