To provide a systematic overview of interventions for stroke related visual impairments.
The purpose of this education is to help you understand how to screen, refer and treat patients related to functional performance.
WE DO NOT DIAGNOSE!
Objectives for Today
■ Identify signs and symptoms that indicate a potential vision problem.
■ Identify the differences amongst the variety of vision problems that can occur
following a neurological event and how it impacts functional performance with
■ Identify how to accurately screen for potential vision problems and when to refer to
an eye specialist.
■ Identify therapeutic approaches used to treat and compensate for problems,
allowing for improved function.
[Purpose] Homonymous hemianopia is one of the most common symptoms following neurologic damage leading to impairments of functional abilities and activities of daily living. There are two main types of restorative
rehabilitation in hemianopia: “border training” which involves exercising vision at the edge of the damaged visual field, and “blindsight training,” which is based on exercising the unconscious perceptual functions deep
inside the blind hemifield. Only border effects have been shown to be facilitated by transcranial direct current stimulation (tDCS). This pilot study represents the first attempt to associate the modulatory effects of tDCS over
the parieto-occipital cortex to blindsight treatment in the rehabilitation of the homonymous hemianopia.
[Subjects and Methods] Patients TA and MR both had chronic hemianopia. TA underwent blindsight treatment which was combined with tDCS followed by blindsight training alone. MR underwent the two training rounds in reverse order.
[Results] The patients showed better scores in clinical-instrumental, functional, and ecological assessments after tDCS combined with blindsight rehabilitation rather than rehabilitation alone. [Conclusion] In this two-case report parietal-occipital tDCS modulate the effects induced by blindsight treatment on hemianopia.
[Conclusion] In this two-case report parietal-occipital tDCS modulate the effects induced by blindsight treatment on hemianopia.
To provide a systematic overview of interventions for stroke related visual impairments.
A systematic review of the literature was conducted including randomized controlled trials, controlled trials, cohort studies, observational studies, systematic reviews, and retrospective medical note reviews. All languages were included and translation obtained. This review covers adult participants (aged 18 years or over) diagnosed with a visual impairment as a direct cause of a stroke. Studies which included mixed populations were included if over 50% of the participants had a diagnosis of stroke and were discussed separately. We searched scholarly online resources and hand searched articles and registers of published, unpublished, and ongoing trials. Search terms included a variety of MESH terms and alternatives in relation to stroke and visual conditions. Article selection was performed by two authors independently. Data were extracted by one author and verified by a second. The quality of the evidence and risk of bias was assessed using appropriate tools dependant on the type of article.
Forty-nine articles (4142 subjects) were included in the review, including an overview of four Cochrane systematic reviews. Interventions appraised included those for visual field loss, ocular motility deficits, reduced central vision, and visual perceptual deficits.
Further high quality randomized controlled trials are required to determine the effectiveness of interventions for treating post-stroke visual impairments. For interventions which are used in practice but do not yet have an evidence base in the literature, it is imperative that these treatments be addressed and evaluated in future studies.
Visual impairments following stroke may include abnormalities of central and/or peripheral vision, eye movements and a variety of visual perception problems such as inattention and agnosia. The visual problems (types of visual impairment) can be complex including ocular as well as cortical damage (Jones & Shinton, 2006; Rowe et al., 2009a). Visual impairments can have wide reaching implications on daily living, independence, and quality of life. Links with depression have also been documented in the literature (Granger, Cotter, Hamilton, & Fiedler, 1993; Nelles et al., 2001; Ramrattan et al., 2001; Tsai et al., 2003; West et al., 2002). The estimation of the overall prevalence of visual impairment is approximately 60% at the acute stage following stroke (Ali et al., 2013; Barrett et al., 2007; Clisby, 1995; Freeman & Rudge, 1987; Isaeff, Wallar, & Duncan, 1974; Rowe et al., 2009b; Rowe et al., 2013). A review of the individual prevalence figures and the recovery rates for each of the possible post-stroke visual impairments has been reported elsewhere in the literature (Hepworth et al., 2016).
In order to treat and manage visual impairments caused by stroke it is important to establish the range and effectiveness of the available treatment options. The aim of this literature review is to provide a comprehensive synthesis of the evidence relating to treatment of visual problems after stroke.
