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Biomedical subjects

G Alarcon

Publications and source records attributed to G Alarcon.

At least 19 recordsLinked to original sources
PubMed2003

VNS in patients with previous unsuccessful resective epilepsy surgery: antiepileptic and psychotropic effects.

OBJECTIVES: To assess the efficacy of vagus nerve stimulation (VNS) in patients with medically and surgically intractable complex partial seizures (CPS). PATIENTS AND METHODS: Sixteen patients with previous temporal [15] and frontal [one] resections were treated with VNS between 1994 and 1999 at King's College Hospital, London, UK. Post-operative video-electroencephalogram telemetry had shown that CPS started from the operated side in 12 patients, contralaterally in three and bilaterally independently in one. RESULTS: Three patients (18.75%) had 50% or more reduction in seizure frequency, but one showed severe worsening of epilepsy, which remitted upon VNS discontinuation. The antiepileptic effect of VNS was not different with respect to the type of operation (anterior temporal lobectomy vs amygdalohippocampectomy), the side of operation, or the side of seizure onset. We observed psychotropic effects in two patients with post-ictal psychosis, in two others with depression, and in a child with severe behavioral disorder. CONCLUSIONS: VNS may have a rather limited antiepileptic role to play in patients with persistent seizures following epilepsy surgery, but may independently possess useful antipsychotic and mood-stabilizing properties.

Adolescent↗
PubMed2001

Sensitivity of recordings at sphenoidal electrode site for detecting seizure onset: evidence from scalp, superficial and deep foramen ovale recordings.

OBJECTIVES: Some authors have recently stressed that the position of the tip of sphenoidal electrodes plays a crucial role in their efficacy in detecting ictal onset. An opportunity to test this hypothesis is provided by recordings from the most superficial contacts of foramen ovale (FO) electrode bundles because these contacts are located at the FO, in a position equivalent to that of optimally located sphenoidal electrodes. To simplify wording, recordings obtained by superficial FO electrodes will hereafter be called sphenoidal recordings, although they have not been obtained with standard sphenoidal electrodes. The sensitivities of simultaneous scalp and sphenoidal recordings for detecting ictal onset have been compared with each other, and with a 'gold standard' provided by simultaneous deep intracranial FO recordings from the mesial aspect of the temporal lobe. METHODS: Three hundred and fourteen seizures obtained from 110 patients under telemetric presurgical assessment for temporal lobe epilepsy have been studied. Scalp electrodes included anterior temporal placements. All scalp electrodes were considered when identifying seizure onset but the anterior temporal electrodes were most frequently involved. RESULTS: Ictal onset time at sphenoidal and scalp recordings: initial ictal changes appeared simultaneously in scalp and sphenoidal recordings in 123 seizures (39.2%). Initial changes occurred earlier in sphenoidal recordings in 63 seizures (20.1%), whereas they were seen earlier on the scalp in 76 seizures (24.2%). Artefacts prevented the comparison between sphenoidal and scalp recordings in 16 seizures (5.1%) and no ictal changes were seen on the scalp and/or sphenoidal recordings in 36 seizures (11.5%). In most of the 63 seizures where ictal changes appeared earlier in sphenoidal recordings, a delayed ipsilateral scalp onset was seen as the signal amplitude increased or scalp changes could be identified retrospectively on the scalp with an onset which appeared simultaneous and ipsilateral to the initial sphenoidal changes. Sphenoidal recordings supplied additional information when compared to scalp recordings in only 22 seizures (7%): in 5 seizures with artefacts on the scalp, in 6 seizures with no changes on the scalp and in 11 seizures with discrepant laterality at onset. Congruence in laterality with respect to deep intracraneal FO recordings: of the 61 seizures with unilateral onset on the scalp, onsets at sphenoidal recordings and deep FO electrodes were ipsilateral in most cases. In only 3 of these 61 seizures (4.9%), sphenoidal recordings lateralized ipsilateral to the deep FO electrodes in the presence of a contralateral onset on the scalp. In 14 among the 122 seizures (11.5%) with bilateral asymmetrical onset on the scalp, sphenoidal recordings lateralized seizure onset ipsilateral to the deep FO electrodes in the presence of a contralateral scalp onset. Thus, when compared with scalp EEG, sphenoidal recordings increased laterality congruence with respect to deep FO electrodes in 17 seizures (5.4%). CONCLUSIONS: Extracranial electrodes located next to the FO at the sphenoidal electrode site yield an improvement over suitable surface electrodes in the identification of ictal onset in only 5.4-7% of seizures. Such improvement derives from the fact that the low amplitude signals often seen at seizure onset may show higher amplitude on sphenoidal than on scalp recordings.

