Frank W. Sharbrough

Correlation of Continuous Electroencephalograms With Cerebral Blood Flow Measurements During Carotid Endarterectomy

During an 11-month period, 81 endarterectomies under a carefully controlled level of general anesthesia were monitored with continuous electroencephalograms (EEG) and intermittent regional cerebral blood flow (CBF) measurements. There was a high correlation between the CBF (milliliter per 100 gm per minute) during carotid occlusion and alterations in the EEG: no EEG change was seen with the flow above 30 ml/100 gm brain per minute, major changes were not seen with a flow between 18 and 30 ml, and changes invariably occurred with a flow below 17 ml. The degree of EEG change reflected the severity of flow reduction but was always reversible with the placement of a shunt. The EEG at the termination of the surgery corresponded with the patients neurological state in that all EEG tracings were normal or unchanged as compared to the preoperative tracing and no neurological worsening occurred in any patients studied. The EEG is a valuable monitoring technique that indicates when a shunt is required and informs the surgeon of the state of cerebral function not only during occlusion but also throughout the entire operative procedure.

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.

Nonpathologic Factors Influencing Brainstem Auditory Evoked Potentials

(1978). Nonpathologic Factors Influencing Brainstem Auditory Evoked Potentials. American Journal of EEG Technology: Vol. 18, No. 4, pp. 177-209.

Cerebral hyperperfusion syndrome: A cause of neurologic dysfunction after carotid endarterectomy

Neurologic deficits evident when patients initially awaken from surgery are generally due to intraoperative embolization or inadequate cerebral protection in patients with marginal cerebral perfusion; neurologic deficits occurring in the immediate postoperative period are usually related to acute carotid occlusion or embolization. However, in a small subset of patients, transient postoperative neurologic dysfunction seems to be related to a syndrome of cerebral hyperperfusion rather than a lack of adequate cerebral blood flow. This study describes the courses of 10 patients with classic findings of cerebral hyperperfusion syndrome. Typically, this syndrome occurred in patients with longstanding severe chronic cerebral ischemia and occurred after correction of a very high-grade carotid stenosis. Intraoperatively, there was often a dramatic increase in xenon-labeled cerebral blood flows, with postocclusion flows sometimes attaining three to four times baseline levels. Postoperatively, the patients initially did well. However, over the next several days, many of them began to complain of unilateral headache on the operated side and subsequently had seizures. Electroencephalography obtained during this period uniformly revealed periodic lateralizing epileptiform discharges on the side of the brain ipsilateral to the endarterectomy. Although neurologic dysfunction fully resolved in all of the patients in this group, it is possible that intracerebral hemorrhage may occur in some patients with hyperperfusion syndrome. The pathophysiology of this syndrome is believed to be related to preoperative loss of cerebral autoregulatory mechanisms caused by chronic cerebral ischemia.

Long-term follow-up of stereotactic lesionectomy in partial epilepsy: predictive factors and electroencephalographic results.

Summary: We performed an extended follow‐up study assessing the efficacy of stereotactic lesionectomy in 23 patients with foreign‐tissue lesions and intractable partial epilepsy. Sixteen lesions involved functional or eloquent cortex as determined by anatomic localization. By definition, the surgical objective in these patients was excision of the lesion, and not the surrounding cerebral cortex. The mean duration of follow‐up was 48.5 months (range 26–69 months). Seventeen patients (74%) had a significant reduction in seizures (90%) after lesionectomy. Thirteen patients (56%) had a class I operative out‐come (seizure‐free, single seizure episode, or auras only). Five of these patients were successfully discontinued from antiepileptic drug (AED) therapy. Patients with temporal lobe lesions were statistically less likely to be rendered seizure‐free (p < 0.05). Age at operation, duration of epilepsy, and underlying pathology were not significant predictors of seizure outcome. The anatomic distribution of extracranial EEG recorded epileptiform activity did not appear to be an important determinant of outcome. The absence of interictal epileptiform activity in the 3‐month postoperative EEG correlated with a significant reduction in seizures. Long‐term follow‐up indicates that lesionectomy may be effective in select patients with medically refractory partial seizure disorders.

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

Summary: 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).

