EEG source imaging concordance with intracranial EEG and epileptologist review in focal epilepsy

EEG source imaging is becoming widely used for the evaluation of medically refractory focal epilepsy. The validity of EEG source imaging has been established in several studies comparing source imaging to the surgical resection cavity and subsequent seizure freedom. We present a cohort of 87 patients and compare EEG source imaging of both ictal and interictal scalp EEG to the seizure onset zone on intracranial EEG. Concordance of EEG source imaging with intracranial EEG was determined on a sublobar level and was quantified by measuring the distance between the source imaging result and the centroid of the active seizure onset zone electrodes. The EEG source imaging results of a subgroup of 26 patients with high density 76-channel EEG were compared with the localization of three experienced epileptologists. Of 87 patients, 95% had at least one analysis concordant with intracranial EEG and 74% had complete concordance. There was a higher rate of complete concordance in temporal lobe epilepsy compared to extratemporal (89.3 and 62.8%, respectively, P = 0.015). Of the total 282 analyses performed on this cohort, higher concordance was also seen in temporal discharges (95%) compared to extratemporal (77%) (P = 0.0012), but no difference was seen comparing high-density EEG with standard (32-channel) EEG. Subgroup analysis of ictal waveforms showed greater concordance for ictal spiking, compared with rhythmic activity, paroxysmal fast activity, or obscured onset. Median distances from the dipole and maximum distributed source to a centroid of seizure onset zone electrodes were 30.0 and 32.5 mm, respectively, and the median distances from dipole and maximum distributed source to nearest seizure onset zone electrode were 22.8 and 21.7, respectively. There were significantly shorter distances in ictal spiking. There were shorter distances in patients with Engel Class 1 outcome from surgical resection compared to patients with worse outcomes. For the subgroup of 26 high-density EEG patients, EEG source localization had a significantly higher concordance (92% versus 65%), sensitivity (57% versus 35%) and positive predictive value (60% versus 36%) compared with epileptologist localization. Our study demonstrates good concordance between ictal and interictal source imaging and intracranial EEG. Temporal lobe discharges have higher concordance rates than extratemporal discharges. Importantly, this study shows that source imaging has greater agreement with intracranial EEG than visual review alone, supporting its role in surgical planning.

Quantitative EEG Analysis, EEG Mapping, and Magnetoencephalography

Quantitative EEG Analysis, EEG Mapping, and Magnetoencephalography reviews quantitative methods that facilitate interpretation of multichannel electroencephalographic (EEG) recordings and extract information not readily obtainable with visual analysis alone. Fourier analysis assesses dominant background frequencies, looks for trends over long periods, and can be applied to epileptic seizures. Pattern recognition can automatically detect epileptiform discharges, seizures, and high-frequency oscillations. Montage reformatting allows the same EEG segment to be viewed using different montages, including the Laplacian montage. Cross-correlation and cross-spectral analysis quantifies time relationships between channels. Interpolation techniques and topographical displays help visualize spatial distributions of cortical activity. Multivariate statistical methods find independent components of a multichannel EEG recording and can be used for artifact removal. Machine learning approaches can facilitate identification of seizures and interictal features. Cortical projection and source localization techniques attempt to resolve underlying generators of EEG activity. Magnetic source imaging finds underlying generators of the magnetoencephalogram.

Instrumentation and Electricity for Clinical Neurophysiology

This chapter reviews the basic principles of electricity and how they apply to electrical circuits. Electrical properties such as electric field, electric potential, electromotive force, electric current, current density, resistance, conductance, resistivity, conductivity, capacitance, inductance, frequency, reactance, and impedance are defined, along with their measurement units and formulas for calculation. Electrical circuit components such as seats of electromotive force (batteries and generators), resistors, capacitors, coils (inductors), diodes, and transistors are described, as well as example circuits containing these components. Key concepts of electrical circuits such as direct current and alternating current, root mean square measurements, time constant, cutoff frequency, resonance, forward and reverse biasing, gain (amplification), and common mode rejection ratio are discussed. High-pass and low-pass filter circuits and differential amplifier circuits are also described and their usage is illustrated.

Volume Conduction in Clinical Neurophysiology

Volume Conduction reviews the theory describing the spread of electrical current throughout the body. The types and polarities of cortically generated potentials and the relationship to excitatory and inhibitory postsynaptic potentials are discussed. Influences of the volume conductor on peripheral nerve and muscle potentials are described. Specific current sources, including monopoles, dipoles, and quadrupoles and the spread of current through the body from each type of source, are described, together with how these influence the spatial distributions of potentials recorded from referential and bipolar montages. The effects of resistive-capacitive properties of the volume conductor medium are discussed, together with how these effects can cause erroneous latency measurements of propagating action potentials. Applications to evoked potentials, electroencephalography (EEG), nerve conduction studies, and needle electromyography are described. Dipole source localization by EEG and magnetoencephalography is also discussed. An Appendix gives formulas for calculating potentials in homogeneous media and discusses how these potentials change in nonhomogeneous volume conductors.

Frequent sleep-related bitemporal focal seizures in transient epileptic amnesia syndrome: Evidence from ictal video-EEG

Two patients who shared similar presenting clinical features of anterograde and retrograde autobiographical amnesia typical of transient epileptic amnesia (TEA) underwent prolonged video electroencephalogram (VEEG) monitoring and were found to have sleep-activated epileptiform activity and frequent subclinical bitemporal seizures predominantly during sleep. Case 1 is a 59-year-old woman whose presenting complaint was memory impairment. Over 18 months, she had three distinct 8-h-long episodes of confusion and disorientation with persistent anterograde and retrograde autobiographical amnesia. VEEG recorded frequent interictal bitemporal sharp waves confined to sleep, and 14 subclinical seizures, also mostly during sleep. Case 2 is a 50-year-old woman with known focal epilepsy also presented with memory complaints. Over the course of 1 year, she had two discrete 2-h-long episodes of amnesia, with ongoing anterograde and retrograde autobiographical amnesia. VEEG recorded independent bitemporal sharp waves, and 14 subclinical seizures during sleep and drowsiness. Memory impairment improved in both patients with successful treatment of their seizures. Although the etiology of accelerated long-term forgetting (ALF) and remote memory impairment (RMI) in transient epileptic amnesia (TEA) is unknown, these cases suggest frequent sleep-related seizures may contribute, and they highlight the importance of video-EEG monitoring.

