HELSINKI, Finland – Sept. 2, 2013 -- According to a study recently published in the journal Science Translational Medicine, Navigated Brain Stimulation (NBS) can be used as a reliable, objective measurement of consciousness in patients who are unable to respond to commands due to brain injury.
A group of international researchers led by a team at University of Milan in Italy used an NBS-EEG technique to effectively measure the amount of information flow occurring in the brain in response to a stimulus. They were able to discriminate between various levels of consciousness and developed a numerical index of human consciousness, the perturbational complexity index (PCI). In the study, measurements were initially made on 32 healthy subjects and subsequently on 20 brain-injured patients who had been in coma and subsequently recovered to various levels of consciousness.
PCI is calculated from principles derived from theoretical neuroscience. When TMS effectively elicits a significant cortical response it directly engages large portions of the thalamocortical system without requiring the patient to perform any sensory, motor, or cognitive task. A patient´s capacity for consciousness can then be assessed based on the complexity of induced cortical interactions, independent of the patient´s ability or willingness to react to external stimuli. PCI´s apparent usefulness, as shown in this study, supports the notion that consciousness is linked to complexity - measured as the information content of distributed causal interactions in the brain.
Because the stimulus needs to be applied to structures of the brain which have not been structurally damaged - in order to avoid damaged tissue potentially impacting the cortical response - the researchers used an Nexstim NBS System. Using the MRI data for guidance, the operator can see, on the NBS System screen, the exact location of the stimulating electric field in the patient´s cortex and confirm that the stimulus targets structures that have not been damaged.
The PCI number derived from the NBS-EEG technique could be particularly useful in assessing recovery of patients in intensive-care units with low levels of consciousness. According to Professor Massimini, one of the study authors, no such objective measure currently exists, resulting in high rates of incorrect diagnoses. Commercial depth-of-anesthesia monitors, now commonly used in the operating room, are difficult to apply in these challenging cases.
In comments accompanying the paper, the researchers emphasized that measures that can reliably distinguish vegetative states from minimally conscious states are crucial and will have an impact on clinical practice. "It will be very important to perform measurements right in the ICU in the acute phase to have an objective marker of what´s happening and to track improvements occurring spontaneously or brought about by treatment," Professor Massimini said in an interview by the news agency Bloomberg. "If you have a number, you can start working towards an evidence-based treatment," he continued.
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