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Editorial |

Cognitive Motor Dissociation Following Severe Brain Injuries

Nicholas D. Schiff, MD1
[+] Author Affiliations
1Feil Family Brain and Mind Research Institute, Weill Cornell Medical College, New York, New York
JAMA Neurol. 2015;72(12):1413-1415. doi:10.1001/jamaneurol.2015.2899.
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A study by Fernández-Espejo et al1 in this issue of JAMA Neurology addresses the important question of what underlying brain mechanisms may account for the sharp dissociation of a retained but unrecognized (covert) cognitive capacity in some severely brain-injured patients with nonpurposeful or absent behavioral responses. During the past 10 years, several studies28 have exposed the challenging problem of understanding the biological basis and meaning of such covert cognition measured by neuroimaging or electrophysiologic techniques in patients who have complex injuries involving the central nervous system. Across many different methods tested in the literature,8 those using motor imagery techniques provide a proxy for command-following behaviors27 and unequivocal evidence, when identified, of higher-level cognitive capacities in patients otherwise fulfilling behavioral criteria for the vegetative state or low-level, minimally conscious state.8 Understanding of the possible mechanisms underlying this marked dissociation of motor and cognitive function is slowly emerging.37

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Cognitive Motor Dissociation (CMD) After Structural Brain Injuries

The distinctions among clinical disorders of consciousness can be best captured on a 2-dimensional axis by comparing the degree of impaired cognitive function against the degree of motor function. The light gray zone encompassing coma, vegetative state (VS), and the left half of the minimally conscious state (MCS) region (corresponding to low-level MCS)8 shows CMD, a clinical syndrome of patients with behavioral examinations consistent with coma, VS, or limited nonreflexive behaviors seen in patients with low-level MCS who nonetheless demonstrate command following using novel neuroimaging technologies. Cognitive motor dissociation marks the wide range of uncertainty regarding the underlying cognitive capacity (marked by inverted bracket at the bottom of the figure) present in patients with no behavioral response. It is not possible to independently judge an individual’s level of consciousness across the range of MCS to the complete locked-in state (CLIS) in any given patient with CMD without methods allowing the patient to both initiate and respond to communication. The bottom left of the figure indicates the functional equivalence of coma and VS as unconscious brain states in which no behavioral evidence of consciousness is present and both cognitive and motor function are absent (with VS differing from coma by the presence of intermittent periods of open eyes). These conditions are reflected by their placement to the left of the vertical broken line, indicating total loss of cognitive function, and below the dashed red line, indicating no motor function. The dark gray oval separating coma and vegetative state from MCS indicates a transition zone where fragments of behavior untied to sensory stimuli may be observed prior to the unequivocal but potentially intermittent behavioral evidence of consciousness demonstrated by patients with MCS; recovery of consistent goal-directed behaviors marks emergence from the MCS above the dashed green line. After emergence from the MCS, recovery patterns include the confusional state (CS) in which patients cannot be formally tested using standard neuropsychometric measures, are disoriented, and exhibit a limited range of cognitive function. The LIS designates normal conscious awareness but severe motor impairment, typically limiting communication channels to eye movements. UWS indicates unresponsive wakefulness syndrome.

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