Routine vs extended outpatient EEG for the detection of interictal epileptiform discharges

OBJECTIVE

To compare the yield of epileptiform abnormalities on 30-minute recordings with those greater than 45 minutes.

METHODS

We performed a prospective observational cross-sectional study of all outpatient routine EEGs comparing the rate of interictal epileptiform discharges (IEDs) and clinical events during the initial 30 minutes (routine) with those occurring in the remaining 30-60 minutes (extended). A relative increase of 10% was considered clinically significant.

RESULTS

EEGs from 1,803 patients were included; overall EEG duration was 59.4 minutes (SD ±6.5). Of 426 patients with IEDs at any time during the EEG, 81 (19.1%, 95% confidence interval 15.6-23) occurred only after the initial 30 minutes. The rate of late IEDs was not associated with age, indication, IED type, or sleep deprivation. Longer recording times also increased event capture rate by approximately 30%.

CONCLUSIONS

The yield of IED and event detection is increased in extended outpatient EEGs compared to 30-minute studies.

Volume Conduction

Electrophysiologic studies involve bioelectric potentials generated by sources inside the body, such as the brain, peripheral nerve, and muscle, which may be some distance away from the recording electrodes. Current sources or sinks (active or passive) generated from these bioelectric potentials lead to widespread extracellular currents flowing in the conducting medium throughout the body, called a volume conductor. This chapter reviews the principles of volume conduction as applied to the potentials recorded in clinical neurophysiological studies. Knowledge of these principles is necessary for proper interpretation of EEG, EMG, NCS, and evoked potential recordings in order to extract information concerning the function and location of the neural structures that generate the recorded activity or waveforms.

Electricity and Electronics in Clinical Neurophysiology

Knowledge of the basic principles of electricity and electronic circuits is necessary for a complete understanding of the proper operation of equipment used in clinical neurophysiology. This chapter reviews the basic principles of electric and electronic circuits that are important to clinical neurophysiology. Topics reviewed include basic fundamentals of electricity (such as electric currents, resistors, and capacitors), circuit analysis, filters, transistors and amplifiers, and the terminology and specifications given in equipment manual, as they relate to clinical neurophysiology, Knowledge of these basic principles and how to solve simple circuit problems is necessary for a complete understanding of the proper operation of equipment used in clinical neurophysiology and of the terminology and specifications given in equipment manuals.

Electrical Safety in the Laboratory and Hospital

The principles of electrical safety are of great importance in clinical neurophysiology. All of the electrophysiological studies that are performed require the application of electrical connections to equipment that, through connections with the patient, pass small amounts of electrical current to the patient. Although small, there is always an inherent risk to the tissue through which current passes. This chapter reviews the principles of electric safety that are relevant to clinical neurophysiologic studies. Knowledge of these principles is necessary both for those involved in evaluating and purchasing test instruments and for those involved in maintaining and using them. All those who order, perform, interpret, or supervise electrophysiologic testing share in the responsibility for patient safety, including electric safety.

Electroencephalographic Special Studies

Digital computers can aid in extracting information from EEG waveforms that is not readily obtainable with visual analysis alone and may also be used for quantitation of key features of waveforms. This may be useful in accurate EEG interpretation and in making serial comparisons between EEGs performed on the same subject at different times. Digital computers may also partially automate the interpretation of EEGs, particularly in prolonged monitoring for epilepsy. This chapter reviews several quantitative analysis techniques that may be applied to digitized EEG data. The technique of magnetoencephalography (MEG) is also discussed. Many of these techniques were primarily used as research tools; but as they have become more widely available, they are having an increasing effect on EEG interpretation and diagnosis.

Electrophysiological Generators in Clinical Neurophysiology

The variety of clinical neurophysiological studies corresponds to a variety of structural generators in the body, including muscles, sweat glands, peripheral nerves, and various components of the central nervous system. Each structural generator may have associated with it several different types of physiological potential. This chapter reviews the generators of electrophysiological potentials in terms of basic cellular electrophysiology and the anatomical structures that generate electrophysiological potentials of clinical interest. Knowledge of the generators of the potentials recorded in clinical neurophysiological studies is helpful in understanding the characteristics and distribution of the recorded potentials and is the first step in correlating the alterations seen in disease states with the pathological changes demonstrated in the underlying generators.

Digital Signal Processing

Digital computers can perform types of signal processing not readily available with analog devices, such as ordinary electrical circuits. This includes making the process of obtaining, storing, retrieving, and viewing clinical neurophysiology data easier; aiding in extracting information from waveforms that is not readily obtainable with visual analysis alone; and improving quantification of key features of waveforms. These processes are useful in accurate clinical diagnosis of electroencephalographic (EEG), electromyographic (EMG), and evoked potential studies, and it also lend themselves to serial comparisons between studies performed on the same subject at different times or between two groups of subjects in scientific investigations. Digital computers may also partially automate the interpretation of clinical neurophysiology studies. This chapter reviews the principles of digitization, the design of digitally based instruments for clinical neurophysiology, and several common uses of digital processing, including averaging, digital filtering, and some types of time-domain and frequency-domain analysis.

Utility of an immunotherapy trial in evaluating patients with presumed autoimmune epilepsy

OBJECTIVE

To evaluate a trial of immunotherapy as an aid to diagnosis in suspected autoimmune epilepsy.

