Noninvasive testing, early surgery, and seizure freedom in tuberous sclerosis complex.

BACKGROUND: The unambiguous identification of the epileptogenic tubers in individuals with tuberous sclerosis complex (TSC) can be challenging. We assessed whether magnetic source imaging (MSI) and coregistration of (18)fluorodeoxyglucose PET (FDG-PET) with MRI could improve the identification of the epileptogenic regions noninvasively in children with TSC. METHODS: In addition to standard presurgical evaluation, 28 children with intractable epilepsy from TSC referred from 2000 to 2007 had MSI and FDG-PET/MRI coregistration without extraoperative intracranial EEG. RESULTS: Based on the concordance of test results, 18 patients with TSC (64%) underwent surgical resection, with the final resection zone confirmed by intraoperative electrocorticography. Twelve patients are seizure free postoperatively (67%), with an average follow-up of 4.1 years. Younger age at surgery and shorter seizure duration were associated with postoperative seizure freedom. Conversely, older age and longer seizure duration were linked with continued seizures postoperatively or prevented surgery because of nonlateralizing or bilateral independent epileptogenic zones. Complete removal of presurgery MSI dipole clusters correlated with postoperative seizure freedom. CONCLUSIONS: Magnetic source imaging and (18)fluorodeoxyglucose PET/MRI coregistration noninvasively localized the epileptogenic zones in many children with intractable epilepsy from tuberous sclerosis complex (TSC), with 67% seizure free postoperatively. Seizure freedom after surgery correlated with younger age and shorter seizure duration. These findings support the concept that early epilepsy surgery is associated with seizure freedom in children with TSC and intractable epilepsy.

Effects of different sensory inputs on the median-derived somatosensory evoked potential.

We studied the effect of a variety of "interfering" stimuli on the median-derived somatosensory evoked potentials recorded over Erb's point, cervical spine, and scalp. We found that the amplitude of N20 and P27 recorded over the scalp was attenuated by active movement, vibration, and tactile stimulation of the ipsilateral hand but not by passive movement. Cervical and peripheral responses were unaffected. The pathophysiologic basis and clinical significance of these findings are discussed.

Effect of reference point on visual evoked potentials: clinical relevance.

For clinical purposes the VEP is generally recorded from the mid-occipital region referenced to the vertex or mid-frontal region. This may lead to interpretive errors that can be avoided if a relatively inactive reference point, such as linked mastoids, is used simultaneously. The additional recording derivation may also be helpful in clarifying aberrant or ambiguous wave forms. The diagnostic yield from the two montages is similar, although the linked-mastoid reference provides a greater number of technically inadequate recordings due to smaller size of P100 and increased contamination by muscle artifact.

Recovery of peripheral and central responses to median nerve stimulation.

We recorded the responses to paired stimuli delivered to the median nerve at the wrist in 8 healthy adult volunteers, in order to characterize the recovery of function after a single conditioning stimulus. Responses were recorded over the nerve at the ipsilateral elbow and in the Erb's point region, over the second cervical spinous process, and over the contralateral 'hand area' of the scalp. The data from 1 subject were discarded because of possible artifactual contamination. In the others, the peripheral responses recovered both in latency and amplitude over a time period that accorded with previously published studies. We found, however, that the recovery periods for latency and amplitude of the responses recorded over the spine and scalp were prolonged compared with the corresponding values for the peripheral responses. Except for the responses recorded over the scalp, the recovery of amplitude either preceded or occurred at the same time as latency. By contrast, for the responses recorded over the scalp, there was a delay in the recovery of amplitude compared with latency. The differences in recovery period that we found at different levels of the nervous system are presumably related to structural and electrophysiological differences in afferent pathways, the presence of interposed synapses, and the intrinsic refractory properties of central neuronal populations.

Sensory discrimination and its relationship to the cerebral processing of infrequent stimuli.

We recorded cerebral evoked potentials, back and forward averaged from the EMG onset of the responding muscle, in three reaction time tasks, each requiring an identical motor response to an identical stimulus but differing in the nature of the sensory discrimination required. Two types of stimuli were presented: a rare one to which the subject responded with finger-extension, and a frequent one to which no response was required. We found a close but variable relationship between the cerebral events associated with performance of a task and the timing of the motor response. As completion of the discrimination process was delayed relative to stimulus occurrence, EMG activity began later relative to the cerebral potentials. Moreover, we were able to record these cerebral events only from the response to the rare (unexpected) stimulus and not when subjects were required to respond to the frequent stimulus, suggesting that the sensory discrimination, in these experiments, is an event that occurred only in the processing of the unexpected stimulus.

Subclasses of event-related potentials: response-locked and stimulus-locked components.

The manner in which information processing proceeds and the relationship of the components of the cerebral "event-related" evoked potential (ERP) to the presumed stages of such processing are uncertain. We studied these relationships by recording long-latency evoked potentials from the scalp in conjunction with electromyographic (EMG) recording in experiments in which subjects were required to make a motor response to one of two auditory stimuli. The cerebral response was averaged from the tone onset (stimulus-synchronized average) and the EMG onset (response-synchronized average). We found that N2 was quite distinct from the other ERP components (P165 and P3) in its lack of a close relationship to the motor response required in a sensory discrimination task. This suggests that, contrary to general belief, the P165-N2-P3 components of the ERP are not generated in an orderly sequence, but rather are generated separately and are possibly related to different cognitive processes.