Various devices are available that can electrically stimulate the brain without the need for surgery or any invasive treatment. There are three main treatment types: repetitive transcranial magnetic stimulation (rTMS) in which the brain is stimulated by a coil applied to the scalp, cranial electrotherapy stimulation (CES) in which electrodes are clipped to the ears or applied to the scalp and transcranial direct current stimulation (tDCS), in which electrodes are applied to the scalp. These have been used to try to reduce pain by aiming to alter the activity of the brain but the efficacy of these treatments is uncertain.
This review included 33 studies, 19 of rTMS, eight of CES and six of tDCS. Only one study was judged as having a low risk of bias. Analysis suggests that low-frequency rTMS is not effective but that a single-dose of high-frequency stimulation of the motor cortex area of the brain provides short-term pain relief. This effect appears to be small. There is limited and conflicting evidence from studies involving multiple doses of rTMS. Most studies of rTMS are small and there is substantial variation between studies in terms of the treatment methods used.
There was insufficient evidence from which to draw strong conclusions regarding CES or tDCS but the available evidence does not suggest that CES is an effective treatment. There is limited evidence that tDCS to the motor cortex may have short-term effects on chronic pain but it is not possible to estimate the size of this effect accurately.
The reporting of side effects varied across the studies. Of the studies that clearly reported side effects only short-lived and minor side effects such as headache, nausea and skin irritation were reported.
More studies of rigorous design and adequate size are required to evaluate all forms of non-invasive brain stimulation for the treatment of chronic pain accurately.