sufficient magnitude to induce neural action potentials in the prefrontal cortex to treat the
symptoms of major depressive disorder without inducing seizure in patients who have failed
at least one antidepressant medication and are currently not on any antidepressant
therapy." The exact details of how TMS functions are still being explored. TMS can be
divided into two types depending on the mode of stimulation:
Single or paired pulse TMS causes neurons in the neocortex under the site of stimulation
to depolarize and discharge an action potential.
Repetitive TMS (“rTMS”) produces longer-lasting effects which persist past the initial
period of stimulation. rTMS can increase or decrease the excitability of the corticospinal
tract. The mechanism of these effects is not clear although it is widely believed to reflect
changes in synaptic efficacy akin to long-term potentiation (LTP) and long-term
depression (LTD).^3
4. EFFICACY
A 2003 depression meta-analysis by Gershon et al.^4 found that most data support an
antidepressant effect of high-frequency repetitive TMS administered to the left
prefrontal cortex. The absence of psychosis, younger age, and certain brain physiological
markers might predict treatment success. Technical parameters possibly affecting
treatment success include intensity and duration of treatment, but these suggestions
require systematic testing.^5
Gershon et al. concluded that: “41% of 139 patients treated with high-frequency rTMS
to the left prefrontal cortex achieved either a 50% decrease in their Hamilton
depression scale scores or a final score of ≤8. Recent studies have pointed to, but not
yet proven, longer treatment courses, more magnetic pulses, and increased field
intensity as likely contributors to treatment success, even when rTMS is the only
antidepressant therapy, and have produced results with rTMS that are comparable to
those of ECT.”^6
