Short-latency somatosensory evoked potentials in brain death

[The use of EEG in the diagnosis of brain death in France]

Scientific and modern conception of brain-death is rather recent, thus making possible organ transplantations. During the past three decades clinical criteria for brain-death have been defined. Electroencephalography (EEG) appears to be a complementary tool as it permits brain-death assessment. In France laws enforced respectively in 1968, 1978, 1991 and 1996 provide a framework for brain-death evaluation before organ transplantation and include EEG recording. Advantages and pitfalls of EEG in the diagnosis of brain-death are considered as this technique may be unreliable. Only evoked potential recordings can allow assessment and confirmation of brain stem death, which is a mandatory criterion. We recommend simultaneous EEG and either brain stem auditory or somatosensory evoked potential recordings to strengthen clinical criteria of brain-death and definitely prove it using objective electrophysiological criteria.

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.

[Importance of evoked potentials in the evolutive prognosis of coma during cerebral anoxia in adults]

Ten cases of postanoxic coma have been studied. A clinical neurological examination with study of brainstem reflexes and the EEG recording were made on the first day (J1), the third day (J3) and the tenth day (J10) after the start of the coma. A recording of the visual evoked potentials, the brainstem evoked potentials and the somatosensory potentials combined was made at the same time. A clinical examination is carried out one month after the coma when the patient survives. According to the initial clinical examination, we distinguished 3 groups of subjects. The results show that in group III the visual evoked potentials such as EEG have a slightly significant prognostic value; frequently the near outcome lead to death whereas EEG activity persists and the visual evoked potentials disappear later. On the other hand, the association of brainstem evoked potentials and somatosensory potentials clearly has a higher prognostic value in this group. The disappearance of the shortest brainstem responses and the cortical somatosensory responses is clearly an unfavourable prognosis. This disappearance associated with the end EEG activity is the absolute proof of brain death. On the other hand, the persistence of these responses is of a better prognosis at least on the survival level, but their degradation during evolution is unfavourable.

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.

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.