Electroencephalography↗
PubMed2001

Multiple subpial transection in Landau-Kleffner syndrome.

We have considered multiple subpial transection (MST) as a treatment option for Landau-Kleffner syndrome (LKS) for the past 6 years. The effect of this technique on language and cognitive ability, behaviour, seizures, and EEG abnormalities is analysed here. Five children (4 males, 1 female; aged 5.5 to 10 years) underwent MST with sufficiently detailed pre- and postoperative data for analysis. Behaviour and seizure frequency improved dramatically after surgery in all children. Improvement in language also occurred in all children, although none improved to an age-appropriate level. All five had electrical status epilepticus in sleep (ESES) before surgery, which was eliminated by the procedure. One child has had an extension of his MST due to the recurrence of ESES and accompanying clinical deterioration with good effect. An attempt is made to set the effect of MST against the natural history of the condition. MST is an important treatment modality in LKS, although the timing of this intervention and its effect on final language outcome remains to be defined.

Cerebral Cortex↗
PubMed2001

Relevance of residual histologic and electrocorticographic abnormalities for surgical outcome in frontal lobe epilepsy.

PURPOSE: To estimate the significance of residual electrocorticographic and neuropathologic abnormalities on seizure control after surgery for frontal lobe epilepsy with the purpose of determining their relevance in deciding the extent of the surgical procedure. METHODS: The presence of epileptiform discharges in intraoperative electrocorticograms (ECoGs) and the nature and extent of neuropathologic abnormalities were reviewed for 35 patients who underwent frontal lobe resections for the treatment of epilepsy at our institution. The relations between surgical outcome and presence of the following features were studied: (a) presence of abnormal tissue at the limits of the resection; (b) presence of sporadic spikes and seizure patterns in the preresection ECoG; (c) their abolition in the postresection ECoG; and (d) the topography of residual discharges with respect to the margins of the resection. RESULTS: On neuropathologic examination, 18 patients showed focal cortical dysplasia (CD), and 17 showed other abnormalities (non-CD). Ten CD patients and 11 non-CD patients experienced a favourable outcome. Seizure patterns were significantly more common in patients with focal cortical dysplasia than in those without, with a sensitivity of 94% and a specificity of 75%. Abolition of seizure patterns was associated with a favourable surgical outcome (p = 0.031). Abolition of sporadic spikes or their presence in the postresection ECoG did not influence outcome. There was no clear relation between outcome and location of residual sporadic discharges. Seizure patterns persisted in the postresection ECoG in three CD patients, were located at the margins of the resection in all three, and these patients had a poor outcome. Incomplete removal of abnormal tissue was not associated with a poorer outcome in either patient group or in the complete sample. CONCLUSIONS: Seizure patterns were significantly more common in patients with cortical dysplasia, and their abolition on postresection ECoG recordings was associated with a favourable surgical outcome. Persistence of sporadic ECoG spikes and incomplete removal of histologic abnormalities did not affect outcome significantly.

Adolescent↗
PubMed2000

A simple algorithm for a digital three-pole Butterworth filter of arbitrary cut-off frequency: application to digital electroencephalography.

Algorithms for low-pass and high-pass three-pole recursive Butterworth filters of a given cut-off frequency have been developed. A band-pass filter can be implemented by sequential application of algorithms for low- and high-pass filters. The algorithms correspond to infinite impulse-response filters that have been designed by applying the bilinear transformation to the transfer functions of the corresponding analog filters, resulting in a recursive digital filter with seven real coefficients. Expressions for filter coefficients as a function of the cut-off frequency and the sampling period are derived. Filter performance is evaluated and discussed. As in the case of their analog counterparts, their transfer function shows marked flattening over the pass band and gradually higher attenuation can be seen at frequencies above or below the cut-off frequency, with a slope of around 60 dB/decade. There is a 3 dB attenuation at the cut-off frequency and a gradual increase in phase shift over one decade above or below the cut-off frequency. Low-pass filters show a maximum overshoot of 8% and high-pass filters show a maximum downwards overshoot of approximately 35%. The filter is mildly under-damped, with a damping factor of 0.5. On an IBM 300GL personal computer at 600 MH with 128 MB RAM, filtering time with MATLAB 5.2 running under Windows 98 is of the order of 50 ms for 60000 samples. This will be adequate for on-line electroencephalography (EEG) applications. The simplicity of the algorithm to calculate filter coefficients for an arbitrary cut-off frequency can be useful to modern EEG laboratories and software designers for electrophysiological applications.

Algorithms↗
PubMed1998

Ictal oroalimentary automatisms with preserved consciousness: implications for the pathophysiology of automatisms and relevance to the international classification of seizures.

