Transcranial Direct Current Stimulation as a Tool in Rehabilitation of Visual Processing after Stroke: A Review

Review Article

Austin J Cerebrovasc Dis & Stroke. 2016; 3(1): 1042.

Transcranial Direct Current Stimulation as a Tool in Rehabilitation of Visual Processing after Stroke: A Review

Cristino ED*, Silva JBS, Andrade MJO, Almeida NL and Santos NA

Department of Psychology, Federal University of Paraíba, Brazil

*Corresponding author: Cristino ED, Laboratory of Perception, Neuroscience and Behavior, Department of Psychology, Federal University of Paraíba, Cidade Universitária, João Pessoa, PB, Zip Code: 58051-900, Brazil

Received: May 15, 2016; Accepted: July 13, 2016; Published: July 15, 2016

Abstract

Stroke is one of the major causes of incapacity worldwide. More specifically, the visual impairments after stroke can lead to substantial losses in the activities of daily living and bring great impact upon an individual’s sense of well-being and independence. The development of novelty rehabilitation strategies to promote the recovery of visual function after stroke is of great importance. Transcranial direct current stimulation (tDCS) is a neuromodulatory technique with increasing popularity in the fields of basic research and rehabilitation. Despite the significant number of studies involving tDCS in rehabilitation after stroke, there are few published studies that specifically involve treatment of visual processing deficits. The aim of this review is to describe and discuss the research using tDCS in visual rehabilitation after stroke and to encourage future investigation on visual processing using tDCS as a tool in rehabilitation after brain lesions. Studies have pointed out that tDCS applied to the occipital cortex has demonstrated good results, with important effects on visual field rehabilitation in hemianopia, motion perception and color discrimination. Although some successes have been achieved in recent years, a lot of questions still need to be understood and others asked. All of this is in order to improve protocols used and, thus, obtain better results.

Keywords: Stroke; Rehabilitation; Visual processing; Transcranial direct current stimulation (tDCS)

Abbreviations

tDCS: Transcranial Direct Current Stimulation; ADL: Activities of Daily Living; HH: Homonymous Hemianopia; tPA: Intravenous Thrombolytic Treatment; TCI: Alteration in Transcallosal Inhibition; VRT: Visual Rehabilitation Training; EEG: Electroencephalography; fMRI: Functional Magnetic Resonance Imaging; HRP: High- Resolution Perimetry; QOL: Quality of Life

Introduction

Every year, about 16.9 million people worldwide suffer their first stroke [1]. This is an alarming number which generates a serious impact on society and on public health. Stroke survivors can experience different kinds of sequelae such as cognitive, motor and sensory perception deficits [2,3]. These consequences can lead to substantial losses in the activities of daily living (ADL) and quality of life [4,5]. More specifically, the visual impairments after stroke include eye movement disorders, perceptual deficits and visual field defects [6]. Visual field defects are a consequence of posterior strokes, and occur on one side of the visual field, usually in both eyes. This condition, called homonymous hemianopia (HH), is reported in up to 57% of patients during 3 months post event. After this period, a complete recovery of visual fields can occurs in up to 44% of cases and the partial recovery in up to 72% [7-9]. In addition, this kind of sequel can profoundly affect many important ADL, including reading, performing visual searches, driving [5,6] and navigating safely within one’s environment [10,11]. Thus, it is possible to realize that all kinds of visual deficits bring great impacts upon an individual’s sense of well-being and independence [12].

Intravenous thrombolytic treatment (tPA) has been successful in reversing visual impairments in the hyperacute phase of ischemic stroke; however, this treatment is only indicated when the neuronal tissue is not yet permanently damaged by the ischemia [13]. Nevertheless, once tissue damage has developed, spontaneous recovery is unpredictable and often incomplete [14]. Therefore, the development of novelty rehabilitation strategies to promote the recovery of visual function after stroke is of great importance [15].

Studies using human clinical trials and animal models have pointed towards evidence of the brain’s potential to reorganize itself within the context of functional recovery after injury [16-18]. Thus, identifying interventions that can promote and modulate these mechanisms seems very important to improving rehabilitation after stroke. In this context, noninvasive cortical stimulation techniques, such as transcranial direct current stimulation (tDCS), have gained prominence in neuro rehabilitation research as a reason of their potential to improve neuro plastic mechanisms associated with functional recovery [19,20].

