Table 2.
Studies reviewed and included in this review.
| Neuroimaging studies | ||||||
|---|---|---|---|---|---|---|
| No | References | Sample | Method | Clinical rating | Task | Results |
| Positron emission tomography (PET) | ||||||
| 1 | Stender et al. (36) | 102 DOC patients (26 VS/UWS, 76 MCS) |
FDG-PET | CRS-R | Motor and visuospatial imagery | VS/UWS showed complete bilateral hypometabolism of the associative frontoparietal cortex and no voxels with preserved metabolism. MCS showed incomplete hypometabolism and partial preservation of activity within the frontoparietal cortex |
| 2 | Stender et al. (36) | 41 DOC patients (14 VS/UWS, 21 MCS, 6EMCS) 29 healthy participants |
FDG-PET | CRS-R | Resting-State | Cerebral metabolic rate of glucose (CMRglc) reduction was observed for both VS/UWS (42%) and MCS (55%) with respect to healthy controls. It was proposed that the majority of the differences between VS/UWS and MCS are concentrated in the fronto-parietal cortex and thalamus |
| 3 | Laureys et al. (7) | 5 DOC patients (5 VS/UWS, 0MCS) 18 healthy participants |
FDG-PET | ANA | Auditory | VS/UWS patients failed to activate the temporoparietal junction cortex but activation in the auditory cortex was preserved bilaterally |
| 4 | Toyoshima et al. (37) |
6 DOC patients (emerged from VS/UWS to MCS) 30 healthy participants |
FDG-PET | CSR-R | Speech | In the patient group, a significant reduction in glucose metabolism was observed in the cerebral peduncles and bilaterally in the cingulum |
| 5 | Hermann et al. (38) | 52 DOC patients (21 VS/UWS, 31 MCS) |
FDG-PET & qEEG | CRS-R | Oddball paradigm | Adoption of qEEG alongside FDG-PET significantly improved diagnostic accuracy compared to using PET or qEEG individually (81% and 88% +/– predictive values for MCS) |
| Event-Related fMRI | ||||||
| 6 | Vogel et al. (39) | 22 DOC patients (10 VS/UWS, 12 MCS) |
fMRI | CRS-R | Mental imagery | Higher BOLD activation was observed in the mental imagery regions of 4 VS/UWS and 9 MCS patients. Improvement of consciousness was observed for all the patients who responded to tasks |
| 7 | Crone et al. (40) | 25 DOC patients (17 VS/UWS, 8MCS) 25 healthy participants |
fMRI | CRS-R | Auditory | No activation of DMN was observed in the medial regions of the VS/UWS patients, while there was a reduction in MCS patients compared to healthy subjects |
| 8 | Okumura et al. (41) | 7 DOC patients (5 VS/UWS, 2 MCS) 21 healthy participants |
fMRI | CRS | Music stimulation | In all healthy and MCS patients, activation was observed in the bilateral superior temporal gyri, but not in the VS/UWS patients |
| 9 | Monti et al. (14) | 24 DOC patients (8 VS/UWS, 16 MCS, 4 EMCS) 16 healthy participants |
fMRI | CRS-R | Auditory and visual | Different BOLD activations were observed for all the patients who have clear connectivity between the prefrontal cortex and the anterior thalamus (3 VS/UWS and 6 MCS) |
| 10 | Stender et al. (13) | 122 DOC patients (41 VS/UWS, 81 MCS) |
fMRI | CRS-R | Motor and visuospatial imagery | It was shown that the use of event-related fMRI to compare activity patterns during motor imagery tasks and rest couldn't be considered as a sensitive tool to distinguish between VS/UWS and MCS patients |
| 11 | Marino et al. (42) | 50 DOC patients (23 VS/UWS, 27 MCS) |
fMRI | GCS | Auditory | Significant BOLD activation in the primary auditory cortex was observed in both MCS and VS/UWS with higher activation in MCS. Among the VS/UWS, 10 patients with responses similar to the MCS group transitioned into MCS later on |
| Resting-State fMRI | ||||||
| 12 | Demertzi et al. (43) | 48 DOC patients (24 VS/UWS, 24 MCS) 57 healthy participants |
fMRI | CRS-R | Resting-State | Default mode network (DMN) was reduced in patients with respect to healthy subjects with 85% of accuracy. However, no obvious difference emerged between VS/UWS and MCS patients |
| 13 | Vanhaudenhuyse et al. (44) | 14 DOC patients (4 VS/UWS, 4 MCS, 5 coma, 1 LIS) 14 healthy participants |
fMRI | CRS-R | Resting-state | Decreased DMN connectivity in DOC correlated with the degree of consciousness impairment. Moreover, impairment in the precuneus connectivity was more significant in VS patients than MCS |
| 14 | Di Perri et al. (45) | 18 DOC patients (11 VS/UWS, 7 MCS) 18 healthy participants |
fMRI | CRS-R | Resting-state | DMN hypoconnectivity associated with hyperconnectivity limbic structures was found in DOC patients. Hyperconnectivity may reinforce neural loops and disrupt the normal pattern of connectivity |
| 15 | Lutkenhoff et al. (46) | 115 DOC patients (38 VS/UWS, 63 MCS, 14 EMCS) 96 healthy participants |
fMRI | CRS-R | Resting-state | In DOC patients, awareness and wakefulness were associated with tissue atrophy within thalamic and basal ganglia nuclei, respectively, which were inversely correlated with the consciousness level. No significant differences found among patient groups |
| 16 | Long et al. (47) | 18 DOC patients (11 VS/UWS, 7 MCS) 13 healthy participants |
fMRI | CRS-R | Resting-state | In DOC patients, the combination of topological patterns observed in the FPN and DMN showed a better ability to predict consciousness compared to other networks |
| 17 | Di Perri et al. (48) | 58 DOC patients (21 VS/UWS, 24 MCS, 13 EMCS) 35 healthy participants |
fMRI | CRS-R | Resting-state | Positive DMN differentiated heathy from DOC patients but not VS/UWS from MCS. Negative DMN was observed only in healthy subjects and EMCS |
| 18 | Fernández-Espejo et al. (49) | 52 DOC patients (19 VS/UWS, 27 MCS,6 EMCS) 23 healthy participants |
fMRI | CRS-R | Resting-state | Congruence between posterior areas of the DMN correlated with the level of consciousness, which was higher in MCS than VS/UWS patients |
| 19 | Kondziella et al. (50) | 7 DOC patients (1 VS/UWS, 3 MCS, 1 EMCS,2C) |
fMRI | CRS-R | Resting-state | Preserved DMN activity correlated with the recovery of DOC patients. The structural integrity of this network correlated with behavioral signs of consciousness. Normal DMN was found in one MCS patient |
| 20 | Demertzi et al. (51) | 101 DOC patients (32 VS/UWS, 59 MCS) 58 healthy participants |
fMRI | CRS-R | Resting-state | Measures of inter-regional activity coordination were used to identify patterns associated with VS/UWS (low coordination pattern) and wakefulness (long-range coherence pattern) |
| 21 | Varley et al. (52) | 18 DOC patients (8VS/UWS, 10 MCS) 15 healthy participants |
fMRI | Not indicated | Resting-state | Fractal values were associated with the level of consciousness, with higher fractal (i.e., more complex) networks being associated with higher levels of consciousness |
The table shows the main results reported in the literature reviewed for the neuroimaging methods.