Is cognitive motor dissociation just a minimally conscious state "plus" by another name?

Abstract

Cognitive Motor Dissociation (CMD) describes a condition whereby brain injury patients can demonstrate response to command through advanced electrophysiology and imaging assessments but are unable to do so through in standard, behavior-based, clinical assessments. Rightfully, significant emphasis has been placed on the fact that despite a similar behavioral phenotype, patients with CMD show better outcomes than patients without CMD. Yet, this finding is not overly surprising when considering that patients with CMD might just be minimally conscious state "plus" patients (MCS+; i.e., patients capable of response to command) who were misdiagnoses due to the known limitations of behavioral assessments in the presence of sensory, cognitive, or motor comorbidities. The present work brings together 131 DOC patients, from two separate longitudinal studies, to assess whether patients able demonstrate response to command via brain responses but not behavioral responses (i.e., CMD patients) are "just" misdiagnosed MCS+ patients, in terms of short-term outcomes, or whether they represent a separate diagnostic entity. Robust general linear modelling reveals that, while CMD patients show greater short-term gains than patients with no evidence of CMD, consistent with prior work, these gains are not different from those seen in patients who can demonstrate response to command behaviorally (i.e., MCS+ patients). This pattern of results remains unchanged when separately analyzing Vegetative State (VS; i.e., entirely unresponsive) and Minimally Conscious State "minus" patients (MCS-; i.e., patients only able to show non-language-mediated response) with and without CMD, and when restricting analyses to traumatic brain injury patients only. These findings suggest that, at least in terms of short-term outcomes, patients with CMD are not meaningfully different from MCS+ patients. Rather, CMD patients are best understood as MCS+ patients who were misdiagnosed likely due to the well-known limitations of behavioral assessments in the presence of comorbidities affecting sensory input, cognitive processing, and/or motor output. These results thus support the suggestion by the European Union practice guidelines to assign diagnoses based on the highest level of response obtained in a patient across behavioral and non-behavioral assessments, as well as the use of advanced assessments not only in behaviorally VS patients, consistent with the US guidelines, but also in MCS- patients. Finally, from a nosological perspective, these findings suggest that patients with CMD might best be described as "MCS+ patients with CMD," to convey at once their true level of consciousness (i.e., MCS+) and the presence of motor output limitations (i.e., CMD).