Spatial characterization of interictal high frequency oscillations in epileptic neocortex

Interictal high frequency oscillations (HFOs), in particular those with frequency components in excess of 200 Hz, have been proposed as important biomarkers of epileptic cortex as well as the genesis of seizures. We investigated the spatial extent, classification and distribution of HFOs using a dense 4 × 4 mm² two dimensional microelectrode array implanted in the neocortex of four patients undergoing epilepsy surgery. The majority (97%) of oscillations detected included fast ripples and were concentrated in relatively few recording sites. While most HFOs were limited to single channels, ∼10% occurred on a larger spatial scale with simultaneous but morphologically distinct detections in multiple channels. Eighty per cent of these large-scale events were associated with interictal epileptiform discharges. We propose that large-scale HFOs, rather than the more frequent highly focal events, are the substrates of the HFOs detected by clinical depth electrodes. This feature was prominent in three patients but rarely seen in only one patient recorded outside epileptogenic cortex. Additionally, we found that HFOs were commonly associated with widespread interictal epileptiform discharges but not with locally generated ‘microdischarges’. Our observations raise the possibility that, rather than being initiators of epileptiform activity, fast ripples may be markers of a secondary local response.

Electrographic seizures and periodic discharges after intracerebral hemorrhage

OBJECTIVE

To determine the frequency and significance of electrographic seizures and other EEG findings in patients with intracerebral hemorrhage (ICH).

METHODS

We reviewed 102 consecutive patients with ICH who underwent continuous electroencephalographic monitoring (cEEG). Demographic, clinical, radiographic, and cEEG findings were recorded. Using multivariate logistic regression analysis, we determined factors associated with 1) electrographic seizures, 2) periodic epileptiform discharges (PEDs), and 3) poor outcome (death, vegetative or minimally conscious state) at hospital discharge.

RESULTS

Seizures occurred in 31% (n = 32) of patients with ICH, prior to cEEG in 19 patients. Eighteen percent (n = 18) of patients had electrographic seizures; only one of these patients also had clinical seizures while on cEEG. After controlling for demographic and clinical predictors, only an increase in ICH volume of 30% or more between admission and 24-hour follow-up CT scan was associated with electrographic seizures (33% vs 15%; OR 9.5, 95% CI 1.7 to 53.8). PEDs were less frequently seen in those with hemorrhages located at least 1 mm from the cortex (8% vs 29%; OR 0.2, 95% CI 0.1 to 0.7). PEDs were independently associated with poor outcome (65% vs 17%; OR 7.6, 95% CI 2.1 to 27.3). In patients with electrographic seizures, the first seizure was detected within the first hour of cEEG monitoring in 56% and within 48 hours in 94%.

CONCLUSIONS

Seizures occurred in one third of patients with intracerebral hemorrhage (ICH) and over half were purely electrographic. Electrographic seizures were associated with expanding hemorrhages, and periodic discharges with cortical ICH and poor outcome. Further research is needed to determine if treating or preventing seizures or PEDs might lead to improved outcome after ICH.

Detection of electrographic seizures with continuous EEG monitoring in critically ill patients

OBJECTIVE

To identify patients most likely to have seizures documented on continuous EEG (cEEG) monitoring and patients who require more prolonged cEEG to record the first seizure.

METHODS

Five hundred seventy consecutive patients who underwent cEEG monitoring over a 6.5-year period were reviewed for the detection of subclinical seizures or evaluation of unexplained decrease in level of consciousness. Baseline demographic, clinical, and EEG findings were recorded and a multivariate logistic regression analysis performed to identify factors associated with 1) any EEG seizure activity and 2) first seizure detected after >24 hours of monitoring.

