The eyes are truly amazing structures. They allow us to interpret light messages from the objects around us and, with the help of our brain, turn those light messages into pictures. Our eyes also allow us to see color and help us in judging the distance we are from objects. This section of our website will define common ocular terms ranging from the structure of the eye to disease processes that can affect our vision. In depth articles concerning the eye can be accessed by visiting the National Eye Institute website.
Cornea – The cornea is the outermost layer of the eye and acts by bending, or refracting, the light rays as they enter the eye. It is clear and dome-shaped. Unlike other tissues of our body, which are nourished by blood, the cornea is nourished by a clear fluid called the aqueous. The cornea must remain transparent to work correctly; any opacity will cause the vision to decrease dramatically.
Iris - The iris is the colored portion of your eye. It is a light regulating muscle that dilates, or enlarges, to let in more light in dim environments and constricts in brighter light. Interestingly, our iris can be an indicator of how much pigment we have in our body. Blue eyes have the least amount of pigmentation; green eyes have more pigment, while brown eyes have the most.
Conjunctiva – The conjunctiva is a thin transparent mucous membrane that covers the outer surface of the eye. It begins at the outer edge of the cornea and covers the sclera and the underside of the eyelids. Its main purpose is to secrete mucous and oils that help to moisten and lubricate our eyes.
Sclera – The sclera is commonly known as “the white of your eye.” It is a tough, opaque tissue that acts as the eye’s outer protective coat.
Pupil – The pupil is simply a hole in the iris, which allows light to enter the eye. The iris regulates the size of the pupil: it is small in bright lighting and large in dim lighting.
Lens – The lens, along with the cornea, bends light rays so that they focus on the retina. It helps protect our retina by absorbing harmful ultraviolet rays from the sun. After years of absorbing these ultraviolet rays the lens can become opaque and a cataract forms. Another primary function of the lens is to change shape so that we can focus on objects that are near to us.
Retina – The retina is composed of about 136 million cells that help us see. These cells are called rods and cones. Rods are “night vision” cells and allow us see objects as black and white. Cones are less numerous and allow us to see color and fine detail. The rods and cones collect the light messages that are directed to the retina by the lens and cornea.
Optic Nerve – The optic nerve is simply an extension of our brain. It serves as a cable through which the light messages collected by the retinal cells are transmitted to the brain.
Nearsightedness (myopia) – Nearsightedness, or myopia, is a condition in which near objects appear clear but distant objects are out of focus. This normally occurs when the eye is too long or the cornea is too curved. In normal vision, light that enters the eye is brought to focus on the retina. In myopia the point of focus is in front of the retina. Spectacles or contact lenses are used to redirect the point of focus to the retina for better distance vision. Myopia affects about 30 percent of the U.S. population.
Farsightedness (hyperopia) – Farsightedness, or hyperopia, is a condition in which distant objects appear clear but near objects are out of focus. This normally occurs when the eye is too short or the cornea has too little curvature. In hyperopia light that enters the eye would come to focus beyond the retina. In school age children, hyperopia is difficult to detect through a vision screening. Hyperopia may manifest itself in decreased concentration, eye fatigue, eye strain, or burning and stinging eyes.
Astigmatism – Astigmatism is a condition in which the cornea of the eye is slightly irregular in shape. Astigmatism causes objects at all distances to be blurred. The more irregular the shape of the cornea the more blurred or distorted objects will appear. Most people have some amount of astigmatism.
Presbyopia – Presbyopia is a condition in which the lens of your eye becomes less flexible causing near objects to appear out of focus. Presbyopia usually occurs in most people in their early to mid forties. It is a gradual process that occurs in everyone and requires a bifocal or progressive spectacle lens to correct. Advancements in contact lens design now allow optometrists to correct presbyopia in this manner.
