Sunday, March 18, 2018

Half a World Away: Visual Field Cuts

a division of United Stroke Alliance
The following is from the StrokeConnection website:

I encourage you to visit their site because they have very good articles on stroke.


Strokes often affect vision and processing of visual information. The most common visual deficit is hemianopia (hem-ee-a-NO-pia) or visual field cut. Our visual field is the whole area that we see in front of us — left to right, top to bottom. Each eye has its own visual field, but the brain combines the information from both eyes so we only see the world as one visual field. Like so many processes in the brain, vision is processed on the opposite side, but it isn’t as simple as the left eye is handled by the right brain. Instead, visual stimulation from the left side of each eye is handled in the right visual cortex. Right-side stimulation of each eye is processed in the left visual cortex. The visual cortex is located in the back part of the brain (see How Vision Works below). A stroke that injures either the optic nerves running from the back of the eyeballs through the brain to the visual cortex or the visual cortex itself will cause a deficit of vision in the same area of both eyes. Thus, a stroke in the visual processing area of the right side of the brain causes a problem with the left visual field of the right eye and the left visual field of the left eye.
If the blindness involves the same half of the visual field of each eye, it is called a homonymous hemianopia. There are variations of field cuts that are much less common. For example, a field cut may involve less than half, say the upper left quadrant of both eyes. The top or bottom of the visual field might be gone or maybe a person can only see the center of the visual field (the edges are missing); or the center of the visual field is absent but the edges can be seen. Further, many of these other types of visual field defects can be caused by problems other than stroke. See our From the Eyes of the Beholder infographic for examples of how some common types of field cuts affect vision from a survivor's perspective.
It is not hard to imagine how vision loss would affect your life. “I think it’s fair to say that vision impacts your ability to be mobile, to be independent, to read, to drive and just be productive,” said neuro-ophthalmologist Adam Cohen, neurology inpatient medical director and teleneurology director at Massachusetts General Hospital. “So having vision problems, particularly severe ones like these, can impact any of those functions.”
If you can only see things in the right or left half of the visual field, why not just move your head to the left or right? Called ‘scanning,’ this is a basic lesson in the rehabilitation of a survivor’s vision, but it is a learned response. The brain of the survivor with a field cut is not receiving visual stimulation from the area of blindness so he may not be stimulated to move his head to take in what he is not seeing. And while scanning training may improve the ability to scan and read, it doesn’t actually improve the field cut.

Hemianopia differs from another stroke deficit known as one-side neglect. “In neglect, visual information is being received by the brain, at least at some level, but it is not available to the conscious state,” Cohen said. “For example, someone has a left neglect and you put a clown to the left of them, the neurons might be firing when the vision information is coming in, but the person is not aware of the clown. If you ask the person ‘What’s in front of you?’ They’ll say, ‘Nothing.’”
Of the two, neglect has a bigger impact on function — and a worse prognosis — than a field cut alone. “Someone who has an isolated field cut is more likely to be aware of their problem and thus, more able to compensate for it,” Cohen said. “Whereas a neglect patient is not aware of it because of that consciousness problem, so they’re unaware of the issue and less able to compensate.”
Returning as much function as possible is the goal of rehabilitation of field cuts, and much of that is accomplished by teaching compensation skills, like scanning. “It’s very much an occupational therapy approach,” Cohen said. “There are a host of devices and potential treatments for people with field cut to actually try to expand the visual field, to cure the root of the problem. But those are not yet widely adopted in the medical fields, like neurology, neuro-ophthalmology or rehabilitation medicine. These ‘expansion’ methods have not yet been validated [by scientific studies].”
Prisms are sometimes used to expand the visual fields. Set into a pair of glasses, the prisms bring in the light and objects from the blind side and make it available to the side with sight. “They can be helpful for some people, but I think most of us have found that patients really aren’t able to use them in a sustainable way,” Cohen said. “Most people actually find them annoying and distracting, which also applies to things like mirrors that direct the light from the blind visual field into the seeing visual field. We tend not to recommend those things.”
In the occupational therapy approach, which is delivered at vision rehabilitation clinics and some eye hospitals, the idea is to help the patient maximize existing function and find new tools and tricks to compensate for their problem. Reading provides an example: The survivor puts her finger on the first word of a line of print and then follows her finger with her eyes as she moves it along the line of type. “It’s a lot easier to direct your eyes doing that,” Cohen said. “The survivor’s function is effectively improved without healing the neurons and the neuron pathways that are the root of the problem.”
Visual rehab specialists focus on safety of the patient so in addition to giving them tools and strategies, they often evaluate their home or work environment. They assess how the survivor’s vision deficit affects their activities of daily living: How do they cook and clean and go to the bathroom?
In the recently published rehab guidelines for stroke, the committee said there is a very low level of evidence to support computer-based expansion of the visual field. Cohen agreed that the jury is still out because vision doctors have not fully verified this treatment or deciphered how it works. It may relate to brain plasticity, and the program is somehow recruiting other areas where vision is processed to take over. The problem for survivors seeking treatment is that “these therapies are not standardized, nor broadly adopted,” he said. “I don’t know that they should rule them out, but I wouldn’t dive into them, particularly when they were unproven, expensive and particularly if there were risks involved,” he said. “Approach them with skepticism.”
“The representation of vision is widespread in the brain, so visual loss after stroke is pretty common,” Cohen said. “Around a third of patients will have that problem. Of those with visual field defects, most are hemianopias . . . but we typically don’t see complete resolution of severe visual field cuts. They tend to be pretty resistant.”

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