RESULTS

Seizures were detected in 19% (n = 110) of patients who underwent cEEG monitoring; the seizures were exclusively nonconvulsive in 92% (n = 101) of these patients. Among patients with seizures, 89% (n = 98) were in intensive care units at the time of monitoring. Electrographic seizures were associated with coma (odds ratio [OR] 7.7, 95% CI 4.2 to 14.2), age <18 years (OR 6.7, 95% CI 2.8 to 16.2), a history of epilepsy (OR 2.7, 95% CI 1.3 to 5.5), and convulsive seizures during the current illness prior to monitoring (OR 2.4, 95% CI 1.4 to 4.3). Seizures were detected within the first 24 hours of cEEG monitoring in 88% of all patients who would eventually have seizures detected by cEEG. In another 5% (n = 6), the first seizure was recorded on monitoring day 2, and in 7% (n = 8), the first seizure was detected after 48 hours of monitoring. Comatose patients were more likely to have their first seizure recorded after >24 hours of monitoring (20% vs 5% of noncomatose patients; OR 4.5, p = 0.018).

CONCLUSIONS

CEEG monitoring detected seizure activity in 19% of patients, and the seizures were almost always nonconvulsive. Coma, age <18 years, a history of epilepsy, and convulsive seizures prior to monitoring were risk factors for electrographic seizures. Comatose patients frequently required >24 hours of monitoring to detect the first electrographic seizure.

Attenuation of somatosensory evoked potentials during positioning in a patient undergoing suboccipital craniectomy for Chiari I malformation with syringomyelia

Intraoperative electrophysiologic monitoring can diminish the risk of neurologic injury by enabling the detection of injury at a time when it can be reversed or minimized. This report describes a 14-year-old girl with a Chiari's malformation type I and syringomyelia who underwent a suboccipital decompression and dural patch grafting with concurrent somatosensory evoked potentials. When the patient was turned into the prone position and the neck was flexed, the left-sided somatosensory evoked potential deteriorated. After the patient's neck was repositioned, the left median nerve potential improved but did not return to baseline. Postoperatively, the patient had decreased proprioception of her left arm, which completely resolved at 2-week follow-up. This single case report does not establish the need for routine somatosensory evoked potential monitoring. Nevertheless, deterioration of the potential in this case led directly to a change in the surgical positioning, which may have significantly reduced the chances of a permanent neurologic injury.

Continuous EEG monitoring and midazolam infusion for refractory nonconvulsive status epilepticus

BACKGROUND

Although cIV-MDZ has emerged as a popular alternative to barbiturate therapy for refractory status epilepticus (RSE), experience with its use for this indication is limited.

OBJECTIVE

- To evaluate the efficacy of continuous intravenous midazolam (cIV-MDZ) for attaining sustained seizure control in patients with RSE.

METHODS

The authors reviewed 33 episodes of RSE treated with cIV-MDZ in their neurologic intensive care unit over 6 years. All patients were monitored with continuous EEG (cEEG). MDZ infusion rates were titrated to eliminate clinical and EEG seizure activity; cIV-MDZ was discontinued once patients were seizure-free for 24 hours. Acute treatment failures (seizures 1 to 6 hours after starting cIV-MDZ), breakthrough seizures (after 6 hours of therapy), post-treatment seizures (within 48 hours of discontinuing therapy), and ultimate treatment failure (frequent seizures that led to treatment with pentobarbital or propofol) were identified.

RESULTS

All patients were in nonconvulsive SE at the time cIV-MDZ was started; the mean duration of SE before treatment was 3.9 days (range 0 to 17 days). In addition to benzodiazepines, 94% of patients had received at least two antiepileptic drugs (AED) before starting cIV-MDZ. The mean loading dose was 0.19 mg/kg, the mean maximal infusion rate was 0.22 mg/kg/h, and the mean duration of cIV-MDZ therapy was 4.2 days (range 1 to 14 days). Acute treatment failure occurred in 18% (6/33) of episodes, breakthrough seizures in 56% (18/32), post-treatment seizures in 68% (19/28), and ultimate treatment failure in 18% (6/33). Breakthrough seizures were clinically subtle or purely electrographic in 89% (16/18) of cases and were associated with an increased risk of developing post-treatment seizures (p = 0.01).

CONCLUSIONS

Although most patients with RSE initially responded to cIV-MDZ, over half developed subsequent breakthrough seizures, which were predictive of post-treatment seizures and were often detectable only with cEEG. Titrating cIV-MDZ to burst suppression, more aggressive treatment with concurrent AED, or a longer period of initial treatment may reduce the high proportion of patients with RSE who relapse after cIV-MDZ is discontinued.