METHOD

We reviewed the charts of 110 patients seen at our autoimmune neurology clinic with seizures as a chief complaint. Twenty-nine patients met the following inclusion criteria: (1) autoimmune epilepsy suspected based on the presence of ≥ 1 neural autoantibody (n = 23), personal or family history or physical stigmata of autoimmunity, and frequent or medically intractable seizures; and (2) initiated a 6- to 12-week trial of IV methylprednisolone (IVMP), IV immune globulin (IVIg), or both. Patients were defined as responders if there was a 50% or greater reduction in seizure frequency.

RESULTS

Eighteen patients (62%) responded, of whom 10 (34%) became seizure-free; 52% improved with the first agent. Of those receiving a second agent after not responding to the first, 43% improved. A favorable response correlated with shorter interval between symptom onset and treatment initiation (median 9.5 vs 22 months; p = 0.048). Responders included 14/16 (87.5%) patients with antibodies to plasma membrane antigens, 2/6 (33%) patients seropositive for glutamic acid decarboxylase 65 antibodies, and 2/6 (33%) patients without detectable antibodies. Of 13 responders followed for more than 6 months after initiating long-term oral immunosuppression, response was sustained in 11 (85%).

CONCLUSIONS

These retrospective findings justify consideration of a trial of immunotherapy in patients with suspected autoimmune epilepsy.

CLASSIFICATION OF EVIDENCE

This study provides Class IV evidence that in patients with suspected autoimmune epilepsy, IVMP, IVIg, or both improve seizure control.

Autoimmune epilepsy: clinical characteristics and response to immunotherapy

OBJECTIVE

To describe clinical characteristics and immunotherapy responses in patients with autoimmune epilepsy.

DESIGN

Observational, retrospective case series.

SETTING

Mayo Clinic Health System.

PATIENTS

Thirty-two patients with an exclusive (n=11) or predominant (n=21) seizure presentation in whom an autoimmune etiology was suspected (on the basis of neural autoantibody [91%], inflammatory cerebrospinal fluid [31%], or magnetic resonance imaging suggesting inflammation [63%]) were studied. All had partial seizures: 81% had failed treatment with 2 or more antiepileptic drugs and had daily seizures and 38% had seizure semiologies that were multifocal or changed with time. Head magnetic resonance imaging was normal in 15 (47%) at onset. Electroencephalogram abnormalities included interictal epileptiform discharges in 20; electrographic seizures in 15; and focal slowing in 13. Neural autoantibodies included voltage-gated potassium channel complex in 56% (leucine-rich, glioma-inactivated 1 specific, 14; contactin-associated proteinlike 2 specific, 1); glutamic acid decarboxylase 65 in 22%; collapsin response- mediator protein 5 in 6%; and Ma2, N-methyl-D-aspartate receptor, and ganglionic acetylcholine receptor in 1 patient each.

INTERVENTION

Immunotherapy with intravenous methylprednisolone; intravenous immune globulin; and combinations of intravenous methylprednisolone, intravenous immune globulin, plasmapheresis, or cyclophosphamide.

MAIN OUTCOME MEASURE

Seizure frequency.

RESULTS

After a median interval of 17 months (range, 3-72 months), 22 of 27 (81%) reported improvement postimmunotherapy; 18 were seizure free. The median time from seizure onset to initiating immunotherapy was 4 months for responders and 22 months for nonresponders (P<.05). All voltage-gated potassium channel complex antibody-positive patients reported initial or lasting benefit (P<.05). One voltage-gated potassium channel complex antibody-positive patient was seizure free after thyroid cancer resection; another responded to antiepileptic drug change alone.

CONCLUSION

When clinical and serological clues suggest an autoimmune basis for medically intractable epilepsy, early-initiated immunotherapy may improve seizure outcome.

Electroencephalographic Special Studies

This chapter reviews several quantitative analysis techniques that may be applied to digitized EEG data. The technique of MEG is also discussed. Many of these techniques were primarily used as research tools; but as they have become more widely available, they are having an increasing effect on EEG interpretation and diagnosis.

Digital Signal Processing

This chapter reviews the principles of digitization, the design of digitally based instruments for clinical neurophysiology, and several common uses of digital processing, including averaging, digital filtering, and some types of time-domain and frequency-domain analysis. An understanding of these principles is necessary to select and use digitally based instruments appropriately and to understand their unique features.

Electrophysiologic Generators in Clinical Neurophysiology

This chapter reviews the generators of electrophysiologic potentials in terms of basic cellular electrophysiology and the anatomical structures that generate electrophysiologic potentials of clinical interest. Knowledge of the generators of the potentials recorded in clinical neurophysiologic studies is helpful in understanding the characteristics and distribution of the recorded potentials and is the first step in correlating the alterations seen in disease states with the pathologic changes demonstrated in the underlying generators.

Volume Conduction

This chapter reviews the principles of volume conduction as applied to the potentials recorded in clinical neurophysiologic studies. Knowledge of these principles is necessary for proper interpretation of EEG, EMG, NCS, SEP, VEP, and BAEP recordings in order to extract information concerning the function and location of the neural structures that generate the recorded activity or waveforms.

Electricity and Electronics for Clinical Neurophysiology

This chapter reviews the basic principles of electric and electronic circuits that are important to clinical neurophysiology. Knowledge of these basic principles and how to solve simple circuit problems is necessary for a complete understanding of the proper operation of equipment used in clinical neurophysiology and of the terminology and specifications given in equipment manuals.

Electric Safety in the Laboratory and Hospital

This chapter reviews the principles of electric safety that are relevant to clinical neurophysiologic studies. Knowledge of these principles is necessary both for those involved in evaluating and purchasing test instruments and for those involved in maintaining and using them. All those who order, perform, interpret, or supervise electrophysiologic testing share the legal responsibility for patient safety, including electric safety.

