He was tested twice, 2 and 6 weeks after the present concussion. His findings were compared to a control group of 12 healthy adults (5 females), aged between 19 and 55 years old (30+-14). We looked for control subjects spanning a wide age range to minimize the potential impact of age-related effects. The age of the patient was included within this age range. Comparison of the results in our patient with a subset of the control subjects closer to his age (mean 44 years of age) did not affect any of the reported findings. The control subjects had no psychiatric or neurological conditions, normal neurological and medical exams, no history of concussions or TBI, were not taking any medications, and had no contraindications to receive TMS.
All participants tolerated the procedures, including TMS, without any side effect or complication. In particular, the patient did not have any headache either just before or at the end of each of the session. No other adverse events (pain, difficulty in concentrating, thinking, change in mood) were reported. All participants gave written informed consent for participation in the study, which had been approved by the local Institutional Review Board.
Brain MRI and DTI studies were obtained using a 3T GE scanner. Anatomical images were acquired using a T1-weighted, three-dimensional, magnetization-prepared, rapid-acquisition, gradientecho (MPRAGE) volume acquisition with a voxel resolution of 1 mm3. Diffusion images were acquired using a diffusion-weighted, single-shot, spin-echo, echo-planar imaging sequence (TE = min, TR = 17,000 ms, FOV 96 mm, voxel size = 2.2 mm 3, 30 nonlinear directions with b-value of 1,000 s/mm2). For precise quantitative measurement of changes in structural MRI, voxel-based morphometry (VBM) was done using FSL-VBM toolbox implemented in the FSL. Brain was extracted and created the study-specific template. All the images were non-linearly registered to the template and GLM using permutation testing was carried out. For diffusion tensor imaging analysis, we used Tract-Based Spatial Statistics (TBSS), which is widely used for whole-brain voxelwise comparison of white matter. First, data were preprocessed for head motion, and distortions due to eddy currents. Brain was extracted and DTI data was calculated for each voxel after fitting the diffusion tensor model to each voxel. This mean image was thresholded at an FA value of 0.2 and skeletonized to generate a white matter tract skeleton representing the center of the tracts common to all subjects. Permutation testing was used to calculate the t-statistics maps (uncorrected p<0.05).
No significantly different areas were found in VBM analysis (uncorrected p<0.05). DTI analysis revealed that changes in fractional anisotropy (FA) in the TBI patient did not differ from the findings in the controls (uncorrected p<0.05) (Fig. 1). Importantly, as part of the acute clinical evaluation after the concussion, the patient underwent a more complete clinical MRI study, including T2 weighted images that ruled out small hemorrhagic intraparenchymal lesions, intracranial hematomas and other signs of pathology.
Neuropsychological function was tested with the Immediate Postconcussion Assessment and Cognitive Test (ImPACT) and the CANTAB battery (Cambridge Cognition) and did not reveal any measurable cognitive deficits in the patient despite his subjective complaints.
Single- and paired-pulse TMS were applied using biphasic and monophasic figure-of-eight coils respectively (each wing, mean diameter 50 mm and outer diameter 70 mm) attached to a Nexstim stimulator (Nexstim Ltd, Helsinki, Finland). Continuous theta burst stimulation (cTBS) was applied using a biphasic figure-of-eight coil (each wing inner diameter 35 mm and outer diameter75 mm) attached to a MagPro stimulator (MagVenture A/S, Farum, Denmark).
Paired-pulse TMS over the left primary motor cortex (M1) was used to assess intracortical facilitation (ICF, tested with 12 ms between pulses) and short/long-interval intracortical inhibition (SICI and LICI, 3 and 100 ms between pulses respectively). The patient had significantly greater ICF at both time points. LICI was absent at week 2, but returned to normal at week 6. SICI in our patient was not different from the control group, which showed expected patterns of SICI, ICF and LICI (Fig. 2A).
Plasticity was evaluated with cTBS, expected to induce inhibition of the stimulated left M1 as indexed by a depression of motor-evoked potentials (MEPs) in contralateral hand muscles in response to subsequent single-pulse TMS. EEG was recorded simultaneously using the Nexstm eXimia TMS-compatible system. Contrary to the findings in controls, our patient had significant MEP facilitation at week 2, at 0, 5, 10 and 20 minutes after cTBS, but displayed normal inhibition of MEPs at week 6 (see Fig. 2B for the values up to 10 minutes after cTBS).
EEG raw data were high-pass filtered (1 Hz), epoched around each TMS pulse (-0.2 to 0.6 s, baseline: pre-TMS). For each subject, an Independent Component Analysis was run to identify and eliminate artifacts and remaining noisy channels were interpolated. Topographical EEG maps, 50 ms after single-pulse TMS, showed that at week 2, the patient had less intense and more widespread activation in response to single-pulse TMS over left M1 than the control group. This activation over left M1 was increased after cTBS, whereas it was decreased for the control group. At week 6, although the activation remained widespread, there was no more massive increase after cTBS (Fig. 2C).
In control subjects, the interhemispheric cross-correlation between temporal signals recorded over the two motor cortices (0 to 600 ms after the single-pulse TMS) tended to decrease 5 min after cTBS compared to baseline, whereas correlation between the stimulated M1 and the ipsilateral parietal and frontal areas tended to increase. The TBI patient significantly differed from the control subjects with a cTBS-induced increase of left M1-left parietal correlation at week 2, but not at week 6 (Fig. 2D and E).