Spike detection II: automatic, perception-based detection and clustering

OBJECTIVES

We developed perception-based spike detection and clustering algorithms.

METHODS

The detection algorithm employs a novel, multiple monotonic neural network (MMNN). It is tested on two short-duration EEG databases containing 2400 spikes from 50 epilepsy patients and 10 control subjects. Previous studies are compared for database difficulty and reliability and algorithm accuracy. Automatic grouping of spikes via hierarchical clustering (using topology and morphology) is visually compared with hand marked grouping on a single record.

RESULTS

The MMNN algorithm is found to operate close to the ability of a human expert while alleviating problems related to overtraining. The hierarchical and hand marked spike groupings are found to be strikingly similar.

CONCLUSIONS

An automatic detection algorithm need not be as accurate as a human expert to be clinically useful. A user interface that allows the neurologist to quickly delete artifacts and determine whether there are multiple spike generators is sufficient.

Evidence for unconscious memory processing during elective cardiac surgery

BACKGROUND

Many anesthetic drugs have been shown to disrupt conscious recall (explicit memory) in volunteers. However, unconscious processing (implicit memory) of intraoperative auditory material may occur during general anesthesia and may provide an opportunity for intraoperative therapeutic intervention. In this study, we examined patients undergoing elective cardiac surgery for evidence of intraoperative implicit and explicit memory.

METHODS AND RESULTS

Twenty-five subjects provided written informed consent and underwent general anesthesia and cardiopulmonary bypass for cardiac surgery. During the operation, patients were randomized to receive 1 of 2 different audiotapes of associated word pairs. Postoperatively, a blinded observer conducted a standardized interview to determine the extent of intraoperative implicit and explicit memory. With the use of free association, significant intraoperative implicit memory was found. In contrast, no patient had spontaneous or directed recall of intraoperative events, and we did not find evidence of intraoperative explicit memory with a recognition task.

CONCLUSIONS

Patients undergoing general anesthesia for cardiac surgery were reliably able to reinforce associations between word pairs solely on the basis of their intraoperative presentation. This provides further evidence that patients are capable of processing intraoperative auditory information.

Evoked potentials in clinical trials for multiple sclerosis

Multiple sclerosis produces disruption of conduction in the central nervous system by a variety of mechanisms, relating, in part, to loss of the myelin sheath. Although often not well correlated with the clinical course of the disease in individual patients, the resulting evoked potential (EP) disturbances can serve as measures of an accumulating disease burden, particularly in longitudinal population studies. Accordingly, EPs can serve as useful instruments for assessing the effectiveness of therapeutic agents which may alter the course of the multiple sclerosis. Furthermore, since EPs measure conduction within the central nervous system, they provide a means of directly assessing symptomatic treatments designed to improve central conduction.

Intraoperative monitoring of motor evoked potentials: a review of 116 cases

We reviewed the results of motor evoked potential (MEP) and somatosensory evoked potential (SEP) monitoring during 116 operations on the spine or spinal cord. We monitored MEPs by electrically stimulating the spinal cord and recording compound muscle action potentials from lower extremity muscles and monitored SEPs by stimulating posterior tibial or peroneal nerves and recording both cortical and subcortical evoked potentials. We maintained anesthesia with an N2O/O2/opioid technique supplemented with a halogenated inhalational agent and maintained partial neuromuscular blockade using a vecuronium infusion. Both MEPs and SEPs could be recorded in 99 cases (85%). Neither MEPs nor SEPs were recorded in eight patients, all of whom had preexisting severe myelopathies. Only SEPs could be recorded in two patients, and only MEPs were obtained in seven cases. Deterioration of evoked potentials occurred during nine operations (8%). In eight cases, both SEPs and MEPs deteriorated; in one case, only MEPs deteriorated. In four cases, the changes in the monitored signals led to major alterations in the surgery. We believe that optimal monitoring during spinal surgery requires recording both SEPs and MEPs. This provides independent verification of spinal cord integrity using two parallel but independent systems, and also allows detection of the occasional insults that selectively affect either motor or sensory systems.

