Changes in cortical plasticity after mild traumatic brain injury.

PURPOSE: Even after a mild traumatic brain injury (TBI) symptoms may be long lasting and never resolve completely. The neurophysiologic substrate for such lasting deficits remains unclear. There is a lack of objective measures of early brain abnormalities following mild TBI, which could shed light on the genesis of these lasting impairments.

METHODS: Here we report findings in a previously healthy man tested 2 and 6 weeks after a well-documented concussion. Findings were compared with 12 control subjects. All subjects underwent brain magnetic resonance imaging (MRI) and diffusion-tensor imaging (DTI). Testing included neuropsychological evaluation and physiological assessment with TMS and EEG, excitatory/inhibitory balance and brain plasticity.

RESULTS: While the MRI, DTI and neuropsychological evaluations showed no abnormalities, neurophysiologic tests revealed subclinical abnormalities in our patient: (1) Significantly higher intracortical facilitation than the control group at both time points; (2) Intracortical inhibition presumably mediated by GABAB receptors was absent at week 2, but returned to normal value at week 6; (3) Abnormal mechanisms of plasticity at week 2, that normalize at week 6.

CONCLUSIONS: These findings demonstrate a transient alteration of brain cortical physiology following concussion independent of anatomical findings and neuropsychological function. This case study suggests that TMS measures may serve as sensitive biomarkers of physiologic brain abnormalities after concussion.