Isoflurane when compared to enflurane and halothane decreases the frequency of cerebral ischemia during carotid endarterectomy

Data from the records of patients who underwent 2223 carotid endarterectomies at the Mayo Clinic between January 1, 1972, and December 31, 1985, were abstracted to compare the effects of isoflurane, enflurane, and halothane on the critical cerebral blood flow (CBF) (i.e., the CBF below which the majority of patients develop EEG ischemic changes within 3 min of carotid occlusion), the incidence of EEG ischemic changes, and the neurologic outcome. In a total of 2196 of these procedures, the patient received one of the three volatile anesthetics and, in 2010 of these, both the EEG and the CBF were monitored. Chronologically, halothane was the primary agent from 1972–1974; enflurane progressively replaced halothane during 1975–1981; and isoflurane was used almost exclusively since 1982. This analysis confirmed a previous study that the critical CBF during isoflurane anesthesia (703 procedures) was approximately 10 ml ± 100 g−1 · min−1, as contrasted to that of approximately 20 ml ± 100 g−1 · min−1 during halothane anesthesia (467 procedures). This analysis also established that the critical CBF during enflurane anesthesia (840 procedures) was approximately 15 ml ± 100 g−1 · min−1. The incidence of EEG ischemic changes was significantly less (P < 0.001) during isoflurane anesthesia (18%) than during either enflurane (26%) or halothane (25%) anesthesia. This difference occurred despite the fact that the preoperative risk status was greater in the patients given isoflurane. There was no difference in neurologic outcome between the three anesthetics, and none was expected, since all patients with EEG changes were immediately shunted, if possible. The authors conclude that relative to halothane and enflurane, isoflurane does offer a degree of cerebral protection for transient incomplete regional cerebral ischemia during carotid endarterectomy.

Low-Grade Glial Neoplasms and Intractable Partial Epilepsy: Efficacy of Surgical Treatment

Summary: We performed a retrospective study of 51 consecutive patients who underwent operation for intractable partial epilepsy related to low‐grade intracerebral neoplasms between 1984 and 1990. All patients had medically refractory partial seizures and a mass lesion identified on neuroimaging studies. Lesionectomy was performed on 17 patients, and 34 had lesion resection and corticectomy. Mean postoperative follow‐up was 4.4 years (range 2–8 years). Sixty‐six percent of patients were seizure‐free, and 88% experienced a significant reduction in seizure frequency. In 16 patients (31%), antiepileptic drugs (AEDs) were successfully discontinued. Twenty‐five of 31 (81%) eligible patients obtained a drivers license after successful operation. Patients with complete tumor resection and no interictal epileptiform activity on postoperative EEG studies had the best operative outcome. Epilepsy surgery can result in long‐term improvement in seizure control and quality of life (QOL) in selected patients with intractable tumor‐related epilepsy. Our results should be useful to clinicians considering treatment options for patients with intractable seizures related to low‐grade intracerebral neoplasms.

MRI in the presurgical evaluation of patients with frontal lobe epilepsy and children with temporal lobe epilepsy" pathologic correlation and prognostic importance*

We performed magnetic resonance imaging (MRI) using a high-field strength magnet (1.5 T) in two series of 53 patients with intractable partial epilepsy of frontal lobe or temporal lobe origin who subsequently received ablative surgery for their seizure disorder. In the first series of patients the pathologic correlation and prognostic importance of an MRI-identified lesion in the frontal lobe were assessed. Twenty-five percent of the patients with negative MRI studies and 67% of patients with neuroimaging abnormalities restricted to the frontal lobe, were seizure-free at a minimum duration of follow-up of 1 year. None of the patients with a multilobar MRI-detected abnormality was seizure-free postoperatively. In the second study the sensitivity and specificity of MRI-based hippocampal volumetry was determined in pediatric patients with partial epilepsy of temporal lobe origin unrelated to foreign-tissue pathology. Hippocampal formation atrophy in the epileptic temporal lobe was identified in 63% of patients. The sensitivity and specificity of hippocampal volumetry was 100% in patients with mesial temporal sclerosis. The presence of an MRI-detected epileptogenic lesion in the frontal lobe and hippocampal formation atrophy in the temporal lobe may correlate with the underlying pathology and affect the identification of potential candidates for epilepsy surgery.

The ictal bradycardia syndrome.

: Purpose: Episodic loss of consciousness presents a diagnostic challenge to the neurologist. A perhaps under‐recognized cause of episodic loss of consciousness, which we call the ictal bradycardia syndrome, occurs when epileptic discharges profoundly disrupt normal cardiac rhythm, resulting in cardiogenic syncope during the ictal event. We attempt to determine whether the presence of the ictal bradycardia syndrome provides localizing information regarding the site of seizure onset and to describe the demographics of patients with this syndrome. We also discuss difficulties in diagnosis and treatment.