No Effect of Anodal Transcranial Direct Current Stimulation on Gamma-Aminobutyric Acid Levels in Patients with Recurrent Mild Traumatic Brain Injury.

In patients in the chronic phase after recurrent mild traumatic brain injury (mTBI), alterations in gamma-aminobutyric acid (GABA) concentration and receptor activity have been reported, possibly mediating subtle but persistent cognitive deficits and increased rate of dementia in older age. We evaluated whether anodal transcranial direct current stimulation (atDCS) over the primary motor cortex reduces GABA concentration and GABAB receptor activity in patients with recurrent mTBI. Seventeen patients (mean age 25, two women) in the chronic phase after recurrent mTBI and 22 healthy control subjects (mean age 26, two women) were included. All participants received comprehensive cognitive testing and detailed questionnaires on post-concussive symptoms at baseline. Subsequently, they participated in four experimental sessions, consisting of either magnetic resonance spectroscopy (MRS)/atDCS/MRS, transcranial magnetic stimulation (TMS)/atDCS/TMS, MRS/sham/MRS, or TMS/sham/TMS to determine GABA concentration (from MRS) and GABAB receptor activity (from TMS) after atDCS and after sham stimulation. Patients with mTBI scored significantly lower on verbal fluency tasks compared with healthy control subjects. GABA concentration at baseline was associated with the number of mTBI, although no group differences in GABA concentration and GABAB receptor activity were found. Moreover, no effects of atDCS on GABA concentration and receptor activity were seen in patients with mTBI or healthy control subjects. GABA concentration may increase with the number of mTBI, but atDCS did not modulate GABA concentration and receptor activity, as has been reported previously. Specifics of experimental design and analysis, but also characteristics of the respective samples, may account for these differential findings, and should be addressed in future larger studies.

Cognitive function and brain structure after recurrent mild traumatic brain injuries in young-to-middle-aged adults.

Recurrent mild traumatic brain injuries (mTBIs) are regarded as an independent risk factor for developing dementia in later life. We here aimed to evaluate associations between recurrent mTBIs, cognition, and gray matter volume and microstructure as revealed by structural magnetic resonance imaging (MRI) in the chronic phase after mTBIs in young adulthood. We enrolled 20 young-to-middle-aged subjects, who reported two or more sports-related mTBIs, with the last mTBI > 6 months prior to study enrolment (mTBI group), and 21 age-, sex- and education matched controls with no history of mTBI (control group). All participants received comprehensive neuropsychological testing, and high resolution T1-weighted and diffusion tensor MRI in order to assess cortical thickness (CT) and microstructure, hippocampal volume, and ventricle size. Compared to the control group, subjects of the mTBI group presented with lower CT within the right temporal lobe and left insula using an a priori region of interest approach. Higher number of mTBIs was associated with lower CT in bilateral insula, right middle temporal gyrus and right entorhinal area. Our results suggest persistent detrimental effects of recurrent mTBIs on CT already in young-to-middle-aged adults. If additional structural deterioration occurs during aging, subtle neuropsychological decline may progress to clinically overt dementia earlier than in age-matched controls, a hypothesis to be assessed in future prospective trials.