Defining Cortical Visual Impairment
Know the Definition of Cortical Visual Impairment
How do we describe the leading cause of pediatric visual impairment in the developed world?
Cortical visual impairment (CVI), which is already the most common cause of vision loss in children in developed countries, will increase in incidence as medical technology and research continue to improve survival.1 Better information and more tools can assist the ophthalmologist in providing patients with care. Let's see how we define CVI, the brain pathways involved in vision and the medical causes of this condition.
Cortical visual impairment is defined as bilateral visual impairment due to posterior visual pathway disease. It is one component of a global neurologic process.
The basic science
The sense of sight is a complex series of processes consisting of sensation followed by integration of sensory information into higher cortical centers. Sensation is commonly measured with visual acuity, visual field studies, contrast sensitivity, stereopsis, and color vision. These processes occur in the occipital lobe cortex of the brain. Integrated visual functions occur when this sensory information is transmitted via white matter pathways to cortices in the temporal, parietal, and frontal lobes of the brain. Integrated or higher order visual functions include motion perception, visual memory, target acquisition with slow and fast eye movements, separation of figure from background based on figure novelty and background complexity, and other integrated visual functions. Cortical visual impairment refers to dysfunction of any of these visual functions due to disease posterior to the lateral geniculate body.
It is now understood that the higher order visual functions occur through two principal pathways. The dorsal stream reaches the parietal lobe and prefrontal cortex and is responsible for spatial orientation and visual target acquisition. The ventral stream reaches the temporal lobe cortex and is responsible for forming perception and visual memory.
Goodale and Milner first described the two-stream hypothesis of neural processing:
1 Information concerning recognition of visual objects travels from the occipital cortex to the inferior temporal cortex (ventral stream). Visual information concerning the location of objects and control of movements to locate objects in space travels from the occipital lobe to the parietal lobe (dorsal stream). Pathophysiology affecting the ventral pathway may cause difficulty with object and facial recognition. Dorsal stream damage may cause difficulty locating objects and visual motor planning (visual guidance of movements).
2 Processing of complex visual information requires connections between the ventral and dorsal streams. A child’s ability to learn is largely vision-based. Accommodations and interventions are necessary so that a child can gain access to educational materials. Activities of daily living are very dependent on vision and require the same interventions.
Brain changes found in premature infants may affect visual pathways resulting in visual characteristics associated with CVI. Christine Roman-Lantzy, PhD, has described characteristic behaviors in children with CVI. These characteristics include: light gazing; photophobia; poor visual attention; preferences for certain colors; visual field abnormalities; difficulty with visual complexity; problems with viewing distance; eccentric viewing; atypical visual reflex behaviors; and variability with contrast. These characteristics can improve.
Periventricular Leukomalacia (PVL)
Cerebral Vascular Accident (CVA)
Trauma, including “Shaken Baby Syndrome”
Congenital Brain Abnormalities
Intraventricular Hemorrhage (IVH)
Table courtesy of Alan Lantzy, MD
Patients may have more than one disability and associated deficits that make the diagnosis and treatment of the patient more challenging. R. Michael Siatkowski, MD, and colleagues describe that premature infants are particularly vulnerable to anterior and posterior visual pathway disease, which may make the identification of the cause of vision loss more difficult.
Classification and prognosis
Roman-Lantzy describes a series of phases of CVI. Phase I is the lowest level of function, where a patient displays rudimentary visual responses. Phase II is a higher level where visual performance is characterized by integration and improved function. Phase III is the highest level of performance where improvement reaches the point near typical visual functioning, though the impact of the characteristics of CVI never fully resolve. Patients may progress from one phase to another. The phases proposed by Roman-Lantzy provide a common language for families and the medical professional to characterize a child’s visual functioning.
CVI has in the past been called “cortical blindness.” The term cortical blindness inferred a poor prognosis. The literature shows that children with CVI can have improvement in visual function. It is essential to communicate this possibility to families of children with CVI. Prognosis for improvement or recovery of vision cannot be made from brain imaging with current technology.
A Primer on CVI
Get the Basics
1 Huo, R., Burden, S.K., Hoyt, C.S., Good, W.V. (1999). Chronic cortical visual impairment in children: Aetiology, prognosis, and associated neurological deficits. Brit J Ophthalmol 83:670-675.
Lehman, S.S. (September 5, 2013). A Primer on Cortical Visual Impairment. Review of Ophthalmology. Retrieved from https://www.reviewofophthalmology.com/article/a-primer-on-cortical-visual-impairment-42791
Legge, R.H. Pediatric Cortical Visual Impairment 101.