Short-latency somatosensory evoked potentials in brain-dead patients

[Determination of irreversibility of clinical brain death. Electroencephalography and evoked potentials]

Principally, in the fourth update of the rules for the procedure to finally determine the irreversible cessation of function of the cerebrum, the cerebellum and the brainstem, the importance of an electroencephalogram (EEG), somatosensory evoked potentials (SEP) and brainstem auditory evoked potentials (BAEP) are confirmed. This paper presents the reliability and validity of the electrophysiological diagnosis, discusses the amendments in the fourth version of the guidelines and introduces the practical application, problems and sources of error.An EEG is the best established supplementary diagnostic method for determining the irreversibility of clinical brain death syndrome. It should be noted that residual brain activity can often persist for many hours after the onset of brain death syndrome, particularly in patients with primary brainstem lesions. The derivation and analysis of an EEG requires a high level of expertise to be able to safely distinguish artefacts from primary brain activity. The registration of EEGs to demonstrate the irreversibility of clinical brain death syndrome is extremely time consuming.The BAEPs can only be used to confirm the irreversibility of brain death syndrome in serial examinations or in the rare cases of a sustained wave I or sustained waves I and II. Very often, an investigation cannot be reliably performed because of existing sound conduction disturbances or failure of all potentials even before the onset of clinical brain death syndrome. This explains why BAEPs are only used in exceptional cases.The SEPs of the median nerve can be very reliably derived, are technically simple and with few sources of error. A serial investigation is not required and the time needed for examination is short. For these reasons SEPs are given preference over EEGs and BAEPs for establishing the irreversibility of clinical brain death syndrome.

Role of short latency evoked potentials in the diagnosis of brain death

OBJECTIVE

The aim of this study is to confirm the effectiveness of auditory brain-stem responses (ABRs) and somatosensory evoked potentials (SEPs) in the diagnosis of brain death (BD).

METHODS

ABRs and SEPs were recorded at the same session in 130 BD patients (age range 8-77 years, 81 male and 49 female). Twenty-four cases were submitted to serial recordings from preterminal conditions through BD.

RESULTS

ABRs were absent in 92 cases (70.8%), only waves I or I-II were present in 32 cases (24.6%), while in the remaining 6 patients (4.6%) waves V and/or III were still present, excluding the death of the brain-stem. In 4 cases (3.1%) SEPs showed the absence of all components following the cervical N9, preventing the diagnosis of BD. Among 126 cases (96.9%) with preserved cervical N9-N13 SEPs confirmed the absence of brain-stem activity in 122 cases (93.7%), in whom no waves following P11 or P13 were recordable. SEPs excluded the diagnosis of BD in the remaining 4 cases (3.2%) showing preserved P14 and/or N18. In all pre terminal patients the far-field P14-N18 were present, and their disappearance was closely related to the onset of BD.

CONCLUSIONS

The combined us of ABRs and SEPs was able to confirm BD in almost all patients, providing an objective confirmation of the diagnosis, and to exclude it in 7 cases, thus improving the reliability of diagnosis.

N18 in median somatosensory evoked potentials: a new indicator of medullary function useful for the diagnosis of brain death

OBJECTIVES

To record N18 in median somatosensory evoked potentials (SEPs) for deeply comatose or brain dead patients and to demonstrate the usefulness of N18 for the diagnosis of brain death in comparison with auditory brain stem responses (ABRs) and P13/14 in median SEPs, which have been conventionally used as complementary tests for the diagnosis of brain death.

METHODS

Subjects were 19 deeply comatose or brain dead patients. Thirteen recordings were performed in deeply comatose but not brain dead conditions, and 12 recordings were performed in brain death. N18 was evaluated in the CPi-C2S lead (or other scalp-C2S leads) to obtain a flat baseline.

RESULTS

N18 was preserved in 12 of 13 non-brain dead comatose recordings whereas it was completely lost for all of the 12 brain death recordings. P13/14 in median SEPs was preserved for all the comatose recordings, whereas apparent P13/14-like potentials, usually of low amplitude, were seen in nine of 12 brain death recordings-that is, frequent false positives. The ABRs already showed features which were characteristic for brain death (loss of components other than wave 1 or small wave 2) for four comatose recordings, in three of which N18 was preserved. The last result not only corresponds with the fact that ABRs can evaluate pontine and midbrain functions and not medullary function, but further supports the medullary origin of N18. In the four patients followed up for the course of progression from coma to brain death, N18s preserved in normal size during the comatose state were completely lost after brain death was established.