Amblyopia (lazy eye) – Amblyopia, often referred to as “lazy eye”, is a condition in which vision in one of the eyes is reduced because communication between the eye and the brain have not fully developed. Amblyopia is often caused by misallignment of the eyes (strabismus), or a difference in prescription between the two eyes which has not been corrected. The visual system develops rapidly between the ages of six and nine, so early detection of amblyopia is essential. Correction of amblyopia usually involves some form of patching and is most effective when done before age seven. The stronger or better eye is patched while the weaker or amblyopic eye is forced to work. It is estimated that about 3 percent of U.S. children suffer from some form of amblyopia.
Strabismus – Strabismus or crossed eyes is a condition where one of the eyes are turned inward (esotropia) or outward (exotropia). Strabismus usually develops by 21 months of age but can occur up to age six. Strabismus can many times be corrected by improving the strength of the eye muscles. It also can be corrected with glasses, eye exercises or surgery.
Allergic Conjunctivitis – Allergic conjunctivitis is the eye’s reaction to allergy-causing substances such as pollen, dust, pet dander, and ragweed, and is usually accompanied by tearing, redness, and itching. When your eyes are exposed to something you’re allergic to, histamine is released and the blood vessels of the conjunctiva become swollen. Reddening of the eyes soon ensue accompanied by itching and tearing. Rubbing the eyes will make the situation worse. Prevention of allergic conjunctivitis is best accomplished by removing the cause, or allergen. This can be difficult since allergens are sometimes located throughout the environment. Treatment includes the use of cold compresses, artificial tears, or prescription medication (oral or eye drops) prescribed by your optometrist.
Cataract - A cataract is a cloudy or opaque area in the lens of the eye. Most cataracts are a result of the normal aging process but are sometimes found in children at birth. Some factors influencing the development of cataracts include: prolonged use of corticosteroids, smoking, diabetes, trauma, radiation exposure, and prolonged exposure to ultraviolet light emitted by the sun. Cataracts usually develop after age 60. By age 75, about 70 percent have visually significant cataracts. Prevention is best achieved by removing those factors associated with the development of cataracts. When outside, one should wear UV protective eyewear. Treatment of cataracts includes surgery and requires the removal of the lens from the eye. The surgeon replaces the cataractous lens with a lens implant known as an intraocular lens (IOL). Sometimes after the primary cataract is removed a secondary cataract can develop. This happens when the capsule, or sac, into which the IOL is implanted, becomes opaque. A laser is used to treat secondary cataracts by removing opacities in the capsule.
Diabetic Retinopathy – Diabetic retinopathy is the leading cause of blindness in working age Americans. It is the progressive deterioration of the retina caused by long-term diabetes. The disease is caused by damage to the blood vessels of the retina. Initially, the blood vessels become porous and leak fluid into the retina causing blurred vision. In later stages of the disease, new growth of blood vessels occurs within the eye. These new blood vessels are fragile and can hemorrhage causing blindness. Treatment of diabetic retinopathy is best accomplished by control of the diabetes and any associated high blood pressure. Sometimes laser surgery is required to seal the leaking blood vessels.
Glaucoma – Glaucoma is the term for several forms of eye disease that damages the optic nerve and can lead to partial vision loss or blindness. Most experts think that increased fluid pressure within the eye reduces the blood flow in the eye. The reduced blood flow damages the optic nerve that sends visual information to the brain. Glaucoma is considered a “thief of sight” because the loss of sight is gradual and often goes unnoticed due to minimal symptoms. Over three million Americans have glaucoma, but half of them don’t know it yet. Glaucoma is usually treated with medication and if detected early enough can result in no loss of vision.
Macular Degeneration – Macular degeneration is a disease that affects the macula, or central portion of the retina, leading to a decrease in visual acuity and possible loss of central vision. The macula is the part of the retina that allows the eye to see fine details in the center of the field of vision. Macular degeneration results from the breakdown of the retinal pigment epithelium (RPE). The RPE is an insulating layer of tissue between the retina and the layer of blood vessels behind the retina. The RPE regulates the nutrients that reach the retina from the blood vessels. Many nutrients in the blood are harmful to the retina and a healthy RPE prevents these harmful nutrients from reaching the retina. There is no treatment for macular degeneration, however, in recent years the use of nutrients specifically formulated for the eye have shown to be effective in reducing the rate of central vision loss caused by the disease.