Treatment of COVID-19 Post-acute Cognitive Impairment Sequelae (PASC) With Portable Brain Stimulation: a Double-blind, Randomized-controlled Trial

As of August 1, 2021, novel coronavirus disease 2019 (COVID-19), an infection caused by Severe Acute Respiratory Syndrome (SARS-CoV-2), had infected over 200 million people and accounted for more than 4 million deaths worldwide. After remission of the acute phase of the disease, many patients report persistent symptoms that go far beyond the consequences of the pulmonary involvement itself and can impose chronic sequelae of COVID-19. According to the World Health Organization, post-COVID-19 syndrome occurs in individuals with a history of probable or confirmed SARS CoV-2 infection, usually 3 months from the onset of COVID-19 with symptoms and lasting at least 2 months and cannot be explained by an alternative diagnosis. The most common complaints are extreme fatigue, inability to perform activities of daily living that involve physical or cognitive exertion, emotional dysregulation, mental fog, and other symptoms of cognitive impairment. Symptoms may be new after initial recovery from an acute episode of COVID-19 or persist from the initial illness. Symptoms can also fluctuate or recur over time.

Given the increasing number of COVID-19 cases, there is an urgent need to develop treatment alternatives for PASC-related cognitive and memory deficits. In this context, transcranial direct current stimulation (tDCS or tDCS) is a non-invasive brain stimulation intervention that may be promising in these cases. It represents an interesting opportunity for targeting the neural circuits underlying neurological, cognitive, and psychiatric disorders. For example, once a target neural circuit has been identified as a related region of interest for a neuropsychiatric disorder, neuromodulation methods could be used to selectively modify activities in the target region. In tDCS, a direct, low-intensity electrical current (e.g., typically 1-4 mA) is passed through at least two electrodes (i.e., anode and cathode), which are applied non-invasively to the scalp. TDCS modulates neuronal activity at the network level by producing current flow around neurons and resulting in an incremental change in neuronal membrane potentials. In turn, such potential changes lead to a series of changes in neuronal function, such as a change in firing rates.

Thus, tDCS can induce long-term changes in brain activity. Learning ability requires functional changes that can be induced by tDCS, which makes this technique a promising tool to improve cognitive performance. In fact, tDCS has already been successfully used in non-COVID-19 samples to improve cognitive performance in several domains, such as working memory, episodic memory, executive function, and language. It is also worth noting that tDCS presents definite or probable evidence of efficacy for several neuropsychiatric disorders that are often accompanied by cognitive deficits such as addiction, attention deficit/hyperactivity disorder, Parkinson's disease (motor and cognitive impairment), epilepsy, schizophrenia, Alzheimer's disease, and depression.

TDCS has attractive advantages for clinical use, such as affordability, lower cost than magnetic stimulation, ease of use and operation, especially for customized devices, and portability. In fact, the development of portable devices with remote control allows for greater ease of use and feasibility in uncontrolled pandemic scenarios. Furthermore, recent evidence suggests that the effects of tDCS may be enhanced by a concomitantly administered intervention. Neurophysiological studies have shown that tDCS parameters used in clinical practice have an unlikely impact on neural spiking, as such modulation is only achieved at doses ≥ 4.5mA. Rather, the effects of tDCS on behavior are best explained by modulating ongoing activity. According to the "selective activity" hypothesis, tDCS preferentially modulates a neuronal network that is already activated, rather than separate inactive neuronal networks. For example, in vitro direct current stimulation (DCS) preferentially increased plasticity in a synaptic pathway previously stimulated at 0.1 Hz, whereas DCS alone, without preactivation, did not influence synaptic efficacy. Another study showed that anodic DCS increased long-term potentiation (LTP) induced by high-frequency stimulation, but did not induce LTP per se. In other words, preclinical studies have shown that tDCS is too nonspecific to alter synaptic efficacy alone, but can enhance Hebbian (associative) plasticity activated by task performance.

The pupillary light reflex (PLR) is a reflex that controls the diameter of the pupil in response to variations in light intensities that reach the retina and is modulated by the autonomic nervous system (ANS) that innervate the circular and radial muscles of the iris, while contracting or relaxing, decrease or increase the size of the pupil, respectively. Studies have shown that a specific selection of intensity and light stimulus, wavelength and duration directly influence the contributions of retinal photoreceptors, making it possible to also assess retinal changes beyond the ANS. Patients with Alzheimer's Disease (AD) are known to have alterations in cholinergic neurotransmission, in a decrease in pupil diameter response to abrupt changes in room lighting or a single flash of light. Therefore, the RPL is a good tool for evaluating patients with AD.