Is the Vegetative Patient Aware?
Coma is a state of pathologic unconsciousness but some comatose patients show evidence of normal sleep-wake cycles and can be awakened. These patients are referred to as being in a persistent vegetative state (PVS) but like patients in a coma, they do not show any awareness of themselves or their environment, do not interact with others, do not display repeatable, predictable, and voluntary actions in response to stimuli, and do not display any awareness or comprehension of language. Another subgroup is referred to as minimally conscious state (MCS) in which patients display some very limited awareness and interaction.
Of course, the diagnosis of PVS and MCS are clinical diagnoses since until recently there was not effective way to determine if the higher cortical areas of the brain were actually functioning enough to enable the patient to be aware.
Functional MRI was developed in the early 1990s and measures tiny and very localized changes in blood flow that reflect increased neural activity. Essentially, more active areas of the brain “light up” on the fMRI scan as blood flow changes second to second. Researchers have now used this tool to evaluate the brains of PVS and MCS patients and compare them to scans of normal control subjects. 54 patients with various causes of brain damage (traumatic, anoxic, stroke, infection) were enrolled in the study.
Of the 54 patients, 5 with traumatic brain injuries were able to modulate their brain activity by generating voluntary, reliable, and repeatable blood-oxygenation-level–dependent responses in predefined neuroanatomical regions when prompted to perform imagery tasks. No such responses were observed in any of the patients with nontraumatic brain injuries. Four of the five patients who were able to generate these responses were admitted to the hospital with a diagnosis of being in a vegetative state. When these four patients were thoroughly retested at the bedside, some behavioral indicators of awareness could be detected in two of them. However, the other two patients remained behaviorally unresponsive at the bedside, even after the functional MRI results were known and despite repeated testing by a multidisciplinary team. Thus, in a minority of cases, patients who meet the behavioral criteria for a vegetative state have residual cognitive function and even conscious awareness.
So it is possible that patients diagnosed as being vegetative are capable of having some awareness and cognitive function but they are in the small minority of PVS patients and all were traumatic brain injury victims. In other words, patients who developed PVS from a lack of blood flow (anoxic brain injury) or other non-traumatic injury mechanism like Terri Schiavo are far less likely than traumatic injury patients to be capable of conscious awareness if at all.
It is possible that more sensitive testing in the future may reveal more patient subgroups who are capable of awareness but this remains highly theoretical. If localized blood flow changes in these patients are too small to be seen on current functional MRI scans then the correlating neural activity may be below a level where consciousness is generated. Much more research needs to be done but this study has already identified a subgroup of PVS patients who may be amendable to treatment once one is developed.