TMS is essentially a method of using brief, intense magnetic pulses to stimulate activity in the brain. A TMS pulse is generated through a coil placed on the scalp. An electric current passes through the coil, which is switched on and off very quickly, generating the magnetic field. The magnetic field passes into our brain without resistance and can induce an electrical field, making nerve cells fire. TMS machines can produce a single pulse or a series of pulses very close together (often referred to as repetitive TMS or rTMS). When repeated bursts of rTMS are applied to the brain, they can cause changes in local brain activity and changes in activity are connected to the target site.

At a minimum, patients need to have a full psychiatric assessment to ensure they have an appropriate diagnosis matched to the type of TMS being used (e.g. depression or obsessive compulsive disorder). In addition, they need to be assessed to ensure that there are no relevant contraindications to treatment such as a seizure disorder. In addition to this, I think it is good practice to use psychometric tools to assess the severity of specific symptoms: for example, something like the Hamilton depression rating scale or the QIDS to assess the severity of depressive symptoms. These should be used before and after treatment to produce some quantification of the degree of improvement achieved. We don’t routinely use broader assessments of personality and the like but these can certainly be helpful as an adjunct to diagnosis and overall assessment.

The mechanism of action of TMS still remains somewhat unclear. It clearly produces local changes in brain activity under the coil but more importantly, seems to produce changes in broader brain regions, most likely those that are structurally or functionally connected to the target site. As such, I think TMS is probably working by making relatively widespread changes in broader networks involved in depression.

In regards to the number of treatment sessions, I think the absolute minimum number of sessions patients should commit to the outset of a course of treatment is 20. If they are not improving after 20 sessions, it is worth having a discussion about whether the type of TMS might be altered. However, many patients will continue for a full 30 sessions to ascertain whether one type of treatment is likely to be effective for them or not.

When treating depression, our expectation is that the core depressive symptoms such as low in mood, anhedonia, lethargy and hopelessness will all improve if the TMS is successful and many patients completely resolved. However, TMS is more specifically a treatment for depression and although there is some evidence that it can help in patients with post-traumatic stress disorder, this is not a proven indication and I would not want to create an expectation that patients are likely to have a relief of trauma related symptoms.

In regard to the treatment of psychotic symptoms, TMS has not really been comprehensively evaluated in patients with depression with psychotic features. There is a considerable body of evidence that it can help some other psychotic symptoms, for example, auditory hallucinations in patients with schizophrenia, but in this case treatment needs to be done in a quite different way from the way it is used in depression.

TMS is generally a very safe and well tolerated treatment. However, it is not completely free of side effects. Many patients will find the tapping sensation that they experience on the front of their forehead uncomfortable and some will even experience stimulation is painful. Some patients will also experience a headache after the end of treatment sessions. However, with careful management, most patients can tolerate the full course of treatment and dropout rates from treatment are extremely low, certainly lower than what you expect with other depression treatments.

When used for depression, TMS is targeted to the prefrontal cortex, which is a fairly large region of the front of the brain which is involved in a wide range of important brain functions. The most common form of TMS will increase activity in this part of the brain, which has been widely shown to be underactive in patients with depression. However, successful treatment also results in changes in other brain regions, presumably because the signal passes through connected brain networks to broader parts of the brain.

There are lots of different definitions but treatment resistant depression is most typically considered a state when somebody has not responded to a number of trials of other therapy, most typically several antidepressant medications. ‘Severe’ depression typically refers to the intensity of patients experiencing and the impact that they are having on person’s life. We most typically offer TMS to patients who have had two or more previous antidepressant medications although many patients coming for treatment have tried many more than this. Unfortunately, once a patient has tried two medications unsuccessfully, the chance that third or fourth medication will work is quite low and certainly seems to be lower than the likelihood that TMS will be successful.

There have only been a number of small trials in patients with bipolar depression. However, quite a number of studies have included patients with both unipolar and bipolar depression and the patients with bipolar depression seemed to do just as well in these studies. In our clinical experience this is certainly the case as well.