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Moving past the eyes and into the brain: It is vital to think beyond the eye in the presence of visual impairment and blindness.
Reviewed by Dr Lotfi Merabet and Jessie Cronan.
Cerebral visual impairment (CVI; also called cortical visual impairment) is a condition related to brain injury and results from damage to the visual pathways in the brain. Unfortunately, CVI is all too common, although not as commonly diagnosed. And therein lies the problem.
CVI is the primary cause of paediatric visual impairment and blindness in developed countries worldwide,1 according to Dr Lotfi Merabet, an associate professor of ophthalmology at Harvard Medical School and director of the Laboratory for Visual Neuroplasticity at Schepens Eye Research Institute and Massachusetts Eye and Ear in Boston, Massachusetts, United States. Dr Merabet is also on the board of directors, education committee and CVI steering committee of the Perkins School for the Blind in Watertown, MA.
“There has been a fundamental shift in the clinical profile of visual impairment in paediatric populations, and the Perkins population mirrors this perfectly,” he commented. He explained that from the 1950s to 1980s, children enrolled in the school were typically profoundly blind due to a disease or pathology at the ocular level: infection, inherited disease or retinopathy of prematurity.
Over the past couple of decades, however, a shift occurred, with children presenting with a more complex profile—they were visually impaired and often had learning disabilities and motor and/or language issues. “This difference in paediatric blindness is important both from clinical and scientific standpoints as well as an educational standpoint,” Dr Merabet emphasised.
Recognition of the source of the brain-based visual disabilities is the key to early intervention in affected children. In light of this fact, Dr Merabet and other researchers are attempting to determine the inherent differences in these children—specifically, how visual impairment caused by damage to the brain is different from visual impairment at the level of the eye.
What has been determined with CVI is that affected children may have sustained a neurological complication or brain injury during the time of the brain’s development: that is, around the time of birth or shortly thereafter. Some children may have had a stroke in utero or were born prematurely. Other causes include seizure disorder, trauma and genetic or metabolic disorders.
Because of medical advancements in neonatal care, many more of these children are surviving today than in the mid-20th century. In addition, today there isgreater recognition of their perceptual and learning issues at the research level, and increasing numbers of clinics are starting to recognise CVI.
In a typical scenario, parents may report that their child cannot locate a favourite toy or cross the street, or becomes overwhelmed in crowds or a busy classroom. In contrast, the child’s vision may be only moderately impaired or evennormal based on a Snellen chart evaluationor visual perimetry, and the eyes appear healthy on slit-lamp evaluation.
When the clinician does not recognise that the visual impairment is localised to the brain, it can often lead the parents into the weeds, with suggestions of possible diagnoses including autism, development delays or psychiatric issues. This is the point at which these children begin to fall through the cracks.
Dr Merabet and his colleagues, in collaboration with Boston Children’s Hospital, Perkins School for the Blind and Boston Medical Center, are currently studying the brain scans of children with CVI to correlate any identified structural changes in the brain to the manifesting developmental and behavioural deficits, and comparing the scans to those obtained from children with normal vision and those with other forms of ocular blindness (see Figure).2 In their study, children are divided based on age: 7 years and older and 14 years and older.
For the younger group, virtual reality and eye tracking arebeing used to obtain an idea of their visual perceptual abilities by assessing functional vision. In this group, the children are tasked with, for example, finding a certain toy in a toy box or a particular person in a crowded hallway, and the complexity of the environment and the child’s reaction to it is assessed. As the environment increases in complexity, despite good visual acuity, the children’s vision becomes a very confusingswirl of colours that prevents the differentiation of objects/persons.
Dr Merabet uses virtual reality to create functional vision assessment tools that surpass the standard eye chart and tests performed in an ophthalmology or optometry clinic. “We found that the children are very sensitive to clutter, crowding, complex motion and, more interestingly, to visual demands. As the task becomes much more difficult—that is, as we make the assessment harder—their visual system breaks down,” he explained.
This point is important because, although a visual assessment during a standard vision test indicates that these children may have normal visual acuity, when put into a situation in which the visual system is taxed, the system can break down and the hidden visual impairment manifests itself.
“This breaking down of the visual system in the setting of high visual demands and complexity is the tell-tale sign of CVI and highlights the difference between visual function and functional vision. We need to gain a better understanding of how these children use their vision in the real world,” Dr Merabet said.
The children who are 14 years and older undergo MRIs of the brain to see how their brains are “wired” (i.e., white matter connectivity) and how theyactivate as a function of performing virtual reality tasks. “We found that their brains are very different from children with typical neurological development and from those who are blind from other causes, emphasising the need for education,” he said.
Another factor under investigation is why some children benefit from intervention and others do not, and the timeline of improvement. Part of the challenge here is determining how the cause of the CVI—brain injury, trauma, infection, seizure or a genetic or metabolic disorder—relates to underlying visual impairment. “This makes for a complicated patient profile, and every child is unique in terms of their challenges,” Dr Merabet pointed out.
No specific treatment currently exists for CVI. Early diagnosis of brain-based visual impairment and implementation of strategies to promote maximal brain development are the current focus, but there is no strong evidence yet to support what does and does not benefit these patients.
“This is a great challenge because much more than vision can be involved. It is important to use a multi-level approach that does not just consider vision, but also language and increasing cognitive function and self-awareness in general. The goal is to promote as much brain development as possible,” Dr Merabet advised.
In his laboratory, the researchers are attempting to develop novel assessment tools and compensatory strategies to better understand the difficulties that the children face and to work around their challenges.
Meanwhile, Perkins has created the CVI Center with the aim of providing resources for researchers, educators, medical professionals and parents on the importance of recognising paediatric brain impairment as quickly as possible and ensuring that such children have access to the effective interventions they need, while driving research in the field. Resources are being focused on how to assess and educate children with CVI and how to identify the ways in which this disorder is fundamentally different from eye-based visual impairment.
52% of the Perkins population has CVI, but there is a huge gap around CVI knowledge and diagnosis, and the conditionis a problem approaching epidemic proportions that is largely unknown even by experts in visual impairment. This recognition has given rise to a comprehensive, digital CVI assessment system called the Perkins CVI Protocol.
This tool is designed to provide more of a “whole child” diagnostic assessment of CVI than previously available and to identify children who may not have been previously diagnosed. As part of the system, an anonymous data medical file review compiles all the basic information about each patient. This review facilitates the gathering of demographic information in the database that can be shared with researchers.
From this database, a lengthy list of questions was compiled to be used with parents. The questions cover 16 visual behaviours; the result is a functional visional assessment (customised student sketch) of the individual’s visual behaviours, what the experienced behaviours may mean, and recommendations for that patient.
The tool includes links to videos/articles about the visual behaviors as well as scientific reports, tutorials and lists of online learning courses. A letter can also be generated, to be sent to patients’ eyecare providers to include them in the assessment of possible CVI.
An education roadmap that will also form part of the CVI Protocol is currently under development and is expected to be rolled out in early 2023. It will create a series of recommendations in the form of in-depth reports that help school teams develop educational programmes for their students with CVI.
“The goal is to identify and diagnose these children earlier and follow them closely to obtain the best benefit,” Dr Merabet said. “Identifying them when they are in their teens may mean that valuable time has been lost, although it is never too late.”