CONCLUSIONS

The N18 potential is generated by the cuneate nucleus in the medulla oblongata in the preceding studies. N18 is suggested to be a promising tool for the diagnosis of brain death because there were no false positives and rare false negatives in the present series for detecting the remaining brain stem function.

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.

Nasopharyngeal electrode recording of somatosensory evoked potentials as an indicator in brain death

Median nerve somatosensory evoked potentials were recorded in 28 comatose patients, eight of whom were progressing from coma to eventual brain death and in 11 brain dead patients using electrodes over the scalp, neck and nasopharynx (nasopharyngeal electrode). This recording technique was used to assess the different derivation of brainstem P14 wave activity. It showed that in the midfrontal scalp to the nasopharynx derivation a clear P14 was present in all comatose patients. This component disappeared during the passage from coma to brain death. In a separate group, simultaneous direct recordings in the vicinity of the dorsal column nuclei and with a nasopharyngeal electrode were made in five patients undergoing neurosurgical procedures at the craniocervical junction with the same somatosensory evoked potential monitor. We found that the P14 recorded with the nasopharyngeal electrode in the neurosurgical patients corresponded in latency and morphology with the P14 recorded directly on the surface of the craniocervical junction and more specifically in the vicinity of the nucleus cuneatus. The nasopharyngeal electrode provides non-invasive access to the ventral brainstem at the medullo-pontine level and the disappearance of the P14 shows a clear sign of involvement of the craniocervical junction in brain dead patients. Our study showed that with a simple montage the nasopharyngeal electrode is an effective non-invasive monitor for brainstem activity and can be used as an early diagnostic indicator of brainstem death.

Multimodality evoked potentials (auditory, somatosensory and motor) in coma

Auditory brainstem responses (ABRs) have proved to be significantly related to outcome, both in severe head injury and brain hemorrhage. Nevertheless, the usefulness of ABR is limited by the anatomic extent of the investigated pathways. The combined use of ABRs and somatosensory evoked potentials (SEPs) improves the outcome prediction in comparison to the use of only one modality. It mainly decreases the rate of false negatives, since patients with severe hemispheric damage sparing the brain stem may have a poor outcome despite normal ABRs. The use of motor evoked potentials (MEPs) from magnetic transcranial stimulation is also significantly related to outcome: it appears to be far superior to the clinical evaluation of motor responses, while the combined use of MEPs and SEPs gives a new opportunity of checking sensorimotor dysfunction. ABRs and SEPs may also be useful tools in the confirmation of brain death, the kernel of which is the assessment of brainstem death: they allow to check lemniscal pathways, which cannot be properly evaluated by clinical examination, and provide an objective confirmation of absence of brain stem activity.

Preserved cortical somatosensory evoked potentials in apnoeic coma with loss of brain-stem reflexes: case report

A comatose patient suffering from diffuse cerebellar haemorrhage developed apnoea and brainstem areflexia, i.e. the clinical signs of brain death. However, median nerve somatosensory evoked potential testing 2.5 h and 22 h after the onset of this clinical syndrome showed cortical potentials partly preserved; these were abolished 46 h after the beginning of the clinical signs of brain death. This case report underlines the need for electrophysiological confirmation of brain death in patients with primarily infratentorial lesions.

Brain death: practicability of evoked potentials

Multimodally evoked potentials were registered in 85 patients who fulfilled the criteria for brain death. While somatosensory and visual evoked potentials have been found to be of limited value for the diagnosis of brain death, the stepwise abolition of brain stem auditory evoked potentials (BAEP) confirmed brain death in 26 out of 85 patients, i.e. 31%. Registration of the abolition of BAEP is concluded to be a safe and acceptable confirmatory test. It is, however, more feasible for institutions, in which BAEP are analysed routinely. In spite of all efforts sequential BAEP could not be used for the diagnosis of brain death in the majority of cases either because of absence of reproducible responses at the initial registration or because the patient was already apnoic at the time of the initial BAEP. Assuming that bilateral preservation of wave I has the same significance as the stepwise abolition of BAEP, since it also proves the integrity of the peripheral receptor, BAEP are relevant for the declaration of brain death in approximately 30% of patients.

