The Eyes and the Nervous System Strokes

By Michael J. Dodd, MD

Maryland Eye Associates

. http://marylandeyeassociates.com

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The Eyes and the Nervous System Strokes

The eyes are intimately involved in the human nervous system. It is estimated that the visual system occupies over 40% of the central nervous system. The visual system not only perceives the external world (in color), but aids in balance, controls the coordinated movement of the eyes and manages focusing for distance and near. Therefore injuries or damage to the central nervous system can often show signs or symptoms in the visual system. In this article we will discuss a common neurologic disorder, which may have manifestations in the visual system. The term “stroke” is medically known as a Cerebro-Vascular Accident, or CVA.

A “stroke” is used to denote one of two different disorders

1. A hemorrhage in the brain substance from a leaky blood vessel or

2. A blood clot (blockage) to a vessel in the brain.

In the first case a hemorrhage into the brain causes damage in the immediate area around the leaky vessel. If the leaky vessel is a tiny capillary, the damage is relatively small and may cause minimal symptoms or damage. If the leaky vessel is large, it can cause considerable damage in a great area, or even cause death. This type of hemorrhagic stroke is seen in elderly patients who often have high blood pressure.

The second type of stroke is from a clot, which moves through the arterial blood vessels until it comes to a narrow enough vessel to stop moving. At this point it blocks blood from passing and the brain tissue “down stream” is starved of oxygen and nutrients. If the clot is tiny and only a small area is damaged, the patient may have little or no symptoms. But if the vessel is large, the damage may be substantial.

So what is the role of the visual system in these disorders? As noted above, if the damaged area of the brain is small there may be no visual symptoms. But patients may have considerable loss of side vision if the area affected is great.

There is a part of the visual system called the optic radiations. These are nerve fibers that start in the retina and go back through the optic nerve; then half of the fibers cross to the opposite side of the brain, while half stay on the same side.

The fibers arc through an area called the parietal lobe and end up in the most posterior part of the brain the occipital lobe (where the brain perceives what the eyes see). If the nerve fibers are injured by a stroke as they pass through the parietal lobe, the patient will notice that half of their field of vision is missing. And the field loss is on the opposite side of the stroke. So a hemorrhage of the right parietal lobe will cause a left field defect of both eyes.

If the stroke is large enough, the patient will also have weakness of the extremities on the left side as well. The blood supply to the parietal lobe is carried by the middle cerebral artery; a relatively large artery. This artery is commonly involved in strokes.

Patients with loss of side vision have difficulty reading and cannot pass a driver's test. Some patients can recover some of their visual field deficit months after their stroke. The eyes can be very important in diagnosing strokes and may aid in determining the degree of recovery.