The reliability of quantitative electroencephalography as an indicator of cerebral ischemia

The electroencephalogram (EEG) has been used to detect episodes of cerebral ischemia during various surgical procedures. Recently, computerized systems for recording and interpreting the quantitative EEG (QEEG) have been used by anesthesiologists because of their ease of application, clarity of display, and reported ability to identify ischemic EEG changes. However, the extent to which automated techniques of QEEG interpretation reliably differentiate cerebral ischemia from the confounding effects of anesthetics and other sources of "artifact" is not completely established. In this study, EEGs were recorded before and after defibrillator testing in patients undergoing implantable cardioverter defibrillator (ICD) placement and during analogous time periods in control patients undergoing abdominal surgery. EEGs were subjected to standard visual inspection by an experienced electroencephalographer and QEEG analysis with a commercially available system was used for automated EEG interpretation in order to evaluate the reliability of this quantitative technique. The CIMON technique identified episodes which met previously defined criteria for QEEG cerebral dysfunction and ischemic pattern in both groups, despite the presumed absence of cerebral ischemia in the control patients. Since there was no evidence of cerebral ischemia in the raw EEGs of either the ICD patients or the controls, these QEEG changes were not confirmed by conventional techniques of EEG interpretation. Our results suggest that caution is warranted when using automated systems for intraoperative interpretation of EEG.

Propagation patterns of temporal spikes

In standard EEG recordings, spikes appear as single events characterized mainly by the scalp location of the their peak voltage. The signal-to-noise ratio of raw EEG is usually too high to permit more detailed analysis. We used spike averaging to improve the resolution of interictal spikes in 40 patients with temporal lobe epilepsy. Spikes were identified visually in raw, digitally stored EEG. When multiple spike types were present in a patient, they were grouped separately. Spikes were synchronized for averaging by aligning their negative peaks in a designated channel. Sixteen patients demonstrated spike propagation from anterior temporal to posterior temporal electrode locations. Thirty-six patients demonstrated spread of spikes from anterior temporal to fronto-polar electrode sites. While anterior temporal and fronto-polar spikes were often synchronous, fronto-polar spikes followed anterior temporal discharges in 25% of cases and preceded them in 13%. Spike averaging revealed propagation patterns not apparent on visual inspection of raw EEG. We speculate that these patterns may reflect inherent physiological properties of temporal and frontal neuronal circuits, possibly utilized by the epileptogenic process.

Implantable cardioverter-defibrillator. Evaluation of clinical neurologic outcome and electroencephalographic changes during implantation

During placement of implantable cardioverter-defibrillators, ventricular arrhythmias are induced to test the function of the devices. Although cerebral hypoperfusion and ischemic electroencephalographic changes occur in patients while implantable cardioverter-defibrillators are being tested, no investigation has assessed neurologic outcome in these patients. Nine patients having either implantation or change of an implantable cardioverter-defibrillator underwent neurologic examination and neuropsychometric tests before and after the operation. After induction of general anesthesia and insertion of implantable cardioverter-defibrillator leads (when needed), ventricular fibrillation, ventricular flutter, or ventricular tachycardia, was induced by means of programmed electrical stimulation. Implantable cardioverter-defibrillator testing continued until satisfactory lead placement was confirmed. The intraoperative electroencephalographic recording was analyzed for evidence of ischemic change. In all, an electroencephalogram was recorded during 50 periods of circulatory arrest. Mean duration of the arrest periods was 13.6 seconds. By means of conventional visual inspection of the raw electroencephalogram, high-amplitude rhythmic delta or theta, voltage attenuation, or loss of fast frequency activity was observed in 30 of the arrests. By means of an automated technique of electroencephalographic interpretation based on power spectral analysis, electroencephalographic changes were correctly identified in 26 of the arrests. The incidence of these electroencephalographic changes was dependent on the arrest duration. The mean interval from arrest onset to electroencephalographic change was 7.5 seconds (standard deviation +/- 1.8 seconds). In patients with electroencephalographic changes during multiple arrests, no downward trend in this interval was detected in later arrests and no evidence of persistent ischemic change was observed in electroencephalograms recorded after the conclusion of implantable cardioverter-defibrillator testing. Postoperative neurologic and neuropsychometric testing was completed in eight patients, none of whom exhibited a new neurologic deficit, exacerbation of a preexisting neurologic condition, or significant deterioration in neuropsychometric performance. We conclude that the brief arrest of cerebral circulation induced during insertion of an implantable cardioverter-defibrillator is not associated with permanent neurologic injury.

