A new diagnostic tool may allow physicians to differentiate vegetative from minimally conscious states in severely brain-damaged patients at their bedside, investigators reported in the May 13 issue of Science.
The clinical diagnosis of these patients has been "extremely difficult," wrote study investigators Dr. Melanie Boly of the University of Liège (Belgium) and her colleagues. Previous neuroimaging experiments have attempted to identify accurate biomarkers of consciousness levels for both vegetative states (defined as preserved arousal without behavioral signs of awareness) and minimally conscious states (defined as nonreflexive and purposeful behaviors in a patient who is not able to communicate).
The findings reported by Dr. Boly and her team suggest that "selective disruption of backward connections, or top-down processes, in the cortical hierarchy can lead to loss of consciousness in brain-damaged patients and can clearly differentiate vegetative and minimally conscious states." The investigators added that the "present approach could constitute a new diagnostic tool to quantify the level of consciousness electrophysiologically at the patients’ bedside."
The researchers compared 22 healthy volunteers with 21 brain-damaged patients; study participants were aged 16-83 years. Overall, 13 brain-damaged patients were in a minimally conscious state, and 8 were in a vegetative state (Science 2011;332:858-62). In the study, the investigators included brain-damaged patients with different etiologies and clinical histories, hoping to find a common deficit that underlies changes in the level of consciousness.
Neuropsychologists performed behavioral assessments of consciousness using the French adaptation of the Coma Recovery Scale–Revised, which was developed to distinguish between patients in VS (vegetative state) and MCS (minimally conscious state), and between MCS patients and those who recovered their ability to communicate functionally.
Using a technique known as dynamic causal modeling, the researchers measured the connectivity of backward and forward connections in the temporal and frontal cortices of study participants, acquiring high-density ERP (event-related potential) recordings.
The only significant difference between patients in a vegetative state and the others was impaired backward connectivity from the frontal to the temporal cortex, the researchers found.
"Our analyses suggest that the (possibly diverse) pathophysiological causes of VS find a common expression in reducing top-down influences from frontal to temporal areas," they wrote. By contrast, patients in an MCS exhibited long-latency components in the scalp ERP and near-normal recurrent effective connectivity with higher-order cortices.
"This group difference cannot be attributable to differences in vigilance, as all patients were maintained in the same state of wakefulness throughout the experiment," they said. "Although forward connections are certainly needed for normal stimulus processing, these results suggest that the integrity of backward connections, or top-down processes, may be necessary for conscious perception."
Additional studies are needed to complement these findings in conditions such as sleep, epilepsy, or anesthesia-induced unconsciousness, the investigators noted.
The researchers received support from the Belgian national Fonds de la Recherche Scientifique (FNRS), European Commission, Mind Science Foundation, McDonnell Foundation, French-Speaking Community Concerted Research Action (ARC 06/11-340), Fondation Léon Frédéricq, and National Institutes of Health. Three authors are supported by the Wellcome Trust.