Short latency somatosensory evoked potentials in brain-dead patients using restricted low cut filter setting.

This paper describes the SSEP patterns in BD when using restricted filter bandpass setting, with special emphasis on the dissociation of some SSEP components when cephalic and non-cephalic references are used. A characteristic SSEP pattern was found: absence of N20 and later responses in the Cc'-Fpz lead and preservation of all or some of the so-called subcortical components in the rest of the derivations. We noted an interesting dissociation in which some SSEP components were not recorded in the scalp-non cephalic derivations, but they were still present in the neck-cephalic and spine channels, with inverse polarity. We selected a restricted bandpass for the scalp-non cephalic, the neck-cephalic and the spine derivations, and we found this dissociation not only for P13-N13 and P14-N14, but for Nm-Pm' and Pm-Nm' components. It is likely that components Nm-Pm' and Pm-Nm' are "far-field potentials" that appear when the slow wave N18 is filtered out by the use of a restricted low filter setting. Our data further suggest that all components after N14-P14, recorded with restricted filter bandpass also recognize two distinct generator sources. Rostral generators are located probably in the brainstem and/or thalamus, but at least a significant part of these SSEP components are generated at the very lower part of the medulla oblongata (dorsal column nuclei) and/or at the upper cervical spine.(ABSTRACT TRUNCATED AT 250 WORDS)

Further differences between brain-stem auditory potentials evoked by rarefaction and condensation clicks as revealed by vector analysis.

Vectorial analysis of the brain-stem auditory evoked potential (BAEP) elicited by either condensation and rarefaction stimuli was performed in 22 healthy subjects. Besides differences in latency of components III and V ('R' longer than 'C'), a significant difference was found between the directions of the vector calculated over the descending limb of wave III, with an ascending direction for 'C' responses and a contralateral direction for 'R' responses. The possible origins of these differences are discussed. It is concluded that the use of alternating or isolated single polarity clicks is not recommended, mainly when assessing integrity of central auditory pathways.

Brain-stem auditory evoked potentials and brain death.

BAEP records were obtained from 30 brain-dead patients. Three BAEP patterns were observed: (1) no identifiable waves (73.34%), (2) an isolated bilateral wave I (16.66%), and (3) an isolated unilateral wave I (10%). When wave I was present, it was always significantly delayed. Significant augmentation of wave I amplitude was present bilaterally in one case and unilaterally in another. On the other hand, in serial records from 3 cases wave I latency tended to increase progressively until this component disappeared. During the same period, wave I amplitude fluctuations were observed. A significant negative correlation was found for wave I latency with heart rate and body temperature in 1 case. Two facts might explain the progressive delay and disappearance of wave I in brain-dead patients: a progressive hypoxic-ischaemic dysfunction of the cochlea and the eighth nerve plus hypothermia, often present in brain-dead patients. Then the incidence of wave I preservation reported by different authors in single BAEP records from brain-dead patients might depend on the moment at which the evoked potential study was done in relation to the onset of the clinical state. It is suggested that, although BAEPs provide an objective electrophysiological assessment of brain-stem function, essential for BD diagnosis, this technique could be of no value for this purpose when used in isolation.

Visual evoked potentials and electroretinography in brain-dead patients.

Visual evoked potentials and electroretinograms were elicited by light-emitting diode stimulation and recorded simultaneously, with cephalic and noncephalic references, in 30 normal subjects and in 30 brain-dead patients. A characteristic pattern was found in the group of patients: when a cephalic reference was used for both visual evoked potentials and the electroretinogram, the a- and b-waves of the electroretinogram were recognized in all patients, and visual evoked responses consisted of waves with inverse polarity, similar morphologic characteristics, the same latency, and less amplitude than those of the electroretinogram. When a noncephalic derivation was chosen for the electroretinogram and visual evoked potentials, electroretinogram did not change in either morphologic features or latency, whereas the visual evoked potential channel showed no response. Only in two cases was it possible to record waves in the visual evoked potential lead with a noncephalic reference, showing a spread of the electroretinogram to the occipital area, with a considerably reduced amplitude. These results suggest that, although contamination of visual evoked potential records by the spread of the electroretinogram to the occipital area could occur, it is easy to confirm the absence of a true cortical visual response in brain-dead patients by means of a noncephalic reference. This pattern clearly confirms that in the visual pathways of brain-dead patients, electrical activity is confined to the retina.