Patients with peripheral field loss complain of colliding with other pedestrians in open-space environments such as shopping malls. Field expansion devices (e.g., prisms) can create artificial peripheral islands of vision. We investigated the visual angle at which these islands can be most effective for avoiding pedestrian collisions, by modeling the collision risk density as a function of bearing angle of pedestrians relative to the patient. Pedestrians at all possible locations were assumed to be moving in all directions with equal probability within a reasonable range of walking speeds. The risk density was found to be highly anisotropic. It peaked at ’458 eccentricity. Increasing pedestrian speed range shifted the risk to higher eccentricities. The risk density is independent of time to collision. The model results were compared to the binocular residual peripheral island locations of 42 patients with forms of retinitis pigmentosa. The natural residual island prevalence also peaked nasally at about 458 but temporally at about 758. This asymmetry resulted in a complementary coverage of the binocular field of view. Natural residual binocular island eccentricities seem well matched to the collision-risk density function, optimizing detection of other walking pedestrians (nasally) and of faster hazards (temporally). Field expansion prism devices will be most effective if they can create artificial peripheral islands at about 458 eccentricities. The collision risk and residual island findings raise interesting questions about normal visual development.
Strokes, or cerebrovascular accidents (CVA) are common, particularly in older people. The problems of motor function and speech are well known. This article explains the common visual problems which can occur with a stroke and gives information about diagnosis and management.
A stroke occurs when there is an interruption to blood flow to the brain either because of a blood clot blocking the blood vessel or a haemorrhage in the brain.1 Strokes can cause signs which are obvious, such as loss of speech, drooping of one side of their face, or weakness or paralysis of the arm and/or leg on one side of the body.1 The vision is affected in about two thirds of people who have had a stroke, but this is often not obvious to the patient or their carers. For example, someone who has weakness down one side may bump into things or not eat all the food on their plate, not realising that this may also be because they have visual field loss.2
A stroke or cerebrovascular accident, (CVA) is the result of a blocked blood vessel in the brain (thrombosis or embolus), or haemorrhage into the brain.1 Strokes are more likely in the elderly, and those who have high blood pressure, diabetes or cardiovascular disease.
There are four ways in which vision can be affected following a stroke:
These may occur in isolation but more frequently occur in combination.3 Problems with central vision are quite common after a stroke. The symptoms include blurred or altered vision. In many the vision improves, but the visual loss can be permanent.
Visual field loss occurs in up to half of people with a stroke, with the commonest defect being homonymous hemianopia in which vision is lost in the right or the left visual fields (Figure 1).4 Patients may not be aware of this, and bump into door frames or trip over things on the affected side. Reading can also be difficult (Figure 2).
Visual perceptual deficits are many and varied affecting about a third of people with a stroke. Problems that may develop include neglect one side of their body; difficulty recognising faces or objects, or difficulties with colour vision, depth perception and motion.5 Eye movement abnormalities can also be varied, including strabismus (misaligned eyes), difficulty in converging the eyes to look at near objects, or double vision due to the cranial nerves which control eye movement being affected.6 Typical symptoms include double vision, or jumbled, blurred and/or juddery vision (Figure 2).
Blurred vision, double vision and lossand loss of visual field are significant symptoms that impair daily functioning.7 The patient or their close relatives may report that they frequently bump into objects such as door frames; have difficulty finding things on surfaces; are unsure of their footing while walking and stumble; may leave food uneaten on one side of the plate and have difficulty with reading. Other impacts on the quality of life include loss of confidence, fear of falling, fear of going out alone, social isolation and loss of independence.8
Examination for visual loss is essential for stroke survivors.9 There are various assessment tools which can be used to examine visual function after a stroke:
Treatment options aim to restore visual function to as normal as possible.10 For eye movement abnormalities,prisms and patching one eye can be effective in reducing double vision.6 For visual field loss a Cochrane systematic review reports favourable evidence of visual scanning training which aims to compensate for the visual field loss.11 It is available as a paper training option (www.strokevision.org.uk) or through computer training (www.eyesearch.ucl.ac.uk; www.readright.ucl.ac.uk.
