EEG and neuroimaging localization in partial epilepsy

Pre-surgical evaluation and surgical treatment in children with extratemporal epilepsy

INTRODUCTION

This review summarizes some patterns of pre-surgical evaluation and surgical treatment of extratemporal epilepsy in pediatric patients with medically refractory seizures, whose ictal behavior is variable. The most effective treatment for intractable partial epilepsy is a focal cortical resection with excision of the epileptogenic zone (the area of ictal onset and initial seizure propagation). This might be risky, though, in the case of a widespread lesion, sometimes encroaching one or more lobes, given the risk to the functional cerebral cortex. An anterior temporal lobectomy might prove more effective then in preventing seizures with fewer potential complications. If partial extratemporal epilepsy is associated with pharmaco-resistant seizures, the preoperative evaluation and operative strategy are determined according to the epileptogenic zone and to the relationship between a substrate-directed disorder and eloquent areas. The pediatric treatment of extratemporal epilepsy is aimed at controlling the seizures, avoiding morbidity, and improving the patient's quality of life through psychosocial integration. Since the immature brain is more plastic than when mature, the recovery of functions after surgery is greater in children than in adults.

RECOMMENDATION

Early surgery is recommended for children with intractable epilepsy, and is now accepted as an important therapeutic modality also for children with chronic epilepsy.

CONCLUSION

Technological advances in the last two decades, mainly in neuroimaging, have led many medical centers to consider surgical treatment of epilepsy, accuracy being granted by MRI-based neuronavigation systems-an interface between the lesion seen in the preoperative magnetic resonance imaging (MRI) and the operative field, often invisible to the surgeon.

Reversible focal MRI abnormalities due to status epilepticus. An EEG, single photon emission computed tomography, transcranial Doppler follow-up study

We demonstrate clinical data and findings of MRI, transcranial Doppler (TCD), single photon emission computed tomography (SPECT) and electroencephalography (EEG) in an 8 month follow-up study of a 15 year old girl who developed focal status epilepticus with sensory and visual illusions. EEG showed right temporal and occipital seizure activity and attenuation of the alpha activity with right predominance. MRI showed a right temporo-parietal hyper signal on the T2 weighted images involving the cortex with sulcal effacement. MRI-angiography suggested insufficient flow in the right transverse sinus. TCD detected an elevated flow velocity in the ipsilateral middle cerebral artery during status epilepticus, corresponding to an increased perfusion of the epileptic area revealed by SPECT. After normalization of the TCD finding, the MRI detected persistent cortical abnormality beyond the 70th day after admission. MRI normalized on the 103rd day of follow-up. Serial EEG frequency analysis demonstrated the recovery of alpha peak frequency on the left side, but the attenuation of rhythmic signals remained persistent on the right. In our case, the restitution of postictal EEG lag behind the consolidation of MRI signal abnormality.

Initial estimation methods for dipole modeling in localization of epileptogenic focus

Although great sensitivities to initial estimates is an inherent feature of iterative dipole optimization algorithms, the study of better initial estimates has been neglected. For convergence to a correct solution, the initial estimates should be extremely close to the desired solution and be attributed to only a single dipole focus. To alleviate the interference of background and multiple foci, the singular value decomposition (SVD) technique is used initially to extract the dominant component of the EEG spike for dipole localization. By observing the three-dimensional topographic mapping, the initial estimates of the dipole parameter set can be computed from the intersection between the null potential plane and from the peak and valley potentials. This work also compares initial estimations of simulation data, including noisy data, noisy data with SVD process and noise-free data. Experimental results confirm that good initial estimates for the dipole parameters are necessary to ensure rapid convergence to the correct solution.

Economic impact of replacing CT with MR imaging for refractory epilepsy

PURPOSE

To evaluate the economic costs of using computed tomography (CT) vs. magnetic resonance (MR) imaging in the preoperative evaluation of refractory epilepsy patients.

METHODS

Preoperative CT and MR imaging findings from 117 patients who underwent surgery for medically refractory epilepsy during a 3.5-year period were reviewed. Cost savings were based on the paradigm that intracranial electroencephalogram monitoring (costing about $50,000) would have been necessary for preoperative localization of the epileptogenic zone in those patients without positive imaging findings. Savings attributed to replacing CT with MR were based on patients with positive MR and normal CT. A similar paradigm was used to calculate savings for replacing MR with CT. National savings were based solely on patients with neoplasms or vascular lesions because paradigms for other lesions vary considerable depending on institutional philosophy.

RESULTS

Replacing CT with MR imaging would have eliminated preoperative intracranial electrode procedures in 29 of 117 patients, with potential savings of $1,450,000 at our institution. In the 37 patients with neoplastic or vascular substrates, MR would have eliminated 10 invasive electrode procedures with estimated savings of $0.5 million institutionally and $3 to $4 million per year nationally. There were no cases to support replacing MR with CT.

