"Brain death"
A lie destined to die


As regards cerebral circulation studies many authors still use the term "cessation or abscence" of' cerebral blood flow in cases  of brainstem death to convey an impression that absent blood flow clearly signifies death. It has already been mentioned that the presence of increased intracranial tension (due to an intracranial haematoma for example) will impede cerebral blood flow. Reducing intracranial tension, however, may correct the condition(5) Moreover, using advanced techniques such as computed tomographic studies with Xenon(45) have revealed that even in cases diagnosed by radionuclide studies as "no flow" revealed the presence of residual flow. These conflicting results can explain why many patients who otherwise fulfilled all the criteria of brainstem death were shown to retain some  intracranial circulation(46)

As regards other tests which depend upon eliciting an electric response as a result of applying a stimulus to one of the cranial nerves relaying in the brain stem, it must be remembered that the elicited response depends on the averaging of electrical events in the brainstem. This averaging requires synchrony of brain activity time-locked to the sensory stimuli(4). Pathological processes (whether traumatic or otherwise) are known to compromise the function of the peripheral receptors or the nerve fibre pathways or the central neurons or all of these. Averaged evoked potentials would be absent in this scenario, since the neurons would either not be receiving a synchronized input or would not fire back simultaneously in a synchronized fashion so as to elicit a recordable response, even though these neurons are viable(47,48). One can conclude that clinical bedside tests whose responses also depend on an intact sensory apparatus may not he conclusive  in this setting.

As for EEG, its reliability and validity are questionable(49). A German survey assessing the validity of' EEG when interpreted by expert diagnosticians described it as being of "limited reliability" as far as diagnosis of brainstem death was concerned. This is because its presence will not rule out the possibility of brainstem failure(5) as it may originate from other irrelevant but viable parts of the brain. The opposite is also true. Its absence does not guarantee the damage of the whole brain. There have been cases where thalamic probing has shown persistent neuronal discharges in the presence of an isoelectric EEG  recorded from the scalp  (50).