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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).
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