No document available.
Abstract :
[en] A known problem in neurophysiological research is the lack of reproducibility. This is especially relevant when the research concerns disorders of consciousness (DoC). Obtaining reliable data is made challenging by the heterogeneity of clinical profiles and brain lesions in DoC patients. As meta-analyses rank among the strongest research designs based on hierarchical levels of evidence, we here use this approach to quantitatively synthesise existing findings based on EEG, MEG and fNIRS in DoC patients.
In January 2022, using databases MEDLINE via Ovid, and Scopus and Embase via Elsevier, we conducted a literature search for resting-state EEG, MEG, and fNIRS studies published from 2000 to 2022, involving adults with prolonged DoC and a primary diagnosis of unresponsive wakefulness syndrome (UWS) or minimally conscious state (MCS) based on a validated behavioural scale. Two referees independently screened abstracts and full texts of potentially relevant studies, followed by extraction of useful data. Authors were contacted when statistics were not available in the published materials. Separate random effect meta-analyses were conducted to compare global metrics between UWS and MCS, and healthy controls (HC) using the R package 'meta'. The full protocol, including the search strategy using controlled vocabulary and keyword terms, is available on PROSPERO (CRD42022327151).
As of January 2023, from a total of 3563 unique studies, 21 were found eligible for inclusion in the meta-analysis, spanning a total of 499 UWS and 508 MCS patients, and 240 HC. We here report only measures appearing in three or more studies, all of these EEG-based. For comparisons between groups of subjects, differences were found across multiple frequency bands for measures of power, connectivity, and graph theory (i.e., participation coefficient). Large effects were only apparent between HC and DoC (both UWS and MCS), with power and connectivity in δ and α bands consistently showing significant differences, along with power in the β band. Still, heterogeneity between studies was often considerable, and larger for UWS than MCS when compared to HC. Power in δ and α bands, as connectivity in α band also differed between UWS and MCS, with medium effect sizes. Furthermore, UWS and MCS showed different power and participation coefficient in θ and connectivity in β band, all with small effect sizes.
Despite heterogeneity, we found major differences in EEG δ and α measures between DoC patients and HC; albeit the effect was smaller, δ and α measures consistently differed also between UWS and MCS, possibly indicating that these measures might be of interest for diagnostic purposes. The relatively larger variability observed in UWS compared to MCS studies supports the notion of heterogeneity of the former group, with part of UWS patients likely presenting covert awareness, i.e., behaviourally unresponsive but with residual brain activity similar to MCS.
