A Systematic Review and Meta- Analysis of the Relationship Between Brain Data and the Outcome in Disorders of Consciousness

A systematic search revealed 68 empirical studies of neurophysiological [EEG, event-related
brain potential (ERP), fMRI, PET] variables as potential outcome predictors in
patients with Disorders of Consciousness (diagnoses Unresponsive Wakefulness
Syndrome [UWS] and Minimally Conscious State [MCS]). Data of 47 publications could
be presented in a quantitative manner and systematically reviewed. Insufficient power
and the lack of an appropriate description of patient selection each characterized about
a half of all publications. In more than 80% studies, neurologists who evaluated the
patients’ outcomes were familiar with the results of neurophysiological tests conducted
before, and may, therefore, have been influenced by this knowledge. In most subsamples
of datasets, effect size significantly correlated with its standard error, indicating
publication bias toward positive results. Neurophysiological data predicted the transition
from UWS to MCS substantially better than they predicted the recovery of consciousness
(i.e., the transition from UWS or MCS to exit-MCS). A meta-analysis was carried
out for predictor groups including at least three independent studies with N > 10 per
predictor per improvement criterion (i.e., transition to MCS versus recovery). Oscillatory
EEG responses were the only predictor group whose effect attained significance for both
improvement criteria. Other perspective variables, whose true prognostic value should
be explored in future studies, are sleep spindles in the EEG and the somatosensory
cortical response N20. Contrary to what could be expected on the basis of neuroscience
theory, the poorest prognostic effects were shown for fMRI responses to stimulation
and for the ERP component P300. The meta-analytic results should be regarded as
preliminary given the presence of numerous biases in the data.