[Effect of anesthetics on sensory evoked potentials]

[French guidelines on electroencephalogram]

Electroencephalography allows the functional analysis of electrical brain cortical activity and is the gold standard for analyzing electrophysiological processes involved in epilepsy but also in several other dysfunctions of the central nervous system. Morphological imaging yields complementary data, yet it cannot replace the essential functional analysis tool that is EEG. Furthermore, EEG has the great advantage of being non-invasive, easy to perform and allows control tests when follow-up is necessary, even at the patient's bedside. Faced with the advances in knowledge, techniques and indications, the Société de Neurophysiologie Clinique de Langue Française (SNCLF) and the Ligue Française Contre l'Épilepsie (LFCE) found it necessary to provide an update on EEG recommendations. This article will review the methodology applied to this work, refine the various topics detailed in the following chapters. It will go over the summary of recommendations for each of these chapters and underline proposals for writing an EEG report. Some questions could not be answered by the review of the literature; in those cases, an expert advice was given by the working and reading groups in addition to the guidelines.

Trauma and impaired consciousness

This article discusses brain trauma and impaired consciousness. It reviews the various states of impaired consciousness related to trauma, with an historical and current literature viewpoint. The causes and pathophysiology of impaired consciousness in concussion, diffuse axonal injury, and focal brain lesions are discussed and management options evaluated.

The prognostic value of evoked responses from primary somatosensory and auditory cortex in comatose patients

OBJECTIVE

To evaluate somatosensory and auditory primary cortices using somatosensory evoked potentials (SEPs) and middle latency auditory evoked potentials (MLAEPs) in the prognosis of return to consciousness in comatose patients.

METHODS

SEPs and MLAEPs were recorded in 131 severe comatose patients. Latencies and amplitudes were measured. Coma had been caused by transient cardiac arrest (n=49), traumatic brain injury (n=22), stroke (n=45), complications of neurosurgery (n=12) and encephalitis (n=3). One month after the onset of coma patients were classified as awake, still comatose or dead. Three months after (M3), they were classified into one of the 5 categories of the Glasgow outcome scale (GOS).

RESULTS

At M3, 41.2% were dead, 47.3% were conscious (GOS 3-5) and 11.5% had not recovered consciousness. None of the patients in whom somatosensory N20 and auditory Pa were absent did return to consciousness and in the post-anoxic group, reduced cortical amplitude too was always associated with bad outcome. Conversely, N20 and Pa were present, respectively, in 33/69 and 34/69 patients who did not recover.

CONCLUSIONS

The prognostic value of SEPs and MLAEPs in comatose patients depends on the cause of coma. Measurement of response amplitudes is informative. Abolition of cortical SEPs and/or cortical MLAEPs precludes post-anoxic comatose patients from returning to consciousness (100% specificity). In any case, the presence of short latency cortical somatosensory or auditory components is not a guarantee for return to consciousness. Late components should then be recorded.

Does wave VI of BAEP pertain to the prognosis of coma?

OBJECTIVES

The significance of brainstem auditory evoked potential (BAEP) wave VI for the outcome of comatose patients was assessed and compared to the prognostic impact of median nerve evoked somatosensory potential (MSEP) recording.

PATIENTS AND METHODS

One hundred sixteen patients in a comatose state due to traumatic brain injury (n = 57) and to intracerebral hemorrhage or tumours (n = 59) were investigated prospectively. Outcome was assessed 3 months after the onset of coma using Glasgow Outcome Score (GOS). Standard BAEP and MSEP recordings were obtained within 24 h after coma onset. The frequency of findings was correlated with outcome by means of non-parametric statistical analysis. Additionally, 27 healthy volunteers were studied to confirm the constant presence of wave VI in normal BAEP.

RESULTS

In healthy controls, wave VI was present in 53/54 ears stimulated. Outcome correlation analysis revealed a significantly (p < 0.01) higher frequency of abnormal wave VI in patients with unfavourable outcome (GOS 1-3) irrespective of the underlying pathology. Similarly, abnormal MSEP findings were significantly more frequent in this outcome group. Diagnostic sensitivity for an unfavourable outcome was low for both parameters, whereas specificity and positive predictive value of abnormal wave VI as well as of abnormal MSEP were equally high.

CONCLUSION

Abnormal BAEP wave VI bears prognostic information in comatose patients.

Brain death and evoked potentials in pediatric patients

OBJECTIVE

To define the evoked potential responses (auditory and somatosensory) obtained from pediatric brain-dead patients.

DESIGN

Prospective study over an 8-yr period (1988-1996).

SETTING

A 14-bed pediatric intensive care unit in a multidisciplinary regional referral center (teaching hospital).

