THE EYE AND HOW IT WORKS

To understand macular degeneration, it is first helpful to know how the eye works.

The eye is a sphere approximately one inch across that comprises multiple layers and internal structures, each of which performs distinct functions.  These operate together as a complex machine.5  At the front of the eye is the cornea, which is responsible for letting light into the eye and bending its rays toward the retina. The retina lines the inside of the back of the eye and is responsible for converting light into signals that the brain can convert to visual images. The layer beneath the retina is the choroid, which carries the blood supply necessary to nourish the outer portion of the retina.

The retina contains a layer of light-receiving photoreceptor cells and several other layers of cells that are directly connected to the brain via the optic nerve. If you think of the eye as a camera receiving images, then the retina is the film where those images are recorded.  The macula is the small central portion of the retina that allows one to see details clearly and perform activities such as reading, driving, and recognizing faces.

There are two types of photoreceptor cells in the retina: cone cells and rod cells. Densely packed within the macula, cone cells are responsible for central vision and color perception. Rod cells predominate in the rest of the retina outside the macula and mainly function in peripheral and night vision.

To achieve vision, light rays must travel through the eye – first through the cornea, the aqueous humor, the pupil, the lens and the vitreous body – to ultimately focus on the retina.6 In the retina, the cone and rod cells convert light into electrical impulses that travel through several types of nerve cells in the retina and then to the optic nerve. The optic nerve then carries these electrical impulses from the eye to the visual cortex of the brain, where “seeing” actually occurs.

When the Eye Doesn’t Work

Visual impairment can be defined as any visual deficit that impairs everyday functioning. Some forms of this are correctable with ordinary eyeglasses, contact lenses or surgery, whereas others are not.   Visual impairment can range from mild or moderate loss of vision to severe loss, to total blindness.

Millions of people have partial or complete loss of vision in the United States, where normal vision is defined as 20/20 and legal blindness is defined as worse than or equal to 20/200 with best correction in the better eye or a visual field extent of less than 20 degrees in diameter. In many states, a visual acuity worse than 20/40 disqualifies people from obtaining a driver’s license or restricts their driving to daytime only, as do some visual field deficits.7

What Causes Visual Impairment?

Although there have been important strides over the past few decades in the treatment and prevention of eye diseases that cause visual impairment, there are still many causes of vision loss for which there are no cures. Even with the best medical treatment, many Americans live with impaired vision due to systemic conditions or specific eye conditions.

Systemic conditions that can cause visual impairment include the following:

• Diabetes
• Hypertension (high blood pressure)
• Cerebrovascular disease or stroke (brain blood vessel occlusion or hemorrhage)
• Atherosclerotic disease (cholesterol deposits in blood vessels, including those of the eye)
• HIV infection (usually due to infection of the retina with cytomegalovirus, a virus.)
• Vitamin A deficiency (most prevalent in third world countries8)
• Infections of the eye – Some eye infections, including those caused by parasites, are more common in developing countries. Infections in a pregnant woman such as toxoplasmosis (from the Toxoplasma gondii organism) can affect the fetus.
• Inflammations – Some systemic inflammatory diseases such as sarcoidosis and rheumatoid arthritis, among others, may have ocular consequences.

Specific eye conditions that commonly cause visual impairment include the following:

• Macular degeneration – The deterioration of the central part of the retina, causing problems with central vision.
• Cataracts – This condition is a loss of transparency of the lens of the eye.
• Glaucoma – A group of eye diseases characterized by progressive damage to the optic nerve connecting the eye to the brain, where increased intraocular pressure (IOP) and other factors contribute to damage.
• Eye injuries