Problem: A biomarker that accurately predicts recovery of ischemic stroke for patients with poor baseline Fugl-Meyer Assessment of the Upper Extremity (FMA-UE) is lacking. Biomarkers that predict recovery while providing functional insight into the underlying neural process are highly desired for optimal clinical care. Objectives: This explorative study aims to determine potential biomarkers from Somatosensory Evoked Potentials (SEP) using ElectroEncephaloGraphy (EEG). Brain asymmetry metrics are derived from high-density EEG recordings of five longitudinal stroke patients during the first six months of stroke recovery. In addition, EEG sources and their interactions, constrained by anatomical information from (d)MRI, are explored for a potential biomarker. Results: Subjects with low baseline FMA-UE show a trend of increased recovery as the Laterality Index (LI) of the infarcted hemisphere increases, although it is not significant within this small group. The LI of the non-infarcted hemisphere shows a significant trend of high LI values at baseline that decrease to lower values during recovery. This effect was tested on a different dataset of 17 longitudinal stroke patients (without dMRI data). The results in this second group showed strong variability between subjects and measurements. The non-infarcted hemisphere is able to significantly predict the FMA-UE for both datasets. On the other hand, estimated effects for the LI of the infarcted hemisphere did not show significant values. Biomarkers were not derived from EEG sources and their interactions. Conclusion: In this study, we have demonstrated the potential of the LI as a biomarker for stroke recovery. Small sample size and absence of controls make hard clinical conclusions impossible. However, our findings show that the LI might predict recovery of the FMA-UE, even at low baseline FMA-UE. The LI of the non-infarcted hemisphere is able to significantly predict the FMA-UE of the affected hand, a surprising discovery that might be attributed to background cortical activity or ipsilateral SEP components. Derivations from EEG sources and their interactions might lead to more sensitive metrics and novel insights into stroke rehabilitation. Significance: If the discovered trends apply to a larger sample size, asymmetry metrics from SEP’s have prognostic value in stroke recovery.

Transcranial Direct Current Stimulation (tDCS) is an upcoming therapeutic tool to improve post-stroke upper extremity motor function by modulating cortical excitability. However, research shows mixed findings on tDCS in stroke patients. The functional and structural brain reorganization may explain the mixed findings: (i) The individual motor function, i.e. the target for tDCS, may be relocated to a non-affected brain region, and (ii) the structural characteristics, i.e. conductivity, of the lesion influences the current propagation in the brain.
This study evaluated the effect of individual optimized tDCS configurations for 21 chronic stroke patients and ten healthy controls using model simulations. We optimized tDCS configurations for two individual targets: (i) a functional target based on EEG during a functional hand task and (ii) an anatomical target based on MRI. The individual optimized configurations are compared with conventional configurations targeting the contralateral and ipsilateral primary motor cortex. The configurations are compared based on the normal component of the simulated electric field at both the functional and anatomical targets. Here we consider a negative field strength inhibitory, therefore undesirable.
The functional target was found ipsilateral in eight of the twenty-one stroke subjects, which indicates the relevance of individual functional targets. The conventional configurations simulated for all healthy subjects a positive field strength in the anatomical target. However, our simulations showed that conventional tDCS can result in negative stimulation for seven stroke subjects in the functional target and two stroke subjects in the anatomical target. The simulations may explain the lack of consistent therapeutic effects of conventional tDCS in stroke patients, and highlight the need for individualized tDCS configurations.