Researchers have developed an inexpensive technique that, they say, can detect covert awareness in people diagnosed as vegetative.
The researchers used electroencephalography (EEG), instead of functional MRI (fMRI), a technology previously shown to detect minimal consciousness in people thought to be vegetative based on an evaluation of their behavioral symptoms (N. Engl. J. Med. 2010;362:579-89).
FMRI works by detecting a patient’s ability to modulate a blood oxygen–level dependent (BOLD) response to mental imagery tasks. Barriers to its wider use include its expense; the difficulty of transporting patients to MRI-equipped facilities; imaging problems when patients cannot remain still; and the technology’s inappropriateness for patients with metal implants in their bodies.
EEG, which measures the activity of groups of cortical neurons from scalp electrodes, can be used at the bedside and is inexpensive, widely available, and unaffected by metal.
For their research, published online Nov. 10 in The Lancet (doi:10.1016/S0140-6736(11)61224-5), Damian Cruse, Ph.D., of the University of Western Ontario, London, and his colleagues in the United Kingdom and Belgium recruited 16 patients diagnosed by behavioral indicators as vegetative according to established criteria. The patients varied in age, gender, cause of injury, and time since injury. Of the 16 patients, 5 had traumatic brain injuries, 2 had had a stroke, and the rest had experienced anoxia. The researchers also recruited a group of 12 healthy individuals to serve as controls.
All of the study participants – people diagnosed as vegetative and healthy controls – were repeatedly asked to imagine the action of squeezing their right hands or wiggling all their toes at the prompt of a beep. EEG responses to the commands were measured.
Dr. Cruse and colleagues found that 3 of 16 patients diagnosed as vegetative were able to repeatedly and reliably generate EEG responses to the two different commands. All 3 responding patients were male, between the ages of 29 and 45; 2 had been diagnosed with traumatic brain injuries and 1 with injury caused by anoxia.
Of the controls, nine produced EEG responses that could be classified as significantly above chance; the researchers could not say definitively why three controls did not. However, they noted, the lack of appropriate response underscored the importance of interpreting only positive results in patients, "because this finding shows unequivocally that a null EEG outcome does not necessarily indicate an absence of awareness."
Dr. Cruse and colleagues called it "extremely unlikely" that the response patterns they detected could have been involuntary or automatic, based on the design of the study and the outcomes recorded. "Successful completion of these EEG tasks represents a substantial cognitive feat, not only for patients who were presumed to be vegetative, but also for control participants," they wrote, noting that one patient’s responses were consistent in over 100 repeated trials, better than the vast majority of controls.
While the median age for subjects with traumatic brain injuries was younger than those with non-traumatic injuries (29 years vs. 44 years), the researchers found no significant relationships between the patients’ ages or clinical histories and their ability to respond.
Dr. Cruse and colleagues wrote in their analysis that their study shows that "this EEG method can identify covert awareness in patients diagnosed in the vegetative state with a similar degree of accuracy to other methods of detection."
The Medical Research Council, James S. McDonnell Foundation, Canada Excellence Research Chairs Program, the European Commission, Fonds de la Recherche Scientifique, the Mind Science Foundation, Belgian French-Speaking Community Concerted Research Action, University Hospital of Liège, and University of Liège all contributed funding to the study. Dr. Cruse and his colleagues disclosed no conflicts of interest.