Transcranial magnetic stimulation in child neurology: current and future directions

Abstract

Transcranial magnetic stimulation (TMS) is a method for focal brain stimulation based on the principle of electromagnetic induction, where small intracranial electric currents are generated by a powerful, rapidly changing extracranial magnetic field. Over the past 2 decades TMS has shown promise in the diagnosis, monitoring, and treatment of neurological and psychiatric disease in adults, but has been used on a more limited basis in children. We reviewed the literature to identify potential diagnostic and therapeutic applications of TMS in child neurology and also its safety in pediatrics. Although TMS has not been associated with any serious side effects in children and appears to be well tolerated, general safety guidelines should be established. The potential for applications of TMS in child neurology and psychiatry is significant. Given its excellent safety profile and possible therapeutic effect, this technique should develop as an important tool in pediatric neurology over the next decade.

Figure 1

TMS device and applications. (A) A block diagram depicting the transcranial magnetic stimulator (TMS) circuit is depicted on the left. The power supply charges the capacitor. An operator or computer then signals for the charge stored in the capacitor to be released into the stimulation coil through a thyristor switch. The current flowing through the stimulating coil (here depicted as a circular coil) produces a perpendicular magnetic field which transverses the skull and induces electrical currents within the cortex underlying the coil. A detectable muscle contraction, typically in a contralateral limb, results if the stimulation coil is placed over the motor cortex. This motor response is quantitatively measured as the motor evoked potential (MEP). (B) Three diagrams depicting various TMS protocols. Single-pulse TMS (sTMS) provides a single stimulation to the cortex. If the coil is positioned over the motor cortex, a motor response is elicited. Paired-pulse TMS (ppTMS) stimulates the motor cortex with 2 pulses separated by a short interstimulus interval (ISI). The resulting motor response is compared to the motor response elicited by signal-pulse TMS. Repetitive TMS (rTMS) uses a train of pulses separated by a regular intertrain interval (ITI) for a specific period of time. The duration of the intertrain interval determines the stimulation frequency.

Figure 2

Reorganization patterns following recovery in acquired hemiplegia organized by age at which the cortical damage occurred. Note that the ipsilateral hemisphere is more often involved in motor control of the affected limb before 2 years of age, suggesting a greater preservation of the capacity for successful motor remapping for brain injury occurring early in life. The hemisphere of origin of the corticospinal projections to the affected limbs as determined by single-pulse TMS is shown. This figure was produced by reviewing all available studies in which the results of single-pulse TMS stimulation for individual cases of acquired hemiplegia were available.