Differences Between Brain Death and Vegetative State

Differences between brain death and vegetative state Glucose metabolism. Ventilation. Brainstem reflexes and hypothalamic function. Spinal reflex. Eye opening. Facial expression. Brain activity. Vegetative state Reduced by 40 – 50%. Brain death Absent. Can breathe spontaneously. Apnoeic and require controlled artificial ventilation. None. Preserved. Rich array of motor activity; reflexive responses to external stimulation (e.g. moving in meaningless ways, and showing startle myoclonus to loud noises). Show spontaneous opening of the eyes (when not in coma). May occasionally smile or cry, utter grunts and sometimes moan or scream. Noxious stimulation causes discrete activation of the thalamus and primary somatosensory cortex. Long-range functional cortico-cortical connectivity is present. Slow body movements generated by residual spinal activity. Never show eye opening, even on noxious stimulation. Never show any facial expression and remain mute. Absent activity. Minimally conscious state  Patients show some limited evidence of awareness of self or environment.  Meaningful stimuli activate specific cortical areas.  Presentation of cries: o Non-verbal universal emotional stimulus. o Activates widespread temporal and insular areas, but not the amygdala. o Regardless of attentional state, cries are known to activate the superior temporal sulcus (auditory cortex), insula and amygdala (involved in emotional processing).  Presentation of the patient’s own name: o Activates the precuneus, anterior cingulate and mesiofrontal cortices, right temporoparietal, left dorsolateral prefrontal cortices and bilateral angular gyri. o This network is believed to be involved in reflective selfconsciousness. o Caused significant positivity (P300) in the event-related cortical potential, though this was delayed. Clinical correlation: locked-in syndrome  Describes patients who are awake and conscious, but selectively deefferented.  Corticospinal and corticobulbar pathways are interrupted so they have no means of producing speech, limb or facial movements.  Acute ventral pontine lesions are its most common cause.  People with such brainstem lesions often: o Remain comatose for some days or weeks, needing artificial respiration. o Then gradually wake up, but remain paralysed and unable to speak.  Usually, but not always, the anatomy of the responsible lesion in the brainstem is such that locked-in patients are left with the capacity to use vertical eye movements to communicate their awareness of internal and external stimuli. o Controlled by the trochlear nerve, which is the only cranial nerve that leaves from the dorsal aspect of the brainstem.  Although they have very poor physical functionality, they report a rich mental life and rate their overall health to be at a level similar to control subjects.  When the patient is at his/her worst, clinicians may often tend to assume that these persons will die anyway, or would choose to die if they only know what the clinicians knew. Neural basis of consciousness  Attention operates in sensory regions, but the top-down control of voluntary or goal-directed attention largely relies on frontal and parietal cortices.  Awareness has also been correlated to activities in these areas.  Activity is abnormal in altered states of consciousness, such as in vegetative patients or during generalised epileptic seizures accompanied by a loss of consciousness. o Frontal regions are also the first areas to deactivate when falling asleep, and the last ones to reactivate after awakening.  Imaging studies of consciousness emphasise the roles of the dorsolateral prefrontal cortex, parietal regions, the inferior frontal cortex, as well as the anterior cingulate cortex. Global workspace theory              Postulates that conscious awareness depends on synchronous activation of several interconnected cortical sites. Unless local sites of cortical activation are “bound together” a sensation is not registered consciously. Synchronous activation occurs at ~300ms after an event (corresponding to the P300 in the event related potential). In the absence of any task or sensory stimulation, the activity in the areas involved in self and external awareness is not constant, but continuously fluctuates. Even in the absence of sensory inputs, structured patterns of ongoing spontaneous activity can be observed in cortical and thalamic neurones. Default-mode network (DMN). o Occurs when subjects are relaxed, engaged in introspection. o Activity decreases when subjects are asked to perform a motor or cognitive task. o Comprises the precuneus and posterior cingulate cortex, the temporaparietal junction, the medial prefrontal cortex, parahippocampal gyri and thalamus. Spontaneous brain activity has been shown to be characterised by the coalescence of different brain rhythms, ranging from very slow to very fast frequencies. Slow fluctuations in the range of 0.01 – 0.1Hz are not random, but coherent within specific neuroanatomical systems, including sensory systems, and self and external awareness networks. Showing a second image <50ms after the first results in masking of the first image; it is not consciously perceived. Non-conscious processing of masked images is observed in multiple cortical areas, mostly within an early time window (~300ms) accompanied by induced gamma band activity, but without coherent long-distance neural activity, suggesting a quickly dissipating feed-forward wave. Conscious processing of unmasked words (interstimulus interval >50ms) is associated with sustained voltage changes at ~300ms, particularly in the prefrontal cortex, large increases in spectral power in the gamma band (30 – 100Hz) and increases in longdistance phase synchrony. Conscious perception of near-threshold sensory stimuli was predicted by increased baseline neural activity in the lateral frontoparietal network 3s before stimulus occurrence, emphasising the importance of activity in the fronto-parietal cortices in awareness of externally applied stimuli. Conversely, sensory stimuli were predicted to be unperceived when preceding baseline activity is increased in the self-awareness network, suggesting competition between conscious access to external stimuli and self-referential processes.  If activity is high 3s prior to stimulation in the self-awareness network, the subject is probably more likely to be in a state of higher introspection of self-orientated thoughts, and will have fewer chances to subsequently report the stimuli as consciously perceived. Conclusions  The vegetative state is not the same as brain death; cerebral metabolism is reduced in the vegetative state but completely absent in brain death.  The locked-in syndrome results from local damage to the ventral pons. Normal cognitive processes are intact but communication is limited to eye movements.  Conscious awareness depends on synchronised activation of multiple cortical areas.