Stroke survivors with persistent impairment of central vision may be helped by low vision services which can include magnifiers, reading aids, computerised adaptations and improved lighting.12 Furthermore, simple adaptations can be made by stroke survivors such as using large print, ensuring good lighting at home, putting labels or coloured stickers on cooking equipment, decluttering areas and having a companion when going out, particularly in busy, crowded places.10
Post-stroke difficulties in visual function are an under-recognised problem that cause significant impact to the quality of life of stroke survivors. Carers and health workers need to be aware that problems with vision are a common consequence of stroke that is not outwardly obvious. Assessment including visual functioning is best provided as part of a multi-disciplinary team on acute stroke units, or in neuro-rehabilitation units. A careful history about visual problems from the patient and carers followed by examination of visual acuity, eye movements and visual field are important in understanding the difficulties in visual functioning.
Management should be tailored to each individual, their visual difficulties and visual needs. With about one quarter of stroke survivors being of working age, rehabilitation in the conext of adaptation of the work place environment is vital if younger people are to return to work after stroke. Rehabilitation requires patience and perseverance on the side of the client, relatives and the health provider.
Despite improvement in stroke prevention and acute stroke management, the increasing ageing population will result in more stroke survivors requiring rehabilitation. Policy makers need to understand the importance of providing post-stroke rehabilitation services including visual functioning.
All webpages accessed 30th January 2017
1 World Health Organization. Stroke and cerebrovascular accident. 2017. http://www.who.int/topics/cerebrovascular_accident/en/
2 Hepworth LR, Rowe FJ, Walker MF, Rockliffe J, Noonan C, Howard C, Currie J. Post-stroke Visual Impairment: A Systematic Literature Review of Types and Recovery of Visual Conditions. Ophthalmology Research: An International Journal. 2015; 5(1). ISSN: 2321-7227
3 Rowe FJ, VIS group. Visual impairment following stroke. Do stroke patients require vision assessment? Age and Ageing. 2009; 38: 188-193
4 Rowe FJ, VIS UK. A Prospective Profile of Visual Field Loss following Stroke: Prevalence, Type, Rehabilitation, and Outcome. BioMed Research International, vol. 2013, Article ID 719096, 1-12, 2013. doi:10.1155/2013/719096.
5 Rowe FJ, VIS group. Visual perceptual consequences of stroke. Strabismus 2009; 17: 24-28
6 Rowe FJ, VIS group. The profile of strabismus in stroke survivors. Eye 2010; 24: 682-5
7 Rowe FJ, VIS Group. Symptoms of stroke related visual impairment. Strabismus, 2013; 21: 150-4
8 Hepworth L, Rowe FJ. Visual impairment following stroke – the impact on quality of life: a systematic review. Ophthalmology Research: an international journal. 2016; 5(2): 1-15
9 Rowe FJ. The importance of accurate visual assessment after stroke: Editorial. Expert Reviews in Ophthalmology. 2011: 6; 133-6
10 Rowe FJ. Care provision and unmet need for post stroke visual impairment; Final report. 2013. http://www.stroke.org.uk/sites/ default/files/final_report_unmet_need_2013.pdf?
11 Pollock A, Hazelton C, Henderson CA, Angilley J, Dhillon B, Langhorne P, Livingstone K, Munro FA, Orr H, Rowe FJ, Shahani U. Interventions for visual field defects in patients with stroke. Cochrane Database of Systematic Reviews 2011, Issue 10. Art. No.: CD008388. DOI: 10.1002/14651858.CD008388.pub2.
12 Virgili G, Rubin G. Orientation and mobility training for adults with low vision. Cochrane Database of Systematic Reviews 2010, Issue 5. Art. No.: CD003925. DOI: 10.1002/14651858.CD003925.pub3
Background: The visual impairments caused by stroke have the potential to affect the ability of an individual to perform activities of daily living. An individual with visual impairment may also have reduced level of independence. The purpose of this review was to investigate the impact on quality of life from stroke related visual impairment, using subjective patient reported outcome measures.
Methods: A systematic search of the literature was performed. The inclusion criteria required studies to have adult participants (aged 18 years or over) with a diagnosis of a visual impairment directly resulting from a stroke. Studies which included visual impairment as a result of other intracranial aetiology, were included if over half of the participants were stroke survivors. Multiple scholarly online databases and registers of published, unpublished and ongoing trials were searched, in addition articles were hand searched. MESH terms and alternatives in relation to stroke and visual conditions were used. Study selection was performed by two authors independently. Data was extracted by one author and verified by a second. The quality of the evidence was assessed using a quality appraisal tool and reporting guidelines.
