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Eye Anatomy

The eye is a complex organ that works much like a camera, focusing light rays and forming an image. On the surface of the eye is the cornea, a thin, spherical layer of tissue that provides a clear window for light to pass through. In a healthy eye, the cornea bends or refracts light rays so they focus precisely on the retina in the back of the eye.

Beneath the cornea is the iris, the colored part of the eye we refer to when we say a person has brown or blue eyes. In the center of the iris is the pupil. The iris functions like a shutter, adjusting pupil size to control the amount of light entering the eye.

Located behind the iris is the lens, which works together with the cornea and vitreous to focus light. Like the lens in a camera, it adjusts light rays as vision shifts between nearby and distant objects in a process called accommodation.

Light then passes through the vitreous, the gelatinous substance that fills most of the eye and gives it its shape.

The back of the eye is lined with a thin layer of tissue containing millions of photoreceptor (light-sensitive) cells. This is the retina, where light rays focus into an upside-down image. In the center of the retina is the macula. Less than 1/4 of an inch in diameter, the macula is responsible for clear central vision. The retina converts the image into an electrical signal that travels down the optic nerve to the brain.

Vision Conditions

Myopia (nearsightedness) is a vision condition affecting nearly a third of people in the United States. A myopic eye focuses properly on nearby objects, but distant objects appear blurry. This imbalance typically occurs because the cornea either has an oblong shape or is excessively curved, so that only some of the light entering the eye focuses on the retina. The most common symptom is difficulty seeing objects in the distance, for example a chalkboard or television screen. The condition usually develops before the age of 20.

Hyperopia (farsightedness) occurs when the cornea is too short or has too little curvature, causing nearby objects to appear blurry while objects in the distance are clear. Common signs of hyperopia include difficulty maintaining a clear focus on nearby objects, and eyestrain or headache after close work.

Presbyopia is a natural change in our eyes' ability to focus. It occurs when the soft crystalline lens of the eye starts to harden. This loss of flexibility affects the lens' ability to focus light in the eye, causing nearby objects to look blurry. Presbyopia happens to everyone starting in about our 40s or 50s -- even in patients who have had laser vision correction. The effects of presbyopia can be corrected with glasses or contact lenses, including bifocals and multifocals; multifocal lens implantation, including Crystalens®, ReZoom® and ReSTOR®; conventional surgery; and monovision LASIK. Laser surgeries such as conventional LASIK, PRK and LASEK cannot correct presbyopia because they reshape the cornea rather than treat the lens.

Astigmatism is the term for when the cornea is oblong rather than spherical in shape. This irregular curvature prevents light from focusing properly on the retina. As a result, objects that are close or at a distance may appear blurry or doubled. Astigmatism can cause headaches, eyestrain and fatigue to blurred or distorted vision. Most people have some degree of astigmatism, which often occurs in combination with myopia or hyperopia. Regular astigmatism is found in 30-40% of people who wear glasses.

Treatment

Eye exams test for myopia, hyperopia and astigmatism and help your optometrist provide a proper prescription if eyeglasses or contact lenses are needed. Eyewear may be used for certain activities, like watching television or driving, or may be worn at all times.

Alternatively, corneal modification procedures such as refractive surgery (LASIK, PRK, CK, etc.) and orthokeratology or ortho-K (a non-invasive procedure that re-shapes the cornea using rigid contact lenses) may improve vision quality. The choice to undergo surgical correction depends on the patient's life and the severity of the condition.

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