What Is a Stroke?
A stroke is an episode of decreased blood flow to the tissues of the brain. This decreased blood flow can be caused by a blockage in a blood vessel (an ischemic stroke) or from damage to a blood vessel that causes bleeding from the blood vessel and prevents the area supplied by that vessel from receiving adequate blood flow (a hemorrhagic stroke).
- "Ischemic" or "ischemia" is from the Greek language, meaning a decrease in blood supply to a part of the body, caused by constriction or blockage of the blood vessels ("isch" = restriction/thinning; "hema" or "haema" = blood).
- "Hemorrhagic" or "hemorrhage" is also from the Greek language, meaning excessive discharge of blood from the blood vessels or, literally, "blood bursting forth" ("hema" or "haema" = blood; rhegnumai = to break forth).
What Happens During a Stroke?
During a stroke, the part of the brain that is affected receives insufficient oxygen and nutrients. This tissue is ischemic, meaning it receives insufficient blood flow to function normally. Ischemic tissue is injured but not dead, and the tissue injury from ischemia can be reversible.
If the ischemia is prolonged or severe, the involved tissue may become infarcted, meaning that some of the tissue has died. Injury from infarction is not reversible, and any damage from infarcted tissue is permanent.
Based on the function provided by the area of the brain involved in a stroke (muscle movement, speech, sense of touch, or sight), a stroke can result in loss of certain normal functions:
- If an area in the brain is infarcted, some brain tissue has died, and the effects of the stroke will be permanent.
- If an area in the brain is ischemic, meaning it is injured but not dead, there may be some recovery of function.
- If the stroke-like episode lasts less than 24 hours and the symptoms resolve, the event is called a transient ischemic attack, or TIA.
Strokes can affect vision in a number of ways, including decreased vision and double vision:
- Decreased vision is caused by damage to the fibers that transmit visual information from the eyes to the brain. This can occur in a variety of locations, since the nerve fibers that transmit vision have a long course from the eyes to the rearmost part of the brain, called the occipital lobe.
- Double vision is caused by damage to nerves responsible for moving the eyes and ensuring that both eyes are aligned (i.e. looking at the same place).
A stroke can also cause problems with the higher order processing of visual input, including neglect, agnosia, agraphia, or alexia. The term "higher-order processing" refers to the strategies required for judgment, decision-making, reasoning, problem-solving, and processing complex information:
- In persons with neglect, the visual pathways and brain are capable of seeing certain areas in space but automatically ignore them. Persons with neglect may not notice people or objects on one side of the room or may not eat food on one half of their plate because they are not aware that the food is there.
- In persons with agnosia, the visual pathways and brain are capable of seeing objects or people, but cannot recognize them).
- Presons with agraphia are unable to write, while those with alexia are unable to read.
How Does a Stroke Decrease Vision?
An understanding of the visual pathways can help clarify how a stroke can affect vision.
Retinal and optic nerve fibers on the right side of each eye see the left side of the world (the left part of the visual field), while fibers on the left side of each eye see the right side of the world (the right part of the visual field).
Similarly, superior retinal and optic nerve fibers on the upper part of each eye see the lower part of the world (the bottom part of the visual field), while the inferior retinal and optic fibers on the lower part of each eye see the upper part of the world (the top part of the visual field).
- In the right eye, the nasal fibers (shown in orange in the diagram above), which see the far right part of the visual field, cross over to the left side of the brain, while the temporal fibers (those on the side of the eye closer to the temple) which see the far left part of the visual field, do not cross over (in purple).
- In the left eye, the nasal fibers (in purple), which see the far left part of the visual field, cross over to the right side of the brain, while the temporal fibers (those on the side of the eye closer to the temple) which see the far right part of the visual field, do not cross over (in orange).
Therefore a stroke, lesion, or injury at the point where the nasal fibers from each eye cross (where the fibers make an "X", called the optic chiasm), would cause loss of the right visual field in the right eye and loss of the left visual field in the left eye.
Further back along the visual pathway (after the fibers have already crossed at the optic chiasm and made an "X"), a stroke, lesion, or injury to the right side of the brain would cause loss of the left visual field of each eye, and an injury to the left side of the brain would cause loss of the right visual field of each eye.
Depending on precisely where in the brain these strokes, lesions, or injuries occur, the effect can involve a smaller or larger area of the visual field and can affect either central (or straight-ahead) vision, peripheral (or side) vision, or both. Multiple strokes or lesions can cause multiple patterns of visual field loss.
How Are the Visual Effects of a Stroke Treated?
Any person with signs or symptoms of a stroke, including loss of visual field, new double vision, weakness of one or more muscles, difficulty speaking, difficulty walking, or other neurological symptoms, should immediately seek care through rescue services or a local emergency department. Early treatment of strokes has been shown to dramatically reduce the severity of the stroke and to improve survival rates.
In the emergency room, the patient will undergo imaging to identify the type of stroke (ischemic or hemorrhagic) and the area(s) of the brain involved, as well as blood tests and other studies to identify any risk factors for stroke.
A patient with an ischemic stroke who is identified soon enough after symptom onset, and who is safe to undergo the procedure, may receive thrombolytic therapy, in which a medication is administered that helps dissolve the blood clot that caused the stroke.
Patients may also be treated with aspirin or blood-thinners. Other medications or interventions may be administered, based on the underlying cause or risk factors identified for the stroke.
Do the Visual Effects From a Stroke Improve Over Time?
In the long term, in some cases, visual field defects from stroke may improve, although there are often permanent deficits. After a stroke, the patient should undergo formal visual field testing by an eye doctor or neurologist to identify precisely the part of the visual field that has been affected. This allows documentation of the deficit and provides a baseline for comparison, should the patient note a change or new visual field loss in the future.
While visual fields that have been lost permanently from stroke cannot be returned via any treatment, there are a variety of strategies that enable patients to adapt to their loss of visual field, and to make the most of the vision they do have:
- A primary strategy is learning to move the head and eye more deliberately and frequently, in order to better see the area of visual field that has been lost.
- People who have double vision from stroke may alleviate this symptom by covering one eye or applying opaque tape to one lens of their eyeglasses.
- Prisms can also help resolve double vision. Prisms move or redirect the direction of light entering the eye. They can be ground into the person's eyeglasses or are available as "stick-on patches" (called Fresnel prisms) that can be placed on the eyeglass lenses.
- If the double vision persists and is significant enough, strabismus surgery can reposition the muscles that are attached to the eye to straighten the eye position, improve eye movement, and treat double vision.
- A comprehensive low vision examination, along with optical and non-optical low vision devices and vision rehabilitation services and training, can be helpful for some individuals with double vision or field loss from stroke.