Results: This review included 11 studies which involved 5646 participants, the studies used a mixture of generic and vision-specific instruments. The seven instruments used by the included studies were the EQ-5D, LIFE-H, SF-36, NEI VFQ-25, VA LV VFQ-48, SRA-VFP and DLTV.
Conclusion: A reduction in quality of life was reported by all studies in stroke survivors with visual impairment. Some studies used generic instruments, therefore making it difficult to extract the specific impact of the visual impairment as opposed to the other deficits caused by stroke. The majority of studies (8/11) primarily had participants with visual field loss. This skew towards visual field loss and no studies investigating the impact ocular motility prevented a comparison of the effects on quality of life due to different visual impairments caused by stroke. In order to fully understand the impact of visual impairment following stroke on quality of life, further studies need to use an appropriate vision-specific outcome measure and include all types of visual impairment which can result from a stroke.
Objective: By means of neuropsychologic tests, to further analyse a specific chiasmal monocular visual testing behaviour, here labelled temporal blocking because of the elective ignorance of optotypes on the temporal side of the chart. Often it is combined with impairment of reading and other cognitive impairments.
Methods: Eighteen patients with lesions to the chiasm and some degree of temporal blocking aged 24 – 76 years underwent:
Results: The temporal blocking in two patients recovered after emergency neurosurgery and their results were normal when subsequently tested. Of the 16 patients with deficiencies, 14 had a poorer left eye (p < 0.01).
Conclusions: The best neuropsychologic tests appeared to be those for visual neglect and the crowded bar test. In most cases, the right cerebral hemisphere’s lack of some crossed information from the left eye, usually needed for normative saccades and adjustment to visual space, may be a factor underlying the specific visual behaviour.
“All great truths begin as blasphemies.”
— George Bernard Shaw
To understand the issues of driving with a homonymous hemianopsia, we have to better define the question. Too often the question is presented as, “Can an individual with homonymous hemianopsia drive safely?” This is the wrong question! The question today should be “Which homonymous hemianopsia patients are safe to drive?” Many research studies have found that even without the kind of clinical patient selection criterion, adaptive devices, therapy and driver’s training that a potential hemianopsia driver should undergo, a significant portion of hemianopsia patients in these studies demonstrated that they may have potential to drive safely.
If we look at the group of all hemianopsia patients, those who are safe to drive will be a very small group. This is owing to the great variability of associated problems of cognition, visual neglect, visual perception, alertness and ability to compensate. No clinician or researcher would ever argue that all hemianopsia patients are safe to drive.
Let us look instead at a limited group of hemianopsia patients for whom the higher order deficits have been screened to rule out cognitive deficits, visual neglect, and poor processing speed. In this group visual field expanders have been prescribed where indicated and the patients trained with these devices and given scanning training. Then these patients have been screened with a behind-the-wheel driving evaluation, we would see a much smaller group. But within that group, would emerge a patients that could have the potential to return to driving.
It is less about the visual field
Another question I see that demonstrates a failure for some to understand where the problem resides is “How much visual field is required to drive safely?” As clinicians that have worked for many decades with hemianopsia patients, we have learned that the visual field defect is only a small part of the driving safety issue. It is usually about the constellation of problems from the brain injury and each individual’s ability to compensate.
While the type and size of visual fields are factors, the higher order cognitive functions are far more important to safe driving than the size of the visual field. These higher cognitive and perceptual functions determine if the patient can safely compensate. The real question should be expanded to, “On a case-by-case basis does this patient with an acquired brain injury from stroke, tumor, trauma or other cause, have the higher-level cognitive skills, compensatory skills, optical devices, experience, stamina driving skills and discipline to drive with a reduced visual field?”
All hemianopsia are not created equal!
Let us look at two patients with identical measurable visual field, both presenting with left homonymous hemianopsias. The first has an isolated stroke in the right occipital lobe no deficits other than the visual field loss. This patient has no visual neglect and no deficits in saccadic eye movements that would impair compensatory scanning and searching into the area of loss. With training and appropriate devices, this patient may have potential to return to safe driving. The second patient has an identical appearing left homonymous hemianopsia but from a stroke in a different location, the right parietal lobe. Thus this patient also has severe left visual neglect, impairments in saccadic eye movements and thus will never return to driving. If we only look at the visual field results, these patients look identical, but they are totally different cases.
