Probing the effects of mild traumatic brain injury with transcranial magnetic stimulation of the primary motor cortex

Primary objective: The present paper systematically reviews studies using transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) to assess cortical excitability, intra-cortical inhibition/facilitation and synaptic plasticity following mild traumatic brain injury (mTBI). Methods: Articles using TMS over M1 in patients with mTBI or sport-related concussion indexed in PubMed and published between 1998 and September 2014 were included in the present review. Main outcomes and results: From the 17 articles that matched search criteria, results from various TMS paradigms were summarized and divided in three main areas of interest: motor cortical excitability/facilitation, motor cortical inhibition and cortical plasticity. Although studies suggest a trend of abnormal intra-cortical inhibition following mTBI, no clear and specific pattern emerges from the surveyed data. Conclusions: At this time and with the possible exception of intra-cortical inhibitory measures, TMS cannot reliably detect changes in M1 excitability in individuals with mTBI or a concussion at both the acute and chronic stages of injury. This may be explained by the small number of studies and large variety of stimulation parameters. Additional longitudinal and multimodal studies are needed to better understand the nature of the excitability changes that may occur within M1 following mTBI.