1. Mild TBI causes a long-lasting elevation of the transcranial magnetic stimulation (TMS) motor threshold Jussi Tallus 1, Pantelis Lioumis 2, Heikki Hämäläinen 3, Seppo Kähkönen 2, Olli Tenovuo* 1 1 Department of Neurology, University of Turku, Finland, 2 Biomag Laboratory, Finland, 3 Department of Psychology, University of Turku, Finland * presenting author

2. Introduction Mild traumatic brain injury (mTBI) is often associated with long lasting symptoms that may be caused partly by diffuse damage to cerebral white matter tracts There is a major clinical need to develop reliable methodology to assess whether prolonged symptoms after mTBI have an organic basis

3. Introduction Cortical excitability can be evaluated by recording electromyographic (EMG) responses elicited by single pulse TMS and measuring the minimum intensity (motor threshold; MT) that can evoke motor evoked potentials (MEPs). MT provides a relatively easy measure of corticospinal tract integrity.

4. Objectives To evaluate whether corticospinal tract integrity measured with TMS elicited MT measurement is altered in subjects with chronic symptoms after mTBI

5. Material and methods Nineteen mTBI patients, 11 with persistent symptoms and 8 recovered The subjects had suffered a mTBI (GCS 13 – 15) on the average five years earlier All subjects had normal MRIs in visual inspection Nobody used CNS affecting medications Nine healthy controls

6. Material and methods Navigated TMS, stimulation on the left primary motor cortex MEPs recorded from the right m. abductor pollicis brevis by EMG MT is defined as the lowest stimulation intensity at which 5 out of 10 pulses evoke 50 µV MEP or greater

7. Results The average MT was 43.0 % (SD ± 0.8) of maximum stimulator output in the control group. Both patient groups had statistically significantly higher MTs, 52.5 % (SD ± 3.1) in the symptomatic and 54.6 % (SD ± 3.4) in the recovered group.

8. Results p = 0.012p = 0.011

9. Conclusions MT may be a useful measure in evaluating brain damage caused by mTBI. This study shows that MT elevation can last until at least several years after the injury, and that it is not necessarily related to subjective complaints.

10. Conclusions The similar results of the symptomatic and recovered groups imply that simple MT by itself is not sufficient as a measure of injury severity or outcome, but it can be useful in combination with other methods. It is noteworthy that also fully recovered mTBI subjects show an abnormal MT, which suggests a persistent injury also in these subjects