Encyclopedia - Retina

What is the retina?
How is the retina examined?
What causes retinal damage?
What are some congenital disorders of the retina?
What are some other retinal diseases and disorders?

What is the retina?

The retina is the light-sensitive membrane that covers the inside of the back of the eye. It receives the images that come through the cornea and lens. The retina contains light-sensitive nerve cells, the rods and cones, which send nerve impulses along the optic nerve to the brain by way of a network of connecting and integrating cells, some of which have very long fibers.

The rods and cones are specialized for different purposes. The rods perceive light, while the cone cells perceive both light and color. The retina has 20 times more rod cells than cone cells. Because there are fewer cone cells and because they need more light to function, it is difficult to discern colors in dim light.

At the center of the retina is an area called the fovea, which has no blood vessels. Light-sensitive cells, primarily cones, are packed tightly in the fovea, so that it has the highest visual resolution. (See Anatomy of the Eye.)

How is the retina examined?

When conducting an eye examination, the examiner uses the ophthalmoscope, a hand-held device with a bright light, to examine the retina. The eye care professional looks for such abnormal conditions as damage to the tiny blood vessels that can be caused by high blood pressure or diabetes, detachment of the retina from the back of the eyeball, or blank spots on surface of the eyeball.

What causes retinal damage?

Heavy drinking and cigarette smoking, along with poor nutrition, can also lead to retinal damage, as can vitamin deficiency and lead poisoning.

What are some congenital disorders of the retina?

Color vision deficiency
Some disorders that affect the retina are inherited. Perhaps the best-known disorder is color vision deficiency. True color blindness, the inability to detect any color so that the world is seen in black and white, is rare.

Color vision deficiency is caused by defect in or a reduced number of cone cells. Most cases are inherited, but some result from injury or certain diseases of the retina or optic nerve. The two common types of hereditary deficiency are a reduced ability to discriminate light wavelengths in the middle (green) and long (red) segments of the light spectrum. A blue deficiency is much less common.

The deficiency is gender-based, so that it is 8 times more common in men of white European origin than in women. About 8 percent of men and 1 percent of women of that ethnic background have either green or red deficiency. The incidence is lower in Asians and Native Americans and lowest of all in the black population.

Color vision is tested with color plates. Most persons with color vision deficiency can function well in everyday life, but the condition can be dangerous for airline pilots or electricians, who work with color-coded wires.

Retinitis pigmentosa
This is a gradual degeneration of the rods and cones that can begin as early as adolescence but most commonly occurs in middle age.

Tay-Sachs disease
Tay-Sachs affects not only the eye but also the brain, causing early death.

What are some other retinal diseases and disorders?

Age-related macular degeneration
An age-related retinal disorder is macular degeneration, gradual deterioration of the innermost part of the retina, the macula. There are two kinds of macular generation, wet and dry. In both kinds, there is a gradual breakdown of cells of the macula and the layer of cells below it, the retinal pigment epithelium.

The "dry" form is most common, accounting for about 90 percent of cases. Progressive breakdown of the cells causes a blind spot in the center of the eye. The "wet" form is caused by an overgrowth of blood cells in the area of the macula. Wet macular degeneration can be treated to some extent, but the dry form is currently untreatable.

Retinopathy
Retinopathy is a term used to describe a variety of disorders of the retina.

One form is diabetic retinopathy, damage to the retina and the blood vessels that serve it when diabetes is not well controlled. The blood vessels can leak or burst, new vessels can grow on the surface of the retina, and there can be abnormal growth of fibrous tissue. Diabetic retinopathy is a major cause of loss of vision.

Hypertensive retinopathy results from high blood pressure, which causes the retinal blood vessels to become abnormally narrow.

Atherosclerosis of the retinal artery, the same kind of blood vessel blockage that leads to heart attack and stroke, can cause severe damage to the retina.

Retinal vein occlusion is a common cause of blindness. It happens when the central vein or artery of the retina is blocked.

Retinal tear and detachment
Retinal tear is a split in the retina that usually is caused by gradual degeneration. It is common in persons with severe nearsightedness, who have thinner-than-normal retinas. It can also be caused by a severe eye injury. A retinal tear often is followed by retinal detachment, in which the vitreous fluid of the eye collects between the delicate nerve membrane and the underlying layer of pigment.

Retinoblastoma
Retinoblastoma is a cancer that occurs early in childhood and has a genetic basis. It occurs in about one of every 20,000 births. It can be treated by radiation but most often requires surgical removal of the affected eye. If both eyes are affected, one may be removed, and the other may receive radiation therapy.

Retinal infections
The retina is also subject to infection.

One common infection is toxoplasmosis, caused by a parasite that is found in raw beef and cat feces. The infection often occurs before birth, and it causes progressive retinal damage over the years.

Other infections include toxocara canis, in which worm larvae lodge in the retina, causing severe damage, and onchocerciasis, another worm infestation.

Bacterial or viral infections elsewhere in the body may also be carried to the retina. They pose a special danger for persons with impaired immune systems.