Cortical current density magnitudes during transcranial direct current stimulation correlate with skull thickness in children, adolescent and young adults

Transcranial direct current stimulation protocols are often applied with a fixed parameter set to all subjects participating in an interventional study. This might lead to considerable effect variation in inhomogeneous subject groups or when transferring stimulation protocols to different age groups. The aim of this study was to evaluate magnitude differences of the electric current density distribution on the gray matter surface in children, adolescent and adults in correlation with the individual volume conductor geometry. We generated individual six compartment finite element models from structural magnetic resonance images of four children (age: 10.95 a±1.32 a), eight adolescents (age: 15.10 a±1.16 a) and eight young adults (age: 21.62 a±2.45 a). We determined the skull thickness in the models as Euclidean distance between the surface of the cerebrospinal fluid compartment and outer skull boundary. For tDCS simulations, we modeled 5×7cm patch electrodes impressing 1mA current intensity as anode and cathode over the left M1 and the right fronto-polar orbit, respectively. The resulting current density was analyzed on the gray matter surface. Our results demonstrate higher cortical current density magnitudes in children compared to adults for a given tDCS current strength. Above the evaluated cortex, the skull thickness increased with age. In conclusion, we underline the importance of age-dependent and individual models in tDCS simulations.