Uncovering ‘Covert Consciousness’ in Brain Injury Patients

Gary Boas

In a paper published in the journal Brain last month Brian Edlow and colleagues reported a study in which they used the imaging techniques functional MRI and EEG to detect ‘covert consciousness’ in the intensive care unit. We checked in with Edlow, associate director of the Center for Neurotechnology and Neurorecovery at Massachusetts General Hospital and an affiliated faculty member in the MGH Martinos Center for Biomedical Imaging, to learn more about the study and its implications for clinical care. Here’s what we found.

What is covert consciousness?

‘Covert consciousness’ is consciousness that cannot be detected by bedside examination. Studies in patients in the chronic stages of recovery from a severe traumatic brain injury (TBI) suggest that approximately 10 percent of those believed to be in a vegetative state or a low-level minimally conscious state based on the bedside exam can actually follow commands during functional MRI or EEG tests.

Why do we need a new approach to detecting covert consciousness?

Today, the bedside neurological exam is the gold standard test for assessing the level of consciousness in a patient with acute severe TBI. Studies have shown, though, that this approach can lead to misclassification of conscious patients as unconscious up to 40 percent of the time. There are a number of possible reasons for this—the patient may not be able to express herself by speaking or writing; she may have arm and leg weakness that prevents her from moving in response to a command; she may be receiving medications that sedate her; or the clinician examiner may misinterpret a purposeful movement as a reflexive, non-purposeful one—all of which underscore the need for a means to measure covert consciousness.

What were the goals of the Brain study?

The investigators set out to determine whether stimulus-based functional MRI and EEG could reveal covert consciousness in patients in the intensive care unit receiving treatment for acute severe traumatic brain injury. They also explored whether these advanced techniques could uncover higher levels of brain function, suggesting a potential for recovery of consciousness, and whether the early brain responses they observed are associated with better long-term functional outcomes.

Interestingly, they used music as well as language and motor imagery stimuli in assessing brain function. They included the music stimulus—a classical music clip with no lyrics—because they believed it would provide more information about function in the right side of the brain than the language stimulus. The latter was expected to provide more information about function in the left side of the brain.

What were the most important findings?

The researchers found evidence of covert consciousness in four patients, including three whose bedside neurological examination suggested a vegetative state. In addition, fMRI and EEG tests identified two other patients whose brains responded to language or music stimuli even though they showed no evidence of language function on bedside examination.

The findings support the idea that early detection of consciousness and brain function in the ICU could help families make more informed decisions about whether or not to continue life-sustaining therapies. Also, since early recovery of consciousness is associated with better long-term functional outcomes, functional MRI and EEG could help patients gain access to rehabilitative care once they are discharged from the intensive care unit.

The Martinos Center’s Ona Wu was co-senior author of the Brain paper, with Eric Rosenthal of the MGH Department of Neurology.