Spatio-temporal source localization and Granger causality in ictal source analysis

We have proposed a new ictal source analysis approach by combining a spatio-temporal source localization approach, and causal interaction estimation technique. The FINE approach is used to identify neural electrical sources from spatio-temporal scalp-EEGs. The Granger causality estimation uses source waveforms estimated by FINE to characterize the causal interaction between the neural electrical sources in order to distinguish primary sources, which initiate ictal events, from secondary sources, which are caused by propagation. In the present study, we applied the proposed analysis approach to an epilepsy patient with symptomatic MRI lesions. It is found that the primary ictal source is within the visible lesion, which gave the consistent presurgical evaluation as MRI for this patient.

Ictal source analysis: localization and imaging of causal interactions in humans

We propose a new integrative approach to characterize the structure of seizures in the space, time, and frequency domains. Such characterization leads to a new technical development of ictal source analysis for the presurgical evaluation of epilepsy patients. The present new ictal source analysis method consists of three parts. First, a three-dimensional source scanning procedure is performed by a spatio-temporal FINE source localization method to locate the multiple sources responsible for the time evolving ictal rhythms at their onsets. Next, the dynamic behavior of the sources is modeled by a multivariate autoregressive process (MVAR). Lastly, the causal interaction patterns among the sources as a function of frequency are estimated from the MVAR modeling of the source temporal dynamics. The causal interaction patterns indicate the dynamic communications between sources, which are useful in distinguishing the primary sources responsible for the ictal onset from the secondary sources caused by the ictal propagation. The present ictal analysis strategy has been applied to a number of seizures from five epilepsy patients, and their results are consistent with observations from either MRI lesions or SPECT scans, which indicate its effectiveness. Each step of the ictal source analysis is statistically evaluated in order to guarantee the confidence in the results.

3D source localization of interictal spikes in epilepsy patients with MRI lesions

The present study aims to accurately localize epileptogenic regions which are responsible for epileptic activities in epilepsy patients by means of a new subspace source localization approach, i.e. first principle vectors (FINE), using scalp EEG recordings. Computer simulations were first performed to assess source localization accuracy of FINE in the clinical electrode set-up. The source localization results from FINE were compared with the results from a classic subspace source localization approach, i.e. MUSIC, and their differences were tested statistically using the paired t-test. Other factors influencing the source localization accuracy were assessed statistically by ANOVA. The interictal epileptiform spike data from three adult epilepsy patients with medically intractable partial epilepsy and well-defined symptomatic MRI lesions were then studied using both FINE and MUSIC. The comparison between the electrical sources estimated by the subspace source localization approaches and MRI lesions was made through the coregistration between the EEG recordings and MRI scans. The accuracy of estimations made by FINE and MUSIC was also evaluated and compared by R(2) statistic, which was used to indicate the goodness-of-fit of the estimated sources to the scalp EEG recordings. The three-concentric-spheres head volume conductor model was built for each patient with three spheres of different radii which takes the individual head size and skull thickness into consideration. The results from computer simulations indicate that the improvement of source spatial resolvability and localization accuracy of FINE as compared with MUSIC is significant when simulated sources are closely spaced, deep, or signal-to-noise ratio is low in a clinical electrode set-up. The interictal electrical generators estimated by FINE and MUSIC are in concordance with the patients' structural abnormality, i.e. MRI lesions, in all three patients. The higher R(2) values achieved by FINE than MUSIC indicate that FINE provides a more satisfactory fitting of the scalp potential measurements than MUSIC in all patients. The present results suggest that FINE provides a useful brain source imaging technique, from clinical EEG recordings, for identifying and localizing epileptogenic regions in epilepsy patients with focal partial seizures. The present study may lead to the establishment of a high-resolution source localization technique from scalp-recorded EEGs for aiding presurgical planning in epilepsy patients.

Use of principal component analysis in the frequency domain for mapping electroencephalographic activities: comparison with phase-encoded Fourier spectral analysis

Principal component analysis (PCA) can separate multichannel electroencephalographic (EEG) epochs into linearly independent (temporally and spatially noncorrelated) components. Results of PCA include component time-series waveforms and factors representing the contribution of each component to each electrode; these factors may be displayed as contour maps representing the topographic distribution of each component. However, PCA often does not achieve the most useful separation of components. PCA may be performed in the frequency domain to potentially improve results. After inspecting principal components of the frequency spectra, spectral values in a selected frequency range are multiplied by a chosen factor to emphasize (or de-emphasize) these frequencies and PCA is redone, promoting the separation of different frequencies into different components. Phase-encoded Fourier spectral analysis (PEFSA) uses multichannel complex Fourier spectra (amplitude and phase) to obtain positive or negative (phase-encoded) potentials at each electrode for any selected frequency. These may be displayed as a contour map representing the topographic distribution of the selected frequency. Applying both techniques, we found that EEG activities of differing frequency were readily separated by PEFSA, while standard PCA often mixed activities with different frequencies into a single component. However, frequency-domain PCA gave a component whose spatial distribution well matched PEFSA results. PCA is superior to PEFSA for separating activities with overlapping frequencies but differing spatial distributions. Preservation of phase information is an advantage of PEFSA and PCA over topographic maps that represent only amplitude (or power) at a given frequency. PCA or PEFSA maps can serve as a starting point for source localization.

Spectral analysis of slow modulation of EEG amplitude and cardiovascular variables in subjects with postural tachycardia syndrome

OBJECTIVE

Previous studies have reported slow (<0.5 Hz) modulation of electroencephalographic (EEG) background amplitude and suggested that this reflects periodic neuronal activity in the brainstem, such as may be recorded from cardiovascular and respiratory centers in animals. We searched for a relationship between EEG amplitude modulation and modulation of simultaneously recorded cardiovascular variables and attempted to determine whether this relationship was altered in subjects with postural tachycardia syndrome (POTS).