Monitoring of intraoperative motor-evoked potentials under conditions of controlled neuromuscular blockade

Motor-evoked potentials were recorded after electrical spinal cord stimulation in 19 patients undergoing neurosurgical or orthopedic procedures. Anesthesia was maintained with nitrous oxide, opioids, and inhaled anesthetics. Vecuronium was infused sufficient to eliminate 90% of twitch tension. The spinal cord was stimulated using either epidural or subarachnoid electrodes. Compound muscle action potentials were recorded from quadriceps and tibialis anterior muscles. Well-formed, stable motor-evoked potentials were recorded in all but one patient, in whom a preexisting myelopathy was felt to preclude recording. Intraoperative deterioration of motor-evoked potentials occurred in one patient who had a postoperative neurologic deficit. This study demonstrates the feasibility and utility of intraoperative motor tract monitoring using direct spinal cord stimulation. Controlled neuromuscular blockade permits recording of compound muscle action potentials while eliminating patient motor activity that could interfere with surgery.

Monitoring during supratentorial surgery

Electroneurophysiological monitoring is employed during various supratentorial surgical procedures. EEG and evoked potential monitoring are used to detect and to facilitate the timely correction of cerebral ischemia during carotid endarterectomy and aneurysm surgery. Direct cortical recording and stimulation is used to identify areas or cortex that would be likely to produce clinical deficits if removed. Electrocorticography is used to identify epileptogenic cortex intraoperatively during surgical treatment of epilepsy.

Normal brain-stem auditory evoked potentials with abnormal latency-intensity studies in patients with acoustic neuromas

Brain-stem auditory evoked potentials (BAEPs) are highly sensitive for detecting acoustic neuromas but false-negative results occur. We studied BAEPs preoperatively in 39 cases of acoustic neuroma. Absolute and interpeak latencies ipsilateral to the tumor, and interaural latency differences, were normal in four patients with small tumors. In three of these, however, results of latency-intensity studies were abnormal. In one patient, the latency-intensity result became normal postoperatively. If acoustic neuroma is suspected, and BAEPs are normal by usual criteria, latency-intensity functions should be examined to maximize chances of detecting a small tumor.

Anatomic and physiologic bases of posterior tibial nerve somatosensory evoked potentials

Following stimulation of the posterior tibial nerve, lumbar electrodes record a response that is the composite of two signals, one (PV) corresponding to the afferent volley in the cauda equina and gracile tract, and another (N22) generated postsynaptically in the gray matter of the lumbar cord. Subcortical structures generate two distinct, widely distributed signals, recordable from scalp electrodes using a noncephalic reference, P31 and N34. P31 is most likely generated by the afferent volley in the caudal medial lemnicus. N34 probably reflects subcortical postsynaptic activity in brain stem and/or thalamus, respectively. The "primary" cortical response, P38/N38, has a complex scalp distribution reflecting the location of the leg area on the mesial aspect of the postcentral gyrus, within the interhemispheric fissure. It is most likely a composite waveform with multiple cortical generators.

State-dependent changes in the N20 component of the median nerve somatosensory evoked potential

Short-latency components of median nerve somatosensory evoked potentials are generally assumed to be unaffected by sleep and level of arousal. We found that sleep prolongs the latency and alters the morphology of the N20 component in normal subjects. These changes may represent differential effects of sleep on various elements contributing to generation of the N20. Failure to control for patient state may degrade the reliability of clinical somatosensory evoked potential testing.