A patient showing seizures presenting ictal automatisms with preserved consciousness is reported. A 30-year-old, right-handed man with normal development and without family history of epilepsy was referred for surgical treatment of epilepsy. At 15 he began to have seizures, starting with an epigastric aura, occasionally developing automatisms (lip-smacking, chewing), sometimes followed by tonic-clonic convulsions. At the time of referral, he averaged six convulsive seizures per year and one nonconvulsive per week. His sleep EEG showed sharpened slow activity over the right anterior quadrant magnetic resonance imaging (MRI) showed a benign lesion in the mesial aspect of the right occipital lobe. Simultaneous video monitoring and intracranial EEG with subdural strips recording from the right temporal and occipital lobes was undertaken. During one seizure, he had pronounced oroalimentary automatisms while holding a conversation with a technician, answering her questions, and explaining details of his seizures. Memory of this event was preserved. At seizure onset, spike activity was seen at the mesial occipital strips. At midseizure, high-voltage sharpened delta was seen throughout the right hemisphere. Left-sided scalp electrodes remained relatively uninvolved. The lesion, a dysembryoplastic neuroepithelial tumour was removed. Surgery was followed by abolition of seizures described. Because it is agreed that complex partial seizures require impaired consciousness, a history of automatisms with retained consciousness usually suggests nonepileptic attacks. This case suggests that automatisms in epileptic seizures can take place with minimal loss of consciousness, particularly if there is widespread but unilateral involvement. The need for a revision of the International Classification is suggested.

Consciousness↗
PubMed1998

Predictive value of interictal epileptiform discharges during non-REM sleep on scalp EEG recordings for the lateralization of epileptogenesis.

PURPOSE: EEG recording during sleep is widely used in the assessment of epilepsy, particularly in candidates for surgery, yet the diagnostic value of this procedure is not well established. We evaluated the predictive reliability of interictal epileptiform discharges (IEDs) for localization in presurgical patients with temporal lobe epilepsy (TLE) during non-REM sleep. METHODS: Preoperative scalp EEG recordings with waking and sleep states were assessed in 83 patients with TLE in whom localization of the epileptogenic zone was subsequently confirmed by successful surgical treatment (patient seizure-free >1 year). RESULTS: The accuracy of EEG recordings for prediction of lateralization significantly changed from 51.8% during waking to 78.3% during sleep. After exclusion of patients who showed no discharges, the predictive value changed from 74.1 to 86.7%. However, in patients in whom the waking scalp EEG lateralized incorrectly, no improvement in reliability was achieved by sleep recording. CONCLUSIONS: Our results suggest that IEDs occurring in non-REM sleep provide more accurate information for lateralization of epileptogenesis than do those occurring during waking. This gain of diagnostic information was obtained in patients who showed either bilateral or no discharges in waking records, because unilateral discharges arising de novo in sleep were always correctly lateralizing. On the other hand, in patients who showed unilateral discharges in the awake state, whether ipsilateral or contralateral to the epileptogenic zone, the findings were generally unchanged during sleep.

Adult↗
PubMed1998

Interictal regional slow activity in temporal lobe epilepsy correlates with lateral temporal hypometabolism as imaged with 18FDG PET: neurophysiological and metabolic implications.

OBJECTIVES: The phenomenon of interictal regional slow activity (IRSA) in temporal lobe epilepsy and its relation with cerebral glucose metabolism, clinical data, MRI, and histopathological findings was studied. METHODS: Interictal 18F-fluorodeoxyglucose positron emission tomography (FDG PET) was performed under continuous scalp EEG monitoring in 28 patients with temporal lobe epilepsy not associated with intracranial foreign tissue lesions, all of whom subsequently underwent resective surgery. Regions of interest (ROIs) were drawn according to a standard template. IRSA was considered lateralised when showing a 4:1 or greater ratio of predominance on one side. RESULTS: Sixteen patients (57%) had lateralised IRSA which was always ipsilateral to the resection and of maximal amplitude over the temporal areas. Its presence was significantly related to the presence of hypometabolism in the lateral temporal neocortex (p=0.0009). Logistic regression of the asymmetry indices for all measured cerebral regions confirmed a strong association between IRSA and decreased metabolism of the posterior lateral temporal neocortex only (p=0.009). No significant relation could be shown between slow activity and age at onset, duration of the epilepsy, seizure frequency, and MRI evidence for hippocampal atrophy. Furthermore, IRSA was not specifically related to mesial temporal sclerosis or any other pathology. CONCLUSIONS: Interictal regional slowing in patients with temporal lobe epilepsy not associated with a mass lesion is topographically related to the epileptogenic area and therefore has a reliable lateralising, and possibly localising, value. Its presence is irrelevant to the severity or chronicity of the epilepsy as well as to lateral deactivation secondary to neuronal loss in the mesial temporal structures. Although slow EEG activity is generally considered as a non-specific sign of functional disturbance, interictal regional slowing in temporal lobe epilepsy should be conceptualised as a distinct electrographic phenomenon which is directly related to the epileptogenic abnormality. The strong correlation between interictal regional slowing and lateral temporal hypometabolism suggests in turn that the second may delineate a field of reduced neuronal inhibition which can receive interictal and ictal propagation.