METHODS

We recorded EEG, blood flow velocity in the middle cerebral artery (MCA), heart rate, respirations, and blood pressure from subjects with POTS and controls during head-up tilt. Time-frequency analysis of 0.512-s epochs of EEG was performed to determine peak alpha amplitude. Spectra were divided into 3 bands: ultraslow, middle, and respiratory.

RESULTS

EEG alpha amplitude modulation in all frequency bands was reduced in POTS subjects while supine. EEG modulation decreased in controls with head-up tilt but not in POTS subjects. Heart rate modulation in the respiratory frequency band decreased with head-up tilt and was significantly less (P<0.02) in ultraslow and respiratory frequency bands in POTS subjects after head-up tilt. Blood pressure and MCA flow velocity modulation in middle and respiratory bands increased with head-up tilt to a greater degree in POTS subjects. Blood pressure and MCA flow velocity modulation frequencies were moderately correlated, but correlations between EEG and cardiovascular variable modulation frequencies were generally low, being highest in the respiratory band but not statistically significant.

CONCLUSION

There are subtle differences in EEG amplitude modulation in subjects with POTS. Altered EEG amplitude modulation in POTS may reflect altered brainstem physiology in this disorder.

Abnormal corticomuscular coherence is associated with the small amplitude cortical myoclonus in Parkinson's disease

Coherence is the degree of time-locked correlation between two signals as a function of frequency. The purpose of this study was to test the following hypotheses: (1) corticomuscular coherence is abnormally increased in those Parkinson's disease (PD) patients with small amplitude cortical myoclonus, and (2) corticomuscular coherence peaks around the time of the myoclonus electromyographic (EMG) discharge. We studied Parkinson's disease patients with and without myoclonus and controls. The data were digitally collected and processed off-line with EMG rectification, creation of 511-msec epochs, Fast-Fourier transform, and coherence analysis. In the 12 to 30 Hz frequency band, but not at 30 to 60 Hz or above, coherence peaks were observed in the PD subjects with myoclonus that were significantly greater than in the control subjects (P < 0.001) and in PD subjects without myoclonus (P < 0.001). The abnormal coherence values are evidence for abnormal rhythmic activity in cortical motor areas in those Parkinson's disease patients with myoclonus. In combination with previous findings on back-averaging, our results show that this myoclonus occurs when neuronal populations are driven to an extreme amount of synchronous activity with higher corticomuscular coherence values. These results have mechanistic implications for cortical dysfunction in Parkinson's disease and for cortical myoclonus in general.

A general method for remontaging based on a singular value decomposition algorithm

The authors developed a general mathematic algorithm to convert any montage (referential, bipolar, or Laplacian) to any other by linear transformation. Input and output montages are described by matrices, and singular value decomposition is used to find the linear transformation. An error signal can be calculated from the input data to monitor remontaging validity. This algorithm also identifies output channels that cannot be obtained from the specified input. The authors tested this algorithm using an instrument that retrieves digitally encoded EEG data from videotape and produces signals in referential or bipolar form. They obtained good agreement when they compared referential and Laplacian data derived from bipolar output with the same montages calculated from referential output for the same EEG segment.

Role and limitations of routine and ambulatory scalp electroencephalography in diagnosing and managing seizures

The scalp electroencephalogram (EEG) is the cornerstone in the diagnosis and treatment of seizure disorders. The EEG, with its excellent temporal resolution, provides a direct measurement of cortical electrophysiology, revealing, for example, the presence of interictal epileptiform discharges that identify regions of an epileptogenic brain. We define the EEG characteristics of focal and generalized epileptiform discharges and provide evidence for their varying diagnostic importance in different patient populations. Identification of nonepileptiform EEG transients, such as wicket waves, small sharp spikes,rhythmic temporal theta activity, and 14- and 6-Hz positive bursts, that can be confused for epileptiform transients is emphasized. A final point is that the clinician must interpret EEG findings within the overall clinical context.

The relationship between quantitative T2 relaxometry and memory in nonlesional temporal lobe epilepsy

PURPOSE

We investigated the relationship between preoperative quantitative magnetic resonance imaging (MRI) T2 relaxometry and volumetry of the hippocampi and pre- and postoperative verbal memory in temporal lobectomy patients who had nonlesional temporal lobe epilepsy.

METHODS

Pre- and postoperative memory data based on the Logical Memory (LM) subtest of the Wechsler Memory Scale-Revised (WMS-R) and the 30-min delayed recall trial of the Rey Auditory Verbal Learning Test (AVLT) were obtained from 26 left and 15 right temporal lobectomy patients. Coronal MRI T2 maps were generated for these 41 temporal lobectomy patients as well as 61 control patients. Hippocampal T2 relaxation times and hippocampal volumes, converted to z scores using control group data, were correlated with neuropsychological performance in the patients.

RESULTS

In left temporal lobe-onset patients, high T2 in the left hippocampal body predicted higher LM performance after surgery. Asymmetrically high T2 in the left hippocampal body (i.e., the right-minus-left difference), compared with the right hippocampal body, also predicted higher LM performance after surgery. In right temporal lobe-onset patients, high T2 in the left hippocampal body predicted relatively lower AVLT performance after surgery. Multiple regression analysis in left temporal-onset patients revealed that high T2 in the left hippocampal body together with higher preoperative LM performance predict higher postoperative LM performance.

CONCLUSIONS

Our findings suggest that elevated (i.e., abnormal) hippocampal T2 signal is associated with memory ability (or hippocampal functional capacity) independent of MRI-determined hippocampal atrophy. Therefore, our findings support the use of quantitative T2 relaxometry as an independent predictor of verbal memory outcome in both left and right TLE patients who are candidates for temporal lobectomy.

