Brain Networks and Depression
— How Does TMS Work and How Is It Enhancing Our Understanding of the Brain?
Unfortunately, depression is a silent killer. Depression and anxiety disorders are the most common mental illnesses in America affecting millions of adults each year. Losing a loved one, going through a divorce, getting fired and other difficult situations can lead a person to feel sad, scared and alone. However, when these feelings are persistent, pervasive and severe the condition is known as “clinical depression.”
TMS is a powerful, non invasive technique for the modulations of brain activity and treatment of clinical depression. It has emerged as a widely used tool in both neuroscience and theraputic settings.using focused, non invasive stimulation of cortical brain regions via rapidy switched magnetic pulses. TMS is an FDA approved treatment for depression (repetitive in 2008 and deep in 2013).
Although we are still actively investigating how the brain works in both healthy and abnormal states such as depression, and how TMS affects the brain network, here’s what we know so far and what we are still exploring:
TMS induces brain-neuronal activity in the area beneath the coil and also induces brain-neuronal functional connectivity in distinct brain regions. The effects can be directly observed and identified after a period of TMS stimulation using functional magnetic resonance imaging (fMRI).
Functional MRIs measure increase in brain activity as a function of blood oxygenation and flow. When a brain area is more active, blood flow increases in that area and it uses more oxygen. This allows us to measure and map both functional anatomy and synchronization between different regions of the brain.
TMS has been studied for functional connectivity change after both excitatory and inhibitory neural stimulation…both of which are essential for effective brain action. TMS can, and has been, delivered to the brain in study cases using different parameters or schedules such as single pulse, repetitive pulses, or in patterns such as theta-bursts. Potential effects of these trials include “virtual lesion” or interference with function, augmentation of function and reduction in neural activity. Both facilitation and inhibition can be seen in fMRIs depending on the exact position and settings or parameters for treatment.
There have been many attempts to better understand the mechanism of action of TMS using fMRI connectivity. Most recently Tik et al (2017) used an unbiased whole-brain resting state network approach to successfully demonstrate that stimulation of the dorsolateral prefrontal cortex (DLPFC) modulates anterior cingulate cortex (ACC) connectivity and one specific cortico-limbic network. It is now clear that DLPFC TMS increases connectivity between ACC and specific fronto-parietal networks.
This information is critical and relevant in clinical TMS practice. Although response rates are increasing, a small percentage of patients do not respond to TMS, thus signaling the importance of continuing to investigate the number, delivery and duration of treatments to determine where/when/how response and remission rates may increase.
Achieve TMS offers treatments to a large number of patients and is using and helping to develop innovative tools and treatments that may allow for an increase in response rates and remission rates for the most challenging, treatment resistant cases in the future.
References available on request.