Functional imaging of severely brain-injured patients: Progress, challenges, and limitations
Despite the wide application of functional neuroimaging techniques to the study of neurological disorders, few reports have examined patterns of brain activity following severe injuries. In this review, we discuss recent functional brain imaging studies of patients in the vegetative state (VS) and pilot studies of patients meeting diagnostic criteria for the newly formulated minimally conscious state (MCS). These neuroimaging studies support the clinical model of VS and provide neurophysiologic correlates of the condition. In addition, the studies suggest a foundation for the conceptual separation of VS and MCS on the basis of distinct physiological underpinnings. We consider the widely varying responses drawn by the definition of MCS and argue for the relevance and importance of nosological distinctions, even within the lowest functional levels observed among patients who remain severely disabled. As further neuroimaging efforts are applied to study the outcomes of severe brain injuries, it is expected that fundamental questions will arise surrounding the underlying mechanisms of similar functional disabilities. Although it is possible that some MCS patients may harbor greater residual cognitive capacities, other MCS patients may never reach a level of reliable communication. Functional imaging studies in conjunction with other physiological and anatomical measurements will help develop an approach to these disorders based on differences in underlying mechanisms, eventually broadening the expertise of the neurologist.