The Effects of Bilateral-Transcranial Electrical Stimulation (tES) on Cortical Activity, Motor Performances, and Cognitive Function in Sub-Acute and Chronic Stroke Individuals

Stroke is a leading cause of death and disability in the world. The prevalence of stroke is high in developing countries; for example in Thailand, stroke is one of a leading cause of death and disability. The estimate prevalence among adults aged 45 years and older is 1.88%, and the mortal rate of stroke is still has continued to increase over the past 5 years. Globally, 87 percent of stroke people have suffered from ischemic stroke while 13% suffered from hemorrhagic stroke. Fifty-seven percent stroke cases remains disability reported by the world stroke organization. Motor impairment is a major limit post-stroke and it affects activities in daily life. Cognitive impairments (i.e., executive function, attention, and memory) are commonly observed in post-stroke survivors which may lead decrease in functional capacity and affects the efficacy of rehabilitation in stroke.

Stroke is defined as a sudden neurological explosion caused by impaired perfusion through the blood vessels to the brain. Ischemic stroke is caused by deficient blood flow and oxygen supply to the brain, while, hemorrhagic stroke is caused by bleeding or leaky blood vessels, resulting in cell death. After-stroke, cortical excitability decreases in the ipsilesional hemisphere and increases in the contralesional hemisphere. Consequently, there is an imbalance in the interhemispheric inhibition (IHI) between both hemispheres. The onset of IHI imbalance in stroke patients remains unclear, although the occurrence of motor impairment since the onset. However, the IHI imbalance is remained throughout stroke life. The imbalance of IHI was found to be negatively correlated with stroke motor recovery; a greater imbalance was associated with a poorer motor performance. IHI imbalance is not a cause of poor motor recovery, but instead might be the consequence of underlying recovery processes.

An electroencephalography (EEG) is the non-invasive measurement tool with high temporal resolution, which can detect a summary of the postsynaptic electrical signals of pyramidal neurons in the cortical layers. EEG is a useful measurement after stroke. Several metrics have been found to be related to recovery across all stroke phases. A recent review in 2022 reported that brain symmetry index (BSI) and spectral power are the most efficacious metrics to use to predict stroke recovery. BSI is one of the more popular EEG-derived parameters use to quantify the mean spectral asymmetry between the two hemispheres. It has a normal range of 0 to 1, where 0 indicates perfect symmetry, while 1 indicates maximal asymmetry. BSI was significantly higher in stroke patients indicating brain asymmetry compared with healthy controls Van Putten and Tavy demonstrated in acute ischemic stroke people that BSI was unchanged within 24 hours post-stroke, however it had a positive correlation between the National Institutes of Health Stroke Scale (NIHSS); a higher impairment, a higher brain asymmetry. It was reported that higher BSI value in the acute to sub-acute phase were associated with lower motor performance in 4 weeks later implying that BSI could possibly aid in prognostication of motor outcome during stroke recovery phase. Moreover, spectral power analysis in stroke individuals showed fluctuation of brain oscillation caused by cell death such as an increase of low frequency power (delta and theta) in the ipsilesional hemisphere that occurred as early as one minute after stroke, whereas high frequency power (alpha and beta) were decreased. An increase in delta activity post-stroke was also negatively corelated to cognitive functioning. Cortical activity in recovery phase can represent motor recovery and cognitive function post-stroke. An increase of high-frequency brain wave (i.e. beta band) is related to improve motor recovery in subacute and chronic phases of stroke. In addition, increase of alpha band and decrease of delta band correlated with improve cognitive function in stroke survivors.

Motor recovery post-stroke rapidly increase within first 3 months (acute to early subacute phase) and less significant recovery subsequently in late-subacute (3-6 months), then, spontaneous recovery is leading to a stable at its limit in chronic phase (>6 months), but the recovery can expand up to 2 years post-stroke after receiving rehabilitation. Therefore, the rehabilitation can enhance stroke recovery beyond spontaneous recovery in the subacute and chronic phases. There are 2 factors influencing stroke recovery: 1) intrinsic factors which are age, sex, stroke onset, stroke type, severity, co-morbidities, socioeconomic status, and genetic profiles, 2) extrinsic factors such as caregiver support, pharmacology, and rehabilitation program in which consists of bottom-up and top-down approaches. Rehabilitation post-stroke is recommended to enhance recovery and reduce long-term disability. The guideline of American Stroke Association (ASA) has recommended that patients with stroke required therapeutic intervention of at least 3 times per week for 20-60 min per session.

Non-invasive brain stimulation (NIBS) i.e., transcranial Electrical stimulation (tES) is a top-down approach that has been recommended to use as an add-on intervention in rehabilitation post-stroke. The most common tES techniques used in research are 1) transcranial Direct Current Stimulation (tDCS) and 2) transcranial Alternating Current Stimulation (tACS). The difference between these two tES techniques is the current forms elicited.