CONCLUSION

Replacing CT with MR decreases health costs associated with preoperative evaluation of intractable epilepsy requiring surgical amelioration.

EEG investigation of temporal lobe epilepsy

We report the results of the study assessing the role of electroencephalogram (EEG) in the evaluation of patients with temporal lobe epilepsy (TLE). A prospective interictal EEG study was performed in 80 patients with TLE, and the results were compared with those of neuroimaging magnetic resonance imaging (MRI) and computed tomography (CT). All patients had interictal scalp-recorded electroencephalographic monitoring with a full array of electrodes placed according to the International 10-20 Placement System, CT and MRI. Scalp EEG had a success rate of 70% in TLE patients, this rate was 50% for MRI and 15% for CT. Epileptiform EEG abnormalities were unilateral in 25 (31%) and bilateral in 31 (39%) patients. In 56% of patients with unilateral interictal activity and 97% of patients with bilateral interictal activity, epileptiform activity was localized at the temporal electrodes. The wave morphology which we most frequently saw in our study was the sharp, sharp-slow wave or spike, or spike-wave. A correlation was observed between the focal MRI-CT abnormalities and the EEG findings. We found abnormal imaging incidence in patients with unilateral EEG findings to be significantly greater than in patients with bilateral EEG findings (chi 2 = 4.62, p = .032). EEG showed abnormality in 28 (70%) of 40 patients whose neuroimaging (NI) tests were found abnormal and also did in 70% of 40 patients whose NI tests were normal. In our study EEG has remained as the most efficient test in the localization of an epileptogenic focus.

Usefulness of ictal and interictal 99mTc ethyl cysteinate dimer single photon emission computed tomography in patients with refractory partial epilepsy

PURPOSE

Ictal perfusion single photon emission computed tomography (SPECT), using HMPAO, has been shown to localize epileptic foci in approximately 90% of studies. Unfortunately, HMPAO decomposes rapidly, precluding the performance of ictal studies. Ethyl cysteinate dimer (ECD) is a SPECT perfusion agent recently approved by the Food and Drug Administration. After preparation, this compound is stable for approximately 6 h. facilitating the performance of ictal studies.

METHODS

In a prospective, open-label, uncontrolled, non randomized study, we evaluated the potential benefits of the use of 99mTc-ECD SPECT for lateralization of the epileptic focus. Ten consecutive adult epilepsy surgery candidates were studied with ictal and interictal 99mTc-ECD SPECT.

RESULTS

The mean delay between seizure onset and ictal SPECT injection was 23.2 s. The mean seizure duration was 84.1 s. Ictal studies agreement between the epilepsy focus and area of hyperperfusion was evident in 8 of 10 cases. In one case, SPECT was lateralized in a patient with bilateral temporal lobe epilepsy (TLE); however, hyperperfusion was observed on the same side of that particular seizure. In another case, there was location disagreement. Interictal SPECT showed focal hypoperfusion in three cases.

CONCLUSIONS

99mTc-ECD proved to be an optimal tracer for ictal studies. Although this is a small series, the results of ictal and interictal findings using 99mTc-ECD are similar to those reported with 99mTc-HMPAO. Because 99mTc-ECD has a longer decomposition time, true ictal studies are easier to obtain. This new tracer will probably allow the use of ictal SPECT to become widely accepted in most epilepsy centers.

Intractable epilepsy in children

Although most children with epilepsy have a good prognosis, a small but significant minority have seizures that either do not respond to conventional antiepileptic drugs (AEDs) or have significant adverse reactions to AEDs. Many children may benefit from epilepsy surgery. Surgical treatment of epilepsy is becoming a well-established therapy for infants and young children with severe, medically intractable seizures. As in older children and adults, the presurgical evaluations of possible surgical candidates typically consist of a detailed history, neurologic and neuropsychologic examination, and anatomic and functional neuroimaging. The "gold standard" test, however, is the recording of ictal events by using simultaneous EEG and videomonitoring. Although temporal lobe resection is the most commonly performed surgery in older children and adults, nontemporal lobe resection, corpus callosotomy, and hemispherectomy are commonly performed in younger children. Efficacy of surgery in children compares favorably with results from adult patients. In addition, because the immature brain is more plastic than the mature brain, recovery of function is often greater after surgery in children than in adults. Early surgery in children with intractable epilepsy is recommended.

