A microTMS system for peripheral nerve stimulation

Transcranial magnetic stimulation (TMS) is a well-established non-invasive technique for the treatment of several neurological disorders. TMS can also be used to stimulate elements of the peripheral nervous system, such as the median and the ulnar nerves. Compared to standard nerve conduction studies, TMS is less painful. However, a limitation is its restricted spatial resolution, which makes it difficult to target precisely any desired stimulation site. To overcome this limitation, we have developed the first prototype of micro TMS (μTMS)system with a non-invasive neurostimulation resolution of 1-5 mm, which is much higher than the conventional (TMS) resolution of 10-30 mm. In preliminary human testing, we established that the stimulation motor threshold of the median nerve is approximately 24% of the maximum output of a clinical TMS system. Based on these measurements, we calibrated the μTMS device to produce a pulse with enough power to stimulate the median nerve. The efficacy of μTMS will be tested as follows: the nerve stimulation site at which maximal response amplitude is obtained will be identified using standard nerve conduction study techniques; then, the same location will be stimulated using the μTMS coil. A focality map of sensory and motor responses relative to the site of peripheral nerve stimulation will be generated. This study aims to demonstrate that the magnetic stimulation elicited by μTMS coils is an effective complement, and potential alternative, to existing electrical stimulation approaches. After demonstrating the ability of the microcoil to direct magnetic stimulation to a much smaller somatic territory than is presently possible with traditional TMS techniques, we plan to apply the technology centrally; the ability to accurately stimulate a smaller region of cortex could translate into even better targeted physiologic assessment and therapies. We believe that μTMS may be a transformational tool in the field of neuromodulation.