Effect of Aging on Cortical Current Flow Due to Transcranial Direct Current Stimulation: Considerations for Safety.

While intracranial volume is thought to be fixed throughout the lifespan, there is little doubt that the brain shrinks with age and it is most precipitous after about the age of 50. This as a consequence reflects an increase in cranial cerebrospinal fluid (CSF) with age. Of the myriad factors influencing brain current flow, these changes in CSF volume are expected to play a profound role given its high electrical conductivity. The aim of this study is to investigate the effects of agerelated morphological changes on brain current flow patterns due to transcranial Direct Current Stimulation (tDCS). Anatomical MRI data were collected for 5 healthy subjects spanning 5 decades of life (ages: 43 to 85). Finite element models derived from the MRI were used to calculate cortical electrical field values during tDCS. The widely used C3-Fp2 (M1-SO) and the F3-F4 montage along with two High Definition-tDCS electrode montages 4X1 (C3-centered) and 4X1 (F3-centered) were simulated. Peak induced electrical field at the intended brain target (assumed to be directly underneath the electrode) and at non-intended brain regions was compared with the individual brain atrophy coefficients. Findings across 4 subjects (ages: 43 to 75) indicate reduced peak electrical field with increasing age. However, this trend reverses for the oldest subject. While age-related morphological changes lead to significant changes in current flow distribution, they are not substantially different than younger adults. The predictions of this study are the first step to assess safety of tDCS in elderly subjects and provide a rational path in customizing stimulation dose for trials involving them.