PATIENTS

Fifty-one pediatric patients with clinically established brain death.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Auditory brainstem and somatosensory evoked potentials were performed with a mean evolution time of 24 hrs after clinical brain death. The first brainstem auditory evoked potential recording was compatible with the diagnosis in 45 patients (90%): 27 patients (53%) did not respond, wave I was patent in 16 (7 bilateral, 6 from the left side, and 3 from the right side), and 2 patients evoked waves I and II in one or both ears. Gross anomalies were found in the remaining six patients. Sixteen patients were tested for somatosensory evoked potentials. N13 identifiable wave (62.5% of the patients) or a flat record were the obtained findings. Electric silence was noted initially on the electroencephalogram (EEG) in only 14 of 29 patients. Later flattening was observed in seven patients. Missing brainstem evoked response was noted earlier than cortical electric silence (range, 12-144 hrs). Any central wave could be pointed out in the evoked potentials of patients with an isoelectric EEG.

CONCLUSIONS

Evoked potential is useful in confirming the diagnosis of brain death in infants and in children as well as in adults. The test can be performed at bedside without interfering with patient care, and results are similar to those obtained in adult patients. Flattening of the EEG requires more time than achieving compatible evoked-potential responses.

[Neuromonitoring and anesthesia in surgery of the spine]

The authors report the main effects of anaesthetic drugs that are used alone or in association with anaesthetic protocols on somatosensory evoked potentials (SEP) and on motor evoked potentials (MEP). In the first part of the article, the effects are analysed on SEPs and MEPs that are obtained from non-invasive methods; in the second part, the effects of anaesthesia are analysed with respect to invasive methods of EP recordings. The current increase of invasive techniques of neuromonitoring by SEPs and MEPs is in relation with the weak effect of anaesthetics on evoked responses. Total intravenous anaesthesia (TIVA) provides stable anaesthesia for non-invasive SEP neuromonitoring only if bolus is avoided. With TIVA and other anaesthetic techniques, the introduction of repetitive stimulation provides new possibilities for non-invasive MEP neuromonitoring.

Dynamics of MLAEP changes in midazolam-induced sedation

This study aimed at assessing the effects of midazolam (MDZ) sedation on auditory brainstem (BAEP) and middle latency (MLAEP) evoked potentials in intensive care conditions. Ten ventilated comatose patients were receiving an intravenous MDZ bolus dose (0.2 mg/kg) followed by a 2 h continuous infusion (0.1 mg/kg/h). MLAEPs and BAEPs elicited by clicks (90 dB HL + masking) were simultaneously and continuously monitored during the first 6 h and for 30 min the next morning. We found no effect of MDZ sedation on BAEPs. Only MLAEP components were modified. However, none of the patients presented any total abolition of the MLAEPs. Bolus injection led to very early alteration of cortical responses, beginning after 5 min and lasting almost 1 h (maximum Pa latency increase, 3.1 ms; maximum Pa-Nb amplitude decrease, 46%). During continuous infusion, MLAEPs remained slightly, although significantly, altered (Pa latency, +1.3 ms; Pa-Nb amplitude, 27%). The Nb wave seemed to be modified earlier and to return to normality later than the Pa wave. These findings incite a careful interpretation of MLAEP tracings acquired during the first hour following MDZ bolus injection. If possible, MDZ should be administered as continuous infusion for reliable interpretation of evoked potential changes in intensive care unit, or during surgery.

[Early and middle latency auditory evoked potentials in vertebrobasilar strokes]

Brainstem auditory evoked potentials (BAEPs) and middle-latency auditory evoked potentials (MLAEPs) have been recorded in 67 patients who had a stroke in well-defined territories of the vertebral and basilar arteries. Either CT scan or MRI have been performed in all cases. BAEPs were abnormal in 41/67 patients and MLAEPs were abnormal in 25/39 patients. BAEPs abnormalities were either bilateral (29/41 cases) or unilateral (12/41 cases). All components of BAEPS were unilaterally absent in four cases and bilaterally in one case. Pa component of MLAEPs was unilaterally delayed or reduced in five cases and bilaterally in 20 cases. Considering the topography of the infarct as shown by CT scan or MRI: medulla oblongata (13 cases): BAEPSs were normal in nine cases; pons (24 cases): BAEPs were abnormal in 16 cases; MLAEPs were abnormal in ten of 15 patients whose BAEPs were abnormal as well; mesencephalon (seven cases): BAEPs were abnormal in only two cases, and MLAEPs were abnormal in two cases one of which BAEPs were normal; in patients with diffuse infarctions either BAEPs or MLAEPs or both were abnormal in all cases. Stimulation of the ear ipsilateral to the lesion disclosed more BAEPs or MLAEPs abnormalities than stimulation of the contralateral ear.

Effects of chronic high serum levels of phenobarbital on evoked potentials in epileptic children

We studied VEP and BAEP in 8 epileptic children with chronic high serum levels of phenobarbital. Records were obtained when the drug serum level was more than 40 mg/l and repeated when serum concentration was within the normal range. During the periods of high levels, P2 latency of the VEP was abnormally increased in all cases but one. The mean P2 latency decreased according to the reduction of the serum level of phenobarbital (139.6 msec vs. 110.1 msec, P = 0.002), and a significant regression coefficient (r = 0.546, P = 0.0271) was also noted between P2 latency and drug serum concentration. BAEPs were normal in all cases but one, who had a coexisting high level of phenytoin. All these findings suggest that the pharmacological effect of phenobarbital may be detected by VEPs and may result in delay of the P2 component.