Localization of the epileptic focus by low-resolution electromagnetic tomography in patients with a lesion demonstrated by MRI

Patients with medically intractable partial epilepsy and well-defined symptomatic MRI lesions were studied using phase-encoded frequency spectral analysis (PEFSA) combined with low-resolution electromagnetic tomography (LORETA). Ten patients admitted to the epilepsy monitoring unit with MRI-identified lesions and intractable partial epilepsy were studied using 31-electrode scalp EEG. The scalp electrodes were located in three-dimensional space using a magnetic digitizer and coregistered with the patient's MRI. PEFSA was used to obtain a phase-encoded scalp map for the ictal frequencies. The ictal generators were obtained from the scalp map using LORETA. In addition, the generators of interictal epileptogenic spikes were identified using time-domain LORETA. The LORETA generators were rostral to the MRI lesion in 87% (7/8) of patients with temporal lobe lesions, but all were located in the mesial temporal lobe in concordance with the patients' MRI lesions. In patients with frontal lobe epilepsy, the ictal generators at the time that the spectral power was maximal localized to the MRI lesions. Eight of 10 patients had interictal spikes, of which 4 were bilateral independent temporal lobe spikes. Only generators of the interictal spikes that were ipsilateral to seizure onset correlated with the ictal generators. LORETA combined with PEFSA of the ictal discharge can localize ictal EEG discharges accurately and improve correlation with brain anatomy by allowing coregistration of the ictal generator with the MRI. Analysis of interictal spikes was less useful than analysis of the ictal discharge.

Cortical potentials at the frequency of absolute wrist velocity become phase-locked during slow sinusoidal tracking movements

A 1-Hz rhythmic event-related potential was recorded at the scalp during performance of a 0.5-Hz tracking task. At cortical motor areas, negative peaks occurred 10-20 ms after peak tracking speeds. Analysis of single sweeps suggested that EEG phase was reset at initiation of the tracking motion and then maintained a constant relationship to wrist speed until task completion. Frequency analysis indicated that rhythm appearance in the averaged potential was predominantly due to phase-locking, because there was no tracking-related increase in 1 Hz amplitude within individual sweeps. While tracking, phase-locking was present over bilateral parieto-occipital and frontal regions, with a slight predominance at the contralateral frontal region. When subjects observed the target motion, phase-locking was localized to parieto-occipital regions. We suggest mental processes such as visual processing, visuomotor coordination and real-time motor planning are reflected in the pacing of localized cortical potential fluctuations.

Clinical neurophysiologic findings in patients with rapidly progressive familial parkinsonism and dementia with pallido-ponto-nigral degeneration

OBJECTIVE

To present clinical electrophysiologic studies performed on the pallido-ponto-nigral degeneration (PPND) family linked to chromosome 17q21-22.

METHODS

Nine patients from this kindred were studied with 11 electroencephalograms (EEGs), 4 electroencephalographic background frequency analysis (BFA) studies, 4 electromyographic recordings (EMGs) including nerve conduction studies (NCSs), 4 electromyographic multichannel surface recordings (MSRs), one pattern visual evoked potential (VEP) study and one median nerve somatosensory evoked potential (SEP) study.

RESULTS

EEGs revealed normal findings early in the disease and diffuse slowing which became more prominent with disease progression. BFA studies demonstrated rapid decrease in mean parietal frequencies with disease progression. EMGs and NCSs showed no abnormalities. MSRs revealed action myoclonus and a dystonic process. Long loop reflexes were absent in resting hand muscles. VEPs and SEPs were normal.

CONCLUSIONS

Clinical neurophysiologic studies were consistent with a cortical and subcortical degenerative process. With clinical deterioration, there is a progressive decline in the mean parietal frequency and background rhythms. Tremor studies were consistent with action myoclonus and a dystonic process and did not show parkinsonian features of resting tremor or agonist-antagonist cocontraction. There was no evidence of peripheral nerve involvement or slowing in central sensory pathways. Electrophysiologic findings are characteristic for this illness.

Scalp-recorded EEG localization in MRI volume data

Scalp-recorded EEG is a noninvasive and widely available tool for studying normal and dysfunctional human neurophysiology with unsurpassed temporal resolution. However, scalp-recorded EEG data is difficult to correlate with anatomy, and most current display and neural source estimation algorithms are based on unrealistic spherical or elliptical models of the head. It is possible to measure the positions of electrodes on the patient's scalp, and to register those electrode positions into the space of a high-resolution MRI volume, and to then use the patient-specific anatomy as the basis for display and estimation of neural sources. We use a surface matching algorithm to register digitized electrode and scalp surface coordinates to a three-dimensional MRI volume. This study uses fiducial markers in phantom and volunteer studies to quantitatively estimate the accuracy of the electrode registration method. Our electrode registration procedure is accurate to 2.21 mm for a realistic head phantom and accurate to 4.16 mm on average for five volunteers. This level of accuracy is considered within acceptable limits for clinical applications.

Spatial filtering of multichannel electroencephalographic recordings through principal component analysis by singular value decomposition

Principal component analysis (PCA) by singular value decomposition (SVD) may be used to analyze an epoch of a multichannel electroencephalogram (EEG) into multiple linearly independent (temporally and spatially noncorrelated) components, or features; the original epoch of the EEG may be reconstructed as a linear combination of the components. The result of SVD includes the components, expressible as time series waveforms, and the factors that determine how much each component waveform contributes to each EEG channel. By omission of some component waveforms from the linear combination, a new EEG can be reconstructed, differing from the original in useful ways. For example, artifacts can be removed and features such as ictal or interictal discharges can be enhanced by suppressing the remainder of the EEG. We developed a variation of this technique in which the factors that reconstruct the modified EEG from the original are stored as a matrix. This matrix is applied to multichannel EEG at successive times to create a new EEG continuously in real time, without redoing the time-consuming SVD. This matrix acts as a spatial filter with useful properties. We successfully applied this method to remove artifacts, including ocular movement and electrocardiographic artifacts. Removal of myogenic artifacts was much less complete, but there was significant improvement in the ability to visualize underlying activity in the presence of myogenic artifacts. The major limitations of the method are its inability to completely separate some artifacts from cerebral activity, especially when both have similar amplitudes, and the possibility that a spatial filter may distort the distribution of activities that overlap with the artifacts being removed.

