Rethinking interhemispheric imbalance as a target for stroke neurorehabilitation.

OBJECTIVE: 

Patients with chronic stroke have been shown to have failure to release interhemispheric inhibition from the intact to the damaged hemisphere prior to movement execution (pre-movement-IHI). This inhibitory imbalance was found to correlate with poor motor performance in the chronic stage after stroke, and has since become a target for therapeutic interventions. The logic of this approach, however, implies that abnormal pre-movement-IHI is causal to poor behavioral outcome, and should therefore be present early after stroke when motor impairment is at its worst. In a longitudinal study, we investigated interhemispheric interactions by tracking patients' pre-movement-IHI for one year following stroke.

METHODS: 

We assessed pre-movement-IHI and motor behavior five times over a one-year period after ischemic stroke in 22 patients, and in 11 healthy participants.

RESULTS: 

We found that pre-movement-IHI was normal during the acute/subacute period, and only became abnormal at the chronic stage; specifically, release of IHI in movement preparation worsened as motor behavior improved. In addition, pre-movement-IHI did not correlate with behavioral measures cross-sectionally, while the longitudinal emergence of abnormal pre-movement-IHI from the acute to the chronic stage was inversely correlated with recovery of finger individuation.

INTERPRETATION: 

These results suggest that interhemispheric imbalance is not a cause of poor motor recovery but instead might be the consequence of underlying recovery processes. These findings call into question the rehabilitation strategy of attempting to rebalance interhemispheric interactions in order to improve motor recovery after stroke.