Magnetic and Direct Current Stimulation for Stroke

Introduction

There has been a recent surge in literature regarding noninvasive techniques that stimulate the brain to better understand plastic changes following stroke, and to modulate neuroplasticity to enhance post-stroke motor recovery. Noninvasive brain stimulation (NIBS) can be broadly categorized into transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) ( Table 169.1 ). The theory behind magnetic and direct current stimulation is that neuronal activity in cortical brain regions underlying the electrodes may be modulated by applied magnetic or electric currents. It has been proposed that modulation of cortical excitability induces neuroplasticity and thereby motor learning. Single- and paired-pulse magnetic stimulation are also used to study cortical excitability, cortical connectivity, and plasticity of the brain. This chapter aims to provide a broad overview of TMS and tDCS, discuss evidence of their effects, and provide considerations and recommendations for future clinical application.

Table 169.1

Summary Table of NIBS Protocols

Type	Description	Use
Single-pulse TMS	0.25 Hz or greater	Measures cortical excitability
Paired-pulse TMS	2 pulses with ISI of 2–14 ms	Measures intracortical inhibition (2–5 ms ISI) or intracortical facilitation (7–14 ms ISI)
Low-frequency rTMS	≤1 Hz	Cortical inhibition
High-frequency rTMS	≥5 Hz	Cortical facilitation
Continuous TBS	3–5 pulses at 50 Hz, every 200 ms for 2 s with a 10 s off period (20 trains)	Cortical inhibition
Intermittent TBS	3–5 pulses at 50 Hz, every 200 ms for 40 s	Cortical facilitation
Anodal tDCS	0.5–2 mA of anodal current	Cortical facilitation
Cathodal tDCS	0.5–2 mA of cathodal current	Cortical inhibition

Hz , Hertz; ISI , interstimulus interval; rTMS , repetitive transcranial magnetic stimulation; TBS , theta burst stimulation; tDCS , transcranial direct current stimulation; TMS , transcranial magnetic stimulation.

Historical Background

The concept of brain stimulation is not new and can be traced back to as early as the 1st century when live torpedo fish was used to deliver strong electric currents to patients suffering from migraines. “Therapeutic electricity” was still prevalent in the 18th century when electric currents were used to elicit different physiological effects, and identify cortical representations of limb movements. However, there was difficulty in focusing electricity to focal areas of the brain. As the skull is a poor conductor, high levels of electrical energy was needed causing widespread activation of the brain resulting in convulsions. Prior to the 1950s, brain stimulation required the electrodes to be directly placed on the exposed brain surface. In the 1960s, Bindman, Leopold, and Redfern performed experiments using low-level direct currents that led to long-lasting brain polarization accompanied by changes in sensory, motor, and emotional abilities. This led to a resurgence of studies exploring the use of direct current stimulation for brain polarization. In the 1980s, Barker, Jalinous, and Freeston first reported the use of magnetic stimulation over the motor cortex to elicit responses in the muscle. And TMS became an important tool in neurophysiology . Meanwhile, the exploration of direct currents continued. In 1998, Priori and colleagues first demonstrated the effects of anodal currents on increased cortical excitability and coined the term “transcranial direct current stimulation.” Subsequent experiments by Nitsche and Paulus demonstrated modulating effects of anodal (increases cortical excitability) and cathodal (decreases cortical excitability) tDCS on brain tissue in which the effects outlasted the duration of stimulation . Since the introduction of single-pulse TMS in the 1980s and the revival of tDCS in the 1990s, there have been numerous forms of TMS and tDCS that have been developed and applied to a variety of neurological and psychiatric conditions.

Brain Plasticity in Stroke

Before addressing the developments using NIBS to enhance post-stroke motor function, it is important to understand the changes in cortical excitability that accompany stroke recovery. Several longitudinal brain imaging (fMRI and PET) and TMS studies have explored neural correlates of stroke recovery. The general consensus is that there is a disruption of interhemispheric balance of cortical excitability after stroke. Particularly, there is decreased activity in the ipsilesional hemisphere and a corresponding increase in activity in the contralesional hemisphere. Many studies have shown that balance of between-hemisphere cortical excitability is associated with improved post-stroke functional recovery . This forms the basis for neuromodulation using stimulation-based priming protocols in stroke. A balance of excitability can be achieved by either upregulating the ipsilesional hemisphere or downregulating the contralesional hemisphere using NIBS.

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