By now you’ve probably realized that cortical visual impairment/cerebral visual impairment (CVI) involves lots of terminology. In fact, you might even hear CVI referred to as both cerebral visual impairment and cortical visual impairment. Some doctors and educators use them interchangeably. Confusing! Over time, though, the lexicon has changed, as we realize that neither term accurately describes the condition fully. Both are anatomical terms that don’t consider the important issue of the brain's connections, which is an important consideration when we discuss CVI.
So, what’s the difference between Cortical Visual Impairment and Cerebral Visual Impairment?
First, a little history. The term cortical visual impairment was coined to differentiate from a condition called cortical blindness, first described in soldiers during World War I. This condition affected adults who suffered bullet injuries to the back of their brain. Due to that specific trauma to the early visual processing parts of the brain, they became blind even though their eyes weren’t injured. Their condition was different from ocular visual impairment, which is due to a problem with the eyes.
Over time, doctors began to notice that children who had early trauma (before birth or shortly thereafter) to the same parts of the brain responsible for visual processing also had problems with their vision. However, unlike individuals that became profoundly and irreversibly blind due to trauma to the brain suffered as adults, we realized that children often had some level of visual function and more often than not, their vision improved over time. Thus, the medical and scientific fields shifted from using the term “cortical blindness” to “cortical visual impairment” to reflect this difference in the case of a brain injury occurring early in development.
“In the case of a child, you have to consider brain development as a work in progress,” says Lotfi Merabet, a clinician-scientist with Massachusetts Eye and Ear and Harvard Medical School. He uses a clock analogy: Consider brain damage in an adult like dropping a mechanical clock on the floor. It will suddenly stop telling the right time, and if we can take the clock apart, we can get an idea as to what parts of the clock were damaged. It’s not as simple when it comes to a child’s brain. It’s like a clock that’s still being built; the parts are still forming and the connections haven’t all been made yet. So dropping a clock that isn’t fully built can lead to a cascade of effects. While it too may stop telling the right time, it’s not as obvious as to why that is the case.
Remember that a child’s brain is constantly adapting and changing; that’s neuroplasticity. As such, we think that’s why a child with CVI is more likely to show an improvement in their vision over time, as the brain continues to develop and adapt to the damage. But to what extent the vision will improve and how best to promote this improvement still remain open questions. Moreover, those strategies will be unique for every child.
But what about cortical or cerebral parts of the brain? Are they the same?
Think of the brain on three levels: cortex, white matter and subcortical structures.
“You can consider the cortex as the executive or business end of the brain; it’s where high-level information gets processed and decisions are made,” Merabet says. Deep inside the brain are what are called subcortical structures. These control the brain’s most basic survival functions: things like breathing, hunger, and fight or flight responses. It’s also where sensory information from the outside world first enters the brain. Connecting the cortex to subcortical structures and other parts of the brain are the white matter connections. This is how information is shuttled back and forth throughout the brain.
Merabet likens the brain’s organization to that of a high-rise office building. The cortex is like all the offices and cubicles; it’s where most of the important decisions are made based on the information available. In contrast, think of the white matter as all the elevators and stairwells that shuttle people to different floors so they can do their work. Finally, subcortical areas are like the lobby of the building; it’s where everyone comes in and out.
Now, think about what would happen if you removed a bunch of cubicles from the building. If all goes well, the people who work next door could be sufficiently cross-trained and take on new tasks and responsibilities. We know the brain can respond in the same way after certain types of damage.
“In certain types of brain damage (such as a stroke), sometimes different parts of the brain can take on new functions. For example, if there is damage to the part of the brain responsible for speech, other areas of the cortex can be recruited to help with recovery of function. Another dramatic example is in the case of individuals who are born with profound ocular blindness. We know that the visual processing areas of the brain become recruited to process non-visual information such as touch and hearing” Merabet says. We can also consider the case when damage occurs to white matter: Going back to our building analogy, if some elevators are out, maybe people can take the stairs if they are available and still travel to other floors. Similarly, if a given pathway in the brain is damaged, there may be an alternative pathway for information to reach its intended destination. Now, if there is extensive damage to the lobby of the building, that’s much more serious, because then nobody can get in or out. Indeed, we know from current research that when damage to subcortical structures is extensive, an individual typically shows a wide range of disabilities and prognosis may be more limited.
