BLOG: MRI helps characterize functional vision loss in stroke survivors
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Visual impairment after a stroke is common. About two-thirds of stoke survivors have visual field loss in half of their field of vision, according to Rowe and colleagues.
The assumption is that the visual field loss is the result of complete damage to the neurons in the cortical region of the brain responsible for vision, so patients are typically counseled to expect the vision loss to be permanent. This can be devastating news to those recovering from a stroke.
But our research shows that in some cases the brain is still responding to visual stimuli, even though the patient can’t consciously “see” that portion of the visual field. Using multimodal imaging, including functional magnetic resonance imaging (fMRI), my colleagues and I have been able to demonstrate a method for characterizing anatomical and functional loss in stroke survivors (Beh et al.). In three out of the four stroke survivors we tested, we found residual functional activity in parts of the brain defined as scotomatous, according to Humphrey perimetry.
Anecdotal instances of “blindsight” have previously been noted. However, the potential to use MRI technology to help analyze the extent of damage and guide rehabilitation efforts is very exciting.
Standard perimetry can identify which parts of the visual field are missing, but it does not provide a complete picture of the deficit. If the vision loss is due to true loss of brain matter, as has always been assumed, it can’t be regained. But if there is merely a “broken link” somewhere in the visual pathway, it may be possible to rehabilitate areas of the brain that are still receiving input, thereby restoring some degree of function.
Of the four individuals with homonymous visual field loss we imaged in this study, two had a hemianopia, and two had lost only one quadrant of their visual field – a quadrantanopia. The patients ranged in age from 33 to 73 years old and had varying lesion sizes, locations and amounts of residual function.
There can be quite a lot of variability in individual responses to stroke, both in terms of the damage sustained and the prognosis, so we wanted our small study population to represent some of that variety.
One stroke survivor showed quite a bit of residual function in the brain compared to the other participants. That participant was the youngest in our cohort, so it is possible that the younger brain was more plastic. However, the much older patient also had significant residual function.
One of the first things we noticed from the MRI scans was that the anatomical change in the brain did not necessarily correlate to the extent of visual field loss. One patient who had a very large lesion in the brain had only a small quadrantanopia, for example. It is clear that our understanding of stroke-related brain damage is not yet sophisticated enough to isolate which anatomic changes “knock out” the conscious vision.
We imaged multiple regions of the brain associated with vision and visual processing to determine which ones were actively responding to visual stimuli. This could help determine which locations to target for rehabilitation and which types of visual tasks or stimuli would be most useful.
For example, activity in a region of the brain that is sensitive to motion might point to a different intervention than activity in areas that are sensitive to color or faces. Multimodal MRI can inform clinicians and researchers so those specific “channels” can be used to build a personalized recovery program.
While much remains to be learned, this research should give stroke survivors some hope. Incorporating MRI – and accessible and relatively inexpensive technology – into diagnosis and prognosis might transform stroke intervention and contribute to more effective, more personalized medicine.
References:
- Beh A, et al. Front Neurosci. 2022;doi.org/10.3389/fnins.2021.737215.
- Rowe FJ, et al. PLoS One. 2019;doi:10.1371/journal.pone.0213035.
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Anthony Beh has an MSc in brain imaging and is pursuing his PhD in psychology at the University of Nottingham in the U.K. His main field of research, under the supervision of Denis Schluppeck, DPhil, and Prof. Paul McGraw, involves investigating homonymous visual field loss in stroke survivors using various brain imaging methodologies. He is also a co-founder of 100 Scientists of Malaysia, a digital platform that features stories of Malaysian scientists in various STEM fields to showcase the diversity in science and inspire science literacy in the public.
Disclaimer: The views and opinions expressed in this blog are those of the authors and do not necessarily reflect the official policy or position of the Neuro-Optometric Rehabilitation Association unless otherwise noted. This blog is for informational purposes only and is not a substitute for the professional medical advice of a physician. NORA does not recommend or endorse any specific tests, physicians, products or procedures. For more on our website and online content, click here.
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