Does the Intensity of Transcranial Magnetic Stimulation Need to be Adjusted to Scalp-Cortex Distance?

To the Editor: Repetitive Transcranial Magnetic Stimulation (rTMS) therapy can be effective in treating drug-resistant depression. However, rTMS therapy has had up until now limited efficacy in extremely treatment-resistant patients and its effects are not as large as electroconvulsive therapy.¹ The efficacy of rTMS may be increased by improving stimulation parameters, in particular by using more precise coil placement on the scalp to stimulate the Dorsolateral Prefrontal Cortex (DLPFC). This cortical area is the target for treatment of depressive disorders with rTMS. This revised position of the coil is supported by recent results which have found that, rather than using the “5 cm method” to position the coil, a more anterior position improves the rTMS response rate.^2,3

The positioning of the coil on the scalp is probably not the only stimulation parameter that can be improved. The efficacy of rTMS therapy could also be improved by measuring the depth of the cortex, in order to adjust the motor threshold (MT) to obtain the same degree of excitability at the DLPFC. The MT, used to determine the intensity of stimulation required for each patient according to their own cortical excitability, corresponds to the minimal intensity required to elicit contraction of the thumb when the coil is positioned on the scalp above the motor cortex. Therefore, it only reflects the intensity required for the magnetic field of the coil to reach the motor cortex at a precise distance from the scalp. As the distance from the scalp to the motor cortex can be different from that to the DLPFC, the intensity needs to be adjusted.

In fact, by examining 20 brain MRIs of patients (mean age 55) eligible for rTMS in our psychiatric department, we found that for 11 of them, the distance from the scalp to the DLPFC (positioned 5 cm anterior to the motor cortex) was in fact greater by 1 mm to 4.5 mm than from the scalp to the motor cortex. Taking into account the fact that every additional millimeter from the stimulating coil required around 3% of additional stimulator output to induce an equivalent cortical effect, the intensity should have been increased by 3 to 13.5%.⁴ We did not use this intensity adjustment because the latest guidelines for the use of rTMS do not recommend it.⁵ However, we hypothesize that the efficacy of the therapy could have been improved by adjusting the intensity in these patients.

This adjustment of the intensity according to the depth of the cortex combined with a more anterior location of the coil on the scalp may further improve the efficacy of rTMS.