Unraveling the mysteries of cerebral/cortical visual impairment

Brain-based vision loss, known as cerebral/cortical visual impairment (CVI) is a leading cause of blindness and visual impairment in children in the United States and other industrialized countries. But our understanding of CVI is limited. Physicians don’t even agree on how to characterize it. Researchers at CHLA intend to change that, and help guide how this disorder is diagnosed. Melinda Chang, MD, Investigator at the Vision Center at Children’s Hospital Los Angeles, is the lead author of "Special Commentary: Cerebral/Cortical Visual Impairment Working Definition: A Report from the National Institutes of Health CVI Workshop," sponsored by the National Eye Institute (NEI) at the National Institutes of Health (NIH). The report, published in the journal Ophthalmology, proposes that CVI is a neurodevelopmental disorder characterized by deficits of visual function and functional vision caused by neurologic damage to visual pathways and processing areas in the brain.  

But first, a standard definition    

Compared to the near total permanent vision loss associated with cortical blindness in adults, which is often due to strokes, children with CVI usually retain some functional vision that may improve over time, possibly due to underlying neuroplasticity in the developing visual system. The report defines CVI based on five key elements:

•  CVI is a spectrum of visual impairments caused by an underlying brain abnormality that affects the development of visual processing pathways and is characterized by deficits in visual function and functional vision. 
• Visual dysfunction is greater than expected by any comorbid ocular conditions alone.  
• Visual dysfunction shows up as lower-order or higher-order visual deficits, or both, leading to characteristic behaviors in affected individuals.  
• Although CVI may accompany other neurodevelopmental disorders it is not primarily a disorder of language, learning, or social communication.   
• Underlying neurologic damage in the developing brain may go unrecognized or undiagnosed until later in life. 
   

“Early detection of CVI is crucial to facilitate early intervention,” says Dr. Chang. “Studies in children with ocular causes of visual impairment consistently show associations between visual impairment and learning challenges, difficulty with social relationships, and lower quality of life. The literature on children with brain-based vision impairment is beginning to show similar associations.” 

The NIH is now developing a CVI registry to collect demographic and clinical data. Future research directions listed in the paper include showing the neurophysiological basis and timing of neuroplasticity in the developing brain of individuals with CVI, standardizing methods of visual assessment, understanding how the functional deficits of individuals with CVI change throughout their lifespan, and evaluating the efficacy of proposed treatments. 

Guidance for clinicians 

Dr. Chang and Larry Yin, MD, MSPH, Chief, Division of General Pediatrics at CHLA are co-authors along with Sharon Lehman, MD, FAAP, a pediatric ophthalmologist at Nemours Children’s Health in Delaware, of the paper, “Diagnosis and Care of Children With Cerebral/Cortical Visual Impairment”, in the journal Pediatrics. 

Routine vision screening may overlook CVI. Parent/caregiver interviews and medical histories can be used to identify children at risk for CVI, who may have sustained early neurologic injury such as preterm birth and hypoxic-ischemic encephalopathy, and result in epilepsy, intellectual disability, cerebral palsy or other neurocognitive impairments.  It can be challenging to assess visual function in children with developmental delays, multiple disabilities, or who are nonverbal. Children at risk for CVI should be referred for pediatric ophthalmologic evaluation.

Behavioral characteristics associated with CVI   

The authors describe how dysfunction of visual processing pathways can result in higher-order visual deficits, including difficulties with complex motion perception, and face and object recognition. Children with CVI may: 

• Experience significant day-to-day variability in visual function  
• Require increased time to respond to a visual stimulus (latency) or may show light gazing (preference for looking at lights), aversion to light 
• Require motion for visual response     
• Turn their head away while reaching for objects  
• Have more difficulty in identifying objects in a complex visual environment, like a particular toy in the toy box or an item of clothing in a pile of clothes 
• Have impaired recognition of specific objects or familiar faces 

While standard neuroimaging such as structural magnetic resonance imaging, electrophysiologic testing, and electroretinography cannot consistently diagnose or predict CVI, the authors suggest that functional MRI and tractography (diffusion tensor imaging) techniques show promise. More accurate measures of visual acuity and visual function in children with CVI are being investigated for diagnosis, including preferential looking tests, sweep visual evoked potentials, functional vision assessments, neuropsychological tests of visual perception, and eye tracking strategies.

“Learning the characteristics and risk factors can enable physicians to identify children with CVI and coordinate effective evaluation, diagnosis, and referrals for vision services,” says Dr. Chang. “While early intervention is ideal, the optimal timing and type of intervention have not been established. Through our work, we hope to determine the best treatments for these children in the future.”