The effect of reference-electrode choice on the spatial resolution of topographical potential maps in the discrimination of deep cerebral sources

Although scalp potential distributions do not uniquely determine the location and configuration of neural generators, they are important because they provide the necessary conditions that any hypothesized sources must satisfy and suggest a basis for testing alternate source hypotheses. One problem that could confound the correct interpretation of scalp potentials is the choice of reference electrode. Changing the reference may make activity patterns and waveform components appear and disappear (Pascual-Marqui et al. (1988) Int. J. Neurosci., 43: 237-249). The cortical imaging technique (CIT), a method for approximating potential fields on the cortical surface, was used to test the effects of the choice of reference electrode on these fields. Simulated and empirical evoked potential scalp-recorded referential data were mathematically analyzed for the case in which the reference (linked-ears) was arbitrarily assumed to be at zero potential, and the case in which the reference was the 'average' electrode, the arithmetic mean of all of the scalp-recorded voltages in the referential montage. The results for the two references were similar. This is encouraging because potential measurements relative to a point at infinity (zero potential) are never available and the assumption that any actual reference used for a recording is at zero potential is therefore suspect.

Long-term electroencephalographic monitoring for diagnosis and management of seizures

Long-term electroencephalographic (EEG) monitoring is the process of recording an EEG for a prolonged period in order to document epileptic seizures or other episodic disturbances of neurologic function. Indications for long-term EEG monitoring include diagnosis of a seizure disorder (epilepsy), classification of seizure types in patients with epilepsy, and localization of the epileptogenic region of the brain. Methods used for long-term EEG monitoring include prolonged analog or digital EEG, prolonged analog or digital ambulatory EEG, and prolonged analog or digital video-EEG monitoring with telemetry. Each of these methods has distinct advantages and disadvantages, particularly relative to storage, retrieval, and manipulation of data. Long-term EEG monitoring is useful in the management of patients with epilepsy and in the diagnosis of a seizure disorder. For most patients, inpatient long-term EEG monitoring is best performed in a specialized epilepsy-monitoring unit, which can provide a safe environment and both educational and psychosocial support. The choice of the most appropriate method of long-term monitoring for a specific clinical situation is best made by an epileptologist or a neurologist at an epilepsy center.

Routine EEG and temporal lobe epilepsy: relation to long-term EEG monitoring, quantitative MRI, and operative outcome

PURPOSE

To investigate the relation among routine EEG, long-term EEG monitoring (LTM), quantitative magnetic resonance imaging (MRI), and surgical outcome in temporal lobe epilepsy (TLE).

METHODS

We evaluated 159 patients with intractable TLE who underwent an anterior temporal lobectomy between 1988 and 1993. The epileptogenic temporal lobe was determined by ictal LTM. A single awake-sleep outpatient EEG with standard activating procedures was performed before LTM. EEGs were analyzed by a blinded investigator.

RESULTS

MRI scans showed unilateral medial temporal atrophy (109 patients) or symmetrical hippocampal volumes (50 patients). The surgically excised epileptogenic brain tissue revealed mesial temporal sclerosis, gliosis, or no histopathologic alteration. Routine EEG revealed temporal lobe epileptiform discharges in 123 patients. Routine EEG findings correlated with the temporal lobe of seizure origin (p < 0.0001) and the results of MRI volumetric studies (p < 0.0001). Interictal epileptiform discharges were seen only during LTM in 24 patients. Routine EEG was disconcordant with interictal LTM in another 20 patients. MRI-identified unilateral medial temporal lobe atrophy was a strong predictor of operative success (p < 0.0001). There was no significant relation between the routine EEG findings and operative outcome (p > 0.20).

CONCLUSIONS

Results of this study modified our approach in patients with TLE. Interictal epileptiform discharges localized to one temporal lobe on serial routine EEGs or during LTM may be adequate to identify the epileptogenic zone in patients with MRI-identified unilateral medial temporal lobe atrophy.

Interelectrode coherences from nearest-neighbor and spherical harmonic expansion computation of laplacian of scalp potential

Interchannel coherence is a measure of spatial extent of and timing relationships among cerebral electroencephalogram (EEG) generators. Interchannel coherence of referentially recorded potentials includes components due to volume conduction and reference site activity. The laplacian of the potential is reference independent and decreases the contribution of volume conduction. Interchannel coherences of the laplacian should, therefore, be less than those of referentially recorded potentials. However, methods used to compute the laplacian involve forming linear combinations of multiple recorded potentials, which may inflate interchannel coherences. WE compared 3 methods of computing the laplacian: (1) modified Hjorth (4 equidistant neighbors to each electrode), (2) Taylor's series (4 nonequidistant neighbors), and (3) spherical harmonic expansion (SHE). Average interchannel coherence introduced by computing the laplacian was less for nearest-neighbor methods (0.0207 +/- 0.0766) but still acceptable for the SHE method (0.0337 +/- 0.0865). Average interchannel coherence for simulated EEG (random data plus a common 10 Hz signal) was less for laplacian than for referential data because of removal of the common referential signal. Interchannel coherences of background EEG and partial seizure activity were less with the laplacian (any method) than with referential recordings. Laplacians calculated from the SHE do not demonstrate excessively large interchannel coherences, as have been reported for laplacians from spherical splines.