SEP testing in deeply comatose and brain dead patients: the role of nasopharyngeal, scalp and earlobe derivations in recording the P14 potential

Median nerve somatosensory evoked potentials (SEPs) were tested in 50 patients (20 brain dead, 18 comatose and in 12 progressing from coma to brain death, i.e., 32 cases with brain death and 30 cases with coma were recorded). Derivations were taken from nasopharynx, earlobes, scalp, and neck using cephalic and non-cephalic references. Cortical and subcortical SEP components were evaluated, focussing on the P14 potential. There is evidence that rostral and caudal parts of the P14 generator (lemniscus medialis) are differently affected in brain death, resulting in an abolition of the rostral part, while occasionally leaving intact for some time the caudal part. Non-cephalic referenced scalp records pick up the whole P14 dipole, whereas nasopharyngeal and earlobe derivations pick up different parts of P14, depending on the reference used. Scalp-to-nasopharynx records derive the most rostral part of P14; this "rostral P14" was bilaterally lost in all brain dead patients, but preserved in all deeply comatose patients with diffuse brain-stem injuries. Scalp-to-earlobe records, in contrast, picked up a P14 dipole segment reaching more caudally, resulting in a P14 potential also in brain dead patients. It is concluded that midfrontal scalp-to-nasopharynx derivations give the most valuable contribution to the electrophysiological assessment of brain death versus deep coma.

Important abnormalities in recordings of somatosensory evoked potentials in coma

We have chosen six illustrations showing how much vital information can be obtained from median nerve SEPs during the first 24 hours in coma. With avulsion of brachial plexus roots there was loss of SEPs at the cervical cord and the scalp from the affected side. In a severe injury of the cervical cord there was preservation of brachial plexus potentials, while SEPs at the cervical cord were absent. After critical deterioration in a case of repeated subarachnoidal hemorrhage, scalp SEPs with very short latency occurred, which is a finding suggestive of destruction of cortical SEP generators heralding a fatal outcome. In a case of brain injury combined with central hyperthermia, there was initially a loss of scalp SEPs probably due to the combined effect of these factors. In a case of brain injury there were bifid peaks at the scalp level. It is important to assess central sensory conduction time only to the first scalp SEP, otherwise an erroneously abnormal state may be inferred. In a patient with clinical and EEG evidence of brain death there was a loss of far-field thalamic potentials at the neck. It is important to be aware of such presentations to be able to provide corroborative assurance for the assessment of prognosis.

[Neurophysiological evaluation using multimodal sensory evoked potentials in patients in coma: general aspects]

Noninvasive sensory evoked potentials (SEP) performed at bedside in the Intensive Care Unit for patients in coma can be helpful in establishing both a diagnosis and a prognosis. Based on a more than 6-year experience on this subject, the authors discuss general aspects concerning these EP, their probable known generators, and propose a classification depicting different aspects observed for flash visual EP (F-VEP), brainstem auditory EP (BAEP), and median nerve somato-sensory EP (SSEP). Isolated, SSEP shows the best diagnostic and prognostic performance. Nevertheless, the authors consider that multimodality SEP are even better than any isolated EP study; cross-correlating information generated through a horizontal (F-VEP), a vertical (SSEP), and a pathway focusing brainstem in greater detail (BAEP) allows the neurophysiological establishment of the level of lesion in the CNS from a better perspective; besides, SEP can help setting the diagnosis of brain (encephalic) death, and the diagnosis of particular problems concerning each pathway. Notwithstanding, most important is prognosis definition, and the findings are summarized. Abnormal BAEP implies bad prognosis, as would be expected considering the severity of a brainstem lesion; on the other hand, a normal BAEP per se does not allow a precise definition, resting on other EP the role prognosis characterization. SSEP if bilaterally normal or only mildly abnormal imply good prognosis; bilateral absence of SSEP thalamo-cortical components has always carried a bad prognosis, since younger patients may at best evolve into a persistent vegetative state; SSEP intermediary results are more often accompanied by variable evolution. FVEP results parallel those of SSEP.

Short latency evoked potentials: new criteria for brain death?

