Evaluation of Corticospinal and Intracortical Excitability After Stroke Using Paired-Pulse Transcranial Magnetic Stimulation: A Narrative Review.

Background

Alterations in corticospinal and intracortical excitability are frequently observed following a stroke. Cortical plasticity and reorganization after stroke can be assessed using transcranial magnetic stimulation (TMS) measures. These include Motor Threshold (MT) and Motor Evoked Potential (MEP) obtained from single-pulse TMS and Short-Interval Intracortical inhibition (SICI) and Intracortical Facilitation (ICF) obtained from Paired-Pulse Paradigms (ppTMS).

Objective

This narrative review aims to provide an update on the most relevant studies and clinically interpret alterations in excitability measures from ppTMS studies following a stroke, focusing on the effects of stroke on MT, MEP, SICI, and ICF in both hemispheres. Additionally, we analyze intracortical disinhibition across different stages of recovery and the implications for inter-hemispheric competition during recovery. Literature search strategy: Studies investigating corticospinal excitability and intracortical inhibition/facilitation after stroke using ppTMS paradigms were reviewed. The main neurophysiological outcomes included MT, MEP amplitude, SICI, and ICF, comparing the affected and unaffected hemispheres and healthy controls.

Results

The reviewed literature consistently demonstrates reduced corticospinal excitability in the affected hemisphere, characterized by an increased MT and decreased MEP amplitude. Conversely, intracortical inhibitory and facilitatory circuits appear to be altered after stroke. Reduced SICI and modified ICF patterns suggest cortical disinhibition.

Conclusions

Assessments based on ppTMS can be considered significant indicators of post-stroke cortical reorganization. Cortical motor inhibition may have an adaptable or stage-specific role, potentially beneficial in the acute and subacute stages, but harmful if it continues beyond the chronic phase.