If a state law looks only at the visual field loss to determine if driving is possible, they would treat both patients the same, denying them both the option of a driver’s license. While the second patient should not drive, this can needlessly devastate the first patient’s life, robbing the patient of independence, ability to get to work, and to lead an otherwise normal life.
How do we predict safety?
The other question we must ask is, “What tests and evaluations best predict safe driving and what are the potential weaknesses that must be addressed in training?” Various neuropsychological tests can give us information on who may have potential to drive safely. More research to establish which tests give us the most effective data is needed. Additionally, behind-the-wheel research studies continue to expand our information on the unique driving behaviors of the hemianopsia driver.
Driving, however, is a complex function. Prior experience, stamina, motivation, and discipline combined with visual status and mental functioning all can shape the impact on safety. After all the testing and treatments are completed to help select those who show potential to drive, a behind-the-wheel driving evaluation with a driving rehabilitator experienced with acquired brain injury and hemianopsia is needed. Only during the behind-the-wheel examination and training can the full complexity of driving be evaluated and training performed to improve specific skills like lane position, use of optical devices and mirrors.
The most important question is, “Have we learned to treat each person as a unique individual, understanding that impairment, disability and handicap are not one in the same?”
Should state laws prevent all Hemianopsia driving?
Setting an arbitrary visual field width to discriminate against all hemianopsia patients is now seen by many current researchers as a needless burden on the portion of hemianopsia patients that have the ability to return to safe driving. Below is what a number of researchers have observed:
As Dr. Eli Peli, Senior Scientist from Harvard’s Schepens Eye Research Institute stated in Driving With Confidence, A Practical Guide to Driving with Low Vision:
“It is clear that not all people with hemianopia function at the same level and many probably could not drive safely. However, a fair percentage of these patients may compensate for their visual loss to such an extent that they can drive as safely as any driver.”
In Automobile Driving Performance of Brain-Injured with Visual Field Defects , T Schulte, H Strasburger, E Muller-Oehring, E Kasten and B Sabel 1999, American Journal of Physical Medicine & Rehabilitation, researchers performed a driving simulator-based study of six hemianopsia patients and a similar size group of normally sighted. They summarized:
“Contrary to our expectations, the findings showed no reliable difference in the performance of visually impaired and the normally sighted subjects on a driving simulator. …Thus on a practical level our results indicate that the suspension of driving privileges for persons having visual field impairments may be unwarranted on the basis of visual field loss alone.”
In a study by Racette & Casson (1999), Visual field loss and driving performance: a retrospective study Abstracts of the Eighth International Conference Vision in Vehicles, they studied 13 homonymous hemianopsia patients and 7 homonymous quadranopsia patients. They determined those who were unsafe, those who need additional assessment, and those who were safe. Only 23% of the hemianopsia patients were found unsafe and none of the quadranopsia patients were deemed unsafe.
“Clearly, the evidence provided by these reports indicate that homonymous visual field defect and homonymous hemianopia by itself can not be an absolute and inevitable contra-indication for practical fitness to drive.”
A 2009 study, On-road driving performance by persons with hemianopia and quadrantanopia, Investigative Ophthalmology Vis Sci 50 (2) 2009, J. Wood, G. McGwin, J. Elgin, M. Vaphiades, R. Braswell, D. DeCarlo, L Kline, G Meek, K Searcy and C. Owsley studied 22 hemianopsia and 8 quadranopsia patients and a normal control group driving over a 14.1 mile course of city and interstate driving. Two back seat evaluators, who were masked to the status of the patient, evaluated the drivers. They found 100% of normal drivers were safe to drive and 73% of hemianopsia and 88% of quadranopsia patients were safe to drive.
The study concluded that:
“Some drivers with hemianopia or quadrantanopia are fit to drive compared with age-matched control drivers. Results call into question the fairness of governmental policies that categorically deny licensure to persons with hemianopia or quadrantanopia without the opportunity for on-road evaluation.”
Continued research is crucial to define all of the parameters of hemianoptic driving. Information from these studies helps us define the best candidate, the areas of weakness and will guide driving rehabilitation specialists in training these patients.
