In the netherworld that lies between death and full consciousness, some grievously injured or ill patients will remain suspended indefinitely. But others, given time, will eke their way out of the twilight and toward recovery. Accurately predicting which group an apparently vegetative patient falls into could bring comfort, solace and sometimes hope to their families -- and also to the patients involved, who may wish to convey they are still "in there," or may feel pain that is not being addressed.
Developing a test that could foretell the long-term outcome of a stricken patient has proved elusive. But a new study finds that scans that look for signs of metabolic activity in certain regions of the brain can improve prediction.
The latest research, published Tuesday in the journal Lancet, tracked for at least a year 102 unconscious subjects, assessing them initially by brain scan, bedside examination and a complex diagnostic test to measure impairments of consciousness. All were diagnosed as either minimally conscious, locked in, or unresponsively wakeful (also called vegetative).
Imaging the subjects' brains with a positron emission tomography, or PET, scan allowed researchers to predict accurately 74 percent of the time whether a patient would show evidence of consciousness a year later. It was a better prognosticator of a poor outcome (continued lack of consciousness), accurately predicting that a patient would continue to be vegetative or minimally conscious in 92 percent of cases. But in 67 percent of cases in which PET scan results suggested a patient would regain some level of lost consciousness, he or she did so.
As other research has suggested, the prognosis for recovery was generally better in patients whose consciousness disorder stemmed from traumatic brain injury than in those harmed by hypoxia -- a prolonged interruption of the supply of oxygenated blood to the brain.
PET imaging of 39 healthy control subjects helped researchers sketch the activity profile of a fully functioning brain and of brains with distinct patterns of impairment: Even when no difference in two patients' awareness levels was evident, the metabolic patterns of a brain in a vegetative state looked very different from those of a brain with intermittent consciousness. And when the PET scanner detected those patterns in an apparently vegetative patient, the researchers suspected that the patient was more likely than one without them to recover some levels of conscious awareness.
Of 41 patients who had been deemed vegetative by existing diagnostic methods, PET scanning detected evidence of minimal but existing consciousness in 13 patients. After a year, nine of those 13 patients had progressed into a minimally conscious state or better, while three had died of complications or because further treatment was withheld. One remained in a vegetative state.
None of the remaining 28 patients (whose PET scans did not suggest hope) had regained a measurable degree of conscious awareness.
That made the PET scan, which detects signs of metabolic activity throughout the brain, a better guide to a patient's outcome than functional magnetic resonance imaging, or fMRI, which detects brain activity in particular brain regions by looking for evidence of oxygenation.
FMRI has been used before to show that some patients in a vegetative state are capable of hearing, understanding and responding appropriately to researchers' commands: In the current study, as in those earlier efforts, researchers asked patients in an apparently vegetative state to navigate through a familiar landscape or to imagine themselves playing tennis, and the fMRI scanner detected clear evidence of responsive spatial navigation or of brain activity in the parts of the brain in which motor activity is planned.
In the current study, however, differences in patients' fMRI activity did not offer any reliable way to discern subtle signs of preserved consciousness.
For vegetative patients who show the surprising cognitive ability to respond to mental imagery tasks in the fMRI, this type of brain scanning could aid in assessing how much mental capacity a patient maintains and might help a patient communicate his or her needs or desires, the authors wrote. But even if an apparently vegetative patient can't play imaginary tennis or walk through his childhood home in an fMRI, a PET scan may still detect evidence of conscious awareness that could be cause for hope, they added. That makes the PET scan a more "sensitive" predictor of improvement.
In the United States, between 9,000 and 37,000 patients lie in hospital beds diagnosed either as minimally conscious or being in a vegetative state (the formal medical diagnosis is "unresponsive wakefulness syndrome"). Those numbers vary greatly -- and are not broken down by condition -- because diagnostic boundaries are ill-defined.
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