Electrophysiologic features of abetalipoproteinemia: functional consequences of vitamin E deficiency

We performed electrophysiologic testing in 10 patients with abetalipoproteinemia (ABL). Peripheral nerve studies implied an axonal disorder. Visual evoked potentials demonstrated prolonged P100 latency in three patients and abnormal electroretinograms in six. Somatosensory evoked potentials indicated dorsal column dysfunction in eight patients. Brainstem auditory evoked potentials were normal. Findings were consistent with the known neuropathology of ABL and of experimental vitamin E deficiency. Stabilization or improvement in electrophysiologic findings occurred with vitamin E supplementation. Neurophysiologic tests document retinal, central somatosensory and peripheral nerve lesions in vitamin E deficiency and provide an objective indication of response to treatment.

Dissociation of frontal N100 from occipital P100 in pattern reversal visual evoked potentials

We studied the relationship between occipital P100 and frontal N100 in visual evoked potentials produced by pattern reversal in normal subjects and two groups of patients. Recording derivation was critical for interpretation since both Fz and Oz electrode sites are active. In 9 patients, but no normal subjects, P100 was absent. In these patients, use of a standard Oz-Fz montage resulted in the erroneous impression of a 'normal' P100 since a downward deflection was produced by the inverting effect of the amplifier on an intact N100 at Fz. When both P100 and N100 were present (at Oz and Fz respectively), their latencies were usually similar but not identical which contributed to apparent latency shifts or W-shaped wave forms in the Oz-Fz derivation. We conclude that use of a non-cephalic or relatively inactive scalp position (such as the mastoid) should be used as a reference site in addition to Fz to reduce interpretive errors.

Effect of cervical spinal cord lesions on early components of the median nerve somatosensory evoked potential

Clinical interpretation of median somatosensory evoked potentials (SEPs) is usually based on latency measurements of selected waveforms. The "cervicomedullary" potential (N14) is commonly recorded by measuring the voltage difference between cervical spine and frontal electrodes. This cervicomedullary potential is actually a composite waveform that is generated by several distinct neural structures. We present evidence that placement of additional recording electrodes to delineate the multiple cervical components of the median SEP enhances ability to detect and localize cervical cord lesions.

Intraoperative BAER monitoring and hearing preservation in the treatment of acoustic neuromas

We treated 20 cases of acoustic neuromas in the last 2 1/2 years using the suboccipital approach with intraoperative monitoring of eighth nerve function in an attempt to preserve hearing. Total tumor removal was accomplished in 18 cases. Three tumors were small (less than 2 cm in size); 3 tumors were moderate sized (2 to 3 cm) and 14 tumors were large (greater than 3 cm). In 15 cases, all BAER components were lost during surgery. These patients had no postoperative hearing. In five patients there was intraoperative preservation of some or all BAER components. These included the three patients with small tumors and two other patients with moderate sized tumors. One patient with a moderate size tumor had preservation of wave 1 only, and had no postoperative speech discrimination. One patient with a small tumor retained all 5 BAER components, but had no postoperative hearing. Three patients in our series retained functional hearing after surgery, including 3 of 5 patients with tumors 2 cm or smaller. Intraoperative BAER monitoring appears to be useful in predicting postoperative hearing. Tumor size seems to be the primary factor in preservation of hearing following acoustic neuroma surgery.

Treatment of cerebrotendinous xanthomatosis with chenodeoxycholic acid

We describe a patient with cerebrotendinous xanthomatosis who was treated for one year with chenodeoxycholic acid. Modest clinical improvement was accompanied by marked improvement in visual and brainstem auditory evoked potentials. Improved central nervous system function coincided with return of plasma and cerebrospinal fluid cholestanol levels to normal.

Real-time reconstruction of evoked potentials using a new two-dimensional filter method

Standard techniques for evoked potential recording extract a stimulus-locked event from accompanying noise by averaging a large number of sequentially obtained responses. This approach is valid only to the extent that the nervous system's electrical response to successive stimuli is identical. The technique is suboptimal for recording unstable evoked potentials which vary with the subject's state and attention. Similarly, standard methods are suboptimal for efficiently analyzing rapid changes such as may be seen in the operating room. We developed an evoked potential recording method that reconstructs the individual evoked responses (or small subaverages of evoked potentials) to a series of stimuli. First, the raw data from an entire series of one to several hundred responses are recorded digitally. Using a frequency domain two-dimensional filter, the data are then filtered twice, once along the data sequence axis for each trial, and again along the cross-trial sequence axis for comparable frequency coefficients in sequential trials. The reconstructed filtered evoked potentials are plotted, with successive responses stacked for easy tracking of component changes.

