Comparison of superficial TMS and deep TMS for major depression

Background:

Traditionally TMS treatment sessions for MDD were administered over the DLPFC using a focal superficial figure-8 coil. Recently the FDA has cleared deep TMS treatment for MDD using Brainsway H-coil, following a large double-blind, placebo-controlled,multi-center study. The induced electric field characteristics, stimulated brain structures and clinical outcomes are very disparate between the two methods.

Objective:

To compare the induced electric field characteristics of a standardfigure 8 coil and the dTMS H1-coil.

Methods:

Electric field characteristics of figure-8 coil and of the H1-coil were measured in a realistic head model filled with physiologic saline concentration, and maps offield distribution wereproduced. We review several studies discussing brain structuresand networks which are relevant for clinical outcome in MDD, anddiscuss the implications of the spatial characteristics of each TMScoil.

Results:

Where using 120% of MT the H1-coil induces supra-thresholdfields at depth of 1.8 cm and significant sub-thresholdfield at depth of 3.2 cm, compared to0.7 cm and 0.7 cm for thefigure-8 coil. Various studies have shown that deeper PFC structures have significantly more connections with other reward system sites compared to superficial PFC. The H1-coil induces alsosignificantly wider stimulation compared to thefigure-8 coil.Recent studies suggest that stimulation of PFC structures withextensive connections to the subgenual cingulate may be crucial forthe antidepressant action of standard TMS. It was shown that theexact location of these structures varies greatly between individuals.

Conclusions:

Since standard TMS coils exert a focal and superficial stimulation, optimal stimulation targets may be easily missed with standard coils. This and the depth differences might contribute to the large efficacy differences found in DBPC multi-center studies with Deep TMS compared to standard TMS.

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