Using magnets to improve health is sometimes hawked in dubious classified ads and “infomercials.” However, a legitimate use of magnetism—repetitive transcranial magnetic stimulation (rTMS)—is showing promise in treating severe depression (Box) 1-4 and other psychiatric disorders.
Patients or their families are likely to ask psychiatrists about rTMS as more becomes known about this investigational technology. Drawing from our experience and the evidence, we offer an update on whether rTMS may be an alternative for treating depression and address issues that must be resolved before it could be used in clinical practice.
WHAT IS RTMS?
rTMS consists of a series of magnetic pulses produced by a stimulator, which can be adjusted for:
- coil type and placement
- stimulation site, intensity, frequency, and number
- amount of time between stimulations
- treatment duration.
rTMS: A new antidepressant technology?
In 1985, Barker and colleagues developed single-pulse transcranial magnetic stimulation to examine motor cortex function.1 The single-pulse mechanism they discovered was subsequently adapted to deliver repetitive pulses and is referred to as repetitive transcranial magnetic stimulation (rTMS).
How rTMS works. Transcranial magnetic stimulation uses an electromagnetic coil applied to the head to produce an intense, localized, fluctuating magnetic field that passes unimpeded into a small area of the brain, inducing an electrical current. This results in neuronal depolarization in a localized area under the coil, and possibly distal effects as well.2 During the neurophysiological studies, it was discovered that subjects also experienced a change in mood.
Antidepressant effects. Similar physiologic effects induced by rTMS, electroconvulsive therapy (ECT), and antidepressants on the endocrine system, sleep parameters, and biochemical measures suggest antidepressant properties.3 In 1993, the first published study examining rTMS in psychiatric patients reported reduced depressive symptoms in two subjects.4 Since then, several clinical trials have examined rTMS’ antidepressive effects. In 2001, Canada’s Health Ministry approved rTMS for treating major depression. In the United States, rTMS remains investigational and is FDA-approved only for clinical trials.
Coil type and placement. Initial studies involved stimulation—typically low-frequency—over the vertex, but most subsequent rTMS trials in depression have stimulated the left dorsolateral prefrontal cortex. Neuroimaging studies have shown prefrontal functioning abnormalities in depressed subjects, and it is hypothesized that stimulating this area (plus possible distal effects) may produce an antidepressant effect.5
Various configurations have been used, but circular and figure-eight-shaped coils are most common. These flat coils are made of tightly wound ferromagnetic material such as copper, enclosed in a heavy plastic cover. With the figure-eight coil, the intersection of the two loops produces the strongest magnetic field.
Stimulation site. Stimulation intensity depends on the individual’s motor threshold, and the site can be determined visually or electrophysiologically.
- With the visual method, the motor threshold over the left primary motor cortex site for the first dorsal interosseous muscle (FDI) or the abductor pollius brevis (APB) is determined by iteration. This involves placing the coil at a progression of sites and increasing stimulation intensity until reliable (in 5 of 10 stimulations) contractions are seen in the right FDI or APB.
- Similarly, the electrophysiologic method uses 5 of 10 motorevoked potentials of 50 microvolts to locate the site.
The only small trial that compared visual and electrophysiologic site determination showed similar results with both methods.6 The most common stimulation site is the left dorsolateral prefrontal cortex, 5 cm anterior and parasagittal to the FDI or APB motor cortex. Alternately, frameless stereotactic systems or the international 10-20 proportional system used in EEG labs have been recommended to target sites more accurately.
Stimulus intensity. Each individual’s motor threshold determines stimulus intensity. Using functional MRI studies, researchers from the Medical University of South Carolina concluded that higher stimulation intensity relative to the motor threshold may have a more robust effect, as the magnetic field declines with distance from the coil.7 However, intensities >120% of the motor threshold are generally avoided because of possible increased seizure risk.9
Frequency of stimulation. Most researchers apply frequencies of 1 to 20 Hz over the left dorsolateral prefrontal cortex, but also use lower frequencies (<1 Hz) over the right dorsolateral prefrontal cortex. Using higher frequencies in major depression is attractive in theory because of:
- the reported association of decreased regional cerebral blood flow with hypometabolism in the left dorsolateral prefrontal cortex
- higher-frequency stimulation’s ability to produce temporary excitation and neuronal depolarization.
Number of stimulations. The number of stimulations is determined by frequency (Hz) and stimulation train duration (for example, 10 Hz for 5 seconds equals 50 stimulations). A typical treatment session incorporates 10 to 30 stimulation trains several seconds apart (the inter-train interval). Thus, a typical session delivers 1,000 to 1,200 stimulations. In studies of unmedicated depressed patients, the total number of stimulations has varied from 8,000 to 32,000 per treatment course.