Adolescent↗
PubMed1997

The Maudsley Family Study, II: Endogenous event-related potentials in familial schizophrenia.

Auditory event-related potentials (ERPs) were obtained from 33 schizophrenics, from 16 families multiply affected with schizophrenia, 57 of their non-schizophrenic first-degree relatives and 32 unrelated healthy controls. Transmission of schizophrenia in these families appeared unilinear with one parent only transmitting the disorder. Consequently, we were able to identify 10 presumed obligate carriers and study eight of them. Schizophrenic patients showed latency prolongation and amplitude reduction of the N100, N200 and P300 waves compared to controls. Their relatives showed similar abnormalities compared to controls. There was a trend for bimodal distribution of the P300 latency in the relatives, with 20 (35%) of them falling outside two standard deviations of the mean of the controls. These included six of the eight obligate carriers. Our results suggest that ERP abnormalities may serve as markers of genetic vulnerability in schizophrenia and may be useful in genetic linkage studies.

Adult↗
PubMed1997

Origin and propagation of interictal discharges in the acute electrocorticogram. Implications for pathophysiology and surgical treatment of temporal lobe epilepsy.

Although acute electrocorticography (ECoG) is routinely performed during epilepsy surgery there is little evidence that the extent of the discharging regions is a useful guide to tailoring the resection or that the findings are predictive of outcome or pathology. Patterns of discharge propagation have, however, rarely been considered in assessing the ECoG. We hypothesize that regions where discharges show earliest peaks ('leading regions') are located in the epileptogenic zone, whereas sites in which late, secondary, propagated activity occurs have less epileptogenic potential and do not need to be excised. To allow intraoperative topographic ECoG analysis, a computer program has been developed to identify leading regions and the sites showing greatest rates or amplitudes of spikes. Their topography has been compared retrospectively with pathology and seizure control in 42 consecutive patients following temporal lobe surgery. Leading regions were most often found in the hippocampus, the subtemporal cortex and the superior temporal gyrus. The most common propagation patterns were from hippocampus to subtemporal cortex and vice versa. There was no association between seizure outcome and the location of regions with greatest incidence or amplitude of spikes or location of leading regions. There was, however, a strong and significant association between poor outcome and non-removal of leading regions other than those in the posterior subtemporal cortex. All leading regions (other than posterior subtemporal) were resected in 27 patients of whom 25 had a favourable outcome. Leading regions (other than posterior subtemporal) remained in 14 patients of whom only four had a good outcome. One patient had no epileptiform activity in the ECoG and good outcome. Persistent posterior subtemporal leading regions remained in nine subjects; all had favourable outcome (Grades I or II) but only three were seizure free. These results suggest that: (i) interictal epileptiform discharges may originate from a complex interaction between separate regions, resulting in propagation and recruitment of neuronal activity along specific neural pathways; (ii) removal of all discharging areas appears unnecessary to achieve seizure control provided that leading regions (other than posterior subtemporal) are removed; and (iii) identification of such leading regions could be used to tailor resections in order to improve seizure control and reduce neurological, neuropsychological and psychiatric post-surgical morbidity.

Adolescent↗
PubMed1995

Power spectrum and intracranial EEG patterns at seizure onset in partial epilepsy.

Intracranial electroencephalographic patterns of seizure onset during preoperative assessment with intracerebral and subdural electrodes have been correlated with surgical outcome in 15 patients with partial epilepsy assessed for surgery. The presence and topography of electrodecremental events, high frequency activity, irregular sharp waves intermixed with slow activity, spike-wave activity and rhythmic ictal transformation at seizure onset were studied in a total of 78 complex partial seizures. Raw traces from intracerebral and subdural recordings were assessed visually in conjunction with changes in the following spectral variables (calculated for consecutive 1.28 sec epochs): amplitude (sum of amplitude of all components within a frequency band), activity, mobility, and complexity. The time course of these variables during preictal and ictal periods was displayed and assessed. This technique proved effective for detecting low-amplitude high-frequency activity and subtle electrodecremental events. It was concluded that: (a) most patients (12/15) showed early electrodecremental events, generalised or local, mainly involving frequencies below 40 Hz; (b) generalised electrodecremental events at onset did not imply poor outcome; (c) localised high-frequency activity, between 20 and 80 Hz, was associated with a good outcome.

