When a Vegetative State Isn’t So Vegetative . . .

There are three distinct stages of impaired consciousness: coma, the vegetative state, and the minimally conscious state.  Vegetative state is defined as wakefulness without conscious awareness of self and environment. Patients in the vegetative state differ from those in a coma because they can be aroused, yet both groups are considered fully unconscious. Functional magnetic resonance imaging (fMRI) studies have been used to show that some of these patients are consciously aware; unfortunately, the procedure is expensive and not readily available to most patients. Now the same researchers have reported a bedside test using electroencephalography (EEG) that can be used to assess the presence of possible residual cognitive function and cognitive awareness in subjects who seem to be entirely vegetative.  Their report is in the Lancet.

Sixteen patients with traumatic and non-traumatic brain injury who were in a vegetative state were enrolled, along with 12 healthy control subjects.  All participants were repeatedly asked to imagine movements of their right hand and toes on command. Their responses on EEG were analyzed for evidence of consistent and appropriate markers of motor imagery, similar to those seen in the healthy controls. 

Three of the 16 patients repeatedly and reliably generated the appropriate EEG responses to two distinct commands, despite being behaviorally entirely unresponsive.  There was no apparent relationship between patients’ clinical histories, age, time since injury, or cause of injury, and their EEG response to commands.

Interestingly, 3 of the 12 control subjects did not produce reliable EEG responses to the commands given.  These were dubbed “so-called brain-computer interface illiterates”.  Their existence emphasizes that only positive results in patients are of value – a negative EEG response in a vegetative patient doesn’t necessarily indicate an absence of awareness.  This latter finding diminishes the usefulness of the bedside EEG test, which otherwise might have become the diagnostic tool of choice for long-term unconscious patients. More research will reveal more useful information about consciousness, we hope.