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Detecting 'Covert Consciousness' in the ICU With TMS-EEG

In This Article

  • Historically, clinicians have used bedside behavioral examination to assess the level of consciousness in patients who experience severe brain injury
  • However, because not all patients are able to respond to the clinicians' commands, bedside behavioral exams may underestimate the level of consciousness in up to 20% of the patients in the intensive care unit
  • More accurate assessments of consciousness are needed, especially because the assessments are often important factors in families' decisions as to whether to continue life support
  • At Massachusetts General Hospital, Brian Edlow, MD, is exploring the use of transcranial magnetic stimulation and electroencephalography for accurate evaluation of 'covert consciousness,' consciousness that cannot be determined using conventional bedside exams

Every year, more than one million people across the globe are impacted by severe brain injury (TBI). Bedside behavioral examination has long been the gold standard test for assessing the level of consciousness in patients who experience such injury. Still, not all patients can respond to the clinician's commands during the test because they cannot express themselves by speaking or writing, they have arm and leg weakness that prevents them from moving in response to a command, or another reason.

Studies have shown that 15%-20% of patients in the intensive care unit (ICU) may have higher levels of consciousness than their bedside behavioral examination suggests. Consciousness that cannot be detected with the bedside exam is labeled "covert consciousness."

At Massachusetts General Hospital, Brian Edlow, MD, associate director of the Center for Neurotechnology and Neurorecovery in the Department of Neurology, a faculty member in the Athinoula A. Martinos Center for Biomedical Imaging, and Chen Institute MGH Research Scholar 2023-2028, is leading the charge in developing a fuller understanding of covert consciousness.

This work is especially important because the bedside behavioral exam results often inform families' decisions on whether to continue life support. Clinicians' assessments of the patient's level of consciousness have been cited as the most significant factor in families' decisions. Without a fuller understanding of what is happening in the brain, they may choose to withdraw the support prematurely. Discontinuation of life-sustaining therapy is the cause of death for up to 80% of patients with acute severe brain injuries.

In recent years, Dr. Edlow and colleagues have shown that task-based functional MRI (fMRI) and EEG could reveal covert consciousness in patients in the ICU receiving treatment for acute severe traumatic brain injury (TBI).

In a study reported in 2017 in Brain, he and collaborators including Ona Wu, PhD, of the Martinos Center for Biomedical Imaging, and Eric Rosenthal, MD, of the Department of Neurology, found evidence of covert consciousness in four patients, including three whose bedside neurological examination suggested a vegetative state.

However, fMRI is both expensive and not commonly available in clinical settings. Seeking a more practical solution, Dr. Edlow explored other neuromonitoring techniques and found a well-suited alternative in transcranial magnetic stimulation electroencephalography (TMS-EEG).

This approach works by inducing electric currents in the cerebral cortex using changing magnetic fields (TMS) and then measuring the complexity of the response with electrodes attached to the scalp (EEG) and quantifying the response as the perturbational complexity index (PCI). In studies reported over the past decade, PCI values above 0.31 have been shown to predict the presence of consciousness with 100% specificity and 100% sensitivity in a dataset of 150 conscious individuals across a broad range of behavioral states, including wakefulness, sleep, and anesthesia. Furthermore, TMS-EEG measurements of PCI detect high brain complexity in approximately 95% of patients with severe brain injuries who have recovered to a minimally conscious state, outperforming other advanced diagnostic tests.

Translation of this technology is now entering an exciting new phase. In March, Dr. Edlow was announced as a 2023 MGH Research Scholar. Funded entirely by philanthropic gifts, the MGH Research Scholars Program provides forward-thinking researchers with the unrestricted support they need to take their work into new and uncharted territories.

The five-year grant accompanying the honor will help advance his project, "Detecting Covert Consciousness in the Intensive Care Unit." In this project, he will lead a multidisciplinary team that will translate TMS-EEG to the ICU, optimize the technique for use in patients with acute severe brain injuries, and assess its ability to detect and predict recovery of consciousness in these patients.

"TMS-EEG has shown unparalleled accuracy at detecting consciousness in patients with chronic brain injuries, providing a compelling clinical and ethical rationale for translating TMS-EEG to the ICU," he says. "This project has the potential to save lives by detecting signs of covert consciousness and preventing premature withdrawal of life-sustaining therapy."

Learn more about the Lab for NeuroImaging of Coma and Consciousness

Learn more about the Athinoula A. Martinos Center for Biomedical Imaging


The rate of misclassification of unconsciousness can be as high as 40%. A Mass General team studied using fMRI and EEG to better identify patients with unconsciousness in the ICU.


Michael J. Young, MD, MPhil, Brian L. Edlow, MD, and colleagues recently published a rich history of the neuroethics of disorders of consciousness (DoC) with special attention to the ethics of using functional MRI and EEG to detect covert consciousness.