Abstracts

The electrical effects of TMS-produced high intensity magnetic fields on the VNS is unknown. Understanding these effects is important before exposing individuals with an implanted VNS to TMS, as could occur in epilepsy research involving TMS. VNS leads and electrodes were embedded in a normal saline gelatin matrix while connected to an operating VNS unit in order to measure the current induced in the leads during maximal intensity biphasic TMS pulses. TMS was delivered by the Magstim model 220 with a 70 mm figure-of-eight coil. We also assessed whether the TMS pulses had a detectable effect on the pulse generator by oscilloscope measurement of the output of the VNS before, during, and after stimulation by the TMS unit. At the highest TMS intensity and with the TMS coil held directly over the VNS wires, a 200 nA, 1.0 msec current was induced by the coil. This translates to an induced charge density of 24 pC/cm²/phase based on the 6 mm² surface area of the VNS electrodes. The VNS pulse generator continued to function normally during and after this stimulation. The VNS was unaffected even when its case was directly stimulated by the TMS coil. The induced charge density measured in this experiment is well below the charge densities that are known to be safe for direct peripheral nerve stimulation, and below the threshold of nerve activation (1). Therefore, using TMS in individuals with VNS should not result in nerve damage. Furthermore, TMS does not affect the function of the VNS pulse generator, including any influence on the stimulation settings. Although the results of these tests under artificial conditions were consistent with low risk, the safety of using TMS in individuals with VNS remains to be systematically studied in a population of patients.
(1) Bullara LA, McCreery DB, Yuen TGH, Agnew WF. A microelectrode for delivery of defined charge densities. J Neurosci Methods 1983;9(1):15-21.