Electrocorticography and temporal lobe epilepsy: relationship to quantitative MRI and operative outcome

We investigated the relationship between electrocorticography (ECoG), quantitative magnetic resonance imaging (MRI), and surgical outcome in 165 patients with intractable nonlesional temporal lobe epilepsy (NLTLE). A standard mesial temporal resection was performed in all patients. Patients with an operative follow-up < 1 year were excluded from the study. The extent of the lateral temporal neocortex resection (LCR) was guided by ECoG and the side of surgery. The extent of the LCR was not predictive of seizure outcome in patients with or without hippocampal formation atrophy (p > 0.5). Patients undergoing a right anterior temporal lobectomy had a larger LCR (p < 0.0001), but the side of surgery was not of predictive value in determining seizure outcome (p > 0.1). The topography of the acute intracranial spikes did not correlate with operative outcome (p > 0.5) and was independent of hippocampal volumetric studies (p > 0.5). The postexcision ECoG was also shown not to be of prognostic importance (p > 0.5). Our results indicates that the extent of the lateral temporal cortical resection and the ECoG findings are not important determinants of surgical outcome in patients with NLTLE.

Hypoxic effect of exogenous insulin on normal and diabetic peripheral nerve

Insulin administration can cause or worsen experimental and human diabetic neuropathy ("insulin neuritis"). In this study, we tested the hypothesis that insulin administration impairs tissue oxygenation. We infused insulin under nonhypoglycemic conditions and evaluated its effect on endoneurial oxygen tension, nerve blood flow, and the oxyhemoglobin dissociation curve of peripheral nerve in normal and diabetic rats. Intravenous insulin infusion resulted in a dose-dependent reduction in endoneurial oxygen tension in normal nerves (from 26% at 0.04 U/kg insulin to 55% at 32 U/kg). The nerves of rats with streptozotocin-induced diabetes were resistant, but with control of hyperglycemia this susceptibility to the endoneurial hypoxic effect of insulin returned. The reduction in endoneurial oxygen tension regressed with glycosylated hemoglobin (Y = 53.8-2.7X, where Y = %reduction in endoneurial oxygen tension and X = HbA1; r = 0.87; P = < 0.001). Diabetes or insulin administration resulted in only minimal and physiologically insignificant alterations in the oxygen dissociation curve and 2,3-diphosphoglycerate of sciatic nerve. Instead, insulin administration resulted in a reduction in nerve nutritive blood flow and an increase in arteriovenous shunt flow. When the latter was eliminated by the closure of arteriovenous shunts (infusion of 5-hydroxytryptamine), endoneurial oxygen reverted to normal. These findings indicate a deleterious vasoactive effect of insulin and may explain the development of insulin neuritis.

Mathematical modeling of hydrogen clearance blood flow measurements in peripheral nerve

The hydrogen clearance technique of blood flow measurement often yields biexponential washout curves. In peripheral nerve, arteriovenous shunt vessels may clear hydrogen gas, causing the fast component of a biexponential curve. We simulated the washout of hydrogen from nerve tissue in the vicinity of a large shunt vessel by modeling the diffusion of hydrogen through tissue to the vessel and its removal by a network of capillaries. We then determined the fast and slow clearance rates and the relative weights or contributions of the fast and slow components and found that they are affected by all of the model parameters.

Alfentanil-induced epileptiform activity in patients with partial epilepsy

We performed a retrospective study investigating the effect of alfentanil hydrochloride on electrocorticography (ECoG) in 23 patients with intractable nonlesional partial epilepsy undergoing anterior temporal lobectomies at this institution. Alfentanil is a short-acting, parenteral, opioid analgesic with a rapid onset of action. Opioid drugs have the potential to induce hippocampal electrographic seizures. Pre-excision ECoG was obtained before and after the administration of 50 micrograms/kg of alfentanil. ECoG was performed using subdural strips placed on the lateral temporal surface and in the suprasylvian region and monopolar depth electrodes implanted into the amygdala and hippocampus. The surgically excised temporal lobes revealed cortical gliosis and varying degrees of hippocampal neuronal loss in all patients. A quantitative assessment of the effect of alfentanil on the ECoG was performed by measuring the frequency of interictal spikes. There was a significant increase in the mesial temporal lobe mean spike frequency after the administration of alfentanil (p < 0.001). One patient had an alfentanil-induced mesial temporal lobe electrographic seizure. Alfentanil did not have a significant effect on spike activity in the suprasylvian region (p = 0.500). Further studies will be necessary to determine the specificity of alfentanil activation in patients with partial seizures of temporal lobe origin.

Effects of antiepileptic drug treatment on the background frequency of EEGs in epileptic patients

The effect of changing antiepileptic drug concentrations within the therapeutic range on the EEGs of epileptic subjects was studied by quantitative EEG analysis. Twenty-seven patients had administration of one or more drugs discontinued on admission to the hospital for prolonged video/EEG monitoring, and drug levels were correlated daily with the simultaneous EEG background. Phenytoin, alone or in combination with other drugs, led to significant changes in the mean EEG background frequency and increased the percentage of power in the theta and delta bands. In the plasma ranges studied, carbamazepine, phenobarbital, and valproic acid did not lead to significant change in the EEG background frequency; however, the number of subjects taking these medications was small.

Mathematical modeling of time-dependent oxygen transport in rat peripheral nerve

We modeled time-dependent transport of oxygen in peripheral nerve. Simulation began with a steady-state oxygen tension field determined by capillary diameter and length, intercapillary distance, blood-flow velocity, oxygen consumption rate, and arterial oxygen tension. One of these parameters was assumed to change rapidly to new constant value, producing time-varying oxygen tensions. A monoexponential or biexponential function characterized the oxygen tension time variation. Rate constants of the slower exponential ranged from 0.017 sec-1 to 0.46 sec-1, implying minimal time lag in response of peripheral nerve oxygen tensions to alterations in blood flow, arterial blood oxygenation, or metabolic demands.