Introduction: Repetitive Transcrainial Magnetic Stimulation (rTMS) involves passing electricity through a coil on the scalp to produce a powerful magnetic field just under the coil which depolarizes superficial cortical neurons. Daily left prefrontal rTMS has a less robust antidepressant effect in elderly subjects. We wondered if rTMS is not reaching the prefrontal cortex in some subjects, and if the distance from the scalp to cortex influenced their antidepressant response.
Methods: We used MRI to measure the distance between coil to cortex in 29 depressed adults enrolled in a randomized, double-blind, placebo-controlled antidepressant trial of left prefrontal rTMS. Medication free subjects were randomized to daily high frequency, low frequency or placebo rTMS at motor threshold stimulation for two weeks. Antidepressant response was defined as a 50 percent decline from baseline in Hamilton Rating Scale for Depression (HRSD) score after two weeks of daily treatment. At baseline, a T1-weighted 3D volumetric MRI scan was obtained with a 1.5 Tesla Picker scanner (142 1 mm sagittal slices). Subjects had markers placed at the site of rTMS stimulation. A trained reader (FAK) blind to treatment arm used MEDx 2.1 to measure the scalp-prefrontal cortex distance. (S-C Distance).
Results:S-C Distance significantly increased with increasing age (r2 = .422, p<0.0001). There was no correlation between S-C Distance and HRSD change, all rTMS antidepressant responders were less than 55 years of age and had S-C Distances less than 17.00 mm. Motor threshold did not significantly increase with age.
These data would imply that in older depressed subjects, rTMS at these parameters might not be adequately stimulating the prefrontal cortex. We are currently measuring the scalp-motor cortex distance in these subjects to determine if there is specific prefrontal atrophy. Higher intensity of rTMS (or more powerful coils) may be needed when using rTMS to treat depressed older adults.
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