Measuring consciousness in coma and related states

Table 1 Key points of the review

Novel neuroimaging techniques in patients with DOC give important key insights into both the understanding of consciousness and the differential diagnosis of clinical DOC entities, given that behavioural assessment alone can sometimes be incorrect and imprecise

Conventional MRI and DTI investigates the structural properties of the brain and the white matter integrity. These studies showed mainly a predictive rather than diagnostic value

PET activations show a critical role of a wide frontoparietal associative network for the emergence of consciousness

fMRI employing active paradigm detects covert awareness in approximately 17% of unresponsive patients at bedside. However, there is a high risk of false negative. fMRI employing passive paradigm shows also a prognostic value. fMRI during resting state shows a broad alteration of brain connectivity, implying both decreased and increased connectivity in patients with DOC

TMS-EEG shows a high diagnostic value even at single subject level

DOC: Disorders of consciousness; MRI: Magnetic resonance imaging; DTI: Diffusion tensor imaging; PET: Positron emission tomography; fMRI: Functional magnetic resonance imaging; TMS-EEG: Transcranial magnetic stimulation coupled with electroencephalography.

Table 2 Main strength and limits of the different techniques

Technique	Strenght	Limits
PET	Relatively direct measure of brain activity	Ionizing, radioactive tracer, low spatial and temporal resolution expensive
MRI	No use of ionizing. Permits both high resolution study of structural brain (DTI) and fMRI employing active, passive and resting state paradigms	Indirect measure of brain activity (functional) Sensitive to movement and artifacts, impractical (application precluded in patients with contraindication), expensive
TMS-EEG	Practical (no important contraindications) gives information at single subject level	Sensitive to muscle artifacts

PET: Positron emission tomography; MRI: Magnetic resonance imaging; TMS-EEG: Transcranial magnetic stimulation coupled with electroencephalography; DTI: Diffusion tensor imaging; fMRI: Functional magnetic resonance imaging.