Functional MRI (fMRI) of Patients in a Vegetative or Minimally Conscious State

After a brain injury, distinction between coma, vegetative state, minimally conscious state, akinetic mutism, and locked-in syndrome is a complex challenge for the neurologist (Giacino 1997, Gaciano 2004). The limits which separates these states, and even the definition of "consciousness" are unclear. Some behavioral tests can help in the differential diagnosis, but are impaired by the lack of input/output caused by lesions in primary areas or in white matter pathways.

Minimally conscious state (MCS), which refers to patients with severe brain damage but who demonstrate unequivocal, but intermittent, behavioral evidence of awareness of self or their environment (Giacino et al. 2002), is the most interesting state to study with functional MRI. Identifying any form of consciousness can have great psychological impact on the patient and its family and potentially influence the therapeutic approach.

Functional MRI has the power to detect brain activation elicited by a stimuli even in the absence of patient output. It could therefore be used as a marker of brain activity or even "consciousness" in patient in a vegetative or minimally conscious state. Shiff et al. found similar activations in 2 patient in a MCS than in a group of healthy volunteers presented with passive language (Wernicke's area) and tactile stimulation (post-central gyrus) paradigms. It suggests that some MCS patients may retain widely distributed cortical systems with potential for cognitive and sensory function despite their inability to follow simple instructions or communicate reliably.

Owen et al. 2006 applied a more complex paradigm to further investigate the state of consciousness of a patient in a vegetative state. The patient was asked to perform 2 mental imagery tasks: tennis playing versus walking in a her house. Brain activations observed were identical to the one obtained in healthy volunteers: supplementary motor area (SMA) while mentally playing tennis, parahippocampal gyrus, posterior parietal cortex, and lateral premotor cortex while visiting her house. Of the 54 patients enrolled in a study performed by the same group (Monti et al. 2010), 5 were able to willfully modulate their brain activity. One patient, without any form of communication at the bedside, was able to modulate his brain activity (tennis playing versus walking in his house) to answer yes or no to questions during functional MRI.

Resting state fMRI, i.e. functional MRI without any stimulus, has also been used to assess patients with disorders of consciousness. The default network is defined as a number of areas including the precuneus, bilateral temporo-parietal junctions and medial prefrontal cortex, which are more active at rest than when the subjects are involved in an attention-demanding cognitive task. Vanhaudenhuyse et al. 2009 have showed that the connectivity of this default network is decreased in severely brain-damaged patients, in proportion to their degree of consciousness impairment.