Potential EEG Biomarker for Presence of Cognition in Patients With Severe Brain Injury
In a paper to be published in the online issue of Current Biology, researchers present evidence of EEG response to natural speech in patients with disorders of consciousness (DOC) that correlates to having a sufficient level of cognition to successfully carry out functional MRI (fMRI) mental imagery tasks. This may eventually provide a new cost-effective, easy-to-administer, and objective method for identifying patients with hidden high-level cognitive capacities. It is hoped that use of this method could lead to improvements in diagnosis, prognosis, and treatment for all patients with DOC.
In healthy subjects, EEG activity follows the natural speech envelope (NSE) (ie, normal fluctuations in the upper/lower decibel levels of narrative speech) with a ~0.10 second delay. In the data presented, the EEG latency to NSE was measured in patients who had been clinically defined with disorders of consciousness (vegetative state [VS; n = 5], MCS [n = 2], or emerging from MCS [EMCS; n = 3]) and compared with healthy subjects (n = 13). Patients with DOC were reassessed with the JFK Coma Recovery Scale—Revised (CRS-R), and all subjects were asked to perform fMRI mental imagery tasks known to be cognitively demanding.
Using the CRS-R, patients were redefined as being in a VS (n = 3), MCS (n = 12) or EMCS (n = 6). Of these patients, 7 with MCS and 3 with EMCS had fMRI activation in response to an active imagery task. These patients’ NSE-EEG latencies did not differ significantly from those seen in healthy control subjects. For the 11 patients who did not have fMRI activation in response to mental imagery tasks, the difference in NSE-EEG compared to healthy subjects, or to those with brain injury who did have fMRI responses, increased progressively with severity of brain injury as measured with the CRS-R (P< .001 for all comparisons).
These findings support the idea that there is a category of patients for whom the presence of cognition does not correlate with behavioral assessments for DOC, and that for some patients, a lack of motor response in behavioral assessment can mask presence of cognition. For these patients with “cognitive-motor dissociation,” this new measure of NSE-EEG may serve as a marker for the presence of cognition that warrants further investigation for more specific diagnosis, prognosis, and treatment planning.