Bulletin of the Los Angeles neurological societies最新文献

Evoked potentials are the electrical potentials of the nervous system which can be evoked by brief stimulations with lights, noises, or other means. Testing for these has become routine in certain settings in neurology. A wide variety of types of such tests are available. The technique usually involves stimulation of the patient with a standardized light, earphones or nerve stimulator. Recordings are taken from the scalp or other sites, and averaged over many separate stimulations. A good working knowledge of the equipment is needed for proper recording and interpretation of these tests. This article reviews the instrumentation, methods and theory behind clinical evoked potentials.

Locating the synapses which generate endogenous potential components recorded at the scalp would permit some aspects of the neural activity directly underlying cognition to be probed non-invasively. Knowing which synapses are functioning abnormally in certain neurological and psychiatric diseases may suggest rational treatment strategies aimed at local brain regions. Although it is not possible to localize the generating synapses using scalp topography, large potentials can be recorded in the hippocampal formation during the same task conditions which evoke scalp endogenous potentials. Phase-reversal of these hippocampal potentials over short distances, accompanied by changes in simultaneously recorded unit-activity, indicates that they are generated locally. Demonstration that they volume-conduct to the scalp would, however, require observation of lesion effects, and recordings from other candidate structures, as well as a realistic model for propagation.

The primary neurologic application of event-related potentials (ERP) to date has been of relatively short-latency (less than 100-200 ms), stimulus-dependent components. These so-called exogenous components are used to evaluate the structural, and to a lesser extent, the functional integrity of primary afferent pathways. Another class of ERP, termed endogenous, and involving longer-latency (greater than 100 ms) stimulus-independent components has been utilized by psychologists and cognitive psychophysiologists in the measuring of neurocognitive processes. Although the experimental specification of these neurocognitial processes is still in its infancy, the empirical applications of endogenous ERP's offers considerable promise for objective non-invasive evaluations of cognitive functions now conducted only behaviorally. Initial neurocognitive ERP applications have focused on global mental status in dementia. A brief review is presented of experimental studies of some endogenous ERPs (mainly P300), their hypothesized underlying neurocognitive processes, and initial and potential clinical applications.

Neurological applications of visual evoked potentials (VEP) have been expanding over the last decade. Stimulation techniques and waveform interpretation are reviewed. Physiologic variables including age, sex, habituation, refraction, cooperation, and factors influencing the amount of light reaching the retina will effect the VEP. Technical variables including luminance, check size, field size and color of check will also have an effect on the VEP. When properly performed, VEP can contribute important information on the visual pathways in patients with optic neuritis, multiple sclerosis, optic nerve and chiasm compressive lesions, non-compressive lesions affecting the visual pathways and neurodegenerative diseases not primarily involving the visual pathways. VEP has been shown to be a very sensitive though non-specific diagnostic tool.

Visual evoked potentials (VEP), brain stem auditory evoked potentials (BAEP), somatosensory evoked potential (SEP), and long latency event related potentials (ERP) have been used to assess various aspects of the central nervous system. Multimodality evoked potential (MEP) testing uses a combination of these tests in the same clinical setting. In the diagnosis of multiple sclerosis and in the diagnosis of brain death, MEP testing has been shown to be more effective than any one evoked potential test used alone. In head trauma, MEP testing is an effective means of localization and prognostication. There are potential uses for MEP testing in Parkinson's Disease and uremia.

Event-related potentials (ERPs) were recorded from 25 elderly patients and 26 normal controls to determine whether ERPs can aid in the differential diagnosis of depression and dementia. Two ERP measures were used: (1) P300 latency, and (2) N1 amplitude increase for a 14db tone intensity increase (i.e. intensity augmenting-reducing). While augmenting-reducing proved of little benefit, P300 latency was highly correlated with Mini-Mental State (MMS) scores and correctly identified 10 of 18 demented patients. None of the seven patients with psychiatric disorders (primarily depression) or 26 normals were misclassified. Better differentiation of depression and dementia resulted from combined use of P300 latency and MMS scores.

The brain stem auditory evoked potentials (BAEP) or, more appropriately, the auditory brain stem response (ABR) is a series of 5-7, vertex-positive, time-locked electric events. They are usually aroused by a brief, rapid onset, high intensity acoustic click or tonal pip. The vertex positive nature of the events is dictated by the placement of surface electrodes, usually featuring an active electrode on the vertex, a referent electrode on the mastoid or earlobe ipsilateral to the signal source, and a ground electrode on the forehead or other neutral tissue. The size of the ABR is sufficiently small, often fractions of a microvolt, to require the use of many signal presentations (1000-2000 stimuli) and digital averaging of the resulting electrical activity for the stimulus time-locked potentials to be elicited from other physiologic "noise." Since the auditory brain stem potentials are known to be elicitable near a subject's behavioral threshold for the stimulus used, and since the various potentials are thought to originate from different points in the brain stem, they have proved useful in hearing evaluations of human beings for two purposes: a) they are useful as a means of measuring hearing sensitivity from patients who cannot or will not give accurate voluntary responses; and b) they are useful as a diagnostic tool for determining the probable cause of an auditory disorder.

Intraoperative monitoring of the brain's electrical function with evoked potentials (EPs) may prove as valuable as cardiac monitoring in detecting abnormalities before permanent damage occurs. To date, evoked potential monitoring has only been employed in select neurological cases, where the significance of electrical changes can be related to postoperative deficits. If EP intraoperative techniques are to be useful in preventing these postoperative deficits, they must be able to predict impending brain dysfunction during surgery. There is strong clinical and experimental data relating EP changes to critical blood flow, brain retraction, and CNS manipulation. However, little is known of the normal fluctuations in electrical activity during routine non-neurological surgery. The data we have collected in control patients (non-neurological) will be a guide to the interpretation of neurological cases (carotid endarterectomy, aneurysm, and cervical spine and cord lesions). We have concentrated on monitoring somatosensory and auditory short-latency EPs since they appear to be the most stable, reproducible and the most applicable to patient monitoring during neurosurgical, vascular and orthopedic procedures.