Correlation of 99mTc-HMPAO SPECT with EEG monitoring: prognostic value for outcome of epilepsy surgery in children

Sixteen children who had focal cortical resections for medically intractable epilepsy were preoperatively evaluated with 99mtechnetium-labelled hexamethylpropyleneamineoxime single photon emission computed tomography (99mTc-HMPAO SPECT). Video-EEG monitoring was performed in all patients. Outcome was assessed according to the criteria of Engel et al. [1], at a mean follow up length of 13.4 +/- 8.7 months, in all patients. Interictal SPECT showed appropriate localization in 11/15 cases, of whom nine had a class 1 outcome and two had class 2 and 4 outcomes. Interictal SPECT did not correlate with ictal EEG in 4/15 patients, of whom two had a class 1 outcome, and two had class 3 and 4 outcomes. Two postictal studies obtained in group I showed good correlation with the area of ictal EEG onset, and both patients had a class 1 outcome. Interictal HMPAO SPECT imaging, when positively correlated with the ictal EEG focus or with the site of surgery determined by other means, may have prognostic value for outcome of cortical resections for epilepsy in children. The use of ictal and post-ictal studies shows promise for further improving prognostic information in this population.

SPECT and epilepsy with continuous spike waves during slow-wave sleep

Ten cases of epilepsy with continuous spike waves in slow-wave sleep (CSWS) were evaluated using single photon emission computed tomography (SPECT); in eight patients the EEG paroxysmal abnormalities showed a predominant localization. SPECT carried out using 99mTc-HMPAO allows study of cerebral blood flow (CBF); the examination was performed during phases of drowsiness and the results compared to the EEG data. In four cases SPECT revealed areas of low CBF in sites corresponding to those of the prevalent EEG discharges; in two cases the areas of hypoperfusion did not correspond to those indicated by the EEG; lastly, in four cases SPECT results were negative. The areas of hypoperfusion were predominantly located in the frontal, temporal, and parietal regions. Furthermore, the percentage of positive SPECT results was significantly higher (five cases out of six) in the group in which the CSWS phase was prolonged for at least 1 year, compared to the group in which this phase lasted less than 1 year. Thus, in this type of epilepsy, SPECT reveals focal cortical areas of decreased CBF which correlated generally to the predominant sites of EEG abnormalities. A longer duration of the CSWS phase seems to be associated with a more significant cortical disorder, documented by the presence of areas of hypoperfusion.

Epilepsy and SPECT

SPECT studies for rCBF imaging in clinical epilepsy are reviewed divided into interictal, ictal and postictal SPECT studies in partial and generalized epilepsies, as compared with the corresponding PET studies, and at times referring to SPECT studies in experimental models of epilepsy. All of the interictal, ictal and postictal SPECT scans in partial epilepsy are useful for the regional determination of epileptic foci, in view of the high appearance rates of focal abnormalities on these SPECT scans and good correlation with epileptic EEG foci. Mechanisms underlying the focal abnormality on interictal SPECT scans (i.e., focal hypoperfusion) image are complicative, and therefore, studies concerning the underlying mechanisms are reviewed with regard to correlations of the SPECT abnormality with the following various clinical factors: focal organic lesions on X-ray CT or MRI scans, frequencies of clinical seizures or interictal seizure discharges on EEGs, background EEG activity, propagation of epileptic activity, antiepileptic drugs, and cognitive functions. Further, new neurochemical SPECT studies using 123I-Iomazenil for benzodiazepine receptor imaging in clinical epilepsy are reviewed and compared with earlier SPECT studies for rCBF imaging with regard to their utility in regional determination of epileptic foci. In addition, the few SPECT or PET studies available on epileptic psychosis are reviewed.

Epilepsy surgery: non-invasive versus invasive focus localization. What is needed from the neurosurgical point of view

This paper discusses the place of non-invasive and invasive methods of investigation in the selection of patients for temporal lobe resection. Two series from the same department are compared and the relationship of the outcome to pathology is also discussed.

Interictal HM-PAO SPECT: a routine investigation in patients with medically intractable complex partial seizures?

Single photon emission computed tomography (SPECT) is increasingly being used as an adjunctive technique in the localisation of epileptogenic foci prior to surgery. As yet, few studies have been undertaken to establish the clinical associations of areas of reduced cerebral perfusion. Sixty-three consecutive patients (15 male, 48 female; median age 30 years, range 14-57 years) with medically intractable complex partial seizures (median/month 8, range 1-36) were investigated as outpatients. All had normal high resolution computerised X-ray tomography (CT) of brain. Twenty-seven (47%) exhibited significant perfusion defects on SPECT scanning with a rotating gamma camera system using Tc-99 HM-PAO. There were no significant differences between patients with normal and abnormal scans in terms of age at scanning or at onset of epilepsy or number of seizures. Patients with perfusion defects did, however, have longer median histories of epilepsy than those with normal scans (normal: 10 years, abnormal: 22 years; P = 0.01). Patients with abnormal scans were no more likely to have suffered febrile convulsions in early childhood. The correlation of abnormal SPECT scans with routine surface EEG recordings was poor with only 41% of cases showing clear agreement between the site of hypoperfusion and focal epileptic activity. It is not yet possible to predict clinically those patients who will exhibit perfusion defects on interictal TC-99 HM-PAO SPECT scanning.