Transcranial direct current stimulation (tDCS)

Studies that involve transcranial direct current stimulation (tDCS) are growing in frequency. In 2000, a PubMed database search using the search terms of tDCS produced only four articles. In 2013, the same search produced approximately 370 references [21]. Nowadays, three years after the study by Barry Hill, et al. [21], it is possible to find more than 2,500 studies involving tDCS in the PubMed database.

The technique

The method tDCS is a non-invasive neuromodulation method for delivering low-intensity polarizing electrical currents to the brain cortex [22]. The use of two electrodes (anode and cathode) placed on the scalp can favor neuronal activity [23]. These electrodes are often large (25–35 cm²), and the current intensity varies between 1 and 2 mA [24]. The current flows from the anode to the cathode electrode, and depending on the electrode positioning, may modulate the resting membrane potential of neurons to be closer or more distant from the firing threshold [22,24]. In general, the anode electrode produces an excitatory modulation while the cathode electrode produces an inhibitory effect. However, some studies point to results with the opposite outcome.

The main mechanism which explains tDCS effects is their capacity to modulate the resting membrane potentials of the stimulated area. In synthesis, is possible to claim that anodal tDCS can causes the resting membrane potential to become more positive, and, as a result, it makes the cell more responsive. On the other hand, the cathodal stimulation will lead to cell hyperpolarization, what difficult the neural impulse [25,26]. To elucidate this action, a study has delivered calcium and sodium channel blockers and noted the in activation of anodal tDCS effects. For more information, see Nitsche, et al [25, 26]. Other studies propose, as mechanism of action, which tDCS alters the levels of gamma-aminobutyric acid (GABA) and glutamate and it can alter the cortical excitability [27-29].

Moreover, it is important to highlight that tDCS effects seem to be site specific, so, moving the electrodes just a few centimeters can dramatically alter the results. Despite this, effects of tDCS are not site limited, what means that the stimulation can affect different areas and not only focus under the electrodes [22-24,30,31]. The safety profiles, reasonable cost, and promising findings have contributed to highlighting the technique [21]. As a therapeutic tool, tDCS has been used, for example, in rehabilitation in Parkinson’s disease [32,33], Alzheimer’s disease [34,35] and chronic pain [36,37].

tDCS in stroke

Studies evaluating the effects of tDCS on stroke have presented well-structured protocols and consistent results. In addition, a variety of sequelae resulting from stroke have shown improvements after different stimulation protocols of tDCS. As an example, it is possible to mention improvements on aphasia [38,39], cognitive functions [40,41] and motor rehabilitation [41-44].

The neurobiological basis of functional recovery after neuro stimulation in stroke patients has their foundation in the interhemispheric imbalance theory that occurs after vascular injury [45]. The interhemispheric imbalance theory suggests an alteration in transcallosal inhibition (TCI), in which inhibition exerted from the ipsilesional hemisphere (lesioned) on the contralesional hemisphere (intact) is weaker than inhibition exerted from the contralesional hemisphere on the ipsilesional hemisphere [46-48]. Restoring the interhemispheric balance by modulating brain activity can be achieved through the use of tDCS [24,26,49,50].

Literature search strategy

Despite the significant number of studies involving tDCS in rehabilitation after stroke, there are only a few published studies that specifically involve treatment of visual processing. Thus, the aim of this review is to describe and discuss the research using tDCS in visual rehabilitation after stroke.

For this purpose, three databases were consulted: PubMed, Scopus and Web of Science for the years 1996-2016. The search terms used were: “tDCS stroke visual function”, “Stroke tDCS visual cortex”, “Stroke tDCS contrast sensitivity” and “Stroke tDCS human color discrimination”. All articles selected were published in English. As a final result, we found 6 studies that involve neuromodulation by tDCS on specific visual losses after stroke. We opted to exclude studies about hemineglect, since this deficiency involves a complexity of functions beyond visual processing. A flowchart of the systematic review (Figure 1) summarizes the literature search strategy.

Citation: Cristino ED, Silva JBS, Andrade MJO, Almeida NL and Santos NA. Transcranial Direct Current Stimulation as a Tool in Rehabilitation of Visual Processing after Stroke: A Review. Austin J Cerebrovasc Dis & Stroke. 2016; 3(1): 1042. ISSN : 2381-9103