-The vegetative state (VS) is a devastating clinical condition characterised by wakefulness without awareness. Functional neuroimaging permits to objectively measure brain responsiveness to external stimuli in VS. The literature on functional magnetic resonance imaging and positron emission tomography studies in these patients has been reviewed. Results from 15 studies were classified in: absent cortical activation or 'typical' activation of 'low level' primary sensory cortices and 'atypical' activation spreading to 'higher level' associative cortices. This descriptive review on 48 published cases suggests that 'atypical' activation patterns seem to herald recovery from VS with a 93% specificity and 69% sensitivity. Passive stimulation paradigms, however, do not permit to make strong claims about the absence or presence of consciousness. Recently proposed mental imagery paradigms permit to identify signs of consciousness in non-communicative brain damaged patients. The clinical application of these functional neuroimaging techniques awaits validation from ongoing multi-centric cohort studies in these challenging patients with chronic disorders of consciousness.
KEY WORDS: brain injury, coma, functional magnetic resonance imaging, minimally conscious state, positron emission tomography, vegetative statePatients in a vegetative state (VS) present sleep-wake cycles but show no sign of awareness of the environment or of self. An accurate and reliable judgment of VS patients' awareness is of paramount importance for their diagnosis and prognosis. In clinical practice, the evidence for the existence of VS patients' awareness comes from bedside behavioural assessment. However, theoretically, awareness is a multifaceted concept. It mainly refers to the subject's own subjective experience, which is not equal to its communicable behavioural expression. Furthermore, for disorders of consciousness like VS, motor dysfunction and arousal fluctuations render the bedside assessment of awareness challenging. 1 Misdiagnosis in VS has been shown to be as high as 37-43%. 2,3,4 Diagnosing the VS is more difficult than diagnosing brain death (ie irreversible coma with absent brainstem reflexes). For the latter, complimentary examinations exist to confirm the clinical diagnosis (eg the absence of electrical cerebral activity as shown by an electroencephalogram (EEG) or of cerebral blood flow as shown by echo Doppler, angiography or scanning techniques). 5 Such objective diagnostic markers are also needed to confirm the clinical diagnosis of VS.Ongoing developments and validation in healthy subjects of brain mapping techniques, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) now permit the use of brain 'activation studies' in clinical settings. Detecting residual brain function in VS by use of functional neuroimaging may provide useful information to the diagnosis and prognosis of these challenging patients. Such studies also provide an opportunity to study the neural correlates of c...