Transcranial Direct Current Stimulation (tDCS)

Developments From Electrosleep to Cranial Electrotherapy Stimulation

ES is the name for which the brain was stimulated to induce a sleep-like state in the subject. The first studies on ES were initiated in 1902 ( ); however, the first clinical report of ES was published 12 years later by Robinovitch ( ). New approaches, such as changing electrode position from covering the eyes to locations around the eyes, presumably to “reduce optic nerve irritation” ( ) were developed, mostly in Europe. ES dose waveform was typically pulsed at 30–100 Hz, but at least one (unsuccessful) case of use of DC current was documented ( ). In a symposium, it was reasoned that ES does not actually induce sleep, rather it is an indirect side effect of the relaxing effects of stimulation. Therefore, the name of ES was changed to CET ( ). This was the first of several changes of the name of ES over the next few decades, often with notable changes in dose. In 1969, TCET was proposed as another alternative name. However, such devices were renamed as CES ( ).

A notable device, produced after the name change to CET, was the Neurotone 101 , which was based on a Russian ES device brought to the United States. Although the Neurotone 101 is no longer in production, it was the first device to be approved by the FDA as a CES device ( ) and all subsequent CES devices approved by the FDA, such as the Alpha-Stim, the Oasis Pro, and the Fisher–Wallace Stimulator ( Fig. 135.2 ), were through a 510k process claiming equivalency, either direct or descendent, to the Neurotone 101.

Modern CES is thus a historical descendent of ES, even as dose and indications have continuously evolved. The FDA sanctioned CES in 1978 through a “grandfather clause” and have not, since then, advanced a regulatory consensus, even as more variants were being developed. CES, in this modern form, excludes DC waveforms, so it is distinct from tDCS, but its historical predicates include DC components.

Developments From Electroanesthesia to Limoge Current and Other Related Methods

EA, in short, was intended to induce anesthesia in the subject using high frequency stimulation so that chemicals did not have to be used, presurgery. EA studies started in 1903, but were first known as Electronarcosis (EN) ( ). Russian scientists used the term “EA” to describe local anesthesia, while “EN” described general anesthesia ( ). However, EA stopped being referred to as local, as applied to the periphery, and began to be known as general anesthesia, as applied to the brain.

Research into EA dosage continued and the term TCES was adopted around 1960–63. Even though the term TCES was not adopted until the early 1960s, similar protocols were used, as early as 1902 by Leduc ( ). In 1951, Denier proposed that high frequency trains of 90 kHz could be used to avoid muscular contraction ( ). Three years later, claimed that alternating currents (AC) at 700 Hz should be applied, but this was abandoned in 1958 due to cardiovascular complications ( ). In 1957, investigators in the Soviet Union attempted to add a Direct Current (DC) component to Leduc’s currents but, as claimed by an American scientist, Robert Smith, it resulted in a collection of undesirable side effects ( ). In 1964, a study claimed that pulsating currents are more effective than DC for the induction of EA ( ). Another study suggested that the use of pure DC for EA required high intensities of approximately 40 mA ( ). EA, and its derivative technologies, are not explicitly integrated into contemporary neuromodulation research and treatment, but represent important historical precedents that include cases of testing of DC waveforms.

Development of Direct Current Stimulation

DCS has been used intermittently as a component in both ES and EA. In 1957, a DC bias was added to ES, which is traditionally applied using only AC or Pulse Current (PC). The advent of TCES, around 1960–63, in the third resurgence of EA research, also incorporated a DC bias. In 1969, pure direct current stimulation was investigated for inducing anesthesia ( ). However, it was not until 1964 that preliminary studies, heralding modern tDCS, were published ( ).

In 1964, Redfearn and Lippold investigated Polarizing Current for the treatment of neuropsychiatric diseases ( ). Their use of prolonged (minutes) of stimulation was motivated by animal studies showing that prolonged DCS could produce lasting changes in excitability ( ). While further open, pilot studies and clinical observations suggested efficacy ( ), a following negative, controlled trial ( ) seems to have halted investigation (at least as published in Western journals) for several decades.

The neurophysiological basis of neuromodulation using short-duration tDCS was investigated by Priori et al. in . Shortly after, Nitsche and Paulus established that prolonged (minutes) tDCS could produce lasting and polarity-specific changes in cortical excitability ( ). This was followed by pilot clinical studies ( ) for indications spanning depression ( ), pain ( ), epilepsy ( ), and a broad range of neuropsychiatric disorders ( ). tDCS is further explored for rehabilitation after stroke ( ). Moreover, due to the perceived safety of tDCS, it was initially validated for neurophysiological changes in healthy subjects and continues to be investigated in healthy individuals for changes in behavior and cognitive performance ( ).

In 2007, HD-tDCS was proposed as a focalized form of tDCS ( ). HD-tDCS electrodes were designed for increased charge-passage capacity through a smaller contact area ( ), arranged in arrays that can be optimized per indication ( ). HD-tDCS montages tested have included the 4 × 1 configuration ( ) as well as individually optimized arrays ( ). The focalization of current with HD-tDCS is an improvement upon tDCS, where previously a broad area would be stimulated, and now specific targets can be stimulated. Some of the devices that have been used are the Schneider (tDCS), Soterix Medical 1 × 1 (tDCS), and the Soterix Medical 4 × 1 (HD-tDCS) ( Fig. 135.2 ).

Transcranial Micropolarization is a technique investigated in Russia which is a modified version of tDCS that uses small electrodes instead of pads and currents up to 1 mA, that are claimed to be “weak” ( ). GVS has been extensively tested for effects on ocular and postural movement ( ). Alongside GVS, Caloric Vestibular Stimulation (CVS) is under investigation due to similar areas being targeted by stimulation. However, CVS does not utilize electricity; rather, it uses irrigation of the ear canal using cold or warm water ( ).