A study by Bower et al, from The Schepens Eye Research Institute, Harvard Medical School, Boston, Massachusetts, Driving with Hemianopia, I: Detection Performance in a Driving Simulator, published November 2009 in Investigative Ophthalmology and Visual Science, tested twelve hemianopsia patients without visual neglect or cognitive loss and twelve matched normals on a simulator test over a two hour period. The hemianopsia patients were tested without visual field expanding systems and they demonstrated significantly more difficulty in detection of suddenly appearing pedestrians on their impaired side inside the simulator.
There was great variability in pedestrian detection among the small group of 12 hemianopsia patients with older driver’s demonstrating lower rates. The authors of this study warned that simulator studies may not match results in real world driving and they further suggested that this also means we must look at each driving candidate individually. They stated:
“In determining fitness to drive for people with HH, the results underscore the importance of individualized assessments including evaluations of blind-side hazard detection.”
The same scientists now plan to do similar tests with patients using visual field expanders. Our years of work would support that the visual field expanders and training can help fill in detection of pedestrians in many patients, but more research is needed.
How could states regulate hemianopsia licensing?
It is clear from the research that we cannot make generalizations about the driving safety of all hemianoptic drivers. Thus simply removing visual field requirements could lead to hemianopsia drivers being licensed who have other cognitive or perceptual problems at make them unsafe.
States that still contain absolute prohibitions against driving with homonymous hemianopsias should consider removing these, and replacing them with a process to judge each patient individually based on current science. The process should include mandatory evaluation with a low vision specialist experienced in hemianopsia for evaluation and treatment followed by additional therapy/training as needed including occupational therapy if indicated. Then a behind-the-wheel driving evaluation and training as appropriate to each case with a certified driving rehabilitation specialist should be completed.
Then, the doctor with the report of the driving rehabilitation specialist would file a special application with the state. The states medical advisory committee would review each case individually. If the application is approved, the patient would have to demonstrate adequate driving skills on an extended state behind-the-wheel test by the state driver’s license bureau. Restrictions on type of driving and time of day could be considered in each case cases.
Please contact us if you have any questions:
The Low Vision Centers of Indiana
Richard L. Windsor, O.D., F.A.A.O., D.P.N.A.P.
Craig Allen Ford, O.D., F.A.A.O.
Laura K. Windsor, O.D., F.A.A.O.
Indianapolis (317) 844-0919
Fort Wayne (260) 432-0575
Hartford City (765) 348-2020
From Hemianopsia.net, The Low Vision Centers of Indiana. Used with permission. www.hemianopsia.net.
Visual field defects after posterior cerebral artery stroke can be improved by vision restoration training (VRT), but when combined with transcranial direct current stimulation (tDCS) which alters brain excitability, vision recovery can be potentiated in the chronic stage. To date the combination of VRT and tDCS has not been evaluated in post-acute stroke rehabilitation.
To determine whether combined tDCS and VRT can be effectively implemented in the early recovery phase following a stroke, we wished to explore the feasibility, safety and efficacy of an early intervention.
Open-label pilot study including a case series of seven tDCS/VRT versus a convenience sample of seven control patients (clinicalTrials.gov ID: NCT02935413).
Patients with homonymous visual field defects following a posterior cerebral artery stroke.
Seven homonymous hemianopia patients were prospectively treated with 10 sessions of combined tDCS (2mA, 10 daily sessions of 20 min) and VRT at 66 (±50) days on average post-stroke. Visual field recovery was compared with retrospective data of 7 controls, whose defect sizes and age of lesions were matched to the experimental subjects and who had received standard rehabilitation with compensatory eye movement and exploration training.
All seven patients of the treatment group completed the treatment protocol. Safety and acceptance were excellent, and patients reported occasional skin itching beneath the electrodes as the only minor side effect. Irrespective of their treatment, both groups (treatment and control) showed improved visual fields as documented by an increased mean sensitivity threshold in dB (decibel) in standard static perimetry. Recovery was significantly greater (p<.05) in tDCS/VRT patients (36.73 ± 37.0%) than in controls (10.74 ± 8.86).
In this open-label pilot study, tDCS/VRT in sub-acute stroke was safe, with excellent applicability and acceptance of the treatment. Preliminary effectiveness calculations show that tDCS/VRT may be superior to standard vision training procedures. A confirmatory, larger-sample, controlled, randomized and double-blind trial is now underway to compare real- vs. sham-tDCS supported visual field training in the early vision rehabilitation phase.