Somatosensory evoked potentials following median nerve stimulation. I. The cervical components

Median somatosensory evoked potentials were studied in 20 normal adult volunteers. Recording electrodes were positioned over posterior, anterior and lateral neck, as well as on the scalp. Three distinct cervical potentials were identified. Immediately after the afferent volley passes Erb's point, a travelling wave is recorded in the lateral cervical electrodes ipsilateral to the side of stimulation. This represents the afferent volley approaching the spinal cord in the proximal brachial plexus and cervical roots and has been designated the 'proximal plexus volley' (PPV). Following PPV, a second travelling wave is recorded which increases in latency from low to high cervical levels. It represents the afferent volley in the dorsal column, and has been designated the 'dorsal column volley' (DCV). Following DCV, a stationary potential, designated CERV N13/P13, is recorded with characteristics of a transverse midline dipole having maximal negativity posteriorly and maximal positivity anteriorly. This potential may be generated by interneurons in the dorsal grey of the cervical cord. Each of these cervical travelling waves is accompanied by a negative far-field potential recorded at the scalp. The PPV is accompanied by a negative scalp deflection with a nominal latency of 10 ms (N10), and the peak of DCV at SC1 is accompanied by a scalp negativity with a nominal latency of 12 ms (N12). In view of these observations, it is necessary to reexamine assumptions regarding the polarity of scalp-recorded potentials generated by remote neural events.

Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the posterior tibial nerve (PTN) in 12 normal adults. Recording using both cephalic and non-cephalic references were obtained from multiple electrodes placed over the spine and scalp. Following PTN stimulation, the fastest recorded potentials of the afferent sensory volley proceeds up the spinal cord at constant velocity. After arrival of the volley at cervical cord levels, 3 widely distributed waves, P28, P31 and N34, are recorded from scalp electrodes. These 'far-field' potentials are followed by a localized positivity (P38) which has a peak voltage either at the vertex or just laterally toward the side of stimulation. A contralateral negativity (N38) was present in most individuals. We propose that P28 arises from medial lemniscus; that P31 is generated by ventrobasal thalamus; and that N34 is probably the result of further activity in thalamus and/or thalamocortical radiations. The P38/N38 complex represents the primary cortical response to PTN stimulation. Its most consistent characteristic is a positivity at the vertex or immediately adjacent scalp areas ipsilateral to the stimulated leg. The topography of the P38/N38 potential varies slightly from individual to individual in a manner consistent with a functional dipole situated in the leg and foot area on the mesial aspect of the postcentral gyrus, whose exact location and orientation changes in accordance with known variations in the location of the leg area.

Toxoplasmosis: a treatable neurologic disease in the immunologically compromised patient

A 10-year-old girl with aplastic anemia developed seizures and a mild hemiparesis following a bone marrow transplant. Based on serologic evidence and a computed tomography scan, which showed a left parietal lucency with ring enhancement, a diagnosis of toxoplasmosis was considered. A brain biopsy of the lucent area demonstrated the inflammation and necrosis but no organisms were seen. During a six-week course of pyrimethamine, sulfadiazine, and folinic acid therapy there was clinical and neuroradiologic resolution. The short course of therapy as well as the inadvertent substitution of folic acid for folinic acid and trimethoprim-sulfamethoxazole for sulfadiazine resulted in the reappearance of neurologic deficits. Reinstitution of appropriate therapy produced gradual improvement over a nine-month period. Serial computer tomography scans correlated with the clinical course. In the immunologically compromised host CNS toxoplasmosis should be considered in the differential diagnosis of an evolving CNS syndrome. Early detection and prolonged therapy with appropriate drugs can result in a favorable outcome. Computed tomography scanning may be helpful in diagnosis and follow-up.