Deep Transcranial Magnetic Stimulation for the treatment of neuropsychiatric disorders in elderly people: Electric field assessment.

In the last years, deep transcranial magnetic stimulation (dTMS) has become the main tool for the treatment of several neuropsychiatric disorders that involve deep brain regions. These pathologies include for example unipolar and bipolar disorders [1], major depressive disorders [2][3], post stroke [4] and vascular [5] depression, memory dysfunctions [6], diabetic neuropathy [7], Parkinson disease [8]. The physical principle that allows the therapeutic use of this technique is based on the induction of an electric field (E), through the coils placed close to the scalp, which safely modulate the activity of the cerebral structure of interest. Depending on the parameter of stimulation, that field can modify the excitability of the brain target. The effectiveness of this stimulation technique is univocally linked to the coil configurations, which should enable the activation of deep targets. This is even more critical for the pathologies listed above, which usually disturb elderly people. That is because they could be affected by the presence of prefrontal atrophy, which implies an increase of the distance between scalp and cortex, making even more difficult to reach deeper structures. Lately, great efforts have been carried on to develop specific coils, such as the family of the so called Hesed (H) coils, that are based on certain design principles and have larger dimension compared to conventional coils, to the specific purpose of improving the penetration depth of the stimulation. Despite clinical studies have attained positive outcomes of dTMS, the mechanism lying below them is not yet completely clear. In order to clarify this issue, the knowledge of the detailed E distribution induced inside cerebral tissues is the main and first step to be performed.