Visual evoked potentials and electroretinography in brain-dead patients

Abstract

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.

Brain Death

“…The subjective dimension of awareness is philosophically impossible to test, but physiologically it seems conceivable that subjective awareness might continue. 295,294,304 Karen Ann Quinlan's brain showed severe damage of the thalamus, with the cerebral hemispheres relatively spared, 298 and other authors have reported similar findings. [305][306][307][308][309] We can ask ourselves if, in a case like this, other activating pathways, projecting to the cerebral cortex without relaying through the thalamus, could stimulate the cerebral cortex to provide internal awareness, even if physicians are unable to detect its manifestations.…”

“…35,37,40,60,223,295,353,354,416,418,475−477 When VEPS and ERG are elicited and recorded simultaneously, using cephalic and noncephalic references in BD, a clear result is found, confirming that in the visual pathways of BD patients, electrical activity is confined to the retina. 35,36,40,298,299 Moreover, the detection of wave I, without any later components, is the most clear-cut BAEP finding compatible with BD. 257,335,337,338 However, a higher proportion of cases with bilateral absence of responses has been reported by most authors.…”

“…17,84,86,99,122,123,284,285 The main finding in PVS is the preservation of arousal with apparent loss of awareness. 14,106,107,121,156,194,283,[296][297][298][299][300][301][302][303]305,308,313 Kinney and Samuels emphasized that the PVS denotes a 'locked-out-syndrome', because the 'cerebral cortex is disconnected from the external world', explained by three main neuropathological patterns: widespread and bilateral lesions of the cerebral cortex, diffuse damage of intra-and subcortical connections in the white matter of the cerebral hemispheres, and necrosis of the thalamus. 106 Can we deny the existence of internal awareness in PVS, because these patients apparently seem to be disconnected from the external world?…”

“…The brain is severely damaged, but not fully and irreversibly destroyed. 106,298 Moreover, it is necessary to consider the potential reversibility of awareness in the PVS, and an important recuperation in MCS. The development of rehabilitation techniques for PVS and MCS patients and others suffering long-lasting effects of brain injury is a crucial challenge for neuroscientists.…”

“…Consequently, consciousness generation is based on anatomy and physiology throughout the brain. 104,105,128,131,134,156,218,298,308,341 Tests Figure 3.13…”