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 Encyclopedia - Perimetry

When is a perimetry test administered?
How is a perimetry test conducted?

Perimetry is a visual field test of the eye that checks for problems of peripheral (side) vision. Peripheral vision is used primarily for detecting objects and for directing central vision so it is possible to see those objects in detail. A loss of peripheral vision results in a condition called tunnel vision and can lead to legal blindness. Severe glaucoma causes the loss of peripheral vision.

When is a perimetry test administered?

The cells that make up the retina are responsible for the ability to see detail, brightness, and color. There are two types of photoreceptor cells in the cornea -- rods and cones. The rods specialize in work at low light levels, and the cones provide sharp vision, color, and contrast discrimination. People with achromatopsia have defective cone cells and must rely on their rod photoreceptors for vision.

In normal eyes, there are about 6 million cone photoreceptors, located mainly in the macula at the center of the retina (See Anatomy of the Eye). These cells are primarily responsible for sharp, straight-ahead vision and also for the ability to distinguish colors. There are 100 million rod receptors, located mostly at the periphery of the retina. The rods are more sensitive to light than cones are, but rods are not able to differentiate among colors, nor can they perceive shades of gray, black, and white.

There are different variations in the severity of symptoms among individuals with achromatopsia. The rarest and most severe is called complete rod monochromatism, where there is a total lack of cone function. People with this disorder are extremely sensitive to light, even in normally lit rooms. They also have symptoms of poor visual acuity and nystagmus, which is involuntary movement of the eyes. Other less severe variations of the disorder are known as incomplete rod monochromatism and blue cone monochromatism. The type depends on which cones are affected.

How is a perimetry test conducted?

A perimetry test is easy and comfortable. Sometimes a doctor administers the test, but usually a trained technician administers it. To take the test, you sit with your head in a chin rest at the edge of a large, bowl-shaped instrument with a fixed spot in the center -- usually a yellow or green light.

There are two types of perimetry tests. In the Goldman kinetic perimeter test, you stare directly at the spot as the technician moves objects or lights of different size and brightness from the side. When you see the object or light, you push a button.

The threshold static automated perimetry, which is the other type of perimeter test, uses stationary objects of light that blink on and off in various parts of the visual field.

With either method, each eye is tested independently. The maps of visual sensitivity, made by either of these methods, are very important in diagnosing diseases of the visual system.

The perimetry test usually takes no more than 30 minutes, and the results are available immediately.

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