Intensity dependent repetitive transcranial magnetic stimulation modulation of blood oxygenation

Background: Repetitive transcranial magnetic stimulation (rTMS) is increasingly being investigated in clinical settings for the treatment of neurological and psychiatric disorders such as dystonia, schizophrenia, and major depressive disorder (MDD). Using near infra-red spectroscopy (NIRS), very short trains of rTMS have previously been shown to modulate cortical blood oxygenation. Methods: In order to investigate the effect of longer, clinically relevant trains of 1 Hz rTMS on oxy-hemoglobin (HbO) at prefrontal cortex, the current study applied ten minute trains of rTMS at both subthreshold and suprathreshold intensities. Results: A similar profile of oxygenation change was observed during the beginning 30-40 s of the trains, however for the remainder, subthreshold rTMS returned to baseline while the suprathreshold TMS resulted in a long period of reduced oxygenation. Limitations: Small sample size. Conclusions: The differences observed may be a product of changes in HbO requirements by inhibitory/excitatory neural circuits, either by reduced HbO demand or by increased HbO consumption, while sustained HbO reduction may be a consequence of a modulation of vaso-motor reactivity. This study has implications for understanding the mechanisms involved in the physiological changes evoked by rTMS and efficacious clinical application of rTMS in disorders such as MDD.