Assessment of Covert Consciousness in the Intensive Care Unit: Clinical and Ethical Considerations

Abstract

Objective: To propose a practical ethical framework for how task-based functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) may be used in the intensive care unit (ICU) to identify covert consciousness in patients with acute severe traumatic brain injury (TBI).

Methods: We present 2 clinical scenarios in which investigational task-based fMRI and EEG were performed in critically ill patients with acute severe TBI who appeared unconscious on the bedside behavioral assessment. From these cases, we consider the clinical and ethical challenges that emerge and suggest how to reconcile them. We also provide recommendations regarding communication with families about ICU patients with covert consciousness.

Results: Covert consciousness was detected acutely in a patient who died in the ICU due to withdrawal of life-sustaining therapy, whereas covert consciousness was not detected in a patient who subsequently recovered consciousness, communication, and functional independence. These cases raise ethical challenges about how assessment of covert consciousness in the ICU might inform treatment decisions, prognostication, and perceptions about the benefits and burdens of ongoing care.

Conclusions: Given that covert consciousness can be detected acutely in the ICU, we recommend that clinicians reconsider evaluative norms for ICU patients. As our clinical appreciation of covert consciousness evolves and its ethical import unfolds, we urge prognostic humility and transparency when clinicians communicate with families in the ICU about goals of care.

Figure 1.. Assessment of consciousness in the intensive care unit.

The standard behavioral assessment used to evaluate a patient’s level of consciousness in the intensive care unit (ICU) is the Glasgow Coma Scale (GCS), as depicted in the left panel. For a patient whose eyes are open and whose limb movements appear reflexive on the GCS, suggesting a vegetative state (VS), there is an approximately 40% chance that consciousness will be detected by a comprehensive behavioral assessment with the Coma Recovery Scale-Revised (CRS-R). The use of a mirror for gaze tracking, as depicted in the middle panel, is the CRS-R procedure that is most likely to detect evidence of consciousness missed by the GCS. For a patient whose CRS-R assessment suggests VS or a low-level minimally conscious state (MCS-), there is an approximately 20% chance that consciousness will be detected by task-based functional MRI (fMRI) or EEG.^{, ,} Task-based fMRI involves providing verbal instructions to a patient via earphones to peform a motor imagery task, which results in an increase in the blood-oxygen level dependent signal within the supplementary motor and premotor cortices, as shown in the right panel.

Figure 2.. Functional MRI detection of covert consciousness in the intensive care unit.

A functional MRI (fMRI) scan was performed for Patient 1 with language stimuli, music stimuli, and a motor imagery task (i.e. imagine squeezing your right hand) on post-injury day 8, at which time the Coma Recovery Scale-Revised assessment suggested a behavioral diagnosis of vegetative state. FMRI revealed association cortex responses to language and music stimuli, as well as volitional command-following during the motor imagery task (arrow), indicating covert consciousness. Unaware of these investigational data, the patient’s surrogates decided to pursue comfort care and he died on post-injury day 11 in the ICU. All fMRI data are shown as Z-statistic images thresholded at cluster-corrected Z scores of 3.1 (inset color bar) and superimposed upon T1-weighted axial images. In the bottom panel, a three-dimensional rendering of fMRI activation during the motor imagery task is shown (arrow). Abbreviations: Ins = insula; LV = lateral ventricle; PMC = premotor cortex. Images adapted with permission from Edlow et al.

Figure 3.. The three dimensions of assessing consciousness.

The patients reported here were evaluated for motor function and overt cognitive function via behavioral assessment with the Coma Recovery Scale-Revised (CRS-R). Covert cognition was assessed with functional MRI (fMRI) and EEG. Levels of consciousness indicated by overt cognition are defined as coma, vegetative state (VS), minimally conscious state without language function (MCS-), minimally conscious state with language function (MCS+), post-traumatic confusional state (PTCS), complete locked-in syndrome (CLIS), locked-in syndrome with preservation of minimal motor function (LIS), and full recovery. Covert consciousness (i.e. cognitive motor dissociation [CMD]) is defined by fMRI or EEG evidence of command-following despite absence of behavioral evidence of language function. Higher-order cortex motor dissociation (HMD) is defined by fMRI and EEG responses within association cortex (e.g. Wernicke’s area) during passive language or music stimuli despite absence of behavioral evidence of language. Using the behavioral diagnosis as the reference standard, patients without behavioral evidence of language (coma, VS, and MCS-) are classified as true negatives (TN) if there are no fMRI or EEG responses. Patients with behavioral evidence of language (MCS+, PTCS, CLIS, LIS, and full recovery) are classified as false negatives (FN) if there are no fMRI or EEG responses, and true positives (TP) if there are fMRI and EEG responses. Abbreviation: CAP = Confusion Assessment Protocol. Figure reproduced with permission from Edlow et al.