Adolescent↗
PubMed1994

Fractal analysis of electroencephalographic signals intracerebrally recorded during 35 epileptic seizures: evaluation of a new method for synoptic visualisation of ictal events.

Traditional electroencephalography (EEG) produces a large volume display of brain electrical activity, which creates problems particularly in assessment of long periods of intracranial, stereoelectroencephalographic (SEEG) recording. A method for fractal analysis that describes 100 SEEG data points in terms of a single estimate of fractal dimension (1 < FD < 2) is reported; the central processing unit time costs amount to approximately 2 min/Mbyte of input signal (using a Sun SPARCstation LX). The diagnostic sensitivity of this method, applied to quantification and synoptic visualisation of SEEG signals recorded during 35 epileptic seizures in 7 patients, is evaluated. It is found that the method consistently defines ictal onset in terms of rapid relative increase in FD across several channels. Clinically severe seizures are characterised by more intense and generalised ictal changes in FD than clinically less severe events. For all 7 patients, and for 75% of individual seizures, "fractal diagnoses" of anatomically defined ictal onset zone coincided closely with ictal onset zone independently determined by inspection of traditional EEG displays of the same data. We conclude that the method is a computationally feasible way to achieve substantial reduction in the volume of SEEG data without undue loss of diagnostically important information in the primary signal.

Brain↗
PubMed1994

Intracerebral propagation of interictal activity in partial epilepsy: implications for source localisation.

The hypothesis that focal scalp EEG and MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The time course and spatial distribution of interictal activity recorded simultaneously by surface electrodes and by electrodes next to mesial temporal structures in 12 patients being assessed for epilepsy surgery have been studied to estimate the degree of confinement of neural activity present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of interictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlations between the amplitudes of deep and surface recordings, together with previous reports on the amplitude of scalp signals produced by artificially implanted dipoles suggest that the ratio of deep to surface activity recorded during interictal epileptiform activity on the scalp is around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but is generated in the underlying neocortex. Also, time delays of up to 220 ms recorded between interictal paroxysms at different recording sites show that interictal epileptiform activity can propagate neuronally within several milliseconds to relatively remote cortex. Large areas of archicortex and neocortex can then be simultaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres that trigger local phenomena which in turn give rise to sharp/slow waves or spikes, and (3) propagation along the neocortex. The low ratio of deep-to-surface signal on the scalp and the simultaneous activation of large neocortical areas can yield spurious equivalent dipoles localised in deeper structures. Frequent interictal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surface is independent of their capacity to trigger seizures. It is concluded that: (1) the deep-to-surface ratios of electromagnetic fields from deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate for surgical assessment; (3) the correct localisation of the onset of interictal activity does not necessarily imply the onset of seizures in the region or in the same hemisphere. It is suggested that, until volume conduction and neurophysiological propagation can be distinguished, semiempirical correlations between symptomatology, surgical outcome, and detailed presurgical modeling of the neocortical projection patterns by combined MEG, EEG, and MRI could be more fruitful than source localization with unrealistic source models.

Adult↗
PubMed1993

MEG and EEG in epilepsy: is there a difference?

Interictal epileptiform activity recorded by scalp EEG, foramen ovale electrodes and MEG is discussed. Gross differences in waveform between the electric and magnetic records are discussed in the light of intracranial depth recordings.

Electroencephalography↗
PubMed1992

A new technique for fractal analysis applied to human, intracerebrally recorded, ictal electroencephalographic signals.

Application of a new method of fractal analysis to human, intracerebrally recorded, ictal electroencephalographic (EEG) signals is reported. 'Frameshift-Richardson' (FR) analysis involves estimation of fractal dimension (1 < FD < 2) of consecutive, overlapping 10-s epochs of digitised EEG data; it is suggested that this technique offers significant operational advantages over use of algorithms for FD estimation requiring preliminary reconstruction of EEG data in phase space. FR analysis was found to reduce substantially the volume of EEG data, without loss of diagnostically important information concerning onset, propagation and evolution of ictal EEG discharges. Arrhythmic EEG events were correlated with relatively increased FD; rhythmic EEG events with relatively decreased FD. It is proposed that development of this method may lead to: (i) enhanced definition and localisation of initial ictal changes in the EEG presumed due to multi-unit activity; and (ii) synoptic visualisation of long periods of EEG data.

Algorithms↗