Over time, doctors and scientists began to realize that visual impairment in CVI was likely the result of developmental damage to the brain on a much broader level, not just the cortex itself. By looking at high detailed brain images (such as MRI) of children with CVI, we realized that there was damage not only involving the cortex, but also the white matter, subcortical areas, as well as other parts of the brain. As such, the term “cerebral” has become more common as a way to reflect this broad and widespread damage implicating many areas of the brain.
Which term should I use?
Some doctors continue using the term “cortical” to reflect that the primary problem is with the child’s brain as opposed to the eyes. Note that this is very different from ocular visual impairment. Your child might have ocular impairment, too, and it’s important to understand if your child’s visual impairment is more likely related to a problem with processing of information at the level of the brain, or how the image is focused in the eye.
“Using the term cortical is convenient on one level to help differentiate between visual impairment due to the eyes, but at the same time, it can also be confusing, because as we learn more and more about the neurophysiology of CVI, we know that damage can occur throughout the brain, and the term cortex has a very specific scientific and medical meaning. This is true not only from a neuro-anatomical standpoint but also from a (re)habilitation point of view as well,” Merabet says. “You have to consider the entire brain in order to understand the extent of the damage and what that child is going through. More often than not, [CVI] will involve damage to many parts of the brain. It’s more a question of location, extent, and timing,” he says.
Reflecting this growing knowledge regarding the neurophysiological basis of CVI, roughly 20 years ago, clinicians and scientists throughout the world started to switch from using the term “cortical” to “cerebral,” as a more holistic and encompassing term to describe the broad extent of brain damage observed in CVI. In the United States, the term cortical is still commonly used.
“More often than not, we see that injury and developmental damage implicates many regions and levels of the brain. By understanding what parts of the brain are injured and their relative importance, we are more likely to better understand the nature of observed visual impairments and how to come up with appropriate developmental strategies,” Merabet explains.
Now we’re at a tipping point; Merabet notes that the medical-scientific literature is shifting more and more from using the term cortical to cerebral, too.
“From a medical and scientific standpoint, this makes sense. The term cerebral also has its limitations, since, by definition, the “cerebrum” does not include other parts of the brain that could also be potentially damaged such as the cerebellum. However, it’s more encompassing than the term “cortical,” as it reflects the more extensive damage we see in CVI. You have to consider all of the brain in order to understand why children have the visual impairments they have,” he says.
Where is my child’s injury?
With all this talk of cerebral visual impairment versus cortical visual impairment, how do you know where your child’s brain injury actually occurred? Many kids with CVI are diagnosed based solely on milestones and functionality. By analyzing your child’s developmental progress, doctors can try to determine where and when the damage occurred to the brain. Doctors can also trace birth history and try to understand when brain damage could have happened. Meanwhile, some kids will get diagnosed using imaging like MRIs and ultrasounds, to pinpoint the site of the trauma.
As we move forward, look for professionals to finalize a common standard of naming convention for CVI. Read Perkins’ statement on why we chose to use “Cortical Visual Impairment/Cerebral Visual Impairment (CVI).”
Watch clips from the 2019 CVI Symposium, during which Dr. Merabet and Dr. Hoyt discussed the name of the condition.
Melnick, M. D., Tadin, D., & Huxlin, K. R. (2016). Relearning to See in Cortical Blindness. The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry, 22(2), 199–212. https://doi.org/10.1177/1073858415621035
Fishman, R.S. Gordon Holmes. (1997). The cortical retina, and the wounds of war. Documenta Ophthalmologica 93, 9–28. https://doi.org/10.1007/BF02569044