The aim of this study was to evaluate whether the auditory brain stem responses (ABR) and short latency somatosensory potentials (SEP) from median nerve stimulation are effective tools in the confirmation of brain death. Thirty six brain dead patients were submitted to ABR and 24 to SEP in the same session. All waves of the ABR were absent in 28 (77.8 per cent) patients, while only wave I was present in the others (22.2 per cent). In SEP recordings the components later than P13 were absent in 17 (70.8 per cent) of cases; in the remaining seven patients (29.2 per cent) a N13/P13 dissociation (namely, retention of the cervical N13 and absence of the far-field P13) was found. The results suggest that SEP and ABR are reliable tools in the diagnosis of brain death and should be included in the criteria: they enable the functional status of two pathways in the brainstem to be checked, which cannot be explored by the clinical examination.

Somatosensory evoked potentials following severe head injury: loss of the thalamic potential with brain death

The thalamic component (P17) of the short-latency somatosensory evoked potential (SEP) was assessed to determine its usefulness in patients with severe head injury. Subjects were a group of patients admitted to the Auckland Hospital Critical Care Unit who subsequently died from head injury. In all instances where brain death was unequivocally established and a SEP recording made in close temporal proximity to the time of brain death the P17 potential was absent. When there was evidence of continuing brainstem activity and particularly where prolonged survival occurred following the last SEP recording the P17 potential remained intact bilaterally. This study shows that the presence or absence of the thalamic component of the short-latency SEP provides a reliable electrophysiological measure of brainstem function in patients where brain death has been suspected.

Far field correlates of thalamic activity in cortical and cervical somatosensory evoked potentials

Median nerve cortical and cervical somatosensory evoked potentials (SEPs) were recorded from a group of normal adults in order to study far field reflections of thalamic activity. In the cortical SEP, the primary response (N20) was preceded by two inflections (N16 and N18). It is shown that these two potentials bear a close temporal relationship to a positive trough on the downward slope of the principal negativity (N13) in the cervical SEP. This potential, which is often bifurcated into two subcomponents, is labeled P17 and it has previously been suggested that it originates in or near the thalamus. The coincidence between the P17 complex and N16 and N18 inflections is consistent with the hypothesis that all three responses are generated in either the thalamic relay nucleus or the thalamo-cortical radiations. Possible clinical applications of recording far field thalamic potentials are discussed.

Somatosensory evoked potentials aiding the diagnosis of brain death

SEP were recorded in 14 patients, who fulfilled the clinical and electroencephalographic criteria of brain death. The results are compared with the respective ones in healthy subjects. Beside the absence of cortical N 20 in each brain dead patient, reduction of amplitude or absence of near field negativity (N 13b) from upper neck regardless of the position of the reference electrode represents the predominant result. The near field potential from the lower neck (N 13a) was unaffected. The counterpart in the far field potential recorded from F z was amplitude reduction of P 13. These results suggest that the dissociation of N 13a and N 13b can confirm the diagnosis of brain death. Moreover these results support the view of two independent generators of N 13a and N 13b despite their identical amplitude and latency.

Serial recording of median nerve stimulated subcortical somatosensory evoked potentials (SEPs) in developing brain death

Subcortical somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded serially in 35 patients during the evolution towards brain death and in brain death. Neuropathological alterations of the central nervous system down to the C1/C2 spinal cord segment in brain death are well known. SEP components supposed to be generated above this level should be lost in brain death, while components generated below should not be altered. Erb's point, scalp and neck potentials were recorded at C3/4, or over the spinous process C7, using an Fz reference. In 10 patients additional montages, including spinous process C2-Fz, a non-cephalic reference (Fz-contralateral shoulder) and a posterior to anterior neck montage (spinous process C7-jugulum) were used. The cephalic referenced N9 and N11 peaks remained unchanged until brain death. N9 and N11 decreased in parallel in amplitude and increased in latency after systemic effects like hypoxia or hypothermia occurred. The cephalic referenced 'N14' decreased in amplitude and increased in latency after the clinical brain death syndrome was observed, while N13 in the posterior to anterior neck montage remained unchanged. The alteration of 'N14' went parallel to the decrease of the P14 amplitude. The subcortical SEPs in the cephalic referenced lead are supposed to be a peak composed by a horizontally orientated dorsal horn generated N13 and a rostrally orientated P14 arising at the level of the foramen magnum. The deterioration of the non-cephalic referenced P14 and of its cephalic referenced reflection 'N14' seems to provide an additional objective criterion for the diagnosis of brain death.