What Is a Stroke?

A stroke is an episode of decreased blood flow to the brain’s tissues. This decreased blood flow can be caused by a blockage in a blood vessel (an ischemic stroke) or 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?

The part of the brain affected by a stroke 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 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 several 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 nerve damage 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 can see 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 can see objects or people but cannot recognize them.
  • Persons with agraphia are unable to write, while those with alexia cannot read.

Master Sgt. Jeffrey Mittman, Wounded by a Roadside Bomb in Iraq

Sgt. Mittman standing at a podium

Master Sgt. Jeffrey Mittman was wounded by a roadside bomb on July 7, 2005 in Baghdad, Iraq. In that attack, along with suffering a traumatic brain injury, his left eye was destroyed, his right arm was badly damaged, and he lost his nose, his lips, and most of his teeth.

“My first concern was how I was going to recover and take care of my family,” he said, since he could no longer lead soldiers in combat. Read about Sgt. Mittman’s long and arduous journey through healing, vision rehabilitation services, and employment.

Jeff adds, “When I decide to go for something, I go! What’s the alternative? You have to be proactive. I set objectives, but the main challenge is to select options for meeting them. There’s a solution to almost everything – you must find it.”

Learn more about brain injury, veterans’ services, and rehabilitation:

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).

diagram of the visual pathways affected by stroke, courtesy of the Encyclopedia Britannica

After the optic nerve exits the eyeball from the back of each eye, it travels a short distance before the nasal fibers (the optic nerve fibers on the side of the eye closer to the nose) from each eye cross over to the opposite side (make an “X”) as they continue traveling toward the brain. Here is what happens in the eye and brain when we look at an object:

  • 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.

Simulation of vision loss from stroke, with half the visual field missing


An example of an injury to the right side of the brain, causing
loss, or unawareness, of the left visual field. Source:
Henry Ford Center for Vision Rehabilitation and Research

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 to see better the area of the visual field that has been lost.
  • People with double vision from a stroke may alleviate this symptom by applying a stick-on opaque spot to the center of one eyeglass lens. This is safer than patching the eye or covering the entire lens, as it allows you to have full peripheral vision.
  • It is necessary to learn to scan from side to side to use the remaining vision you to access the missing area. Training in scanning until it becomes natural is usually effective. For some, particularly younger individuals, prisms may be helpful as a prompt to scan to the missing side. Fresnel prisms which cover half the lens and Peli prisms which cover less of the lens are both available as a stick-ons, and, if found to be effective, can be ground into the lenses.
  • Suppose the double vision persists and is significant enough. In that case, strabismus surgery can reposition the muscles attached to the eye to straighten the eye position, improve eye movement, and treat double vision.
  • A comprehensive low vision examination, optical and non-optical common vision devices, vision rehabilitation services and training can be helpful for some individuals with double vision or field loss from a stroke.

By Mrinali Patel Gupta, M.D. updated by Lylas Mogk, M.D., 9/23