TRANSCRANIAL DIRECT CURRENT STIMULATION

What is tDCS?

Transcranial Direct Current Stimulation, or tDCS, is a form of non-invasive brain stimulation that typically delivers a fixed low electric current between 1 and 2 milliamperes (mA) to a specific part of the brain. It is a portable and wearable device that applies a positive or negative current between two electrodes that pass through the brain to create a circuit. A positive (anodal) current is assumed to stimulate the firing of neurons and facilitate certain behaviors. In contrast, a negative (cathodal) current decreases the firing of neurons. It inhibits behaviors.¹ tDCS has been used to reduce symptoms of depression, decrease hallucinations in schizophrenia, and improve language skills in autism. Examples of tDCS devices include Soterix Medical, Activadose, Focus, Brain Premier, LIFTiD, and PlatoWork.

How can tDCS help alleviate certain long COVID symptoms?

Many long COVID patients have reported cognitive symptoms of fatigue and brain fog that can impact activities of daily living. The mechanism behind this is not fully understood but is believed to be due to a dysregulated immune system that causes ongoing damage to the brain.

An acute COVID infection can cause an overreactive immune response called systemic inflammatory response syndrome or SIRS. Inflammatory mediators are excessively released at this time, which can cause fatigue. The body tries to create a compensatory response, which can lead to more dysregulation and persistent inflammation. Inflammation can also cause reduced blood flow throughout the brain, which can also cause fatigue.²

In a state of fatigue, there may be alterations or reduced connectivity between different nodes and more frontal regions of the brain and increased connectivity in more posterior areas. The frontal lobes of our brain are typically associated with executive functions involved in cognition. As a result, various regions of the frontal lobes have been targeted for brain stimulation with tDCS because of their proposed involvement in physical and cognitive fatigue.³  It has also been proposed that tDCS may help relieve post-COVID loss of smell by modulating connectivity in the brain’s orbitofrontal cortex.

What evidence supports the use of tDCS for long COVID?

One study involving 48 long-COVID patients randomized to either tDCS at 2 mA or sham (control treatment) found a statistically significant improvement in physical fatigue but not cognitive fatigue. tDCS was also associated with an improvement in depressive symptoms.³ Participants underwent eight sessions for eight consecutive days (excluding weekends) for two weeks. Treatment was localized to the left dorsolateral prefrontal cortex. 

Another study of 12 subjects with post-COVID fatigue received tDCS at 4 mA or sham treatment over the primary motor cortex in four daily 20-minute sessions. Results found a significant decrease in subjective, but not objective, performance fatigue from performing a task involving muscle contractions.⁴ Additionally, a similar study assessing high-definition tDCS at 3 mA targeting the left primary motor cortex found a significantly more significant reduction in fatigue than sham treatment, particularly in the cognitive and psychosocial domains of fatigue. The study also found a significant anxiety reduction.⁵ 

In terms of its effect on loss of smell due to COVID, a small study of 7 patients with persistent loss of smell due to COVID for at least six months received a two-week session of combined olfactory training and anodal tDCS. Both subjective and objective measures showed statistically significant improvement in smell performance.⁶

Based on the available evidence, tDCS positively impacts certain measures of fatigue and, potentially, loss of smell. Compared to other neuromodulation methods, tDCS has greater evidence for its use in long COVID. More studies are currently being conducted to better understand the additional benefits of tDCS and other neuromodulation types for long COVID symptoms.

What are some things to watch out for with tDCS?

tDCS is a safe and well-tolerated procedure that can be done for multiple sessions. When administered for up to 20 minutes at weak direct currents, no serious adverse events were reported, such as seizure or psychotic symptoms. Some side effects may include a slight tingling sensation, light itching sensation, sensation of a short light flash, slight skin redness under the electrode, and moderate fatigue. Skin-related adverse events were the most common. Other reported adverse events included headache, nausea, and insomnia.⁷

References:

1)      Thair H, Holloway AL, Newport R, Smith AD. Transcranial Direct Current Stimulation (tDCS): A Beginner’s Guide for Design and Implementation. Front Neurosci. 2017 Nov 22;11:641. doi: 10.3389/fnins.2017.00641

2)      Linnhoff S, Koehler L, Haghikia A, Zaehle T. The therapeutic potential of non-invasive brain stimulation for the treatment of Long-COVID-related cognitive fatigue. Front Immunol. 2023 Jan 9;13:935614. doi: 10.3389/fimmu.2022.935614

3)      Oliver-Mas S, Delgado-Alonso C, Delgado-Álvarez A, Díez-Cirarda M, Cuevas C, Fernández-Romero L, Matias-Guiu A, Valles-Salgado M, Gil-Martínez L, Gil-Moreno MJ, Yus M, Matias-Guiu J, Matias-Guiu JA. Transcranial direct current stimulation for post-COVID fatigue: a randomized, double-blind, controlled pilot study. Brain Commun. 2023 Apr 10;5(2):fcad117. doi: 10.1093/braincomms/fcad117

4)      Workman C, Boles-Ponto L, Kamholz J, Bryant A, Rudroff T. Transcranial Direct Current Stimulation and Post-COVID-19-Fatigue. Brain Stimul. 2021 November-December;14(6):1672–3. doi: 10.1016/j.brs.2021.10.268

5)      Vestito L, Mori L, Trompetto C, Bagnasco D, Canevari RF, Ponzano M, Subbrero D, Cecchella E, Barbara C, Clavario P, Bandini F. Impact of tDCS on persistent COVID-19 olfactory dysfunction: a double-blind sham-controlled study. J Neurol Neurosurg Psychiatry. 2023 Jan;94(1):87-88. doi: 10.1136/jnnp-2022-329162

6)      Santana K, França E, Sato J, Silva A, Queiroz M, de Farias J, Rodrigues D, Souza I, Ribeiro V, Caparelli-Dáquer E, Teixeira AL, Charvet L, Datta A, Bikson M, Andrade S. Non-invasive brain stimulation for fatigue in post-acute sequelae of SARS-CoV-2 (PASC). Brain Stimul. 2023 Jan-Feb;16(1):100-107. doi: 10.1016/j.brs.2023.01.1672

7)      Matsumoto H, Ugawa Y. Adverse events of tDCS and tACS: A review. Clin Neurophysiol Pract. 2016 Dec 21;2:19-25. doi: 10.1016/j.cnp.2016.12.003

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